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Nathaniel Southgate Shaler
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Title: Outlines of the Earth's History
A Popular Study in Physiography
Author: Nathaniel Southgate Shaler
Release Date: June 12, 2006 [EBook #18562]
Language: English
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[Illustration: _Dunes at Ipswich Light, Massachusetts. Note the
effect of bushes in arresting the movement of the wind-blown sand._]
OUTLINES OF THE
EARTH'S HISTORY
A POPULAR STUDY
IN PHYSIOGRAPHY
BY
NATHANIEL SOUTHGATE SHALER
PROFESSOR OF GEOLOGY IN HARVARD UNIVERSITY
DEAN OF LAWRENCE SCIENTIFIC SCHOOL
ILLUSTRATED WITH INDEX
NEW YORK AND LONDON
D. APPLETON AND COMPANY
1898, 1910
PREFACE.
The object of this book is to provide the beginner in the study of the
earth's history with a general account of those actions which can be
readily understood and which will afford him clear understandings as
to the nature of the processes which have made this and other
celestial spheres. It has been the writer's purpose to select those
series of facts which serve to show the continuous operations of
energy, so that the reader might be helped to a truer conception of
the nature of this sphere than he can obtain from ordinary text-books.
In the usual method of presenting the elements of the earth's history
the facts are set forth in a manner which leads the student to
conceive that history as in a way completed. The natural prepossession
to the effect that the visible universe represents something done,
rather than something endlessly doing, is thus re-enforced, with the
result that one may fail to gain the largest and most educative
impression which physical science can afford him in the sense of the
swift and unending procession of events.
It is well known to all who are acquainted with the history of geology
that the static conception of the earth--the idea that its existing
condition is the finished product of forces no longer in action--led
to prejudices which have long retarded, and indeed still retard, the
progress of that science. This fact indicates that at the outset of a
student's work in this field he should be guarded against such
misconceptions. The only way to attain the end is by bringing to the
understanding of the beginner a clear idea of successions of events
which are caused by the forces operating in and on this sphere. Of all
the chapters of this great story, that which relates to the history of
the work done by the heat of the sun is the most interesting and
awakening. Therefore an effort has been made to present the great
successive steps by which the solar energy acts in the processes of
the air and the waters.
The interest of the beginner in geology is sure to be aroused when he
comes to see how very far the history of the earth has influenced the
fate of men. Therefore the aim has been, where possible, to show the
ways in which geological processes and results are related to
ourselves; how, in a word, this earth has been the well-appointed
nursery of our kind.
All those who are engaged in teaching elementary science learn the
need of limiting the story they have to tell to those truths which can
be easily understood by beginners. It is sometimes best, as in stating
such difficult matters as those concerning the tides, to give
explanations which are far from complete, and which, as to their mode
of presentation, would be open to criticism were it not for the fact
that any more elaborate statements would most likely be
incomprehensible to the novice, thus defeating the teacher's aim.
It will be observed that no account is here given of the geological
ages or of the successions of organic life. Chapters on these subjects
were prepared, but were omitted for the reason that they made the
story too long, and also because they carried the reader into a field
of much greater difficulty than that which is found in the physical
history of the earth.
N.S.S.
_March, 1898._
CONTENTS.
CHAPTER PAGE
I.--INTRODUCTION TO THE STUDY OF NATURE 1
II.--WAYS AND MEANS OF STUDYING NATURE 9
III.--THE STELLAR REALM 31
IV.--THE EARTH 81
V.--THE ATMOSPHERE 97
VI.--GLACIERS 207
VII.--THE WORK OF UNDERGROUND WATER 250
VIII.--THE SOIL 313
IX.--THE ROCKS AND THEIR ORDER 349
LIST OF FULL-PAGE ILLUSTRATIONS.
FACING PAGE
Dunes at Ipswich Light, Massachusetts _Frontispiece_
Seal Rocks near San Francisco, California 33
Lava stream, in Hawaiian Islands, flowing into the sea 72
Waterfall near Gadsden, Alabama 90
South shore, Martha's Vineyard, Massachusetts 121
Pocket Creek, Cape Ann, Massachusetts 163
Muir Glacier, Alaska 207
Front of Muir Glacier 240
Mount Ætna, seen from near Catania 201
Mountain gorge, Himalayas, India 330
OUTLINES OF THE EARTH'S HISTORY.
CHAPTER I.
AN INTRODUCTION TO THE STUDY OF NATURE.
The object of this book is to give the student who is about to enter
on the study of natural science some general idea as to the conditions
of the natural realm. As this field of inquiry is vast, it will be
possible only to give the merest outline of its subject-matter, noting
those features alone which are of surpassing interest, which are
demanded for a large understanding of man's place in this world, or
which pertain to his duties in life.
In entering on any field of inquiry, it is most desirable that the
student should obtain some idea as to the ways in which men have been
led to the knowledge which they possess concerning the world about
them. Therefore it will be well briefly to sketch the steps by which
natural science has come to be what it is. By so doing we shall
perceive how much we owe to the students of other generations; and by
noting the difficulties which they encountered, and how they avoided
them, we shall more easily find our own way to knowledge.
The primitive savages, who were the ancestors of all men, however
civilized they may be, were students of Nature. The remnants of these
lowly people who were left in different parts of the world show us
that man was not long in existence before he began to devise some
explanation concerning the course of events in the outer world.
Seeing the sun rise and set, the changes of the moon, the alternation
of the seasons, the incessant movement of the streams and sea, and the
other more or less orderly successions of events, our primitive
forefathers were driven to invent some explanation of them. This,
independently, and in many different times and places, they did in a
simple and natural way by supposing that the world was controlled by a
host of intelligent beings, each of which had some part in ordering
material things. Sometimes these invisible powers were believed to be
the spirits of great chieftains, who were active when on earth, and
who after death continued to exercise their power in the larger realms
of Nature. Again, and perhaps more commonly, these movements of Nature
were supposed to be due to the action of great though invisible
beasts, much like those which the savage found about him. Thus among
our North American Indians the winds are explained by the supposition
that the air is fanned by the wings of a great unseen bird, whose duty
it is to set the atmosphere into motion. That no one has ever seen the
bird doing the work, or that the task is too great for any conceivable
bird, is to the simple, uncultivated man no objection to this view. It
is long, indeed, before education brings men to the point where they
can criticise their first explanations of Nature.
As men in their advance come to see how much nobler are their own
natures than those of the lower animals, they gradually put aside the
explanation of events by the actions of beasts, and account for the
order of the world by the supposition that each and every important
detail is controlled by some immortal creature essentially like a man,
though much more powerful than those of their own kind. This stage of
understanding is perhaps best shown by the mythology of the Greeks,
where there was a great god over all, very powerful but not
omnipotent; and beneath him, in endless successions of command,
subordinate powers, each with a less range of duties and capacities
than those of higher estate, until at the bottom of the system there
were minor deities and demigods charged with the management of the
trees, the flowers, and the springs--creatures differing little from
man, except that they were immortal, and generally invisible, though
they, like all the other deities, might at their will display
themselves to the human beings over whom they watched, and whose path
in life they guided.
Among only one people do we find that the process of advance led
beyond this early and simple method of accounting for the processes of
Nature, bringing men to an understanding such as we now possess. This
great task was accomplished by the Greeks alone. About twenty-five
hundred years ago the philosophers of Greece began to perceive that
the early notion as to the guidance of the world by creatures
essentially like men could not be accepted, and must be replaced by
some other view which would more effectively account for the facts.
This end they attained by steps which can not well be related here,
but which led them to suppose separate powers behind each of the
natural series--powers having no relation to the qualities of mankind,
but ever acting to a definite end. Thus Plato, who represents most
clearly this advance in the interpretation of facts, imagined that
each particular kind of plant or animal had its shape inevitably
determined by something which he termed an idea, a shape-giving power
which existed before the object was created, and which would remain
after it had been destroyed, ever ready again to bring matter to the
particular form. From this stage of understanding it was but a short
step to the modern view of natural law. This last important advance
was made by the great philosopher Aristotle, who, though he died about
twenty-two hundred years ago, deserves to be accounted the first and
in many ways the greatest of the ancient men of science who were
informed with the modern spirit.
With Aristotle, as with all his intellectual successors, the
operations of Nature were conceived as to be accounted for by the
action of forces which we commonly designate as natural laws, of which
perhaps the most familiar and universal is that of gravitation, which
impels all bodies to move toward each other with a degree of intensity
which is measured by their weight and the distance by which they are
separated.
For many centuries students used the term law in somewhat the same way
as the more philosophical believers in polytheism spoke of their gods,
or as Plato of the ideas which he conceived to control Nature. We see
by this instance how hard it is to get rid of old ways of thinking.
Even when the new have been adopted we very often find that something
of the ancient and discarded notions cling in our phrases. The more
advanced of our modern philosophers are clear in their mind that all
we know as to the order of Nature is that, given certain conditions,
certain consequences inevitably follow.
Although the limitations which modern men of science perceive to be
put upon their labours may seem at first sight calculated to confine
our understanding within a narrow field of things which can be seen,
or in some way distinctly proved to exist, the effect of this
limitation has been to make science what it is--a realm of things
known as distinct from things which may be imagined. All the
difference between ancient science and modern consists in the fact
that in modern science inquirers demand a businesslike method in the
interpretation of Nature. Among the Greeks the philosopher who taught
explanations of any feature in the material world which interested him
was content if he could imagine some way which would account for the
facts. It is the modern custom now to term the supposition of an
explanation a _working hypothesis_, and only to give it the name of
theory after a very careful search has shown that all the facts which
can be gathered are in accordance with the view. Thus when Newton made
his great suggestion concerning the law of gravitation, which was to
the effect that all bodies attracted each other in proportion to
their masses, and inversely as the square of their distance from each
other, he did not rest content, as the old Greeks would have done,
with the probable truth of the explanation, but carefully explored the
movements of the planets and satellites of the solar system to see if
the facts accorded with the hypothesis. Even the perfect
correspondence which he found did not entirely content inquirers, and
in this century very important experiments have been made which have
served to show that a ball suspended in front of a precipice will be
attracted toward the steep, and that even a mass of lead some tons in
weight will attract toward itself a small body suspended in the manner
of a pendulum.
It is this incessant revision of the facts, in order to see if they
accord with the assumed rule or law, which has given modern science
the sound footing that it lacked in earlier days, and which has
permitted our learning to go on step by step in a safe way up the
heights to which it has climbed. All explanations of Nature begin with
the work of the imagination. In common phrase, they all are guesses
which have at first but little value, and only attain importance in
proportion as they are verified by long-continued criticism, which has
for its object to see whether the facts accord with the theory. It is
in this effort to secure proof that modern science has gathered the
enormous store of well-ascertained facts which constitutes its true
wealth, and which distinguishes it from the earlier imaginative and to
a great extent unproved views.
In the original state of learning, natural science was confounded with
political and social tradition, with the precepts of duty which
constitute the law of the people, as well as with their religion, the
whole being in the possession of the priests or wise men. So long as
natural action was supposed to be in the immediate control of numerous
gods and demigods, so long, in a word, as the explanation of Nature
was what we term polytheistic, this association of science with other
forms of learning was not only natural but inevitable. Gradually,
however, as the conception of natural law replaced the earlier idea as
to the intervention of a spirit, science departed from other forms of
lore and came to possess a field to itself. At first it was one body
of learning. The naturalists of Aristotle's time, and from his day
down to near our own, generally concerned themselves with the whole
field of Nature. For a time it was possible for any one able and
laborious man to know all which had been ascertained concerning
astronomy, chemistry, geology, as well as the facts relating to living
beings. The more, however, as observation accumulated, and the store
of facts increased, it became difficult for any one man to know the
whole. Hence it has come about that in our own time natural learning
is divided into many distinct provinces, each of which demands a
lifetime of labour from those who would know what has already been
done in the field, and what it is now important to do in the way of
new inquiries.
The large divisions which naturalists have usually made of their tasks
rest in the main on the natural partitions which we may readily
observe in the phenomenal world. First of all comes astronomy,
including the phenomena exhibited in the heavens, beyond the limits of
the earth's atmosphere. Second, geology, which takes account of all
those actions which in process of time have been developed in our own
sphere. Third, physics, which is concerned with the laws of energy, or
those conditions which affect the motion of bodies, and the changes
which are impressed upon them by the different natural forces. Fourth,
chemistry, which seeks to interpret the principles which determine the
combination of atoms and the molecules which are built of them under
the influence of the chemical affinities. Fifth, biology, or the laws
of life, a study which pertains to the forms and structures of animals
and plants, and their wonderful successions in the history of the
world. Sixth, mathematics, or the science of space and number, that
deals with the principles which underlie the order of Nature as
expressed at once in the human understanding and in the material
universe. By its use men were made able to calculate, as in
arithmetic, the problems which concern their ordinary business, as
well as to compute the movements of the celestial bodies, and a host
of actions which take place on the earth that would be inexplicable
except by the aid of this science. Last of all among the primary
sciences we may name that of psychology, which takes account of mental
operations among man and his lower kindred, the animals.
In addition to the seven sciences above mentioned, which rest in a
great measure on the natural divisions of phenomena, there are many,
indeed, indefinitely numerous, subdivisions which have been made to
suit the convenience of students. Thus astronomy is often separated
into physical and mathematical divisions, which take account either of
the physical phenomena exhibited by the heavenly bodies or of their
motions. In geology there are half a dozen divisions relating to
particular branches of that subject. In the realm of organic life, in
chemistry, and in physics there are many parts of these sciences which
have received particular names.
It must not be supposed that these sciences have the independence of
each other which their separate names would imply. In fact, the
student of each, however, far he may succeed in separating his field
from that of the other naturalists, as we may fitly term all students
of Nature, is compelled from time to time to call in the aid of his
brethren who cultivate other branches of learning. The modern
astronomer needs to know much of chemistry, or else he can not
understand many of his observations on the sun. The geologists have to
share their work with the student of animal and vegetable life, with
the physicists; they must, moreover, know something of the celestial
spheres in order to interpret the history of the earth. In fact, day
by day, with the advance of learning, we come more clearly to
perceive that all the processes of Nature are in a way related to each
other, and that in proportion as we understand any part of the great
mechanism, we are forced in a manner to comprehend the whole. In other
words, we are coming to understand that these divisions of the field
of science depend upon the limitations of our knowledge, and not upon
the order of Nature itself. For the purposes of education it is
important that every one should know something of the great truths
which each science has disclosed. No mortal man can compass the whole
realm of this knowledge, but every one can gain some idea of the
larger truths which may help him to understand the beauty and grandeur
of the sphere in which he dwells, which will enable him the better to
meet the ordinary duties of life, that in almost all cases are related
to the facts of the world about us. It has been of late the custom to
term this body of general knowledge which takes account of the more
evident facts and important series of terrestrial actions
physiography, or, as the term implies, a description of Nature, with
the understanding that the knowledge chosen for the account is that
which most intimately concerns the student who seeks information that
is at once general and important. Therefore, in this book the effort
is made first to give an account as to the ways and means which have
led to our understanding of scientific problems, the methods by which
each person may make himself an inquirer, and the outline of the
knowledge that has been gathered since men first began to observe and
criticise the revelations the universe may afford them.
CHAPTER II.
WAYS AND MEANS OF STUDYING NATURE.
It is desirable that the student of Nature keep well in mind the means
whereby he is able to perceive what goes on in the world about him. He
should understand something as to the nature of his senses, and the
extent to which these capacities enable him to discern the operations
of Nature. Man, in common with his lower kindred, is, by the mechanism
of the body, provided with five somewhat different ways by which he
may learn something of the things about him. The simplest of these
capacities is that of touch, a faculty that is common to the general
surface of the body, and which informs us when the surface is affected
by contact with some external object. It also enables us to discern
differences of temperature. Next is the sense of taste, which is
limited to the mouth and the parts about it. This sense is in a way
related to that of touch, for the reason that it depends on the
contact of our body with material things. Third is the sense of smell,
so closely related to that of taste that it is difficult to draw the
line between the two. Yet through the apparatus of the nose we can
perceive the microscopically small parts of matter borne to us through
the air, which could not be appreciated by the nerves of the mouth.
Fourth in order of scope comes the hearing, which gives us an account
of those waves of matter that we understand as sound. This power is
much more far ranging than those before noted; in some cases, as in
that of the volcanic explosions from the island of Krakatoa, in the
eruption of 1883, the convulsions were audible at the distance of
more than a thousand miles away. The greater cannon of modern days may
be heard at the distance of more than a hundred miles, so that while
the sense of touch, taste, and smell demand contact with the bodies
which we appreciate, hearing gives us information concerning objects
at a considerable distance. Last and highest of the senses, vastly the
most important in all that relates to our understanding of Nature, is
sight, or the capacity which enables us to appreciate the movement of
those very small waves of ether which constitute light. The eminent
peculiarity of sight is that it may give us information concerning
things which are inconceivably far away; it enables us to discern the
light of suns probably millions of times as remote from us as is the
centre of our own solar system.
Although much of the pleasure which the world affords us comes through
the other senses, the basis of almost all our accurate knowledge is
reported by sight. It is true that what we have observed with our eyes
may be set forth in words, and thus find its way to the understanding
through the ears; also that in many instances the sense of touch
conveys information which extends our perceptions in many important
ways; but science rests practically on sight, and on the insight that
comes from the training of the mind which the eyes make possible.
The early inquirers had no resources except those their bodies
afforded; but man is a tool-making creature, and in very early days he
began to invent instruments which helped him in inquiry. The earliest
deliberate study was of the stars. Science began with astronomy, and
the first instruments which men contrived for the purpose of
investigation were astronomical. In the beginning of this search the
stars were studied in order to measure the length of the year, and
also for the reason that they were supposed in some way to control the
fate of men. So far as we know, the first pieces of apparatus for this
purpose were invented in Egypt, perhaps about four thousand years
before the Christian era. These instruments were of a simple nature,
for the magnifying glass was not yet contrived, and so the telescope
was impossible. They consisted of arrangements of straight edges and
divided circles, so that the observers, by sighting along the
instruments, could in a rough way determine the changes in distance
between certain stars, or the height of the sun above the horizon at
the various seasons of the year. It is likely that each of the great
pyramids of Egypt was at first used as an observatory, where the
priests, who had some knowledge of astronomy, found a station for the
apparatus by which they made the observations that served as a basis
for casting the horoscope of the king.
In the progress of science and of the mechanical invention attending
its growth, a great number of inventions have been contrived which
vastly increase our vision and add inconceivably to the precision it
may attain. In fact, something like as much skill and labour has been
given to the development of those inventions which add to our learning
as to those which serve an immediate economic end. By far the greatest
of these scientific inventions are those which depend upon the lens.
By combining shaped bits of glass so as to control the direction in
which the light waves move through them, naturalists have been able to
create the telescope, which in effect may bring distant objects some
thousand times nearer to view than they are to the naked eye; and the
microscope, which so enlarges minute objects as to make them visible,
as they were not before. The result has been enormously to increase
our power of vision when applied to distant or to small objects. In
fact, for purposes of learning, it is safe to say that those tools
have altogether changed man's relation to the visible universe. The
naked eye can see at best in the part of the heavens visible from any
one point not more than thirty thousand stars. With the telescope
somewhere near a hundred million are brought within the limits of
vision. Without the help of the microscope an object a thousandth of
an inch in diameter appears as a mere point, the existence of which we
can determine only under favourable circumstances. With that
instrument the object may reveal an extended and complicated structure
which it may require a vast labour for the observer fully to explore.
Next in importance to the aid of vision above noted come the
scientific tools which are used in weighing and measuring. These
balances and gauges have attained such precision that intervals so
small as to be quite invisible, and weights as slight as a
ten-thousandth of a grain, can be accurately measured. From these
instruments have come all those precise examinations on which the
accuracy of modern science intimately depends. All these instruments
of precision are the inventions of modern days. The simplest
telescopes were made only about two hundred and fifty years ago, and
the earlier compound microscopes at a yet later date. Accurate
balances and other forms of gauges of space, as well as good means of
dividing time, such as our accurate astronomical clocks and
chronometers, are only about a century old. The instruments have made
science accurate, and have immensely extended its powers in nearly all
the fields of inquiry.
Although the most striking modern discoveries are in the field which
was opened to us by the lens in its manifold applications, it is in
the chemist's laboratory that we find that branch of science, long
cultivated, but rapidly advanced only within the last two centuries,
which has done the most for the needs of man. The ancients guessed
that the substances which make up the visible world were more
complicated in their organization than they appear to our vision. They
even suggested the great truth that matter of all kinds is made up of
inconceivably small indivisible bits which they and we term atoms. It
is likely that in the classic days of Greece men began to make simple
experiments of a chemical nature. A century or two after the time of
Mohammed, the Arabians of his faith, a people who had acquired Greek
science from the libraries which their conquests gave them, conducted
extensive experiments, and named a good many familiar chemical
products, such as alcohol, which still bears its Arabic name.
These chemical studies were continued in Europe by the alchemists, a
name also of Arabic origin, a set of inquirers who were to a great
extent drawn away from scientific studies by vain though unending
efforts to change the baser metals into gold and silver, as well as to
find a compound which would make men immortal in the body. By the
invention of the accurate balance, and by patient weighing of the
matters which they submitted to experiment, by the invention of
hypotheses or guesses at truth, which were carefully tested by
experiment, the majestic science of modern chemistry has come forth
from the confused and mystical studies of the alchemists. We have
learned to know that there are seventy or more primitive or apparently
unchangeable elements which make up the mass of this world, and
probably constitute all the celestial spheres, and that these elements
in the form of their separate atoms may group themselves in almost
inconceivably varied combinations. In the inanimate realm these
associations, composed of the atoms of the different substances,
forming what are termed molecules, are generally composed of but few
units. Thus carbonic-acid gas, as it is commonly called, is made up of
an aggregation of molecules, each composed of one atom of carbon and
two of oxygen; water, of two atoms of hydrogen and one of oxygen;
ordinary iron oxide, of two atoms of iron and three of oxygen. In the
realm of organic life, however, these combinations become vastly more
complicated, and with each of them the properties of the substance
thus produced differ from all others. A distinguished chemist has
estimated that in one group of chemical compounds, that of carbon, it
would be possible to make such an array of substances that it would
require a library of many thousand ordinary volumes to contain their
names alone.
It is characteristic of chemical science that it takes account of
actions which are almost entirely invisible. No contrivances have been
or are likely to be invented which will show the observer what takes
place when the atoms of any substance depart from their previous
combination and enter on new arrangements. We only know that under
certain conditions the old atomic associations break up, and new ones
are formed. But though the processes are hidden, the results are
manifest in the changes which are brought about upon the masses of
material which are subjected to the altering conditions. Gradually the
chemists of our day are learning to build up in their laboratories
more and more complicated compounds; already they have succeeded in
producing many of the materials which of old could only be obtained by
extracting them from plants. Thus a number of the perfumes of flowers,
and many of the dye-stuffs which a century ago were extracted from
vegetables, and were then supposed to be only obtainable in that way,
are now readily manufactured. In time it seems likely that important
articles of food, for which we now depend upon the seeds of plants,
may be directly built up from the mineral kingdom. Thus the result of
chemical inquiry has been not only to show us much of the vast realm
of actions which go on in the earth, but to give us control of many of
these movements so that we may turn them to the needs of man.
Animals and plants were at an early day very naturally the subjects of
inquiry. The ancients perceived that there were differences of kind
among these creatures, and even in Aristotle's time the sciences of
zoölogy and botany had attained the point where there were
considerable treatises on those subjects. It was not, however, until a
little more than a century ago that men began accurately to describe
and classify these species of the organic world. Since the time of
Linnæus the growth of our knowledge has gone forward with amazing
swiftness. Within a century we have come to know perhaps a hundred
times as much concerning these creatures as was learned in all the
earlier ages. This knowledge is divisible into two main branches: in
one the inquirers have taken account of the different species, genera,
families, orders, and classes of living forms with such effect that
they have shown the existence at the present time of many hundred
thousand distinct species, the vast assemblage being arranged in a
classification which shows something as to the relationship which the
forms bear to each other, and furthermore that the kinds now living
have not been long in existence, but that at each stage in the history
of the earth another assemblage of species peopled the waters and the
lands.
At first naturalists concerned themselves only with the external forms
of living creatures; but they soon came to perceive that the way in
which these organisms worked, their physiology, in a word, afforded
matters for extended inquiry. These researches have developed the
science of physiology, or the laws of bodily action, on many accounts
the most modern and extensive of our new acquisitions of natural
learning. Through these studies we have come to know something of the
laws or principles by which life is handed on from generation to
generation, and by which the gradations of structure have been
advanced from the simple creatures which appear like bits of animated
jelly to the body and mind of man.
The greatest contribution which modern naturalists have made to
knowledge concerns the origin of organic species. The students of a
century ago believed that all these different kinds had been suddenly
created either through natural law or by the immediate will of God. We
now know that from the beginning of organic life in the remote past to
the present day one kind of animal or plant has been in a natural and
essentially gradual way converted into the species which was to be its
successor, so that all the vast and complicated assemblage of kinds
which now exists has been derived by a process of change from the
forms which in earlier ages dwelt upon this planet. The exact manner
in which these alterations were produced is not yet determined, but in
large part it has evidently been brought about by the method indicated
by Mr. Darwin, through the survival of the fittest individuals in the
struggle for existence.
Until men came to have a clear conception as to the spherical form of
the earth, it was impossible for them to begin any intelligent
inquiries concerning its structure or history. The Greeks knew the
earth to be a sphere, but this knowledge was lost among the early
Christian people, and it was not until about four hundred years ago
that men again came to see that they dwelt upon a globe. On the basis
of this understanding the science of geology, which had in a way been
founded by the Greeks, was revived. As this science depends upon the
knowledge which we have gained of astronomy, physics, chemistry, and
biology, all of which branches of learning have to be used in
explaining the history of the earth, the advance which has been made
has been relatively slow. Geology as a whole is the least perfectly
organized of all the divisions of learning. A special difficulty
peculiar to this science has also served to hinder its development.
All the other branches of learning deal mainly, if not altogether,
with the conditions of Nature as they now exist. In this alone is it
necessary at every step to take account of actions which have been
performed in the remote past.
It is an easy matter for the students of to-day to imagine that the
earth has long endured; but to our forefathers, who were educated in
the view that it had been brought from nothingness into existence
about seven thousand years ago, it was most difficult and for a time
impossible to believe in its real antiquity. Endeavouring, as they
naturally did, to account for all the wonderful revolutions, the
history of which is written in the pages of the great stone book, the
early geologists supposed this planet to have been the seat of
frequent and violent changes, each of which revolutionized its shape
and destroyed its living tenants. It was only very gradually that
they became convinced that a hundred million years or more have
elapsed since the dawn of life on the earth, and that in this vast
period the march of events has been steadfast, the changes taking
place at about the same rate in which they are now going on. As yet
this conception as to the history of our sphere has not become the
general property of the people, but the fact of it is recognised by
all those who have attentively studied the matter. It is now as well
ascertained as any of the other truths which science has disclosed to
us.
It is instructive to note the historic outlines of scientific
development. The most conspicuous truth which this history discloses
is that all science has had its origin and almost all its development
among the peoples belonging to the Aryan race. This body of folk
appears to have taken on its race characteristics, acquired its
original language, its modes of action, and the foundations of its
religion in that part of northern Europe which is about the Baltic
Sea. Thence the body of this people appear to have wandered toward
central Asia, where after ages of pastoral life in the high table
lands and mountains of their country it sent forth branches to India,
Asia Minor and Greece, to Persia, and to western Europe. It seems ever
to have been a characteristic of these Aryan peoples that they had an
extreme love for Nature; moreover, they clearly perceived the need of
accounting for the things that happened in the world about them. In
general they inclined to what is called the pantheistic explanation of
the universe. They believed a supreme God in many different forms to
be embodied in all the things they saw. Even their own minds and
bodies they conceived as manifestations of this supreme power. Among
the Aryans who came to dwell in Europe and along the eastern
Mediterranean this method of explaining Nature was in time changed to
one in which humanlike gods were supposed to control the visible and
invisible worlds. In that marvellous centre of culture which was
developed among the Greeks this conception of humanlike deities was in
time replaced by that of natural law, and in their best days the
Greeks were men of science essentially like those of to-day, except
that they had not learned by experience how important it was to
criticise their theories by patiently comparing them with the facts
which they sought to explain. The last of the important Greek men of
science, Strabo, who was alive when Christ was born, has left us
writings which in quality are essentially like many of the able works
of to-day. But for the interruption in the development of Greek
learning, natural science would probably have been fifteen hundred
years ahead of its present stage. This interruption came in two ways.
In one, through the conquest of Greece and the destruction of its
intellectual life by the Romans, a people who were singularly
incapable of appreciating natural science, and who had no other
interest in it except now and then a vacant and unprofitable curiosity
as to the processes of the natural world. A second destructive
influence came through the fact that Christianity, in its energetic
protest against the sins of the pagan civilization, absolutely
neglected and in a way despised all forms of science.
The early indifference of Christians to natural learning is partly to
be explained by the fact that their religion was developed among the
Hebrews, a people remarkable for their lack of interest in the
scientific aspects of Nature. To them it was a sufficient explanation
that one omnipotent God ruled all things at his will, the heavens and
the earth alike being held in the hollow of his hand.
Finding the centre of its development among the Romans, Christianity
came mainly into the control of a people who, as we have before
remarked, had no scientific interest in the natural world. This
condition prolonged the separation of our faith from science for
fifteen hundred years after its beginning. In this time the records of
Greek scientific learning mostly disappeared. The writings of
Aristotle were preserved in part for the reason that the Church
adopted many of his views concerning questions in moral philosophy and
in politics. The rest of Greek learning was, so far as Europe was
concerned, quite neglected.
A large part of Greek science which has come down to us owes its
preservation to a very singular incident in the history of learning.
In the ninth century, after the Arabs had been converted to
Mohammedanism, and on the basis of that faith had swiftly organized a
great and cultivated empire, the scholars of that folk became deeply
interested in the remnants of Greek learning which had survived in the
monastic and other libraries about the eastern Mediterranean. So
greatly did they prize these records, which were contemned by the
Christians, that it was their frequent custom to weigh the old
manuscripts in payment against the coin of their realm. In astronomy,
mathematics, chemistry, and geology the Arabian students, building on
the ancient foundations, made notable and for a time most important
advances. In the tenth century of our era they seemed fairly in the
way to do for science what western Europe began five centuries later
to accomplish. In the fourteenth century the centre of Mohammedan
strength was transferred from the Arabians to the Turks, from a people
naturally given to learning to a folk of another race, who despised
all such culture. Thenceforth in place of the men who had treasured
and deciphered with infinite pains all the records of earlier
learning, the followers of Mohammed zealously destroyed all the
records of the olden days. Some of these records, however, survived
among the Arabs of Spain, and others were preserved by the Christian
scholars who dwelt in Byzantium, or Constantinople, and were brought
into western Europe when that city was captured by the Turks in the
fifteenth century.
Already the advance of the fine arts in Italy and the general tendency
toward the study of Nature, such as painting and sculpture indicate,
had made a beginning, or rather a proper field for a beginning, of
scientific inquiry. The result was a new interest in Greek learning in
all its branches, and a very rapid awakening of the scientific spirit.
At first the Roman Church made no opposition to this new interest
which developed among its followers, but in the course of a few years,
animated with the fear that science would lead men to doubt many of
the dogmas of the Church, it undertook sternly to repress the work of
all inquirers.
The conflict between those of the Roman faith and the men of science
continued for above two hundred years. In general, the part which the
Church took was one of remonstrance, but in a few cases the spirit of
fanaticism led to the persecution of the men who did not obey its
mandates and disavow all belief in the new opinions which were deemed
contrary to the teachings of Scripture. The last instance of such
oppression occurred in France in the year 1756, when the great Buffon
was required to recant certain opinions concerning the antiquity of
the earth which he had published in his work on Natural History. This
he promptly did, and in almost servile language withdrew all the
opinions to which the fathers had objected. A like conflict between
the followers of science and the clerical authorities occurred in
Protestant countries. Although in no case were the men of science
physically tortured or executed for their opinions, they were
nevertheless subjected to great religious and social pressure: they
were almost as effectively disciplined as were those who fell under
the ban of the Roman Church.
Some historians have criticised the action of the clerical authorities
toward science as if the evil which was done had been performed in our
own day. It should be remembered, however, that in the earlier
centuries the churches regarded themselves as bound to protect all men
from the dangers of heresy. For centuries in the early history of
Christianity the defenders of the faith had been engaged in a
life-and-death struggle with paganism, the followers of which held all
that was known of Nature. Quite naturally the priestly class feared
that the revival of scientific inquiry would bring with it the evils
from which the world had suffered in pagan times. There is no doubt
that these persecutions of science were done under what seemed the
obligations of duty. They may properly be explained particularly by
men of science as one of the symptoms of development in the day in
which they were done. It is well for those who harshly criticise the
relations of the Church to science to remember that in our own
country, about two centuries ago, among the most enlightened and
religious people of the time, Quakers were grievously persecuted, and
witches hanged, all in the most dutiful and God-fearing way. In
considering these relations of science to our faith, the matter should
be dealt with in a philosophical way, and with a sense of the
differences between our own and earlier ages.
To the student of the relations between Christianity and science it
must appear doubtful whether the criticism or the other consequences
which the men of science had to meet from the Church was harmful to
their work. The early naturalists, like the Greeks whom they followed,
were greatly given to speculations concerning the processes of Nature,
which, though interesting, were unprofitable. They also showed a
curious tendency to mingle their scientific speculations with ancient
and base superstitions. They were often given to the absurdity
commonly known as the "black art," or witchcraft, and held to the
preposterous notions of the astrologists. Even the immortal astronomer
Kepler, who lived in the sixteenth century, was a professional
astrologer, and still held to the notion that the stars determined the
destiny of men. Many other of the famous inquirers in those years
which ushered in modern science believed in witchcraft. Thus for a
time natural learning was in a way associated with ancient and
pernicious beliefs which the Church was seeking to overthrow. One
result of the clerical opposition to the advancement of science was
that its votaries were driven to prove every step which led to their
conclusions. They were forced to abandon the loose speculation of
their intellectual guides, the Greeks, and to betake themselves to
observation. Thus a part of the laborious fact-gathering habit on
which the modern advance of science has absolutely depended was due to
the care which men had to exercise in face of the religious
authorities.
In our own time, in the latter part of the nineteenth century, the
conflict between the religious authority and the men of science has
practically ceased. Even the Roman Church permits almost everywhere an
untrammelled teaching of the established learning to which it was at
one time opposed. Men have come to see that all truth is accordant,
and that religion has nothing to fear from the faithful and devoted
study of Nature.
The advance of science in general in modern times has been greatly due
to the development of mechanical inventions. Among the ancients, the
tools which served in the arts were few in number, and these of
exceeding simplicity. So far as we can ascertain, in the five hundred
years during which the Greeks were in their intellectual vigour, not
more than half a dozen new machines were invented, and these were
exceedingly simple. The fact seems to be that a talent for mechanical
invention is mainly limited to the peoples of France, Germany, and of
the English-speaking folk. The first advances in these contrivances
were made in those countries, and all our considerable gains have come
from their people. Thus, while the spirit of science in general is
clearly limited to the Aryan folk, that particular part of the motive
which leads to the invention of tools is restricted to western and
northern Europe, to the people to whom we give the name of Teutonic.
Mechanical inventions have aided the development of our sciences in
several ways. They have furnished inquirers with instruments of
precision; they have helped to develop accuracy of observation; best
of all, they have served ever to bring before the attention of men a
spectacle of the conditions in Nature which we term cause and effect.
The influence of these inventions on the development of learning has
been particularly great where the machines, such as our wind and water
mills, and our steam engine, make use of the forces of Nature,
subjugating them to the needs of man. Such instruments give an
unending illustration as to the presence in Nature of energy. They
have helped men to understand that the machinery of the universe is
propelled by the unending application of power. It was, in fact,
through such machines that men of science first came to understand
that energy, manifested in the natural forces, is something that
eternally endures; that we may change its form in our arts as its form
is changed in the operations of Nature, but the power endures forever.
It is interesting to note that the first observation which led to this
most important scientific conclusion that energy is indestructible
however much it may change its form, was made by an American, Benjamin
Thompson, who left this country at the time of the Revolution, and
after a curious life became the executive officer, and in effect king,
of Bavaria. While engaged in superintending the manufacture of cannon,
he observed that in boring out the barrel of the gun an amount of heat
was produced which evaporated a certain amount of water. He therefore
concluded that the energy required to do the boring of the metal
passed into the state of heat, and thus only changed its state, in no
wise disappearing from the earth. Other students pursuing the same
line of inquiry have clearly demonstrated what is called the law of
the conservation of energy, which more than anything has helped us to
understand the large operations of Nature. Through these studies we
have come to see that, while the universe is a place of ceaseless
change, the quantities of energy and of matter remain unaltered.
The foregoing brief sketch, which sets forth some of the important
conditions which have affected the development of science, may in a
way serve to show the student how he can himself become an interpreter
of Nature. The evidence indicates that the people of our race have
been in a way chosen among all the varieties of mankind to lead in
this great task of comprehending the visible universe. The facts,
moreover, show that discovery usually begins with the interest which
men feel in the world immediately about them, or which is presented to
their senses in a daily spectacle. Thus Benjamin Franklin, in the
midst of a busy life, became deeply interested in the phenomena of
lightning, and by a very simple experiment proved that this wonder of
the air was due to electrical action such as we may arouse by rubbing
a stick of sealing-wax or a piece of amber with a cloth. All
discoveries, in a word, have had their necessary beginnings in an
interest in the facts which daily experience discloses. This desire to
know something more than the first sight exhibits concerning the
actions in the world about us is native in every human soul--at least,
in all those who are born with the heritage of our race. It is
commonly strong in childhood; if cultivated by use, it will grow
throughout a lifetime, and, like other faculties, becomes the stronger
and more effective by the exertions which it inspires. It is therefore
most important that every one should obey this instinctive command to
inquiry, and organize his life and work so that he may not lose but
gain more and more as time goes on of this noble capacity to
interrogate and understand the world about him.
It is best that all study of Nature should begin not in laboratories,
nor with the things which are remote from us, but in the field of
Nature which is immediately about us. The student, even if he dwell in
the unfavourable conditions of a great city, is surrounded by the
world which has yielded immeasurable riches in the way of learning,
which he can appropriate by a little study. He can readily come to
know something of the movements of the air; the buildings will give
him access to a great many different kinds of stone; the smallest
park, a little garden, or even a few potted plants and captive
animals, may tell him much concerning the forms and actions of living
beings. By studying in this way he can come to know something of the
differences between things and their relations to each other. He will
thus have a standard by which he can measure and make familiar the
body of learning concerning Nature which he may find in books. From
printed pages alone, however well they be written, he can never hope
to catch the spirit that animates the real inquirer, the true lover of
Nature.
On many accounts the most attractive way of beginning to form the
habit of the naturalist is by the study of living animals and plants.
To all of us life adds interest, and growth has a charm. Therefore it
is well for the student to start on the way of inquiry by watching the
actions of birds and insects or by rearing plants. It is fortunate if
he can do both these agreeable things. When the habit of taking an
account of that most important part of the world which is immediately
about him has been developed in the student, he may profitably proceed
to acquire the knowledge of the invisible universe which has been
gathered by the host of inquirers of his race. However far he
journeys, he should return to the home world that lies immediately and
familiarly about him, for there alone can he acquire and preserve that
personal acquaintance with things which is at once the inspiration and
the test of all knowledge.
Along with this study of the familiar objects about us the student may
well combine some reading which may serve to show him how others have
been successful in thus dealing with Nature at first hand. For this
purpose there are, unfortunately, but few works which are well
calculated to serve the needs of the beginner. Perhaps the best
naturalist book, though its form is somewhat ancient, is White's
Natural History of Selborne. Hugh Miller's works, particularly his Old
Red Sandstone and My Schools and Schoolmasters, show well how a man
may become a naturalist under difficulties. Sir John Lubbock's studies
on Wasps, and Darwin's work on Animals and Plants under Domestication
are also admirable to show how observation should be made. Dr. Asa
Gray's little treatise on How Plants Grow will also be useful to the
beginner who wishes to approach botany from its most attractive
side--that of the development of the creature from the seed to seed.
There is another kind of training which every beginner in the art of
observing Nature should obtain, and which many naturalists of repute
would do well to give themselves--namely, an education in what we may
call the art of distance and geographical forms. With the primitive
savage the capacity to remember and to picture to the eye the shape of
a country which he knows is native and instinctive. Accustomed to
range the woods, and to trust to his recollection to guide him through
the wilderness to his home, the primitive man develops an important
art which among civilized people is generally dormant. In fact, in our
well-trodden ways people may go for many generations without ever
being called upon to use this natural sense of geography. The easiest
way to cultivate the geographic sense is by practising the art of
making sketch maps. This the student, however untrained, can readily
do by taking first his own dwelling house, on which he should practise
until he can readily from memory make a tolerably correct and
proportional plan of all its rooms. Then on a smaller scale he should
begin to make also from recollection a map showing the distribution of
the roads, streams, and hills with which his daily life makes him
familiar. From time to time this work from memory should be compared
with the facts. At first the record will be found to be very poor, but
with a few months of occasional endeavour the observer will find that
his mind takes account of geographic features in a way it did not
before, and, moreover, that his mind becomes enriched with
impressions of the country which are clear and distinct, in place of
the shadowy recollections which he at first possessed.
When the student has attained the point where, after walking or riding
over a country, he can readily recall its physical features of the
simpler sort, he will find it profitable to undertake the method of
mapping with contour lines--that is, by pencilling in indications to
show the exact shape of the elevations and depressions. The principle
of contour lines is that each of them represents where water would
come against the slope if the area were sunk step by step below the
sea level--in other words, each contour line marks the intersection of
a horizontal plane with the elevation of the country. Practice on this
somewhat difficult task will soon give the student some idea as to the
complication of the surface of a region, and afford him the basis for
a better understanding of what geography means than all the reading he
can do will effect. It is most desirable that training such as has
been described should be a part of our ordinary school education.
Very few people have clear ideas of distances. Even the men whose
trade requires some such knowledge are often without that which a
little training could give them. Without some capacity in this
direction, the student is always at a disadvantage in his contact with
Nature. He can not make a record of what he sees as long as the
element of horizontal and vertical distance is not clearly in mind. To
attain this end the student should begin by pacing some length of road
where the distances are well known. In this way he will learn the
length of his step, which with a grown man generally ranges between
two and a half and three feet. Learning the average length of his
stride by frequent counting, it is easy to repeat the trial until one
can almost unconsciously keep the count as he walks. Properly to
secure the training of this sort the observer should first attentively
look across the distance which is to be determined. He should notice
how houses, fences, people, and trees appear at that distance. He will
quickly perceive that each hundred feet of additional interval
somewhat changes their aspect. In training soldiers to measure with
the eye the distances which they have to know in order effectively to
use the modern weapons of war, a common device is to take a squad of
men, or sometimes a company, under the command of an officer, who
halts one man at each hundred yards until the detachment is strung out
with that interval as far as the eye can see them. The men then walk
to and fro so that the troops who are watching them may note the
effects of increased distance on their appearance, whether standing or
in motion. At three thousand yards a man appears as a mere dot, which
is not readily distinguishable. Schoolboys may find this experiment
amusing and instructive.
After the student has gained, as he readily may, some sense of the
divisions of distance within the range of ordinary vision, he should
try to form some notion of greater intervals, as of ten, a hundred,
and perhaps a thousand miles. The task becomes more difficult as the
length of the line increases, but most persons can with a little
address manage to bring before their eyes a tolerably clear image of a
hundred miles of distance by looking from some elevation which
commands a great landscape. It is doubtful, however, whether the
best-trained man can get any clear notion of a thousand miles--that
is, can present it to himself in imagination as he may readily do with
shorter intervals.
The most difficult part of the general education which the student has
to give himself is begun when he undertakes to picture long intervals
of time. Space we have opportunities to measure, and we come in a way
to appreciate it, but the longest lived of men experiences at most a
century of life, and this is too small a measure to give any notion as
to the duration of such great events as are involved in the history of
the earth, where the periods are to be reckoned by the millions of
years. The only way in which we can get any aid in picturing to
ourselves great lapses of time is by expressing them in units of
distance. Let a student walk away on a straight road for the distance
of a mile; let him call each step a year; when he has won the first
milestone, he may consider that he has gone backward in time to the
period of Christ's birth. Two miles more will take him to the station
which will represent the age when the oldest pyramids were built. He
is still, however, in the later days of man's history on this planet.
To attain on the scale the time when man began, he might well have to
walk fifty miles away, while a journey which would thus by successive
steps describe the years of the earth's history since life appeared
upon its surface would probably require him to circle the earth at
least four times. We may accept it as impossible for any one to deal
with such vast durations save with figures which are never really
comprehended. It is well, however, to enlarge our view as to the age
of the earth by such efforts as have just been indicated.
When we go beyond the earth into the realm of the stars all efforts
toward understanding the ranges of space or the durations of time are
quite beyond the efforts of man. Even the distance of about two
hundred and forty thousand miles which separates us from the moon can
not be grasped by even the greater minds. No human intelligence,
however cultivated, can conceive the distance of about ninety-five
million miles which separates us from the sun. In the celestial realm
we can only deal with relations of space and time in a general and
comparative way. We can state the distances if we please in millions
of miles, or we can reckon the ampler spaces by using the interval
which separates the earth from the sun as we do a foot rule in our
ordinary work, but the depths of the starry spaces can only be sounded
by the winged imagination.
Although the student has been advised to begin his studies of Nature
on the field whereon he dwells, making that study the basis of his
most valuable communications with Nature, it is desirable that he
should at the same time gain some idea as to the range and scope of
our knowledge concerning the visible universe. As an aid toward this
end the following chapters of this book will give a very brief survey
of some of the most important truths concerning the heavens and the
earth which have rewarded the studies of scientific men. Of remoter
things, such as the bodies in the stellar spaces, the account will be
brief, for that which is known and important to the general student
can be briefly told. So, too, of the earlier ages of the earth's
history, although a vast deal is known, the greater part of the
knowledge is of interest and value mainly to geologists who cultivate
that field. That which is most striking and most important to the mass
of mankind is to be found in the existing state of our earth, the
conditions which make it a fit abode for our kind, and replete with
lessons which he may study with his own eyes without having to travel
the difficult paths of the higher sciences.
Although physiography necessarily takes some account of the things
which have been, even in the remote past, and this for the reason that
everything in this day of the world depends on the events of earlier
days, the accent of its teaching is on the immediate, visible, as we
may say, living world, which is a part of the life of all its
inhabitants.
CHAPTER III.
THE STELLAR REALM.
Even before men came to take any careful account of the Nature
immediately about them they began to conjecture and in a way to
inquire concerning the stars and the other heavenly bodies. It is
difficult for us to imagine how hard it was for students to gain any
adequate idea of what those lights in the sky really are. At first men
imagined the celestial bodies to be, as they seemed, small objects not
very far away. Among the Greeks the view grew up that the heavens were
formed of crystal spheres in which the lights were placed, much as
lanterns may be hung upon a ceiling. These spheres were conceived to
be one above the other; the planets were on the lower of them, and the
fixed stars on the higher, the several crystal roofs revolving about
the earth. So long as the earth was supposed to be a flat and
limitless expanse, forming the centre of the universe, it was
impossible for the students of the heavens to attain any more rational
view as to their plan.
The fact that the earth was globular in form was understood by the
Greek men of science. They may, indeed, have derived the opinion from
the Egyptian philosophers. The discovery rested upon the readily
observed fact that on a given day the shadow of objects of a certain
height was longer in high latitude than in low. Within the tropics,
when the sun was vertical, there would be no shadow, while as far
north as Athens it would be of considerable length. The conclusion
that the earth was a sphere appears to have been the first large
discovery made by our race. It was, indeed, one of the most important
intellectual acquisitions of man.
Understanding the globular form of the earth, the next and most
natural step was to learn that the earth was not the centre of the
planetary system, much less of the universe, but that that centre was
the sun, around which the earth and the other planets revolved. The
Greeks appear to have had some idea that this was the case, and their
spirit of inquiry would probably have led them to the whole truth but
for the overthrow of their thought by the Roman conquest and the
spread of Christianity. It was therefore not until after the revival
of learning that astronomers won their way to our modern understanding
concerning the relation of the planets to the sun. With Galileo this
opinion was affirmed. Although for a time the Church, resting its
opposition on the interpretation of certain passages of Scripture,
resisted this view, and even punished the men who held it, it
steadfastly made its way, and for more than two centuries has been the
foundation of all the great discoveries in the stellar realm. Yet long
after the fact that the sun was the centre of the solar system was
well established no one understood why the planets should move in
their ceaseless, orderly procession around the central mass. To Newton
we owe the studies on the law of gravitation which brought us to our
present large conception as to the origin of this order. Starting with
the view that bodies attracted each other in proportion to their
weight, and in diminishing proportion as they are removed from each
other, Newton proceeded by most laborious studies to criticise this
view, and in the end definitely proved it by finding that the motions
of the moon about the earth, as well as the paths of the planets,
exactly agreed with the supposition.
The last great path-breaking discovery which has helped us in our
understanding of the stars was made by Fraunhofer and other
physicists, who showed us that substances when in a heated, gaseous,
or vaporous state produced, in a way which it is not easy to explain
in a work such as this, certain dark lines in the spectrum, or streak
of divided light which we may make by means of a glass prism, or, as
in the rainbow, by drops of water. Carefully studying these very
numerous lines, those naturalists found that they could with singular
accuracy determine what substances there were in the flame which gave
the light. So accurate is this determination that it has been made to
serve in certain arts where there is no better means of ascertaining
the conditions of a flaming substance except by the lines which its
light exhibits under this kind of analysis. Thus, in the manufacture
of iron by what is called the Bessemer process, it has been found very
convenient to judge as to the state of the molten metal by such an
analysis of the flame which comes forth from it.
[Illustration: _Seal Rocks near San Francisco, California, showing
slight effect of waves where there is no beach._]
No sooner was the spectroscope invented than astronomers hastened by
its aid to explore the chemical constitution of the sun. These studies
have made it plain that the light of our solar centre comes forth from
an atmosphere composed of highly heated substances, all of which are
known among the materials forming the earth. Although for various
reasons we have not been able to recognise in the sun all the elements
which are found in our sphere, it is certain that in general the two
bodies are alike in composition. An extension of the same method of
inquiry to the fixed stars was gradually though with difficulty
attained, and we now know that many of the elements common to the sun
and earth exist in those distant spheres. Still further, this method
of inquiry has shown us, in a way which it is not worth while here to
describe, that among these remoter suns there are many aggregations of
matter which are not consolidated as are the spheres of our own solar
system, but remain in the gaseous state, receiving the name of nebulæ.
Along with the growth of observational astronomy which has taken place
since the discoveries of Galileo, there has been developed a view
concerning the physical history of the stellar world, known as the
nebular hypothesis, which, though not yet fully proved, is believed by
most astronomers and physicists to give us a tolerably correct notion
as to the way in which the heavenly spheres were formed from an
earlier condition of matter. This majestic conception was first
advanced, in modern times at least, by the German philosopher Immanuel
Kant. It was developed by the French astronomer Laplace, and is often
known by his name. The essence of this view rests upon the fact
previously noted that in the realm of the fixed stars there are many
faintly shining aggregations of matter which are evidently not solid
after the manner of the bodies in our solar system, but are in the
state where their substances are in the condition of dustlike
particles, as are the bits of carbon in flame or the elements which
compose the atmosphere. The view held by Laplace was to the effect
that not only our own solar system, but the centres of all the other
similar systems, the fixed stars, were originally in this gaseous
state, the material being disseminated throughout all parts of the
heavenly realm, or at least in that portion of the universe of which
we are permitted to know something. In this ancient state of matter we
have to suppose that the particles of it were more separated from each
other than are the atoms of the atmospheric gases in the most perfect
vacuum which we can produce with the air-pump. Still we have to
suppose that each of these particles attract the other in the
gravitative way, as in the present state of the universe they
inevitably do.
Under the influence of the gravitative attraction the materials of
this realm of vapour inevitably tended to fall in toward the centre.
If the process had been perfectly simple, the result would have been
the formation of one vast mass, including all the matter which was in
the original body. In some way, no one has yet been able to make a
reasonable suggestion of just how, there were developed in the
process of concentration a great many separate centres of aggregation,
each of which became the beginning of a solar system. The student may
form some idea of how readily local centres may be produced in
materials disseminated in the vaporous state by watching how fog or
the thin, even misty clouds of the sunrise often gather into the
separate shapes which make what we term a "mackerel" sky. It is
difficult to imagine what makes centres of attraction, but we readily
perceive by this instance how they might have occurred.
When the materials of each solar system were thus set apart from the
original mass of star dust or vapour, they began an independent
development which led step by step, in the case of our own solar
system at least, and presumably also in the case of the other suns,
the fixed stars, to the formation of planets and their moons or
satellites, all moving around the central sun. At this stage of the
explanation the nebular hypothesis is more difficult to conceive than
in the parts of it which have already been described, for we have now
to understand how the planets and satellites had their matter
separated from each other and from the solar centre, and why they came
to revolve around that central body. These problems are best
understood by noting some familiar instances connected with the
movement of fluids and gases toward a centre. First let us take the
case of a basin in which the water is allowed to flow out through a
hole in its centre. When we lift the stopper the fluid for a moment
falls straight down through the opening. Very quickly, however, all
the particles of the water start to move toward the centre, and almost
at once the mass begins to whirl round with such speed that, although
it is working toward the middle, it is by its movement pushed away
from the centre and forms a conical depression. As often as we try the
experiment, the effect is always the same. We thus see that there is
some principle which makes particles of fluid that tend toward a
centre fail directly to attain it, but win their way thereto in a
devious, spinning movement.
Although the fact is not so readily made visible to the eye, the same
principle is illustrated in whirling storms, in which, as we shall
hereafter note with more detail, the air next the surface of the earth
is moving in toward a kind of chimney by which it escapes to the upper
regions of the atmosphere. A study of cyclones and tornadoes, or even
of the little air-whirls which in hot weather lift the dust of our
streets, shows that the particles of the atmosphere in rushing in
toward the centre of upward movement take on the same whirling motion
as do the molecules of water in the basin--in fact, the two actions
are perfectly comparable in all essential regards, except that the
fluid is moving downward, while the air flows upward. Briefly stated,
the reason for the movement of fluid and gas in the whirling way is as
follows: If every particle on its way to the centre moved on a
perfectly straight line toward the point of escape, the flow would be
directly converging, and the paths followed would resemble the spokes
of a wheel. But when by chance one of the particles sways ever so
little to one side of the direct way, a slight lateral motion would
necessarily be established. This movement would be due to the fact
that the particle which pursued the curved line would press against
the particles on the out-curved side of its path--or, in other words,
shove them a little in that direction--to the extent that they
departed from the direct line they would in turn communicate the
shoving to the next beyond. When two particles are thus shoving on one
side of their paths, the action which makes for revolution is doubled,
and, as we readily see, the whole mass may in this way become quickly
affected, the particles driven out of their path, moving in a curve
toward the centre. We also see that the action is accumulative: the
more curved the path of each particle, the more effectively it shoves;
and so, in the case of the basin, we see the whirling rapidly
developed before our eyes.
In falling in toward the centre the particles of star dust or vapour
would no more have been able one and all to pursue a perfectly
straight line than the particles of water in the basin. If a man
should spend his lifetime in filling and emptying such a vessel, it is
safe to say that he would never fail to observe the whirling movement.
As the particles of matter in the nebular mass which was to become a
solar system are inconceivably greater than those of water in the
basin, or those of air in the atmospheric whirl, the chance of the
whirling taking place in the heavenly bodies is so great that we may
assume that it would inevitably occur.
As the vapours in the olden day tended in toward the centre of our
solar system, and the mass revolved, there is reason to believe that
ringlike separations took place in it. Whirling in the manner
indicated, the mass of vapour or dust would flatten into a disk or a
body of circular shape, with much the greater diameter in the plane of
its whirling. As the process of concentration went on, this disk is
supposed to have divided into ringlike masses, some approach to which
we can discern in the existing nebulæ, which here and there among the
farther fixed stars appear to be undergoing such stages of development
toward solar systems. It is reasonably supposed that after these rings
had been developed they would break to pieces, the matter in them
gathering into a sphere, which in time was to become a planet. The
outermost of these rings led to the formation of the planet farthest
from the sun, and was probably the first to separate from the parent
mass. Then in succession rings were formed inwardly, each leading in
turn to the creation of another planet, the sun itself being the
remnant, by far the greater part of the whole mass of matter, which
did not separate in the manner described, but concentrated on its
centre. Each of these planetary aggregations of vapour tended to
develop, as it whirled upon its centre, rings of its own, which in
turn formed, by breaking and concentrating, the satellites or moons
which attend the earth, as they do all the planets which lie farther
away from the sun than our sphere.
[Illustration: Fig. 1.--Saturn, Jan. 26, 1889 (Antoniadi).]
As if to prove that the planets and moons of the solar system were
formed somewhat in the manner in which we have described it, one of
these spheres, Saturn, retains a ring, or rather a band which appears
to be divided obscurely into several rings which lie between its group
of satellites and the main sphere. How this ring has been preserved
when all the others have disappeared, and what is the exact
constitution of the mass, is not yet well ascertained. It seems clear,
however, that it can not be composed of solid matter. It is either in
the form of dust or of small spheres, which are free to move on each
other; otherwise, as computation shows, the strains due to the
attraction which Saturn itself and its moons exercise upon it would
serve to break it in pieces. Although this ring theory of the
formation of the planets and satellites is not completely proved, the
occurrence of such a structure as that which girdles Saturn affords
presumptive evidence that it is true. Taken in connection with what we
know of the nebulæ, the proof of Laplace's nebular hypothesis may
fairly be regarded as complete.
It should be said that some of the fixed stars are not isolated suns
like our own, but are composed of two great spheres revolving about
one another; hence they are termed double stars. The motions of these
bodies are very peculiar, and their conditions show us that it is not
well to suppose that the solar system in which we dwell is the only
type of order which prevails in the celestial families; there may,
indeed, be other variations as yet undetected. Still, these
differences throw no doubt on the essential truth of the theory as to
the process of development of the celestial systems. Though there is
much room for debate as to the details of the work there, the general
truth of the theory is accepted by nearly all the students of the
problem.
A peculiar advantage of the nebular hypothesis is that it serves to
account for the energy which appears as light and heat in the sun and
the fixed stars, as well as that which still abides in the mass of our
earth, and doubtless also in the other large planets. When the matter
of which these spheres were composed was disseminated through the
realms of space, it is supposed to have had no positive temperature,
and to have been dark, realizing the conception which appears in the
first chapter of Genesis, "without form, and void." With each stage of
the falling in toward the solar centres what is called the "energy of
position" of this original matter became converted into light and
heat. To understand how this took place, the reader should consider
certain simple yet noble generalizations of physics. We readily
recognise the fact that when a hammer falls often on an anvil it heats
itself and the metal on which it strikes. Those who have been able to
observe the descent of meteoric stones from the heavens have remarked
that when they came to the earth they were, on their surfaces at
least, exceedingly hot. Any one may observe shining meteors now and
then flashing in the sky. These are known commonly to be very small
bits of matter, probably not larger than grains of sand, which,
rushing into our atmosphere, are so heated by the friction which they
encounter that they burn to a gas or vapour before they attain the
earth. As we know that these particles come from the starry spaces,
where the temperature is somewhere near 500° below 0° Fahr., it is
evident that the light and heat are not brought with them into the
atmosphere; it can only be explained by the fact that when they enter
the air they are moving at an average speed of about twenty miles a
second, and that the energy which this motion represents is by the
resistance which the body encounters converted into heat. This fact
will help us to understand how, as the original star dust fell in
toward the centre of attraction, it was able to convert what we have
termed the energy of position into temperature. We see clearly that
every such particle of dust or larger bit of matter which falls upon
the earth brings about the development of heat, even though it does
not actually strike upon the solid mass of our sphere. The conception
of what took place in the consolidation of the originally disseminated
materials of the sun and planets can be somewhat helped by a simple
experiment. If we fit a piston closely into a cylinder, and then
suddenly drive it down with a heavy blow, the compressed air is so
heated that it may be made to communicate fire. If the piston should
be slowly moved, the same amount of heat would be generated, or, as we
may better say, liberated by the compression, though the effect would
not be so striking. A host of experiments show that when a given mass
of matter is brought to occupy a less space the effect is in
practically all cases to increase the temperature. The energy which
kept the particles apart is, when they are driven together, converted
into heat. These two classes of actions are somewhat different in
their nature; in the case of the meteors, or the equivalent star dust,
the coming together of the particles is due to gravitation. In the
experiment with the cylinder above described, the compression is due
to mechanical energy, a force of another nature.
There is reason for believing that all our planets, as well as the sun
itself, and also the myriad other orbs of space, have all passed
through the stages of a transition in which a continually
concentrating vapour, drawn together by gravitation, became
progressively hotter and more dense until it assumed the condition of
a fluid. This fluid gradually parted with its heat to the cold spaces
of the heavens, and became more and more concentrated and of a lower
temperature until in the end, as in the case of our earth and of other
planets, it ceased to glow on the outside, though it remained
intensely heated in the inner parts. It is easy to see that the rate
of this cooling would be in some proportion to the size of the sphere.
Thus the earth, which is relatively small, has become relatively cold,
while the sun itself, because of its vastly greater mass, still
retains an exceedingly high temperature. The reason for this can
readily be conceived by making a comparison of the rate of cooling
which occurs in many of our ordinary experiences. Thus a vial of hot
water will quickly come down to the temperature of the air, while a
large jug filled with the fluid at the same temperature will retain
its heat many times as long. The reason for this rests upon the simple
principle that the contents of a sphere increase with its enlargement
more rapidly than the surface through which the cooling takes place.
The modern studies on the physical history of the sun and other
celestial bodies show that their original store of heat is constantly
flowing away into the empty realms of space. The rate at which this
form of energy goes away from the sun is vast beyond the powers of the
imagination to conceive; thus, in the case of our earth, which viewed
from the sun would appear no more than a small star, the amount of
heat which falls upon it from the great centre is enough each day to
melt, if it all could be put to such work, about eight thousand cubic
miles of ice. Yet the earth receives only 1/2,170,000,000 part of the
solar radiation. The greater part of this solar heat--in fact, we may
say nearly all of it--slips by the few and relatively small planets
and disappears in the great void.
The destiny of all the celestial spheres seems in time to be that
they shall become cooled down to a temperature far below anything
which is now experienced on this earth. Even the sun, though its heat
will doubtless endure for millions of years to come, must in time, so
far as we can see, become dark and cold. So far as we know, we can
perceive no certain method by which the life of the slowly decaying
suns can be restored. It has, however, been suggested that in many
cases a planetary system which has attained the lifeless and lightless
stage may by collision with some other association of spheres be by
the blow restored to its previous state of vapour, the joint mass of
the colliding systems once again to resume the process of
concentration through which it had gone before. Now and then stars
have been seen to flash suddenly into great brilliancy in a way which
suggests that possibly their heat had been refreshed by a collision
with some great mass which had fallen into them from the celestial
spaces. There is room for much speculation in this field, but no
certainty appears to be attainable.
The ancients believed that light and heat were emanations which were
given off from the bodies that yielded them substantially as odours
are given forth by many substances. Since the days of Newton inquiry
has forced us to the conviction that these effects of temperature are
produced by vibrations having the general character of waves, which
are sent through the spaces with great celerity. When a ray of light
departs from the sun or other luminous body, it does not convey any
part of the mass; it transmits only motion. A conception of the action
can perhaps best be formed by suspending a number of balls of ivory,
stone, or other hard substance each by a cord, the series so arranged
that they touch each other. Then striking a blow against one end of
the line, we observe that the ball at the farther end of the line is
set in motion, swinging a little away from the place it occupied
before. The movement of the intermediate balls may be so slight as to
escape attention. We thus perceive that energy can be transmitted
from one to another of these little spheres. Close observation shows
us that under the impulse which the blow gives each separate body is
made to sway within itself much in the manner of a bell when it is
rung, and that the movement is transmitted to the object with which it
is in contact. In passing from the sun to the earth, the light and
heat traverse a space which we know to be substantially destitute of
any such materials as make up the mass of the earth or the sun. Judged
by the standards which we can apply, this space must be essentially
empty. Yet because motions go through it, we have to believe that it
is occupied by something which has certain of the properties of
matter. It has, indeed, one of the most important properties of all
substances, in that it can vibrate. This practically unknown thing is
called ether.
The first important observational work done by the ancients led them
to perceive that there was a very characteristic difference between
the planets and the fixed stars. They noted the fact that the planets
wandered in a ceaseless way across the heavens, while the fixed stars
showed little trace of changing position in relation to one another.
For a long time it was believed that these, as well as the remoter
fixed stars, revolved about the earth. This error, though great, is
perfectly comprehensible, for the evident appearance of the movement
is substantially what would be brought about if they really coursed
around our sphere. It was only when the true nature of the earth and
its relations to the sun were understood that men could correct this
first view. It was not, indeed, until relatively modern times that the
solar system came to be perceived as something independent and widely
detached from the fixed stars system; that the spaces which separate
the members of our own solar family, inconceivably great as they are,
are but trifling as compared with the intervals which part us from the
nearer fixed stars. At this stage of our knowledge men came to the
noble suggestion that each of the fixed stars was itself a sun, each
of the myriad probably attended by planetary bodies such as exist
about our own luminary.
It will be well for the student to take an imaginary journey from the
sun forth into space, along the plane in which extends that vast
aggregation of stars which we term the Milky Way. Let him suppose that
his journey could be made with something like the speed of light, or,
say, at the rate of about two hundred thousand miles a second. It is
fit that the imagination, which is free to go through all things,
should essay such excursions. On the fancied outgoing, the observer
would pass the interval between the sun and the earth in about eight
minutes. It would require some hours before he attained to the outer
limit of the solar system. On his direct way he would pass the orbits
of the several planets. Some would have their courses on one side or
the other of his path; we should say above or below, but for the fact
that we leave these terms behind in the celestial realm. On the margin
of the solar system the sun would appear shrunken to the state where
it was hardly greater than the more brilliant of the other fixed
stars. The onward path would then lead through a void which it would
require years to traverse. Gradually the sun which happened to lie
most directly in his path would grow larger; with nearer approach, it
would disclose its planets. Supposing that the way led through this
solar system, there would doubtless be revealed planets and satellites
in their order somewhat resembling those of our own solar family, yet
there would doubtless be many surprises in the view. Arriving near the
first sun to be visited, though the heavens would have changed their
shape, all the existing constellations having altered with the change
in the point of view, there would still be one familiar element in
that the new-found planets would be near by, and the nearest fixed
stars far away in the firmament.
With the speed of light a stellar voyage could be taken along the path
of the Milky Way, which would endure for thousands of years. Through
all the course the journeyer would perceive the same vast girdle of
stars, faint because they were far away, which gives the dim light of
our galaxy. At no point is it probable that he would find the separate
suns much more aggregated or greatly farther apart than they are in
that part of the Milky Way which our sun now occupies. Looking forth
on either side of the "galactic plane," there would be the same
scattering of stars which we now behold when we gaze at right angles
to the way we are supposing the spirit to traverse.
As the form of the Milky Way is irregular, the mass, indeed, having
certain curious divisions and branches, it well might be that the
supposed path would occasionally pass on one or the other side of the
vast star layer. In such positions the eye would look forth into an
empty firmament, except that there might be in the far away, tens of
thousands of years perhaps at the rate that light travels away from
the observer, other galaxies or Milky Ways essentially like that which
he was traversing. At some point the journeyer would attain the margin
of our star stratum, whence again he would look forth into the
unpeopled heavens, though even there he might discern other remote
star groups separated from his own by great void intervals.
* * * * *
The revelations of the telescope show us certain features in the
constitution and movements of the fixed stars which now demand our
attention. In the first place, it is plain that not all of these
bodies are in the same physical condition. Though the greater part of
these distant luminous masses are evidently in the state of
aggregation displayed by our own sun, many of them retain more or less
of that vaporous, it may be dustlike, character which we suppose to
have been the ancient state of all the matter in the universe. Some of
these masses appear as faint, almost indistinguishable clouds, which
even to the greatest telescope and the best-trained vision show no
distinct features of structure. In other cases the nebulous
appearance is hardly more than a mist about a tolerably distinct
central star. Yet again, and most beautifully in the great nebula of
the constellation of Orion, the cloudy mass, though hardly visible to
the naked eye, shows a division into many separate parts, the whole
appearing as if in process of concentration about many distinct
centres.
The nebulas are reasonably believed by many astronomers to be examples
of the ancient condition of the physical universe, masses of matter
which for some reason as yet unknown have not progressed in their
consolidation to the point where they have taken on the
characteristics of suns and their attendant planets.
Many of the fixed stars, the incomplete list of which now amounts to
several hundred, are curiously variable in the amount of light which
they send out to the earth. Sometimes these variations are apparently
irregular, but in the greater number of cases they have fixed periods,
the star waxing and waning at intervals varying from a few months to a
few years. Although some of the sudden flashings forth of stars from
apparent small size to near the greatest brilliancy may be due to
catastrophes such as might be brought about by the sudden falling in
of masses of matter upon the luminous spheres, it is more likely that
the changes which we observe are due to the fact that two suns
revolving around a common centre are in different stages of
extinction. It may well be that one of these orbs, presumably the
smaller, has so far lost temperature that it has ceased to glow. If in
its revolution it regularly comes between the earth and its luminous
companion, the effect would be to give about such a change in the
amount of light as we observe.
The supposition that a bright sun and a relatively dark sun might
revolve around a common centre of gravity may at first sight seem
improbable. The fact is, however, that imperfect as our observations
on the stars really are, we know many instances in which this kind of
revolution of one star about another takes place. In some cases these
stars are of the same brilliancy, but in others one of the lights is
much brighter than the other. From this condition to the state where
one of the stars is so nearly dark as to be invisible, the transition
is but slight. In a word, the evidence goes to show that while we see
only the luminous orbs of space, the dark bodies which people the
heavens are perhaps as numerous as those which send us light, and
therefore appear as stars.
Besides the greater spheres of space, there is a vast host of lesser
bodies, the meteorites and comets, which appear to be in part members
of our solar system, and perhaps of other similar systems, and in part
wanderers in the vast realm which intervenes between the solar
systems. Of these we will first consider the meteors, of which we know
by far the most; though even of them, as we shall see, our knowledge
is limited.
From time to time on any starry night, and particularly in certain
periods of the year, we may behold, at the distance of fifty or more
miles above the surface of the earth, what are commonly called
"shooting stars." The most of these flashing meteors are evidently
very small, probably not larger than tiny sand grains, possibly no
greater than the fragments which would be termed dust. They enter the
air at a speed of about thirty miles a second. They are so small that
they burn to vapour in the very great heat arising from their friction
on the air, and do not attain the surface of the earth. These are so
numerous that, on the average, some hundreds of thousands probably
strike the earth's atmosphere each day. From time to time larger
bodies fall--bodies which are of sufficient bulk not to be burned up
in the air, but which descend to the ground. These may be from the
smallest size which may be observed to masses of many hundred pounds
in weight. These are far less numerous than the dust meteorites; it is
probable, however, that several hundred fragments each year attain the
earth's surface. They come from various directions of space, and
there is as yet no means of determining whether they were formed in
some manner within our planetary system or whether they wander to us
from remoter realms. We know that they are in part composed of
metallic iron commingled with nickel and carbon (sometimes as very
small diamonds) in a way rarely if ever found on the surface of our
sphere, and having a structure substantially unknown in its deposits.
In part they are composed of materials which somewhat resemble certain
lavas. It is possible that these fragments of iron and stone which
constitute the meteorites have been thrown into the planetary spaces
by the volcanic eruption of our own and other planets. If hurled forth
with a sufficient energy, the fragments would escape from the control
of the attraction of the sphere whence they came, and would become
independent wanderers in space, moving around the sun in varied orbits
until they were again drawn in by some of the greater planets.
As they come to us these meteorites often break up in the atmosphere,
the bits being scattered sometimes over a wide area of country. Thus,
in the case of the Cocke County meteorite of Tennessee, one of the
iron species, the fragments, perhaps thousands in number, which came
from the explosion of the body were scattered over an area of some
thousand square miles. When they reach the surface in their natural
form, these meteors always have a curious wasted and indented
appearance, which makes it seem likely that they have been subject to
frequent collisions in their journeys after they were formed by some
violent rending action.
In some apparent kinship with the meteorites may be classed the
comets. The peculiarity of these bodies is that they appear in most
cases to be more or less completely vaporous. Rushing down from the
depths of the heavens, these bodies commonly appear as faintly
shining, cloudlike masses. As they move in toward the sun long trails
of vapour stream back from the somewhat consolidated head. Swinging
around that centre, they journey again into the outer realm. As they
retreat, their tail-like streamers appear to gather again upon their
centres, and when they fade from view they are again consolidated. In
some cases it has been suspected that a part at least of the cometary
mass was solid. The evidence goes to show, however, that the matter is
in a dustlike or vaporous condition, and that the weight of these
bodies is relatively very small.
[Illustration: Fig. 2.--The Great Comet of 1811, one of the many
varied forms of these bodies.]
Owing to their strange appearance, comets were to the ancients omens
of calamity. Sometimes they were conceived as flaming swords; their
forms, indeed, lend themselves to this imagining. They were thought to
presage war, famine, and the death of kings. Again, in more modern
times, when they were not regarded as portents of calamity, it was
feared that these wanderers moving vagariously through our solar
system might by chance come in contact with the earth with disastrous
results. Such collisions are not impossible, for the reason that the
planets would tend to draw these errant bodies toward them if they
came near their spheres; yet the chance of such collisions happening
to the earth is so small that they may be disregarded.
MOTIONS OF THE SPHERES.
Although little is known of the motions which occur among the
celestial bodies beyond the sphere of our solar family, that which has
been ascertained is of great importance, and serves to make it likely
that all the suns in space are upon swift journeys which in their
speed equal, if they do not exceed, the rate of motion among the
planetary spheres, which may, in general, be reckoned at about twenty
miles a second. Our whole solar system is journeying away from certain
stars, and in the direction of others which are situated in the
opposite part of the heavens. The proof of this fact is found in the
observations which show that on one side of us the stars are
apparently coming closer together, while on the other side they are
going farther apart. The phenomenon, in a word, is one of perspective,
and may be made real to the understanding by noting what takes place
when we travel down a street along which there are lights. We readily
note that these lights appear to close in behind us, and widen their
intervals in the direction in which we journey. By such evidence
astronomers have become convinced that our sphere, along with the sun
which controls it, is each second a score of miles away from the point
where it was before.
There is yet other and most curious evidence which serves to show that
certain of the stars are journeying toward our part of the heavens at
great speed, while others are moving away from us by their own proper
motion. These indications are derived from the study of the lines in
the light which the spectrum reveals to us when critically examined.
The position of these cross lines is, as we know, affected by the
motion of the body whence the light comes, and by close analysis of
the facts it has been pretty well determined that the distortion in
their positions is due to very swift motions of the several stars. It
is not yet certain whether these movements of our sun and of other
solar bodies are in straight lines or in great circles.
It should be noted that, although the evidence from the spectroscope
serves to show that the matter in the stars is akin to that of our own
earth, there is reason to believe that those great spheres differ much
from each other in magnitude.
We have now set forth some of the important facts exhibited by the
stellar universe. The body of details concerning that realm is vast,
and the conclusions drawn from it important; only a part, however, of
the matter with which it deals is of a nature to be apprehended by the
student who does not approach it in a somewhat professional way. We
shall therefore now turn to a description of the portion of the starry
world which is found in the limits of our solar system. There the
influences of the several spheres upon our planet are matters of vital
importance; they in a way affect, if they do not control, all the
operations which go on upon the surface of the earth.
THE SOLAR SYSTEM.
We have seen that the matter in the visible universe everywhere tends
to gather into vast associations which appear to us as stars, and that
these orbs are engaged in ceaseless motion in journeys through space.
In only one of these aggregations--that which makes our own solar
system--are the bodies sufficiently near to our eyes for us, even with
the resources of our telescopes and other instruments, to divine
something of the details which they exhibit. In studying what we may
concerning the family of the sun, the planets, and their satellites,
we may reasonably be assured that we are tracing a history which with
many differences is in general repeated in the development of each
star in the firmament. Therefore the inquiry is one of vast range and
import.
Following, as we may reasonably do, the nebular hypothesis--a view
which, though not wholly proved, is eminently probable--we may regard
our solar system as having begun when the matter of which it is
composed, then in a finely divided, cloudy state, was separated from
the similar material which went to make the neighbouring fixed stars.
The period when our solar system began its individual life was remote
beyond the possibility of conception. Naturalists are pretty well
agreed that living beings began to exist upon the earth at least a
hundred million years ago; but the beginnings of our solar system must
be placed at a date very many times as remote from the present day.[1]
[Footnote 1: Some astronomers, particularly the distinguished Professor
Newcomb, hold that the sun can not have been supplying heat as at
present for more than about ten million years, and that all geological
time must be thus limited. The geologist believes that this reckoning is
far too short.]
According to the nebular theory, the original vapour of the solar
system began to fall in toward its centre and to whirl about that
point at a time long before the mass had shrunk to the present limits
of the solar system as defined by the path of the outermost planets.
At successive stages of the concentration, rings after the manner of
those of Saturn separated from the disklike mass, each breaking up and
consolidating into a body of nebulous matter which followed in the
same path, generally forming rings which became by the same process
the moons or satellites of the sphere. In this way the sun produced
eight planets which are known, and possibly others of small size on
the outer verge of the system which have eluded discovery. According
to this view, the planetary masses were born in succession, the
farthest away being the oldest. It is, however, held by an able
authority that the mass of the solar system would first form a rather
flat disk, the several rings forming and breaking into planets at
about the same time. The conditions in Saturn, where the inner ring
remains parted, favours the view just stated.
Before making a brief statement of the several planets, the asteroids,
and the satellites, it will be well to consider in a general way the
motions of these bodies about their centres and about the sun. The
most characteristic and invariable of these movements is that by which
each of the planetary spheres, as well as the satellites, describes an
orbit around the gravitative centre which has the most influence upon
it--the sun. To conceive the nature of this movement, it will be well
to imagine a single planet revolving around the sun, each of these
bodies being perfect spheres, and the two the only members of the
solar system. In this condition the attraction of the two bodies would
cause them to circle around a common centre of gravity, which, if the
planet were not larger or the sun smaller than is the case in our
solar system, would lie within the mass of the sun. In proportion as
the two bodies might approach each other in size, the centre of
gravity would come the nearer to the middle point in a line connecting
the two spheres. In this condition of a sun with a single planet,
whatever were the relative size of sun and planet, the orbits which
they traverse would be circular. In this state of affairs it should be
noted that each of the two bodies would have its plane of rotation
permanently in the same position. Even if the spheres were more or
less flattened about the poles of their axes, as is the case with all
the planets which we have been able carefully to measure, as well as
with the sun, provided the axes of rotation were precisely parallel to
each other, the mutual attraction of the masses would cause no
disturbance of the spheres. The same would be the case if the polar
axis of one sphere stood precisely at right angles to that of the
other. If, however, the spheres were somewhat flattened at the poles,
and the axes inclined to each other, then the pull of one mass on the
other would cause the polar axes to keep up a constant movement which
is called nutation, or nodding.
The reason why this nodding movement of the polar axes would occur
when these lines were inclined to each other is not difficult to see
if we remember that the attraction of masses upon each other is
inversely as the square of the distance; each sphere, pulling on the
equatorial bulging of the other, pulls most effectively on the part of
it which is nearest, and tends to draw it down toward its centre. The
result is that the axes of the attracted spheres are given a wobbling
movement, such as we may note in the spinning top, though in the toy
the cause of the motion is not that which we are considering.
If, now, in that excellent field for the experiment we are essaying,
the mind's eye, we add a second planet outside of the single sphere
which we have so far supposed to journey about the sun, or rather
about the common centre of gravity, we perceive at once that we have
introduced an element which leads to a complication of much
importance. The new sphere would, of course, pull upon the others in
the measure of its gravitative value--i.e., its weight. The centre of
gravity of the system would now be determined not by two distinct
bodies, but by three. If we conceive the second planet to journey
around the sun at such a rate that a straight line always connected
the centres of the three orbs, then the only effect on their
gravitative centre would be to draw the first-mentioned planet a
little farther away from the centre of the sun; but in our own solar
system, and probably in all others, this supposition is inadmissible,
because the planets have longer journeys to go and also move slower,
the farther they are from the sun. Thus Mercury completes the circle
of its year in eighty-eight of our days, while the outermost planet
requires sixty thousand days (more than one hundred and sixty-four
years) for the same task. The result is not only that the centre of
gravity of the system is somewhat displaced--itself a matter of no
great account--but also that the orbit of the original planet ceases
to be circled and becomes elliptical, and this for the evident reason
that the sphere will be drawn somewhat away from the sun when the
second planet happens to lie in the part of its orbit immediately
outside of its position, in which case the pull is away from the solar
centre; while, on the other hand, when the new planet was on the other
side of the sun, its pull would serve to intensify the attraction
which drew the first sphere toward the centre of gravity. As the
pulling action of the three bodies upon each other, as well as upon
their equatorial protuberances, would vary with every change in their
relative position, however slight, the variations in the form of their
orbits, even if the spheres were but three in number, would be very
important. The consequences of these perturbations will appear in the
sequel.
In our solar system, though there are but eight great planets, the
group of asteroids, and perhaps a score of satellites, the variety of
orbital and axial movement which is developed taxes the computing
genius of the ablest astronomer. The path which our earth follows
around the sun, though it may in general and for convenience be
described as a variable ellipse, is, in fact, a line of such
complication that if we should essay a diagram of it on the scale of
this page it would not be possible to represent any considerable part
of its deviations. These, in fact, would elude depiction, even if the
draughtsman had a sheet for his drawing as large as the orbit itself,
for every particle of matter in space, even if it be lodged beyond the
limits of the farthest stars revealed to us by the telescope,
exercises a certain attraction, which, however small, is effective on
the mass of the earth. Science has to render its conclusions in
general terms, and we can safely take them as such; but in this, as in
other instances, it is well to qualify our acceptance of the
statements by the memory that all things are infinitely more
complicated than we can possibly conceive or represent them to be.
We have next to consider the rotations of the planetary spheres upon
their axes, together with the similar movement, or lack of it, in the
case of their satellites. This rotation, according to the nebular
hypothesis, may be explained by the movements which would set up in
the share of matter which was at first a ring of the solar nebula, and
which afterward gathered into the planetary aggregation. The way of it
may be briefly set forth as follows: Such a ring doubtless had a
diameter of some million miles; we readily perceive that the particles
of matter in the outer part of the belt would have a swifter movement
around the sun than those on the inside. When by some disturbance, as
possibly by the passage of a great meteoric body of a considerable
gravitative power, this ring was broken in two, the particles
composing it on either side would, because of their mutual attraction,
tend to draw away from the breach, widening that gap until the matter
of the broken ring was aggregated into a sphere of the star dust or
vapour. When the nebulous matter originally in the ring became
aggregated into a spherical form, it would, on account of the
different rates at which the particles were moving when they came
together, be the surer to fall in toward the centre, not in straight
lines, but in curves--in other words, the mass would necessarily take
on a movement of rotation essentially like that which we have
described in setting forth the nebular hypothesis.
In the stages of concentration the planetary nebulæ might well repeat
those through which the greater solar mass proceeded. If the volume of
the material were great, subordinate rings would be formed, which when
they broke and concentrated would constitute secondary planets or
satellites, such as our moon. For some reason as yet unknown the outer
planets--in fact, all those in the solar system except the two inner,
Venus and Mercury and the asteroids--formed such attendants. All these
satellite-forming rings have broken and concentrated except the inner
of Saturn, which remains as an intellectual treasure of the solar
system to show the history of its development.
To the student who is not seeking the fulness of knowledge which
astronomy has to offer, but desires only to acquaint himself with the
more critical and important of the heavenly phenomena which help to
explain the earth, these features of planetary movement should prove
especially interesting for the reason that they shape the history of
the spheres. As we shall hereafter see, the machinery of the earth's
surface, all the life which it bears, its winds and rains--everything,
indeed, save the actions which go on in the depths of the sphere--is
determined by the heat and light which come from the sun. The
conditions under which this vivifying tide is received have their
origin in the planetary motion. If our earth's path around the centre
of the system was a perfect circle, and if its polar axis lay at right
angles to the plane of its journey, the share of light and heat which
would fall upon any one point on the sphere would be perfectly
uniform. There would be no variations in the length of day or night;
no changes in the seasons; the winds everywhere would blow with
exceeding steadiness--in fact, the present atmospheric confusion would
be reduced to something like order. From age to age, except so far as
the sun itself might vary in the amount of energy which it radiated,
or lands rose up into the air or sunk down toward the sea level, the
climate of each region would be perfectly stable. In the existing
conditions the influences bring about unending variety. First of all,
the inclined position of the polar axis causes the sun apparently to
move across the heavens, so that it comes in an overhead position once
or twice in the year in quite half the area of the lands and seas.
This apparent swaying to and fro of the sun, due to the inclination of
the axis of rotation, also affects the width of the climatal belts on
either side of the equator, so that all parts of the earth receive a
considerable share of the sun's influence. If the axis of the earth's
rotation were at right angles to the plane of its orbit, there would
be a narrow belt of high temperature about the equator, north and
south of which the heat would grade off until at about the parallels
of fifty degrees we should find a cold so considerable and uniform
that life would probably fade away; and from those parallels to the
poles the conditions would be those of permanent frost, and of days
which would darken into the enduring night or twilight in the realm
of the far north and south. Thus the wide habitability of the earth is
an effect arising from the inclination of its polar axis.
[Illustration: Fig. 3.--Inclination of Planetary Orbits (from
Chambers).]
As the most valuable impression which the student can receive from his
study of Nature is that sense of the order which has made possible all
life, including his own, it will be well for him to imagine, as he may
readily do, what would be the effect arising from changes in relations
of earth and sun. Bringing the earth's axis in imagination into a
position at right angles to the plane of the orbit, he will see that
the effect would be to intensify the equatorial heat, and to rob the
high latitudes of the share which they now have. On moving the axis
gradually to positions where it approaches the plane of the orbit, he
will note that each stage of the change widens the tropic belt.
Bringing the polar axis down to the plane of the orbit, one hemisphere
would receive unbroken sunshine, the other remaining in perpetual
darkness and cold. In this condition, in place of an equatorial line
we should have an equatorial point at the pole nearest the sun; thence
the temperatures would grade away to the present equator, beyond which
half the earth would be in more refrigerating condition than are the
poles at the present day. In considering the movements of our planet,
we shall see that no great changes in the position of the polar axis
can have taken place. On this account the suggested alterations of the
axis should not be taken as other than imaginary changes.
It is easy to see that with a circular orbit and with an inclined axis
winter and summer would normally come always at the same point in the
orbit, and that these seasons would be of perfectly even length. But,
as we have before noted, the earth's path around the sun is in its
form greatly affected by the attractions which are exercised by the
neighbouring planets, principally by those great spheres which lie in
the realm without its orbit, Jupiter and Saturn. When these attracting
bodies, as is the case from time to time, though at long intervals,
are brought together somewhere near to that part of the solar system
in which the earth is moving around the sun, they draw our planet
toward them, and so make its path very elliptical. When, however, they
are so distributed that their pulling actions neutralize each other,
the orbit returns more nearly to a circular form. The range in its
eccentricity which can be brought about by these alterations is very
great. When the path is most nearly circular, the difference in the
major and minor axis may amount to as little as about five hundred
thousand miles, or about one one hundred and eighty-sixth of its
average diameter. When the variation is greatest the difference in
these measurements may be as much as near thirteen million miles, or
about one seventh of the mean width of the orbit.
The first and most evident effect arising from these changes of the
orbit comes from the difference in the amount of heat which the earth
may receive according as it is nearer or farther from the sun. As in
the case of other fires, the nearer a body is to it the larger the
share of light and heat which it will receive. In an orbit made
elliptical by the planetary attraction the sun necessarily occupies
one of the foci of the ellipse. The result is, of course, that the
side of the earth which is toward the sun, while it is thus brought
the nearer to the luminary, receives more energy in the form of light
and heat than come to any part which is exposed when the spheres are
farther away from each other in the other part of the orbit.
Computations clearly show that the total amount of heat and the
attendant light which the earth receives in a year is not affected by
these changes in the form of its path. While it is true that it
receives heat more rapidly in the half of the ellipse which is nearest
the source of the inundation, it obtains less while it is farther
away, and these two variations just balance each other.
Although the alterations in the eccentricity of its orbit do not vary
the annual supply of heat which the earth receives, they are capable
of changing the character of the seasons, and this in the way which we
will now endeavour to set forth, though we must do it at the cost of
considerable attention on the part of the reader, for the facts are
somewhat complicated. In the first place, we must note that the
ellipticity of the earth's orbit is not developed on fixed lines, but
is endlessly varied, as we can readily imagine it would be for the
reason that its form depends upon the wandering of the outer planetary
spheres which pull the earth about. The longer axis of the ellipse is
itself in constant motion in the direction in which the earth travels.
This movement is slow, and at an irregular rate. It is easy to see
that the effect of this action, which is called the revolution of the
apsides, or, as the word means, the movement of the poles of the
ellipse, is to bring the earth, when a given hemisphere is turned
toward the sun, sometimes in the part of the orbit which is nearest
the source of light and heat, and sometimes farther away. It may thus
well come about that at one time the summer season of a hemisphere
arrives when it is nearest the sun, so that the season, though hot,
will be very short, while at another time the same season will arrive
when the earth is farthest from the sun, and receives much less heat,
which would tend to make a long and relatively cool summer. The reason
for the difference in length of the seasons is to be found in the
relative swiftness of the earth's revolution when it is nearest the
sun, and the slowness when it is farther away.
There is a further complication arising from that curious phenomenon
called the precession of the equinoxes, which has to be taken into
account before we can sufficiently comprehend the effect of the
varying eccentricity of the orbit on the earth's seasons. To
understand this feature of precession we should first note that it
means that each year the change from the winter to the summer--or, as
we phrase it, the passage of the equinoctial line--occurs a little
sooner than the year before. The cause of this is to be found in the
attraction which the heavenly bodies, practically altogether the moon,
exercises on the equatorial protuberance of the earth. We know that
the diameter of our sphere at the equator is, on the average,
something more than twenty-six miles greater than it is through the
poles. We know, furthermore, that the position of the moon in relation
to the earth is such that it causes the attraction on one half of this
protuberance to be greater than it is upon the other. We readily
perceive that this action will cause the polar axis to make a certain
revolution, or, what comes to the same thing, that the plane of the
equator will constantly be altering its position. Now, as the
equinoctial points in the orbit depend for their position upon the
attitude of the equatorial plane, we can conceive that the effect is a
change in position of the place in that orbit where summer and winter
begin. The actual result is to bring the seasonal points backward,
step by step, through the orbit in a regular measure until in
twenty-two thousand five hundred years they return to the place where
they were before. This cycle of change was of old called the Annus
Magnus, or great year.
If the earth's orbit were an ellipse, the major axis of which remained
in the same position, we could readily reckon all the effects which
arise from the variations of the great year. But this ellipse is ever
changing in form, and in the measure of its departure from a circle
the effects on the seasons distributed over a great period of time are
exceedingly irregular. Now and then, at intervals of hundreds of
thousands or millions of years, the orbit becomes very elliptical;
then again for long periods it may in form approach a circle. When in
the state of extreme ellipticity, the precession of the equinoxes will
cause the hemispheres in turn each to have their winter and summer
alternately near and far from the sun. It is easily seen that when the
summer season comes to a hemisphere in the part of the orbit which is
then nearest the sun the period will be very hot. When the summer
came farthest from the sun that part of the year would have the
temperature mitigated by its removal to a greater distance from the
source of heat. A corresponding effect would be produced in the winter
season. As long as the orbit remained eccentric the tendency would be
to give alternately intense seasons to each hemisphere through periods
of about twelve thousand years, the other hemisphere having at the
same time a relatively slight variation in the summer and winter.
At first sight it may seem to the reader that these studies we have
just been making in matters concerning the shape of the orbit and the
attendant circumstances which regulate the seasons were of no very
great consequence; but, in the opinion of some students of climate, we
are to look to these processes for an explanation of certain climatal
changes on the earth, including the Glacial periods, accidents which
have had the utmost importance in the history of man, as well as of
all the other life of the planet.
It is now time to give some account as to what is known concerning the
general conditions of the solar bodies--the planets and satellites of
our own celestial group. For our purpose we need attend only to the
general physical state of these orbs so far as it is known to us by
the studies of astronomers. The nearest planet to the sun is Mercury.
This little sphere, less than half the diameter of our earth, is so
close to the sun that even when most favourably placed for observation
it is visible for but a few minutes before sunrise and after sunset.
Although it may without much difficulty be found by the ordinary eye,
very few people have ever seen it. To the telescope when it is in the
_full moon_ state it appears as a brilliant disk; it is held by most
astronomers that the surface which we see is made up altogether of
clouds, but this, as most else that has been stated concerning this
planet, is doubtful. The sphere is so near to the sun that if it were
possessed of water it would inevitably bear an atmosphere full of
vapour. Under any conceivable conditions of a planet placed as
Mercury is, provided it had an atmosphere to retain the heat, its
temperature would necessarily be very high. Life as we know it could
not well exist upon such a sphere.
Next beyond Mercury is Venus, a sphere only a little less in diameter
than the earth. Of this sphere we know more than we do of Mercury, for
the reason that it is farther from the sun and so appears in the
darkened sky. Most astronomers hold that the surface of this planet
apparently is almost completely and continually hidden from us by what
appears to be a dense cloud envelope, through which from time to time
certain spots appear of a dark colour. These, it is claimed, retain
their place in a permanent way; it is, indeed, by observing them that
the rotation period of the planet has, according to some observers,
been determined. It therefore seems likely that these spots are the
summits of mountains, which, like many of our own earth, rise above
the cloud level.
Recent observations on Venus made by Mr. Percival Lowell appear to
show that the previous determinations of the rotation of that planet,
as well as regards its cloud wrap, are in error. According to these
observations, the sphere moves about the sun, always keeping the same
side turned toward the solar centre, just as the moon does in its
motion around the earth. Moreover, Mr. Lowell has failed to discover
any traces of clouds upon the surface of the planet. As yet these
results have not been verified by the work of other astronomers;
resting, however, as they do on studies made with an excellent
telescope and in the very translucent and steady air of the Flagstaff
Station, they are more likely to be correct than those obtained by
other students. If it be true that Venus does not turn upon its axis,
such is likely to be the case also with the planet Mercury.
Next in the series of the planets is our own earth. As the details of
this planet are to occupy us during nearly all the remainder of this
work, we shall for the present pass it by.
Beyond the earth we pass first to the planet Mars, a sphere which has
already revealed to us much concerning its peculiarities of form and
physical state, and which is likely in the future to give more
information than we shall obtain from any other of our companions in
space, except perhaps the moon. Mars is not only nearer to us than any
other planet, but it is so placed that it receives the light of the
sun under favourable conditions for our vision. Moreover, its sky
appears to be generally almost cloudless, so that when in its orbital
course the sphere is nearest our earth it is under favourable
conditions for telescopic observation. At such times there is revealed
to the astronomer a surface which is covered with an amazing number of
shadings and markings which as yet have been incompletely interpreted.
The faint nature of these indications has led to very contradictory
statements as to their form; no two maps which have been drawn agree
except in their generalities. There is reason to believe that Mars has
an atmosphere; this is shown by the fact that in the appropriate
season the region about either pole is covered by a white coating,
presumably snow. This covering extends rather less far toward the
planet's equator than does the snow sheet on our continents. Taking
into account the colour of the coating, and the fact that it
disappears when the summer season comes to the hemisphere in which it
was formed, we are, in fact, forced to believe that the deposit is
frozen water, though it has been suggested that it may be frozen
carbonic acid. Taken in connection with what we have shortly to note
concerning the apparent seas of this sphere, the presumption is
overwhelmingly to the effect that Mars has seasons not unlike our own.
The existence of snow on any sphere may safely be taken as evidence
that there is an atmosphere. In the case of Mars, this supposition is
borne out by the appearance of its surface. The ruddy light which it
sends back to us, and the appearance on the margin of the sphere,
which is somewhat dim, appears to indicate that its atmosphere is
dense. In fact, the existence of an atmosphere much denser than that
of our own earth appears to be demanded by the fact that the
temperatures are such as to permit the coming and going of snow. It is
well known that the temperature of any point on the earth, other
things being equal, is proportionate to the depth of atmosphere above
its surface. If Mars had no more air over its surface than has an
equal area of the earth, it would remain at a temperature so low that
such seasonal changes as we have observed could not take place. The
planet receives one third less heat than an equal area of the earth,
and its likeness to our own temperature, if such exists, is doubtless
brought about by the greater density of its atmosphere, that serves to
retain the heat which comes upon its surface. The manner in which this
is effected will be set forth in the study of the earth's atmosphere.
[Illustration: Fig. 4.--Mars, August 27, 1892 (Guiot), the white patch
is the supposed Polar Snow Cap.]
As is shown by the maps of Mars, the surface is occupied by shadings
which seem to indicate the existence of water and lands. Those
portions of the area which are taken to be land are very much divided
by what appear to be narrow seas. The general geographic conditions
differ much from those of our own sphere in that the parts of the
planet about the water level are not grouped in great continents, and
there are no large oceans. The only likeness to the conditions of our
earth which we can perceive is in a general pointing of the somewhat
triangular masses of what appears to be land toward one pole. As a
whole, the conditions of the Martial lands and seas as regards their
form, at least, is more like that of Europe than that of any other
part of the earth's surface. Europe in the early Tertiary times had a
configuration even more like that of Mars than it exhibits at present,
for in that period the land was very much more divided than it now is.
If the lands of Mars are framed as are those of our own earth, there
should be ridges of mountains constituting what we may term the
backbones of the continent. As yet such have not been discerned, which
may be due to the fact that they have not been carefully looked for.
The only peculiar physical features which have as yet been discerned
on the lands of Mars are certain long, straight, rather narrow
crevicelike openings, which have received the name of "canals." These
features are very indistinct, and are just on the limit of visibility.
As yet they have been carefully observed by but few students, so that
their features are not yet well recorded; as far as we know them,
these fissures have no likeness in the existing conditions of our
earth. It is difficult to understand how they are formed or preserved
on a surface which is evidently subjected to rainfalls.
It will require much more efficient telescopes than we now have before
it will be possible to begin any satisfactory study on the geography
of this marvellous planet. We can not hope as yet to obtain any
indications as to the details of its structure; we can not see closely
enough to determine whether rivers exist, or whether there is a
coating which we may interpret as vegetation, changing its hues in the
different seasons of the year. An advance in our instruments of
research during the coming century, if made with the same speed as
during the last, will perhaps enable us to interpret the nature of
this neighbour, and thereby to extend the conception of planetary
histories which we derive from our own earth.
[Illustration: Fig. 5.--Comparative Sizes of the Planets (Chambers).]
Beyond Mars we find one of the most singular features of our solar
system in a group of small planetary bodies, the number of which now
known amounts to some two hundred, and the total may be far greater.
These bodies are evidently all small; it is doubtful if the largest is
three hundred and the smaller more than twenty miles in diameter. So
far as it has been determined by the effect of their aggregate mass in
attracting the other spheres, they would, if put together, make a
sphere far less in diameter than our earth, perhaps not more than five
hundred miles through. The forms of these asteroids is as yet unknown;
we therefore can not determine whether their shapes are spheroidal, as
are those of the other planets, or whether they are angular bits like
the meteorites. We are thus not in a position to conjecture whether
their independence began when the nebulous matter of the ring to which
they belonged was in process of consolidation, or whether, after the
aggregation of the sphere was accomplished, and the matter solidified,
the mass was broken into bits in some way which we can not yet
conceive. It has been conjectured that such a solid sphere might have
been driven asunder by a collision with some wandering celestial body;
but all we can conceive of such actions leads us to suppose that a
blow of this nature would tend to melt or convert materials subjected
to it into the state of vapour, rather than to drive them asunder in
the manner of an explosion.
The four planets which lie beyond the asteroids give us relatively
little information concerning their physical condition, though they
afford a wide field for the philosophic imagination. From this point
of view the reader is advised to consult the writings of the late R.A.
Proctor, who has brought to the task of interpreting the planetary
conditions the skill of a well-trained astronomer and a remarkable
constructive imagination.
The planet Jupiter, by far the largest of the children of the sun,
appears to be still in a state where its internal heat has not so far
escaped that the surface has cooled down in the manner of our earth.
What appear to be good observations show that the equatorial part of
its area, at least, still glows from its own heat. The sphere is
cloud-wrapped, but it is doubtful whether the envelope be of watery
vapour; it is, indeed, quite possible that besides such vapour it may
contain some part of the many substances which occupy the atmosphere
of the sun. If the Jovian sphere were no larger than the earth, it
would, on account of its greater age, long ago have parted with its
heat; but on account of its great size it has been able,
notwithstanding its antiquity, to retain a measure of temperature
which has long since passed away from our earth.
In the case of Saturn, the cloud bands are somewhat less visible than
on Jupiter, but there is reason to suppose in this, as in the
last-named planet, that we do not behold the more solid surface of the
sphere, but see only a cloud wrap, which is probably due rather to the
heat of the sphere itself than to that which comes to it from the sun.
At the distance of Saturn from the centre of the solar system a given
area of surface receives less than one ninetieth of the sun's heat as
compared with the earth; therefore we can not conceive that any
density of the atmosphere whatever would suffice to hold in enough
temperature to produce ordinary clouds. Moreover, from time to time
bright spots appear on the surface of the planet, which must be due to
some form of eruptions from its interior.
Beyond Saturn the two planets Uranus and Neptune, which occupy the
outer part of the solar system, are so remote that even our best
telescopes discern little more than their presence, and the fact that
they have attendant moons.
From the point of view of astronomical science, the outermost planet
Neptune, of peculiar interest for the reason that it was, as we may
say, discovered by computation. Astronomers had for many years
remarked the fact that the next inner planetary sphere exhibited
peculiarities in its orbit which could only be accounted for on the
supposition that it was subjected to the attraction of another
wandering body which had escaped observation. By skilful computation
the place in the heavens in which this disturbing element lay was so
accurately determined that when the telescope was turned to the given
field a brief study revealed the planet. Nothing else in the history
of the science of astronomy, unless it be the computation of eclipses,
so clearly and popularly shows the accuracy of the methods by which
the work of that science may be done.
As we shall see hereafter, in the chapters which are devoted to
terrestrial phenomena, the physical condition of the sun determines
the course of all the more important events which take place on the
surface of the earth. It is therefore fit that in this preliminary
study of the celestial bodies, which is especially designed to make
the earth more interpretable to us, we should give a somewhat special
attention to what is known under the title of "Solar Physics."
The reader has already been told that the sun is one of many million
similar bodies which exist in space, and, furthermore, that these
aggregations of matter have been developed from an original nebulous
condition. The facts indicate that the natural history of the sun, as
well as that of its attendant spheres, exhibits three momentous
stages: First, that of vapour; second, that of igneous fluidity;
third, that in which the sphere is so far congealed that it becomes
dark. Neither of these states is sharply separated from the other; a
mass may be partly nebulous and partly fluid; even when it has been
converted into fluid, or possibly into the solid state, it may still
retain on the exterior some share of its original vaporous condition.
In our sun the concentration has long since passed beyond the limits
of the nebulous state; the last of the successively developed rings
has broken, and has formed itself into the smallest of the planets,
which by its distance from the sun seems to indicate that the process
of division by rings long ago attained in our solar system its end,
the remainder of its nebulous material concentrating on its centre
without sign of any remaining tendency to produce these planet-making
circles.
THE CONSTITUTION OF THE SUN.
Before the use of the telescope in astronomical work, which was begun
by the illustrious Galileo in 1608, astronomers were unable to
approach the problem of the structure of the sun. They could discern
no more than can be seen by any one who looks at the great sphere
through a bit of smoked glass, as we know this reveals a disklike body
of very uniform appearance. The only variation in this simple aspect
occurs at the time of a total eclipse, when for a minute or two the
moon hides the whole body of the sun. On such occasions even the
unaided eye can see that there is about the sphere a broad, rather
bright field, of an aspect like a very thin cloud or fog, which rises
in streamer like projections at points to a quarter of a million miles
or more above the surface of the sphere. The appearance of this
shining field, which is called the corona, reminds one of the aurora
which glows in the region about either pole of the earth.
One of the first results of the invention of the telescope was the
revelation of the curious dark objects on the sun's disk, known by the
name of spots from the time of their discovery, or, at least, from the
time when it was clearly perceived that they were not planets, but
really on the solar body. The interest in the constitution of the
sphere has increased during the last fifty years. This interest has
rapidly grown until at the present time a vast body of learning has
been gathered for the solution of the many problems concerning the
centre of our system. As yet there is great divergence in the views of
astronomers as to the interpretation of their observations, but
certain points of great general interest have been tolerably well
determined. These may be briefly set forth by an account of what would
meet the eye if an observer were able to pass from the surface of the
earth to the central part of the sun.
[Illustration: _Lava stream, in Hawaiian Islands, flowing into the
sea. Note the "ropy" character of the half-frozen rock on the sides of
the nearest rivulet of the lava._]
In passing from the earth to a point about a quarter of a million
miles from the sun's surface--a distance about that of the moon from
our sphere--the observer would traverse the uniformly empty spaces of
the heavens, where, but for the rare chance of a passing meteorite or
comet, there would be nothing that we term matter. Arriving at a point
some two or three hundred thousand miles from the body of the sun, he
would enter the realm of the corona; here he would find scattered
particles of matter, the bits so far apart that there would perhaps be
not more than one or two in the cubic mile; yet, as they would glow
intensely in the central light, they would be sufficient to give the
illumination which is visible in an eclipse. These particles are most
likely driven up from the sun by some electrical action, and are
constantly in motion, much as are the streamers of the aurora.
Below the corona and sharply separated from it the observer finds
another body of very dense vapour, which is termed the chromosphere,
and which has been regarded as the atmosphere of the sun. This layer
is probably several thousand miles thick. From the manner in which it
moves, in the way the air of our own planet does in great storms, it
is not easy to believe that it is a fluid, yet its sharply defined
upper surface leads us to suppose that it can not well be a mere mass
of vapour. The spectroscope shows us that this chromosphere contains
in the state of vapour a number of metallic substances, such as iron
and magnesium. To an observer who could behold this envelope of the
sun from the distance at which we see the moon, the spectacle would be
more magnificent than the imagination, guided by the sight of all the
relatively trifling fractures of our earth, can possibly conceive.
From the surface of the fiery sea vast uprushes of heated matter rise
to the height of two or three hundred thousand miles, and then fall
back upon its surface. These jets of heated matter have the aspect of
flames, but they would not be such in fact, for the materials are not
burning, but merely kept at a high temperature by the heat of the
great sphere beneath. They spring up with such energy that they at
times move with a speed of one hundred and fifty miles a second, or at
a rate which is attained by no other matter in the visible universe,
except that strange, wandering star known to astronomers as
"Grombridge, 1830," which is traversing the firmament with a speed of
not less than two hundred miles a second.
Below the chromosphere is the photosphere, the lower envelope of the
sun, if it be not indeed the body of the sphere itself; from this
comes the light and heat of the mass. This, too, can not well be a
firm-set mass, for the reason that the spots appear to form in and
move over it. It may be regarded as an extremely dense mass of gas, so
weighed down by the vast attraction of the great sphere below it that
it is in effect a fluid. The near-at-hand observer would doubtless
find this photosphere, as it appears in the telescope, to be sharply
separated from the thinner and more vaporous envelopes--the
chromosphere and the corona--which are, indeed, so thin that they are
invisible even with the telescope, except when the full blaze of the
sun is cut off in a total eclipse. The fact that the photosphere,
except when broken by the so-called spots, lies like a great smooth
sea, with no parts which lie above the general line, shows that it has
a very different structure from the envelope which lies upon it. If
they were both vaporous, there would be a gradation between them.
On the surface of the photosphere, almost altogether within thirty
degrees of the equator of the sun, a field corresponding approximately
to the tropical belt of the earth, there appear from time to time the
curious disturbances which are termed spots. These appear to be
uprushes of matter in the gaseous state, the upward movement being
upon the margins of the field and a downward motion taking place in
the middle of the irregular opening, which is darkened in its central
part, thus giving it, when seen by an ordinary telescope, the aspect
of a black patch on the glowing surface. These spots, which are from
some hundred to some thousand miles in diameter, may endure for
months before they fade away. It is clear that they are most abundant
at intervals of about eleven years, the last period of abundance being
in 1893. The next to come may thus be expected in 1904. In the times
of least spotting more than half the days of a year may pass without
the surface of the photosphere being broken, while in periods of
plenty no day in the year is likely to fail to show them.
[Illustration: Fig. 6.--Ordinary Sun-spot, June 22, 1885.]
It is doubtful if the closest seeing would reveal the cause of the
solar spots. The studies of the physicists who have devoted the most
skill to the matter show little more than that they are tumults in the
photosphere, attended by an uprush of vapours, in which iron and other
metals exist; but whether these movements are due to outbreaks from
the deeper parts of the sun or to some action like the whirling storms
of the earth's atmosphere is uncertain. It is also uncertain what
effect these convulsions of the sun have on the amount of the heat and
light which is poured forth from the orb. The common opinion that the
sun-spot years are the hottest is not yet fully verified.
Below the photosphere lies the vast unknown mass of the unseen solar
realm. It was at one time supposed that the dark colour of the spots
was due to the fact that the photosphere was broken through in those
spaces, and that we looked down through them upon the surface of the
slightly illuminated central part of the sphere. This view is
untenable, and in its place we have to assume that for the eight
hundred and sixty thousand miles of its diameter the sun is composed
of matter such as is found in our earth, but throughout in a state of
heat which vastly exceeds that known on or in our planet. Owing to its
heat, this matter is possibly not in either the solid or the fluid
state, but in that of very compressed gases, which are kept from
becoming solid or even fluid by the very high temperature which exists
in them. This view is apparently supported by the fact that, while the
pressure upon its matter is twenty-seven times greater in the sun than
it is in the earth, the weight of the whole mass is less than we
should expect under these conditions.
As for the temperature of the sun, we only know that it is hot enough
to turn the metals into gases in the manner in which this is done in a
strong electric arc, but no satisfactory method of reckoning the scale
of this heat has been devised. The probabilities are to the effect
that the heat is to be counted by the tens of thousands of degrees
Fahrenheit, and it may amount to hundreds of thousands; it has,
indeed, been reckoned as high as a million degrees. This vast
discharge is not due to any kind of burning action--i.e., to the
combustion of substances, as in a fire. It must be produced by the
gradual falling in of the materials, due to the gravitation of the
mass toward its centre, each particle converting its energy of
position into heat, as does the meteorite when it comes into the air.
It is well to close this very imperfect account of the learning which
relates to the sun with a brief tabular statement showing the relative
masses of the several bodies of the solar system. It should be
understood that by mass is meant not the bulk of the object, but the
actual amount of matter in it as determined by the gravitative
attraction which it exercises on other celestial bodies. In this test
the sun is taken as the measure, and its mass is for convenience
reckoned at 1,000,000,000.
TABLE OF RELATIVE MASSES OF SUN AND PLANETS.[2]
+------------------------------------------------------------+
| The sun 1,000,000,000 |
| Mercury 200 |
| Venus 2,353 |
| Earth 3,060 |
| Mars 339 |
| Asteroids ? |
| Saturn 285,580 |
| Jupiter 954,305 |
| Uranus 44,250 |
| Neptune 51,600 |
| Combined mass of the four inner planets 5,952 |
| Combined mass of all the planets 1,341,687 |
+------------------------------------------------------------+
[Footnote 2: See Newcomb's Popular Astronomy, p. 234. Harper Brothers,
New York.]
It thus appears that the mass of all the planets is about one seven
hundredth that of the sun.
Those who wish to make a close study of celestial geography will do
well to procure the interesting set of diagrams prepared by the late
James Freeman Clarke, in which transparencies placed in a convenient
lantern show the grouping of the important stars in each
constellation. The advantage of this arrangement is that the little
maps can be consulted at night and in the open air in a very
convenient manner. After the student has learned the position of a
dozen of the constellations visible in the northern hemisphere, he can
rapidly advance his knowledge in the admirable method invented by Dr.
Clarke.
Having learned the constellations, the student may well proceed to
find the several planets, and to trace them in their apparent path
across the fixed stars. It will be well for him here to gain if he can
the conception that their apparent movement is compounded of their
motion around the sun and that of our own sphere; that it would be
very different if our earth stood still in the heavens. At this stage
he may well begin to take in mind the evidence which the planetary
motion supplies that the earth really moves round the sun, and not
the sun and planets round the earth. This discovery was one of the
great feats of the human mind; it baffled the wits of the best men for
thousands of years. Therefore the inquirer who works over the evidence
is treading one of the famous paths by which his race climbed the
steeps of science.
The student must not expect to find the evidence that the sun is the
centre of the solar system very easy to interpret; and yet any youth
of moderate curiosity, and that interest in the world about him which
is the foundation of scientific insight, can see through the matter.
He will best begin his inquiries by getting a clear notion of the fact
that the moon goes round the earth. This is the simplest case of
movements of this nature which he can see in the solar system. Noting
that the moon occupies a different place at a given hour in the
twenty-four, but is evidently at all times at about the same distance
from the earth, he readily perceives that it circles about our sphere.
This the people knew of old, but they made of it an evidence that the
sun also went around our sphere. Here, then, is the critical point.
Why does the sun not behave in the same manner as the moon? At this
stage of his inquiry the student best notes what takes place in the
motions of the planets between the earth and the sun. He observes that
those so-called inferior planets Mercury and Venus are never very far
away from the central body; that they appear to rise up from it, and
then to go back to it, and that they have phases like the moon. Now
and then Venus may be observed as a black spot crossing the disk of
the sun. A little consideration will show that on the theory that
bodies revolve round each other in the solar system these movements of
the inner planets can only be explained on the supposition that they
at least travel around the great central fire. Now, taking up the
outer planets, we observe that they occasionally appear very bright,
and that they are then at a place in the heavens where we see that
they are far from the solar centre. Gradually they move down toward
the sunset and disappear from view. Here, too, the movement, though
less clearly so, is best reconcilable with the idea that these bodies
travel in orbits, such as those which are traversed by the inner
planets. The wonder is that with these simple facts before them, and
with ample time to think the matter over, the early astronomers did
not learn the great truth about the solar system--namely, that the sun
is the centre about which the planets circled. Their difficulty lay
mainly in the fact that they did not conceive the earth as a sphere,
and even after they attained that conception they believed that our
globe was vastly larger than the planets, or even than the sun. This
misconception kept even the thoughtful Greeks, who knew that the earth
was spherical in form, from a clear notion as to the structure of our
system. It was not, indeed, until mathematical astronomy attained a
considerable advance, and men began to measure the distances in the
solar system, and until the Newtonian theory of gravitation was
developed, that the planetary orbits and the relation of the various
bodies in the solar system to each other could be perfectly discerned.
Care has been taken in the above statements to give the student
indices which may assist him in working out for himself the evidence
which may properly lead a person, even without mathematical
considerations of a formal kind, to construct a theory as to the
relation of the planets to the sun. It is not likely that he can go
through all the steps of this argument at once, but it will be most
useful to him to ponder upon the problem, and gradually win his way to
a full understanding of it. With that purpose in mind, he should avoid
reading what astronomers have to say on the matter until he is
satisfied that he has done as much as he can with the matter on his
own account. He should, however, state his observations, and as far as
possible draw the results in his note-book in a diagrammatic form. He
should endeavour to see if the facts are reconcilable with any other
supposition than that the earth and the other planets move around the
sun. When he has done his task, he will have passed over one of the
most difficult roads which his predecessors had to traverse on their
way to an understanding of the heavens. Even if he fail he will have
helped himself to some large understandings.
The student will find it useful to make a map of the heavens, or
rather make several representing their condition at different times in
the year. On this plot he should put down only the stars whose places
and names he has learned, but he should plot the position of the
planets at different times. In this way, though at first his efforts
will be very awkward, he will soon come to know the general geography
of the heavens.
Although the possession or at least the use of a small astronomical
telescope is a great advantage to a student after he has made a
certain advance in his work, such an instrument is not at all
necessary, or, indeed, desirable at the outset of his studies. An
ordinary opera-glass, however, will help him in picking out the stars
in the constellations, in identifying the planets, and in getting a
better idea as to the form of the moon's surface--a matter which will
be treated in this work in connection with the structure of the earth.
CHAPTER IV.
THE EARTH.
In beginning the study of the earth it is important that the student
should at once form the habit of keeping in mind the spherical form of
the planet. Many persons, while they may blindly accept the fact that
the earth is a sphere, do not think of it as having that form. Perhaps
the simplest way of securing the correct image of the shape is to
imagine how the earth would appear as seen from the moon. In its full
condition the moon is apt to appear as a disk. When it is new, and
also when in its waning stages it is visible in the daytime, the
spherical form is very apparent. Imagining himself on the surface of
the moon, the student can well perceive how the earth would appear as
a vast body in the heavens; its eight thousand miles of diameter,
about four times that of the satellite, would give an area sixteen
times the size which the moon presents to us. On this scale the
continents and oceans would appear very much more plain than do the
relatively slight irregularities on the lunar surface.
With the terrestrial globe in hand, the student can readily construct
an image which will represent, at least in outline, the appearance
which the sphere he inhabits would present when seen from a distance
of about a quarter of a million miles away. The continent of
Europe-Asia would of itself appear larger than all the lunar surface
which is visible to us. Every continent and all the greater islands
would be clearly indicated. The snow covering which in the winter of
the northern hemisphere wraps so much of the land would be seen to
come and go in the changes of the seasons; even the permanent ice
about either pole, and the greater regions of glaciers, such as those
of the Alps and the Himalayas, would appear as brilliant patches of
white amid fields of darker hue. Even the changes in the aspect of the
vegetation which at one season clothes the wide land with a green
mantle, and at another assumes the dun hue of winter, would be, to the
unaided eye, very distinct. It is probable that all the greater rivers
would be traceable as lines of light across the relatively dark
surface of the continents. By such exercises of the constructive
imagination--indeed, in no other way--the student can acquire the
habit of considering the earth as a vast whole. From time to time as
he studies the earth from near by he should endeavour to assemble the
phenomena in the general way which we have indicated.
The reader has doubtless already learned that the earth is a slightly
flattened sphere, having an average diameter of about eight thousand
miles, the average section at the equator being about twenty-six miles
greater than that from pole to pole. In a body of such large
proportions this difference in measurement appears not important; it
is, however, most significant, for it throws light upon the history of
the earth's mass. Computation shows that the measure of flattening at
the poles is just what would occur if the earth were or had been at
the time when it assumed its present form in a fluid condition. We
readily conceive that a soft body revolving in space, while all its
particles by gravitation tended to the centre, would in turning
around, as our earth does upon its axis, tend to bulge out in those
parts which were remote from the line upon which the turning took
place. Thus the flattening of our sphere at the poles corroborates the
opinion that its mass was once molten--in a word, that its ancient
history was such as the nebular theory suggests.
Although we have for convenience termed the earth a flattened
spheroid, it is only such in a very general sense. It has an infinite
number of minor irregularities which it is the province of the
geographer to trace and that of the geologist to account for. In the
first place, its surface is occupied by a great array of ridges and
hollows. The larger of these, the oceans and continents, first deserve
our attention. The difference in altitude of the earth's surface from
the height of the continents to the deepest part of the sea is
probably between ten and eleven miles, thus amounting to about two
fifths of the polar flattening before noted. The average difference
between the ocean floor and the summits of the neighbouring continents
is probably rather less than four miles. It happens, most fortunately
for the history of the earth, that the water upon its surface fills
its great concavities on the average to about four fifths of their
total depth, leaving only about one fifth of the relief projecting
above the ocean level. We have termed this arrangement fortunate, for
it insures that rainfall visits almost all the land areas, and thereby
makes those realms fit for the uses of life. If the ocean had only
half its existing area, the lands would be so wide that only their
fringes would be fertile. If it were one fifth greater than it is, the
dry areas would be reduced to a few scattered islands.
From all points of view the most important feature of the earth's
surface arises from its division into land and water areas, and this
for the reason that the physical and vital work of our sphere is
inevitably determined by this distribution. The shape of the seas and
lands is fixed by the positions at which the upper level of the great
water comes against the ridges which fret the earth's surface. These
elevations are so disposed that about two thirds of the hard mass is
at the present time covered with water, and only one third exposed to
the atmosphere. This proportion is inconstant. Owing to the endless
up-and-down goings of the earth's surface, the place of the shore
lines varies from year to year, and in the geological ages great
revolutions in the forms and relative area of water and land are
brought about.
Noting the greater divisions of land and water as they are shown on a
globe, we readily perceive that those parts of the continental ridges
which rise above the sea level are mainly accumulated in the northern
hemisphere--in fact, far more than half the dry realm is in that part
of the world. We furthermore perceive that all the continents more or
less distinctly point to the southward; they are, in a word,
triangles, with their bases to the northward, and their apices,
usually rather acute, directed to the southward. This form is very
well indicated in three of the great lands, North and South America
and Africa; it is more indistinctly shown in Asia and in Australia. As
yet we do not clearly understand the reason why the continents are
triangular, why they point toward the south pole, or why they are
mainly accumulated in the northern hemisphere. As stated in the
chapter on astronomy, some trace of the triangular form appears in the
land masses of the planet Mars. There, too, these triangles appear to
point toward one pole.
Besides the greater lands, the seas are fretted by a host of smaller
dry areas, termed islands. These, as inquiry has shown, are of two
very diverse natures. Near the continents, practically never more than
a thousand miles from their shores, we find isles, often of great
size, such as Madagascar, which in their structure are essentially
like the continents--that is, they are built in part or in whole of
non-volcanic rocks, sandstones, limestones, etc. In most cases these
islands, to which we may apply the term continental, have at some time
been connected with the neighbouring mainland, and afterward separated
from it by a depression of the surface which permitted the sea to flow
over the lowlands. Geologists have traced many cases where in the past
elevations which are now parts of a continent were once islands next
its shore. In the deeper seas far removed from the margins of the
continents the islands are made up of volcanic ejections of lava,
pumice, and dust, which has been thrown up from craters and fallen
around their margin or are formed of coral and other organic remains.
Next after this general statement as to the division of sea and land
we should note the peculiarities which the earth's surface exhibits
where it is bathed by the air, and where it is covered by the water.
Beginning with the best-known region, that of the dry land, we observe
that the surface is normally made up of continuous slopes of varying
declivity, which lead down from the high points to the sea. Here and
there, though rarely, these slopes centre in a basin which is occupied
by a lake or a dead sea. On the deeper ocean floors, so far as we may
judge with the defective information which the plumb line gives us,
there is no such continuity in the downward sloping of the surface,
the area being cast into numerous basins, each of great extent.
When we examine in some detail the shape of the land surface, we
readily perceive that the continuous down slopes are due to the
cutting action of rivers. In the basin of a stream the waters act to
wear away the original heights, filling them into the hollows, until
the whole area has a continuous down grade to the point where the
waters discharge into the ocean or perhaps into a lake. On the bottom
of the sea, except near the margin of the continent, where the floor
may in recent geological times have been elevated into the air, and
thus exposed to river action, there is no such agent working to
produce continuous down grades.
Looking upon a map of a continent which shows the differences in
altitude of the land, we readily perceive that the area is rather
clearly divided into two kinds of surface, mountains and plains, each
kind being sharply distinguished from the other by many important
peculiarities. Mountains are characteristically made up of distinct,
more or less parallel ridges and valleys, which are grouped in very
elongated belts, which, in the case of the American Cordilleras,
extend from the Arctic to the Antarctic Circle. Only in rare instances
do we find mountains occupying an area which is not very distinctly
elongated, and in such cases the elevations are usually of no great
height. Plains, on the other hand, commonly occupy the larger part of
the continent, and are distributed around the flanks of the mountain
systems. There is no rule as to their shape; they normally grade away
from the bases of the mountains toward the sea, and are often
prolonged below the level of the water for a considerable distance
beyond the shore, forming what is commonly known as the continental
shelf or belt of shallows along the coast line. We will now consider
some details concerning the form and structure of mountains.
In almost any mountain region a glance over the surface of the country
will give the reader a clew to the principal factor which has
determined the existence of these elevations. Wherever the bed rocks
are revealed he will recognise the fact that they have been much
disturbed. Almost everywhere the strata are turned at high angles;
often their slopes are steeper than those of house roofs, and not
infrequently they stand in attitudes where they appear vertical. Under
the surface of plains bedded rocks generally retain the nearly
horizontal position in which all such deposits are most likely to be
found. If the observer will attentively study the details of position
of these tilted rocks of mountainous districts, he will in most cases
be able to perceive that the beds have been flexed or folded in the
manner indicated by the diagram. Sometimes, though rarely, the tops of
these foldings or arches have been preserved, so that the nature of
the movement can be clearly discerned. More commonly the upper parts
of the upward-arching strata have been cut off by the action of the
decay-bringing forces--frost, flowing water, or creeping ice in
glaciers--so that only the downward pointing folds which were formed
in the mountain-making are well preserved, and these are almost
invariably hidden within the earth.
[Illustration: Fig. 7.--Section of mountains. Rockbridge and Bath
counties, Va. (from Dana). The numbers indicate the several
formations.]
By walking across any considerable mountain chain, as, for instance,
that of the Alleghanies, it is generally possible to trace a number of
these parallel up-and-down folds of the strata, so that we readily
perceive that the original beds had been packed together into a much
less space than they at first occupied. In some cases we could prove
that the shortening of the line has amounted to a hundred miles or
more--in other words, points on the plain lands on either side of the
mountain range which now exists may have been brought a hundred miles
or so nearer together than they were before the elevations were
produced. The reader can make for himself a convenient diagram showing
what occurred by pressing a number of leaves of this book so that the
sheets of paper are thrown into ridges and furrows. By this experiment
he also will see that the easiest way to account for such foldings as
we observe in mountains is by the supposition that some force residing
in the earth tends to shove the beds into a smaller space than they
originally occupied. Not only are the rocks composing the mountains
much folded, but they are often broken through after the manner of
masonry which has been subjected to earthquake shocks, or of ice which
has been strained by the expansion that affects it as it becomes
warmed before it is melted. In fact, many of our small lakes in New
England and in other countries of a long winter show in a miniature
way during times of thawing ice folds which much resemble mountain
arches.
At first geologists were disposed to attribute all the phenomena of
mountain-folding to the progressive cooling of the earth. Although
this sphere has already lost a large part of the heat with which it
was in the beginning endowed, it is still very hot in its deeper
parts, as is shown by the phenomena of volcanoes. This internal heat,
which to the present day at the depth of a hundred miles below the
surface is probably greater than that of molten iron, is constantly
flowing away into space; probably enough of it goes away on the
average each day to melt a hundred cubic miles or more of ice, or, in
more scientific phrase, the amount of heat rendered latent by melting
that volume of frozen water. J.R. Meyer, an eminent physicist,
estimated the quantity of heat so escaping each day of the year to be
sufficient to melt two hundred and forty cubic miles of ice. The
effect of this loss of heat is constantly to shrink the volume of the
earth; it has, indeed, been estimated that the sphere on this account
contracts on the average to the amount of some inches each thousand
years. For the reason that almost all this heat goes from the depths
of the earth, the cool outer portion losing no considerable part of
it, the contraction that is brought about affects the interior
portions of the sphere alone. The inner mass constantly shrinking as
it loses heat, the outer, cold part is by its weight forced to settle
down, and can only accomplish this result by wrinkling. An analogous
action may be seen where an apple or a potato becomes dried; in this
case the hard outer rind is forced to wrinkle, because, losing no
water, it does not diminish in its extent, and can only accommodate
itself to the interior by a wrinkling process. In one case it is water
which escapes, in the other heat; but in both contraction of the part
which suffers the loss leads to the folding of the outside of the
spheroid.
Although this loss of heat on the part of the earth accounts in some
measure for the development of mountains, it is not of itself
sufficient to explain the phenomena, and this for the reason that
mountains appear in no case to develop on the floors of the wide sea.
The average depth of the ocean is only fifteen thousand feet, while
there are hundreds, if not thousands, of mountain crests which exceed
that height above the sea. Therefore if mountains grew on the sea
floor as they do upon the land, there should be thousands of peaks
rising above the plain of the waters, while, in fact, all of the
islands except those near the shores of continents are of volcanic
origin--that is, are lands of totally different nature.
Whenever a considerable mountain chain is formed, although the actual
folding of the beds is limited to the usually narrow field occupied by
these disturbances, the elevation takes place over a wide belt of
country on one or both sides of the range. Thus if we approach the
Rocky Mountains from the Mississippi Valley, we begin to mount up an
inclined plane from the time we pass westward from the Mississippi
River. The beds of rock as well as the surface rises gradually until
at the foot of the mountain; though the rocks are still without
foldings, they are at a height of four or five thousand feet above the
sea. It seems probable--indeed, we may say almost certain--that when
the crust is broken, as it is in mountain-building, by extensive folds
and faults, the matter which lies a few score miles below the crust
creeps in toward those fractures, and so lifts up the country on which
they lie. When we examine the forms of any of our continents, we find
that these elevated portions of the earth's crust appear to be made up
of mountains and the table-lands which fringe those elevations. There
is not, as some of our writers suppose, two different kinds of
elevation in our great lands--the continents and the mountains which
they bear--but one process of elevation by which the foldings and the
massive uplifts which constitute the table-lands are simultaneously
and by one process formed.
Looking upon continents as the result of mountain growth, we may say
that here and there on the earth's crust these dislocations have
occurred in such association and of such magnitude that great areas
have been uplifted above the plain of the sea. In general, we find
these groups of elevations so arranged that they produce the
triangular form which is characteristic of the great lands. It will be
observed, for instance, that the form of North America is in general
determined by the position of the Appalachian and Cordilleran systems
on its eastern and western margins, though there are a number of
smaller chains, such as the Laurentians in Canada and the ice-covered
mountains of Greenland, which have a measure of influence in fixing
its shore lines.
[Illustration: _Waterfall near Gadsden, Alabama. The upper shelf of
rock is a hard sandstone, the lower beds are soft shale. The
conditions are those of most waterfalls, such as Niagara._]
The history of plains, as well as that of mountains, will have further
light thrown upon it when in the next chapter we come to consider the
effect of rain water on the land. We may here note the fact that the
level surfaces which are above the seashores are divisible into two
main groups--those which have been recently lifted above the sea
level, composed of materials laid down in the shallows next the shore,
and which have not yet shared in mountain-building disturbances, and
those which have been slightly tilted in the manner before indicated
in the case of the plains which border the Rocky Mountains on the
east. The great southern plain of eastern and southern United States,
extending from near New York to Mexico, is a good specimen of the
level lands common on all the continents which have recently emerged
from the sea. The table-lands on either side of the Mississippi
Valley, sloping from the Alleghanies and the Cordilleras, represent
the more ancient type of plain which has already shared in the
elevation which mountain-building brings about. In rarer cases plains
of small area are formed where mountains formerly existed by the
complete moving down of the original ridges.
There is a common opinion that the continents are liable in the course
of the geologic ages to very great changes of position; that what is
now sea may give place to new great lands, and that those already
existing may utterly disappear. This opinion was indeed generally held
by geologists not more than thirty years ago. Further study of the
problem has shown us that while parts of each continent may at any
time be depressed beneath the sea, the whole of its surface rarely if
ever goes below the water level. Thus, in the case of North America,
we can readily note very great changes in its form since the land
began to rise above the water. But always, from that ancient day to
our own, some portion of the area has been above the level of the sea,
thus providing an ark of refuge for the land life when it was
disturbed by inundations. The strongest evidence in favour of the
opinion that the existing continents have endured for many million
years is found in the fact that each of the great lands preserves many
distinct groups of animals and plants which have descended from
ancient forms dwelling upon the same territory. If at any time the
relatively small continent of Australia had gone beneath the sea, all
of the curious pouched animals akin to the opossum and kangaroo which
abound in that country--creatures belonging in the ancient life of the
world--would have been overwhelmed.
We have already noted the fact that the uplifting of mountains and of
the table-lands about them, which appears to have been the basis of
continental growth, has been due to strains in the rocks sufficiently
strong to disturb the beds. At each stage of the mountain-building
movement these compressive strains have had to contend with the very
great weight of the rocks which they had to move. These lands are not
to be regarded as firm set or rigid arches, but as highly elastic
structures, the shapes of which may be determined by any actions which
put on or take off burden. We see a proof of this fact from numerous
observations which geologists are now engaged in making. Thus during
the last ice epoch, when almost all the northern part of this
continent, as well as the northern part of Europe, was covered by an
ice sheet several thousand feet thick, the lands sank down under their
load, and to an extent roughly proportional to the depth of the icy
covering. While the northern regions were thus tilted down by the
weight which was upon them, the southern section of this land, the
region about the Gulf of Mexico, was elevated much above its present
level; it seems likely, indeed, that the peninsula of Florida rose to
the height of several hundred feet above its present shore line. After
the ice passed away the movements were reversed, the northern region
rising and the southern sinking down. These movements are attested by
the position of the old shore lines formed during the later stages of
the Glacial epoch. Thus around Lake Ontario, as well as the other
Great Lakes, the beaches which mark the higher positions of those
inland seas during the closing stages of the ice time, and which, of
course, were when formed horizontal, now rise to the northward at the
rate of from two to five feet for each mile of distance. Recent
studies by Mr. G.K. Gilbert show that this movement is still in
progress.
Other evidence going to show the extent to which the movements of the
earth's crust are affected by the weight of materials are found in the
fact that wherever along the shores thick deposits of sediments are
accumulated the tendency of the region where they lie is gradually to
sink downward, so that strata having an aggregate thickness of ten
thousand feet or more may be accumulated in a sea which was always
shallow. The ocean floor, in general, is the part of the earth's
surface where strata are constantly being laid down. In the great
reservoir of the waters the _débris_ washed from the land, the dust
from volcanoes, and that from the stellar spaces, along with the vast
accumulation of organic remains, almost everywhere lead to the
steadfast accumulation of sedimentary deposits. On the other hand, the
realms of the surface above the ocean level are constantly being worn
away by the action of the rivers and glaciers, of the waves which beat
against the shores, and of the winds which blow over desert regions.
The result is that the lands are wearing down at the geologically
rapid average rate of somewhere about one foot in five thousand years.
All this heavy matter goes to the sea bottoms. Probably to this cause
we owe in part the fact that in the wrinklings of the crust due to the
contraction of the interior the lands exhibit a prevailing tendency to
uprise, while the ocean floors sink down. In this way the continents
are maintained above the level of the sea despite the powerful forces
which are constantly wearing their substance away, while the seas
remain deep, although they are continually being burdened with
imported materials.
[Illustration: Fig. 8.--Diagram showing the effect of the position of
the fulcrum point in the movement of the land masses. In diagrams I
and II, the lines _a b_ represent the land before the movement, and
_a' b'_ its position after the movement; _s_, _s_, the position of the
shore line; _p_, _p_, the pivotal points; _l_, _s_, the sea line. In
diagram III, the curved line designates a shore; the line _a b_,
connecting the pivotal points _p_, _p_, is partly under the land and
partly under the sea.]
It is easy to see that if the sea floors tend to sink downward, while
the continental lands uprise, the movements which take place may be
compared with those which occur in a lever about a fulcrum point. In
this case the sea end of the bar is descending and the land end
ascending. Now, it is evident that the fulcrum point may fall to the
seaward or to the landward of the shore; only by chance and here and
there would it lie exactly at the coast line. By reference to the
diagram (Fig. 8), it will be seen that, while the point of rotation is
just at the shore, a considerable movement may take place without
altering the position of the coast line. Where the point of no
movement is inland of the coast, the sea will gain on the continent;
where, however, the point is to seaward, beneath the water, the land
will gain on the ocean. In this way we can, in part at least, account
for the endless changes in the attitude of the land along the coastal
belt without having to suppose that the continents cease to rise or
the sea floors to sink downward. It is evident that the bar or section
of the rocks from the interior of the land to the bottoms of the seas
is not rigid; it is also probable that the matter in the depths of the
earth, which moves with the motions of this bar, would change the
position of the fulcrum point from time to time. Thus it may well come
about that our coast lines are swaying up and down in ceaseless
variation.
In very recent geological times, probably since the beginning of the
last Glacial period, the region about the Dismal Swamp in Virginia has
swayed up and down through four alternating movements to the extent of
from fifty to one hundred feet. The coast of New Jersey is now sinking
at the rate of about two feet in a hundred years. The coast of New
England, though recently elevated to the extent of a hundred feet or
more, at a yet later time sank down, so that at some score of points
between New York and Eastport, Me., we find the remains of forests
with the roots of their trees still standing below high-tide mark in
positions where the trees could not have grown. Along all the marine
coasts of the world which have been carefully studied from this point
of view there are similar evidences of slight or great modern changes
in the level of the lands. At some points, particularly on the coast
of Alaska and along the coast of Peru, these uplifts of the land have
amounted to a thousand feet or more. In the peninsular district of
Scandinavia the swayings, sometimes up and sometimes down, which are
now going on have considerably changed the position of the shore lines
since the beginning of the historical period.
There are other causes which serve to modify the shapes and sizes of
the continents which may best be considered in the sequel; for the
present we may pass from this subject with the statement that our
great lands are relatively permanent features; their forms change from
age to age, but they have remained for millions of years habitable to
the hosts of animals and plants which have adapted their life to the
conditions which these fields afford them.
CHAPTER V.
THE ATMOSPHERE.
The firm-set portion of the earth, composed of materials which became
solid when the heat so far disappeared from the sphere that rocky
matter could pass from its previous fluid condition to the solid or
frozen state, is wrapped about by two great envelopes, the atmosphere
and the waters. Of these we shall first consider the lighter and more
universal air; in taking account of its peculiarities we shall have to
make some mention of the water with which it is greatly involved;
afterward we shall consider the structure and functions of that fluid.
Atmospheric envelopes appear to be common features about the celestial
spheres. In the sun there is, as we have noted, a very deep envelope
of this sort which is in part composed of the elements which form our
own air; but, owing to the high temperature of the sphere, these are
commingled with many substances which in our earth--at least in its
outer parts--have entered in the solid state. Some of the planets, so
far as we can discern their conditions, seem also to have gaseous
wraps; this is certainly the case with the planet Mars, and even the
little we know of the other like spheres justifies the supposition
that Jupiter and Saturn, at least, have a like constitution. We may
regard an atmosphere, in a word, as representing a normal and
long-continued state in the development of the heavenly orbs. In only
one of these considerable bodies of the solar system, the moon, do we
find tolerably clear evidence that there is no atmosphere.
The atmosphere of the earth is composed mainly of very volatile
elements, known as nitrogen and argon. This is commingled with oxygen,
also a volatile element. Into this mass a number of other substances
enter in varying but always relatively very small proportions. Of
these the most considerable are watery vapour and carbon dioxide; the
former of these rarely amounts to one per cent of the weight of the
air, considering the atmosphere as a whole, and the latter is never
more than a small fraction of one per cent in amount. As a whole, the
air envelope of the earth should be regarded as a mass of nitrogen and
argon, which only rarely, under the influence of conditions which
exist in the soil, enters into combinations with other elements by
which it assumes a solid form. The oxygen, though a permanent element
in the atmosphere, tends constantly to enter into combinations which
fix it temporarily or permanently in the earth, in which it forms,
indeed, in its combined state about one half the weight of all the
mineral substances we know. The carbon dioxide, or carbonic-acid gas,
as it is commonly termed, is a most important substance, as it affords
plants all that part of their bodies which disappear on burning. It is
constantly returned to the atmosphere by the decay of organic matter,
as well as by volcanic action.
In addition to the above-noted materials composing the air, all of
which are imperatively necessary to the wonderful work accomplished by
that envelope, we find a host of other substances which are
accidentally, variably, and always in small quantities contained in
this realm. Thus near the seashores, and indeed for a considerable
distance into the continent, we find the air contains a certain amount
of salt so finely divided that it floats in the atmosphere. So, too,
we find the air, even on the mountain tops amid eternal snows, charged
with small particles of dust, which, though not evident to the
unassisted eye, become at once visible when we permit a slender ray of
light to enter a dark chamber.
It is commonly asserted that the atmosphere does not effectively
extend above the height of forty-five miles; we know that it is
densest on the surface of the earth, the most so in those depressions
which lie below the level of the sea. This is proved to us by the
weight which the air imposes upon the mercury at the open end of a
barometric tube. If we could deepen these cavities to the extent of a
thousand miles, the pressure would become so great that if the pit
were kept free from the heat of the earth the gaseous materials would
become liquefied. Upward from the earth's surface at the sea level the
atoms and molecules of the air become farther apart until, at the
height of somewhere between forty and fifty miles, the quantity of
them contained in the ether is so small that we can trace little
effect from them on the rays of light which at lower levels are
somewhat bent by their action. At yet higher levels, however, meteors
appear to inflame by friction against the particles of air, and even
at the height of eighty miles very faint clouds have at times been
discerned, which are possibly composed of volcanic dust floating in
the very rarefied medium, such as must exist at this great elevation.
The air not only exists in the region where we distinctly recognise
it; it also occupies the waters and the under earth. In the waters it
occurs as a mechanical mixture which is brought about as the rain
forms and falls in the air, as the streams flow to the sea, and as the
waves roll over the deep and beat against the shores. In the realm of
the waters, as well as on the land, the air is necessary for the
maintenance of all animal forms; but for its presence such life would
vanish from the earth.
Owing to certain peculiarities in its constitution, the atmosphere of
our earth, and that doubtless of myriad other spheres, serves as a
medium of communication between different regions. It is, as we know,
in ceaseless motion at rates which may vary from the speed in the
greatest tempests, which may move at the rate of somewhere a hundred
and fifty miles an hour, to the very slow movements which occur in
caverns, where the transfer is sometimes effected at an almost
microscopic rate in the space of a day. The motion of the atmosphere
is brought about by the action of heat here and there, and in a
trifling way, by the heat from the interior of the earth escaping
through hot springs or volcanoes, but almost altogether by the heat of
the sun. If we can imagine the earth cut off from the solar radiation,
the air would cease to move. We often note how the variable winds fall
away in the nighttime. Those who in seeking for the North Pole have
spent winters in the long-continued dark of that region have noted
that the winds almost cease to blow, the air being disturbed only when
a storm originated in the sunlit realm forced its way into the
circumpolar darkness.
The sun's heat does not directly disturb the atmosphere; if we could
take the solid sphere of the world away, leaving the air, the rays
would go straight through, and there would be no winds produced. This
is due to the fact that the air permits the direct rays of heat, such
as come from the sun, to pass through it with very slight resistance.
In an aërial globe such as we have imagined, the rays impinging upon
its surface would be slightly thrown out of their path as they are in
passing through a lens, but they would journey on in space without in
any considerable measure warming the mass. Coming, however, upon the
solid earth, the heat rays warm the materials on which they are
arrested, bringing them to a higher temperature than the air. Then
these heated materials radiate the energy into the air; it happens,
however, that this radiant heat can not journey back into space as
easily as it came in; therefore the particles of air next the surface
acquire a relatively high temperature. Thus a thermometer next the
ground may rise to over a hundred degrees Fahrenheit, while at the
same time the fleecy clouds which we may observe floating at the
height of five or six miles above the surface are composed of frozen
water.
The effect of the heated air which acquires its temperature by
radiation from the earth's surface is to produce the winds. This it
brings about in a very simple manner, though the details of the
process have a certain complication. The best illustration of the mode
in which the winds are produced is obtained by watching what takes
place about an ordinary fire at the bottom of a chimney. As soon as
the fire is lit, we observe that the air about it, so far as it is
heated, tends upward, drawing the smoke with it. If the air in the
chimney be cold, it may not draw well at first; but in a few minutes
the draught is established, or, in other words, the heated lower air
breaks its way up the shaft, gradually pushing the cooler matter out
at the top. In still air we may observe the column from the flue
extending about the chimney-top, sometimes to the height of a hundred
feet or more before it is broken to pieces. It is well here to note
the fact that the energy of the draught in a chimney is, with a given
heat of fire and amount of air which is permitted to enter the shaft,
directly proportionate to the height; thus in very tall flues, between
two and three hundred feet high, which are sometimes constructed, the
uprush is at the speed of a gale.
Whenever the air next the surface is so far heated that it may
overcome the inertia of the cooler air above, it forces its way up
through it in the general manner indicated in the chimney flue. When
such a place of uprush is established, the hot air next the surface
flows in all directions toward the shaft, joining the expedition to
the heights of the atmosphere. Owing to the conditions of the earth's
surface, which we shall now proceed to trace, these ascents of heated
air belong in two distinct classes--those which move upward through
more or less cylindrical chimneys in the atmosphere, shafts which are
impermanent, which vary in diameter from a few feet to fifty or
perhaps a hundred miles, and which move over the surface of the earth;
and another which consists of a broad, beltlike shaft in the
equatorial regions, which in a way girdles the earth, remains in
about the same place, continually endures, and has a width of hundreds
of miles. Of these two classes of uprushes we shall first consider the
greatest, which occurs in the central portions of the tropical realm.
Under the equator, owing to the fact that the sun for a considerable
belt of land and sea maintains the earth at a high temperature, there
is a general updraught which began many million years ago, probably
before the origin of life, in the age when our atmosphere assumed its
present conditions. Into this region the cooler air from the north and
south necessarily flows, in part pressed in by the weight of the cold
air which overlies it, but aided in its motion by the fact that the
particles which ascend leave place for others to occupy. Over the
surfaces of the land within the tropical region this draught toward
what we may term the equatorial chimney is perturbed by the
irregularities of the surface and many local accidents. But on the
sea, where the conditions are uniform, the air moving toward the point
of ascent is marked in the trade winds, which blow with a steadfast
sweep down toward the equator. Many slight actions, such as the
movement of the hot and cold currents of the sea, the local air
movements from the lands or from detached islands, somewhat perturb
the trade winds, but they remain among the most permanent features in
this changeable world. It is doubtful if anything on this sphere
except the atoms and molecules of matter have varied as little as the
trade winds in the centre of the wide ocean. So steadfast and uniform
are they that it is said that the helm and sails of a ship may be set
near the west coast of South America and be left unchanged for a
voyage which will carry the navigator in their belt across the width
of the Pacific.
Rising up from the earth in the tropical belt, the air attains the
height of several thousand feet; it then begins to curve off toward
the north and south, and at the height of somewhere about three to
five miles above the surface is again moving horizontally toward
either pole; attaining a distance on that journey, it gradually
settles down to the surface of the earth, and ceases to move toward
higher latitudes. If the earth did not revolve upon its axis the
course of these winds along the surface toward the equator, and in the
upper air back toward the poles, would be made in what we may call a
square manner--that is, the particles of air would move toward the
point where they begin to rise upward in due north and south lines,
according as they came from the southern or northern hemisphere, and
the upper currents or counter trades would retrace their paths also
parallel with the meridians or longitude lines. But because the earth
revolves from west to east, the course of the trade winds is oblique
to the equator, those in the northern hemisphere blowing from
northeast to southwest, those in the southern from southeast to
northwest. The way in which the motion of the earth affects the
direction of these currents is not difficult to understand. It is as
follows:
Let us conceive a particle of air situated immediately over the
earth's polar axis. Such an atom would by the rotation of the sphere
accomplish no motion except, indeed, that it might turn round on its
own centre. It would acquire no velocity whatever by virtue of the
earth's movement. Then let us imagine the particle moving toward the
equator with the speed of an ordinary wind. At every step of its
journey toward lower latitudes it would come into regions having a
greater movement than those which it had just left. Owing to its
inertia, it would thus tend continually to lag behind the particles of
matter about it. It would thus fall off to the westward, and, in place
of moving due south, would in the northern hemisphere drift to the
southwest, and in the southern hemisphere toward the northwest. A good
illustration of this action may be obtained from an ordinary
turn-table such as is used about railway stations to reverse the
position of a locomotive. If the observer will stand in the centre of
such a table while it is being turned round he will perceive that his
body is not swayed to the right or left. If he will then try to walk
toward the periphery of the rotating disk, he will readily note that
it is very difficult, if not impossible, to walk along the radius of
the circle; he naturally falls behind in the movement, so that his
path is a curved line exactly such as is followed by the winds which
move toward the equator in the trades. If now he rests a moment on the
periphery of the table, so that his body acquires the velocity of the
disk at that point, and then endeavours to walk toward the centre, he
will find that again he can not go directly; his path deviates in the
opposite direction--in other words, the body continually going to a
place having a less rate of movement by virtue of the rotation of the
earth, on account of its momentum is ever moving faster than the
surface over which it passes. This experiment can readily be tried on
any small rotating disk, such as a potter's wheel, or by rolling a
marble or a shot from the centre to the circumference and from the
circumference to the centre. A little reflection will show the
inquirer how these illustrations clearly account for the oblique
though opposite sets of the trade winds in the upper and lower parts
of the air.
The dominating effect of the tropical heat in controlling the
movements of the air currents extends, on the ocean surface, in
general about as far north and south as the parallels of forty
degrees, considerably exceeding the limits of the tropics, those lines
where the sun, because of the inclination of the earth's axis, at some
time of the year comes just overhead. Between these belts of trade
winds there is a strip or belt under the region where the atmosphere
is rising from the earth, in which the winds are irregular and have
little energy. This region of the "doldrums" or frequent calms is one
of much trouble to sailing ships on their voyages from one hemisphere
to another. In passing through it their sails are filled only by the
airs of local storms, or winds which make their way into that part of
the sea from the neighbouring continents. Beyond the trade-wind belt,
toward the poles, the movements of the atmosphere are dependent in
part on the counter trades which descend to the surface of the earth
in latitudes higher than that in which the surface or trade winds
flow. Thus along our Atlantic coast, and even in the body of the
continent, at times when the air is not controlled by some local
storm, the counter trade blows with considerable regularity.
The effect of the trade and counter-trade movements of the air on the
distribution of temperature over the earth's surface is momentous. In
part their influence is due to the direct heat-carrying power of the
atmosphere; in larger measure it is brought about by the movement of
the ocean waters which they induce. Atmospheric air, when deprived of
the water which it ordinarily contains, has very little
heat-containing capacity. Practically nearly all the power of
conveying heat which it possesses is due to the vapour of water which
it contains. By virtue of this moisture the winds do a good deal to
transfer heat from the tropical or superheated portion of the earth's
surface to the circumpolar or underheated realms. At first, the
relatively cool air which journeys toward the equator along the
surface of the sea constantly gains in heat, and in that process takes
up more and more water, for precisely the same reason that causes
anything to dry more rapidly in air which has been warmed next a fire.
The result is that before it begins to ascend in the tropical
updraught, being much moisture-laden, the atmosphere stores a good
deal of heat. As it rises, rarefies, and cools, the moisture descends
in the torrential rains which ordinarily fall when the sun is nearly
vertical in the tropical belt.
Here comes in a very interesting principle which is of importance in
understanding the nature of great storms, either the continuous storm
of the tropics or the local and irregular whirlings which occur in
various parts of the earth. When the moisture-laden air starts on its
upward journey from the earth it has, by virtue of the watery vapour
which it contains, a store of energy which becomes applied to
promoting the updraught. As it rises, the moisture in the air gathers
together or condenses, and in so doing parts with the heat which
caused it to evaporate from the ocean surface. For a given weight of
water, the amount of heat required to effect the evaporation is very
great; this we may roughly judge by observing what a continuous fire
is required to send a pint of water into the state of steam. This
energy, when it is released by the condensation of water into rain or
snow, becomes again heat, and tends somewhat, as does the fire in the
chimney, to accelerate the upward passage of the air. The result is
that the water which ascends in the equatorial updraught becomes what
we may term fuel to promote this important element in the earth's
aërial circulation. Trades and counter trades would doubtless exist
but for the efficiency of this updraught, which is caused by the
condensation of watery vapour, but the movement would be much less
than it is.
WHIRLING STORMS.
In the region near the equator, or near the line of highest
temperature, which for various reasons does not exactly follow the
equator, there is, as we have noticed, a somewhat continuous uprushing
current where the air passes upward through an ascending chimney,
which in a way girdles the sea-covered part of the earth. In this
region the movements of the air are to a great extent under the
control of the great continuous updraught. As we go to the north and
south we enter realms where the air at the surface of the earth is, by
the heat which it acquires from contact with that surface, more or
less impelled upward; but there being no permanent updraught for its
escape, it from time to time breaks through the roof of cold air which
overlies it and makes a temporary channel of passage. Going polarward
from the equator, we first encounter these local and temporary
upcastings of the air near the margin of the tropical belt. In these
districts, at least over the warmer seas, during the time of the year
when it is midsummer, and in the regions where the trade winds are not
strong enough to sweep the warm and moisture-laden air down to the
equatorial belt, the upward tending strain of the atmosphere next the
earth often becomes so strong that the overlying air is displaced,
forming a channel through which the air swiftly passes. As the
moisture condenses in the way before noted, the energy set free serves
to accelerate the updraught, and a hurricane is begun. At first the
movement is small and of no great speed, but as the amount of air
tending upward is likely to be great, as is also the amount of
moisture which it contains, the aërial chimney is rapidly enlarged,
and the speed of the rising air increased. The atmosphere next the
surface of the sea flows in toward the channel of escape; its passage
is marked by winds which are blowing toward the centre. On the
periphery of the movement the particles move slowly, but as they win
their way toward the centre they travel with accelerating velocity. On
the principle which determines the whirling movement of the water
escaping through a hole in the bottom of a basin, the particles of the
air do not move on straight lines toward the centre, but journey in
spiral paths, at first along the surface, and then ascending.
We have noted the fact that in a basin of water the direction of the
whirling is what we may term accidental--that is, dependent on
conditions so slight that they elude our observation--but in
hurricanes a certain fact determines in an arbitrary way the direction
in which the spin shall take place. As soon as such a movement of the
air attains any considerable diameter, although in its beginning it
may have spun in a direction brought about by local accidents, it will
be affected by the diverse rates of travel, by virtue of the earth's
rotation, of the air on its equatorial and polar sides. On the
equatorial side this air is moving more rapidly than it is on the
polar side. By observing the water passing from a basin this
principle, with a few experiments, can be made plain. The result is to
cause these great whirlwinds of the hurricanes of higher latitudes to
whirl round from right to left in the northern hemisphere and in the
reverse way in the southern. The general system of the air currents
still further affects these, as other whirling storms, by driving
their centres or chimneys over the surface of the earth. The principle
on which this is done may be readily understood by observing how the
air shaft above a chimney, through which we may observe the smoke to
rise during a time of calm, is drawn off to one side by the slight
current which exists even when we feel no wind; it may also be
discerned in the little dust whirls which form in the streets on a
summer day when the air is not much disturbed. While they spin they
move on in the direction of the air drift. In this way a hurricane
originating in the Gulf of Mexico may gradually journey under the
influence of the counter trades across the Antilles, or over southern
Florida, and thence pursue a devious northerly course, generally near
the Atlantic coast and in the path of the Gulf Stream, until it has
travelled a thousand miles or more toward the North Atlantic. The
farther it goes northward the less effectively it is fed with warm and
moisture-laden air, the feebler its movement becomes, until at length
it is broken up by the variable winds which it encounters.
A very interesting and, from the point of view of the navigator,
important peculiarity of these whirls is that at their centre there is
a calm, similar in origin and nature to the calm under the equator
between the trade-wind belts. Both these areas are in the field where
the air is ascending, and therefore at the surface of the earth does
not affect the sails of ships, though if men ever come to use flying
machines and sail through the tropics at a good height above the sea
it will be sensible enough. The difference between the doldrum of the
equator and that of the hurricane, besides their relative areas, is
that one is a belt and the other a disk. If the seafarer happens to
sail on a path which leads him through the hurricane centre, he will
first discern, as from the untroubled air and sea he approaches the
periphery of the storm, the horizon toward the disturbance beset by
troubled clouds, all moving in one direction. Entering beneath this
pall, he finds a steadily increasing wind, which in twenty miles of
sailing may, and in a hundred miles surely will, compel him to take in
all but his storm sails, and is likely to bring his ship into grave
peril. The most furious winds the mariner knows are those which he
encounters as he approaches the still centre. These trials are made
the more appalling by the fact that in the furious part of the whirl
the rain, condensing from the ascending air, falls in torrents, and
the electricity generated in the condensation gives rise to vivid
lightning. If the storm-beset ship can maintain her way, in a score or
two of miles of journey toward the centre, generally very quickly, it
passes into the calm disk, where the winds, blowing upward, cease to
be felt. In this area the ship is not out of danger, for the waves,
rolling in from the disturbed areas on either side, make a torment of
cross seas, where it is hard to control the movements of a sailing
vessel because the impulse of the winds is lost. Passing through this
disk of calm, the ship re-encounters in reverse order the furious
portion of the whirl, afterward the lessening winds, until it escapes
again into the airs which are not involved in the great torment.
In the old days, before Dove's studies of storms had shown the laws of
hurricane movement, unhappy shipmasters were likely to be caught and
retained in hurricanes, and to battle with them for weeks until their
vessels were beaten to pieces. Now the "Sailing Directions," which are
the mariner's guide, enable him, from the direction of the winds and
the known laws of motion of the storm centre, to sail out of the
danger, so that in most cases he may escape calamity. It is otherwise
with the people who dwell upon the land over which these atmospheric
convulsions sweep. Fortunately, where these great whirlwinds trespass
on the continent, they quickly die out, because of the relative lack
of moisture which serves to stimulate the uprush which creates them.
Thus in their more violent forms hurricanes are only felt near the
sea, and generally on islands and peninsulas. There the hurricane
winds, by the swiftness of their movement, which often attains a speed
of a hundred miles or more, apply a great deal of energy to all
obstacles in their path. The pressure thus produced is only less
destructive than that which is brought about by the tornadoes, which
are next to be described.
There is another effect from hurricanes which is even more destructive
to life than that caused by the direct action of the wind. In these
whirlings great differences in atmospheric pressure are brought about
in contiguous areas of sea. The result is a sudden elevation in the
level of one part of the water. These disturbances, where the shore
lands are low and thickly peopled, as is the case along the western
coast of the Bay of Bengal, may produce inundations which are terribly
destructive to life and property. They are known also in southern
Florida and along the islands of the Caribbean, but in that region are
not so often damaging to mankind.
Fortunately, hurricanes are limited to a very small part of the
tropical district. They occur only in those regions, on the eastern
faces of tropical lands, where the general westerly set of the winds
favours the accumulation of great bodies of very warm, moist air next
the surface of the sea. The western portion of the Gulf of Mexico and
the Caribbean, the Bay of Bengal, and the southeastern portion of Asia
are especially liable to their visitations. They sometimes develop,
though with less fury, in other parts of the tropics. On the western
coast of South America and Africa, where the oceans are visited by the
dry land winds, and where the waters are cooled by currents setting
in from high latitudes, they are unknown.
Only less in order of magnitude than the hurricanes are the circular
storms known as cyclones. These occur on the continents, especially
where they afford broad plains little interrupted by mountain ranges.
They are particularly well exhibited in that part of North America
north of Mexico and south of Hudson Bay. Like the hurricanes, they
appear to be due to the inrush of relatively warm air entering an
updraught which had been formed in the overlying, cooler portions of
the atmosphere. They are, however, much less energetic, and often of
greater size than the hurricane whirl. The lack of energy is probably
due to the comparative dryness of the air. The greater width of the
ascending column may perhaps be accounted for by the fact that,
originating at a considerable height above the sea, they have a less
thickness of air to break through, and so the upward setting column is
readily made broad.
The cyclones of North America appear generally to originate in the
region of the Rocky Mountains, though it is probable that in some
instances, perhaps in many, the upward set of the air which begins the
storm originates in the ocean along the Pacific coast. They gather
energy as they descend the great sloping plain leading eastward from
the Rocky Mountains to the central portion of the great continental
valley. Thence they move on across the country to the Atlantic coast.
Not infrequently they continue on over the ocean to the European
continent. The eastward passage of the storm centre is due to the
prevailing eastward movement of the air in its upper part throughout
that portion of the northern hemisphere. Commonly they incline
somewhat to the northward of east in their journey. In all cases the
winds appear to blow spirally into the common storm centre. There is
the same doldrum area or calm field in the centre of the storm that we
note between the trade winds and in the middle of a hurricane disk,
though this area is less defined than in the other instances, and the
forward motion of the storm at a considerable speed is in most cases
characteristic of the disturbance. On the front of one of these storms
in North America the winds commonly begin in the northeast, thence
they veer by the east to the southwest. At this stage in the movement
the storm centre has passed by, the rainfall commonly ceases, and
cold, dry winds setting to the northwestward set in. This is caused by
the fact that the ascending air, having attained a height above the
earth, settles down behind the storm, forming an anticyclone or mass
of dry air, which presses against the retreating side of the great
whirlwind.
In front of the storm the warm and generally moist relatively warm
air, pressing in toward the point of uprise and overlaid by the upper
cold air, is brought into a condition where it tends to form small
subordinate shafts up through which it whirls on the same principle,
but with far greater intensity than the main ascending column. The
reason for the violence of this movement is that the difference in
temperature of the air next the surface and that at the height of a
few thousand feet is great. As might be expected, these local
spinnings are most apt to occur in the season when the air next the
earth is relatively warm, and they are aptest to take place in the
half of the advancing front lying between the east and south, for the
reason that there the highest temperatures and the greatest humidity
are likely to coexist. In that part of the field, during the time when
the storm is advancing from the Rocky Mountains to the Atlantic, a
dozen or more of these spinning uprushes may be produced, though few
of them are likely to be of large size or of great intensity.
The secondary storms of cyclones, such as are above noted, receive the
name of tornadoes. They are frequent and terrible visitations of the
country from northern Texas, Florida, and Alabama to about the line of
the Great Lakes; they are rarely developed in the region west of
central Kansas, and only occasionally do they exhibit much energy in
the region east of the plain-lands of the Ohio Valley. Although known
in other lands, they nowhere, so far as our observations go, exhibit
the paroxysmal intensity which they show in the central portion of the
North American continent. There the air which they affect acquires a
speed of movement and a fury of action unknown in any other
atmospheric disturbances, even in those of the hurricanes.
The observer who has a chance to note from an advantageous position
the development of a tornado observes that in a tolerably still air,
or at least an air unaffected by violent winds--generally in what is
termed a "sultry" state of the atmosphere--the storm clouds in the
distance begin to form a kind of funnel-shaped dependence, which
gradually extends until it appears to touch the earth. As the clouds
are low, this downward-growing column probably in no case is observed
for the height of more than three or four thousand feet. As the funnel
descends, the clouds above and about it may be seen to take on a
whirling movement around the centre, and under favourable
circumstances an uprush of vapours may be noted in the centre of the
swaying shaft. As the whirl comes nearer, the roar of the disturbance,
which at a distance is often compared to the sound made by a threshing
machine or to that of distant musketry, increases in loudness until it
becomes overwhelming. When a storm such as this strikes a building, it
is not only likely to be razed by the force of the wind, but it may be
exploded, as by the action of gunpowder fired within its walls,
through the sudden expansion of the air which it contains. In the
centre of the column, although it rarely has a diameter of more than a
few hundred feet, the uprush is so swift that it makes a partial
vacuum. The air, striving to get into the space which it is eager to
occupy, is whirling about at such a rate that the centrifugal motion
which it thus acquires restrains its entrance. In this way there may
be, as the column rapidly moves by, a difference of pressure
amounting probably to what the mercury of a barometer would indicate
by four or five inches of fall. Unless the structure is small and its
walls strong, its roof and sides are apt to be blown apart by this
difference of pressure and the consequent expansion of the contained
air. In some cases where wooden buildings have withstood this curious
action the outer clapboards have been blown off by the expansion of
the small amount of air contained in the interspaces between that
covering and the lath and plaster within (see Fig. 9).
[Illustration: Fig. 9.--Showing effect of expansion of air contained
in a hollow wall during the passage of the storm.]
The blow of the air due to its rotative whirling has in several cases
proved sufficient to throw a heavy locomotive from the track of a
well-constructed railway. In all cases where it is intense it will
overturn the strongest trees. The ascending wind in the centre of the
column may sometimes lift the bodies of men and of animals, as well as
the branches and trunks of trees and the timber of houses, to the
height of hundreds of feet above the surface. One of the most striking
exhibitions of the upsucking action in a tornado is afforded by the
effect which it produces when it crosses a small sheet of water. In
certain cases where, in the Northwestern States of this country, the
path of the storm lay over the pool, the whole of the water from a
basin acres in extent has been entirely carried away, leaving the
surface, as described by an observer, apparently dry enough to plough.
Fortunately for the interests of man, as well as those of the lower
organic life, the paths of these storms, or at least the portion of
their track where the violence of the air movement makes them very
destructive, often does not exceed five hundred feet in width, and is
rarely as great as half a mile in diameter. In most cases the length
of the journey of an individual tornado does not exceed thirty miles.
It rarely if ever amounts to twice that distance.
In every regard except their small size and their violence these
tornadoes closely resemble hurricanes. There is the same broad disk of
air next the surface spirally revolving toward the ascending centre,
where its motion is rapidly changed from a horizontal to a vertical
direction. The energy of the uprush in both cases is increased by the
energy set free through the condensation of the water, which tends
further to heat and thus to expand the air. The smaller size of the
tornado may be accounted for by the fact that we have in their
originating conditions a relatively thin layer of warm, moist air next
the earth and a relatively very cold layer immediately overlying it.
Thus the tension which serves to start the movement is intense, though
the masses involved are not very great. The short life of a tornado
may be explained by the fact that, though it apparently tends to grow
in width and energy, the central spout is small, and is apt to be
broken by the movements of the atmosphere, which in the front of a
cyclone are in all cases irregular.
On the warmer seas, but often beyond the limits of the tropics,
another class of spinning storms, known as waterspouts, may often be
observed. In general appearance these air whirls resemble tornadoes,
except that they are in all cases smaller than that group of
whirlings. As in the tornadoes, the waterspout begins with a funnel,
which descends from the sky to the surface of the sea. Up the tube
vapours may be seen ascending at great speed, the whole appearing like
a gigantic pillar of swiftly revolving smoke. When the whirl reaches
the water, it is said that the fluid leaps up into the tube in the
form of dense spray, an assertion which, in view of the fact of the
action of a tornado on a lake as before described, may well be
believed. Like the tornadoes and dust whirls, the life of a waterspout
appears to be brief. They rarely endure for more than a few minutes,
or journey over the sea for more than two or three miles before the
column appears to be broken by some swaying of the atmosphere. As
these peculiar storms are likely to damage ships, the old-fashioned
sailors were accustomed to fire at them with cannon. It has been
claimed that a shot would break the tube and end the little
convulsion. This, in view of the fact that they appear to be easily
broken up by relatively trifling air currents, may readily be
believed. The danger which these disturbances bring to ships is
probably not very serious.
The special atmospheric conditions which bring about the formation of
waterspouts are not well known; they doubtless include, however, warm,
moist air next the surface of the sea and cold air above. Just why
these storms never attain greater size or endurance is not yet known.
These disturbances have been seen for centuries, but as yet they have
not been, in the scientific sense, observed. Their picturesqueness
attracts all beholders; it is interesting to note the fact that
perhaps the earliest description of their phenomena--one which takes
account in the scientific spirit of all the features which they
present--was written by the poet Camoëns in the Lusiad, in which he
strangely mingles fancy and observation in his account of the great
voyage of Vasco da Gama. The poet even notes that the water which
falls when the spout is broken is not salt, but fresh--a point which
clearly proves that not much of the water which the tube contains is
derived from the sea. It is, in fact, watery vapour drawn from the air
next the surface of the ocean, and condensed in its ascent through the
tube. In this and other descriptions of Nature Camoëns shows more of
the scientific spirit than any other poet of his time. He was in this
regard the first of modern writers to combine a spiritual admiration
for Nature with some sense of its scientific meaning.
In treating of the atmosphere, meteorologists base their studies
largely on changes in the weight of that medium, which they determine
by barometric observations. In fact, the science of the air had its
beginning in Pascal's admirable observation on the changes in the
height of a column of mercury contained in a bent tube as he ascended
the volcanic peak known as Puy de Dome, in central France. As before
noted, it is to the disturbances in the weight of the air, brought
about mainly by variations in temperature, that we owe all its
currents, and it is upon these winds that the features we term climate
in largest measure depend. Every movement of the winds is not only
brought about by changes in the relative weight of the air at certain
points, but the winds themselves, owing to the momentum which the air
attains by them, serve to bring about alterations in the quantity of
air over different parts of the earth, which are marked most
distinctly by barometric variations. These changes are exceedingly
complicated; a full account of them would demand the space of this
volume. A few of the facts, however, should be presented here. In the
first place, we note that each day there is normally a range in the
pressure which causes the barometer to be at the lowest at about four
o'clock in the morning and four o'clock in the afternoon, and highest
at about ten o'clock in those divisions of the day. This change is
supposed to be due to the fact that the motes of dust in the
atmosphere in the night, becoming cooled, condense the water vapour
upon their surfaces, thus diminishing the volume of the air. When the
sun rises the water evaporated by the heat returns from these little
storehouses into the body of the atmosphere. Again in the evening the
condensation sets in; at the same time the air tends to drift in from
the region to the westward, where the sun is still high, toward the
field where the barometer has been thus lowered; the current gradually
attains a certain volume, and so brings about the rise of the
barometer about ten o'clock at night.
In the winter time, particularly on the well-detached continent of
North America, we find a prevailing high barometer in the interior of
the country and a corresponding low state of pressure on the Atlantic
Ocean. In the summer season these conditions are on the whole
reversed.
Under the tropics, in the doldrum belt, there is a zone of low
barometer connected to the ascending currents which take place along
that line. This is a continuous manifestation of the same action which
gives a large area of a disklike form in the centre or eye of the
hurricane and in the middle portion of the tornado's whirl. In
general, it may be said that the weight of the air is greatest in the
regions from which it is blowing toward the points of upward escape,
and least in and about those places where the superincumbent air is
rising through a temporary or permanent line of escape. In other
words, ascending air means generally a relatively low barometer, while
descending air is accompanied by greater pressure in the field upon
which it falls.
In almost every part of the earth which is affected by a particular
physiography we find that the movements of the atmosphere next the
surface are qualified by the condition which it encounters. In fact,
if a person were possessed of all the knowledge which could be
obtained concerning winds, he could probably determine as by a map the
place where he might chance to find himself, provided he could extend
his observations over a term of years. In other words, the regimen of
the winds--at least those of a superficial nature--is almost as
characteristic of the field over which they go as is a map of the
country. Of these special winds a number of the more important have
been noted, only a few of which we can advert to. First among these
may well come the land and sea breezes which are remarked about all
islands which are not continuously swept by permanent winds. One of
the most characteristic instances of these alternate winds is perhaps
that afforded on the island of Jamaica.
The island of Jamaica is so situated within the basin of the Caribbean
that it does not feel the full influence of the trades. It has a range
of high mountains through its middle part. In the daytime the surface
of the land, which has the sun overhead twice each year, and is always
exposed to nearly vertical radiation, becomes intensely hot, so that
an upcurrent is formed. The formation of this current is favoured by
the mountains, which apply a part of the heat at the height of about a
mile above the surface of the sea. This action is parallel to that we
notice when, in order to create a draught in the air of a chimney, we
put a torch some distance up above the fireplace, thus diminishing the
height of the column of air which has to be set in motion. It is
further shown by the fact that when miners sought to make an upcurrent
in a shaft, in order to lead pure air into the workings through other
openings, they found after much experience that it was better to have
the fire near the top of the shaft rather than at the bottom.
The ascending current being induced up the mountain sides of Jamaica,
the air is forced in from the sea to the relatively free space. Before
noon the current, aided in its speed by a certain amount of the
condensation of the watery vapour before described, attains the
proportions of a strong wind. As the sun begins to sink, the earth's
surface pours forth its heat; the radiation being assisted by the
extended surfaces of the plants, cooling rapidly takes place.
Meanwhile the sea, because of the great heat-storing power of water,
is very little cooled, the ascent of the air ceases, the temporary
chimney with its updraught is replaced by a downward current, and the
winds blow from the land until the sun comes again to reverse the
current. In many cases these movements of the daily winds flowing into
and from islands induce a certain precipitation of moisture in the
form of rain. Generally, however, their effect is merely to ameliorate
the heat by bringing alternately currents from the relatively cool sea
and from the upper atmosphere to lessen the otherwise excessive
temperature of the fields which they traverse.
Although characteristic sea and land winds are limited to regions
where the sun's heat is great, they are traceable even in high
latitudes during the periods of long-continued calm attended with
clear skies. Thus on the island of Martha's Vineyard, in
Massachusetts, the writer has noted, when the atmosphere was in such a
state, distinct night and day, or sea and land, breezes coming in
their regular alternation. During the night when these alternate winds
prevail the central portion of the island, at the distance of three
miles from the sea, is remarkably cold, the low temperature being due
to the descending air current. To the same physical cause may be
attributed the frequent insets of the sea winds toward midday along
the continental shores of various countries. Thus along the coast of
New England in the summer season a clear, still, hot day is certain to
lead to the creation of an ingoing tide of air, which reaches some
miles into the interior. This stream from the sea enters as a thin
wedge, it often being possible to note next the shore when the
movement begins a difference of ten degrees of temperature between the
surface of the ground to which the point of the wedge has attained,
and a position twenty feet higher in the air. This is a beautiful
example to show at once how the relative weight of the atmosphere,
even when the differences are slight, may bring about motion, and also
how masses of the atmosphere may move by or through the rest of the
medium in a way which we do not readily conceive from our observations
on the transparent mass. Very few people have any idea how general is
the truth that the air, even in continuous winds, tends to move in
more or less individualized masses. This, however, is made very
evident by watching the gusts of a storm or the wandering patches of
wind which disturb the surface of an otherwise smooth sea.
[Illustration: _South shore, Martha's Vineyard, Massachusetts, showing
a characteristic sand beach with long slope and low dunes. Note the
three lines of breakers and the splash flows cutting little bays in
the sand._]
Among the notable local winds are those which from their likeness to
the Föhn of the Swiss valleys receive that name. Föhns are produced
where a body of air blowing against the slope of a continuous mountain
range is lifted to a considerable height, and, on passing over the
crest, falls again to a low position. In its ascent the air is cooled,
rarefied, and to a great extent deprived of its moisture. In
descending it is recondensed, and by the process by which its atoms
are brought together its latent heat is made sensible. There being but
little watery vapour in the mass, this heat is not much called for by
that heat-storing fluid, and so the air is warmed. So far Föhn winds
have only been remarked as conspicuous features in Switzerland and on
the eastern face of the Rocky Mountains. In the region about the head
waters of the Missouri and to the northward their influence in what
are called the Chinook winds is distinctly to ameliorate the severe
winter climate of the country.
In almost all great desert regions, particularly in the typical
Sahara, we find a variety of storm belonging to the whirlwind group,
which, owing to the nature of the country, take on special
characteristics. These desert storms take up from the verdureless
earth great quantities of sand and other fine _débris_, which often so
clouds the air as to bring the darkness of night at midday. Their
whirlings appear in size to be greater than those which produce
tornadoes or waterspouts, but less than hurricanes or cyclones.
Little, however, is known about them. They have not been well
observed by meteorologists. In some ways they are important, for the
reason that they serve to carry the desert sand into regions
previously verdure-clad, and thus to extend the bounds of the desolate
fields in which they originate. Where they blow off to the seaward,
they convey large quantities of dust into the ocean, and thus serve to
wear down the surface of the land in regions where there are no rivers
to effect that action in the normal way.
Notwithstanding its swift motion when impelled by differences in
weight, the movements of the air have had but little direct and
immediate influence on the surface of the earth. The greater part of
the work which it does, as we shall see hereafter, is done through the
waters which it impels and bears about. Yet where winds blow over
verdureless surfaces the effect of the sand which they sweep before
them is often considerable. In regions of arid mountains the winds
often drive trains of sand through the valleys, where the sharp
particles cut the rocks almost as effectively as torrents of water
would, distributing the wearing over the width of the valley. The dust
thus blown, from a desert region may, when it attains a country
covered with vegetation, gradually accumulate on its surface, forming
very thick deposits. Thus in northwestern China there is a wide area
where dust accumulations blown from the arid districts of central Asia
have gradually heaped up in the course of ages to the depth of
thousands of feet, and this although much of the _débris_ is
continually being borne away by the action of the rain waters as they
journey toward the sea. Such dust accumulations occur in other parts
of the world, particularly in the districts about the upper
Mississippi and in the valleys of the Rocky Mountains, but nowhere are
they so conspicuous as in the region first mentioned.
Where prevailing winds from the sea, from great lakes, and even from
considerable rivers, blow against sandy shores or cliffs of the same
nature, large quantities of sand and dust are often driven inland
from the coast line. In most cases these wind-borne materials take on
the form of dunes, or heaps of sand, varying from a few feet to
several hundred feet in height. It is characteristic of these hills of
blown sand that they move across the face of the country. Under
favourable conditions they may journey scores of miles from the shore.
The marching of a dune is effected through the rolling up of the sand
on the windward side of the elevation, when it is impelled by the
current of air to the crest where it falls into the lee or shelter
which the hill makes to the wind. In this way in the course of a day
the centre of the dune, if the wind be blowing furiously, may advance
a measurable distance from the place it occupied before. By fits and
starts this ongoing may be indefinitely continued. A notable and
picturesque instance of the march of a great dune may be had from the
case in which one of them overwhelmed in the last century the village
of Eccles in southeastern England. The advancing sand gradually crept
into the hamlet, and in the course of a decade dispossessed the people
by burying their houses. In time the summit of the church spire
disappeared from view, and for many years thereafter all trace of the
hamlet was lost. Of late years, however, the onward march of the sands
has disclosed the church spire, and in the course of another century
the place may be revealed on its original site, unchanged except that
the marching hill will be on its other side.
In the region about the head of the Bay of Biscay the quantity of
these marching sands is so great that at one time they jeopardized the
agriculture of a large district. The French Government has now
succeeded, by carefully planting the surface of the country with
grasses and other herbs which will grow in such places, in checking
the movement of the wind-blown materials. By so doing they have merely
hastened the process by which Nature arrests the march of dunes. As
these heaps creep away from the sea, they generally come into regions
where a greater variety of plants flourish; moreover, their sand
grains become decayed, so that they afford a better soil. Gradually
the mat of vegetation binds them down, and in time covers them over so
that only the expert eye can recognise their true nature. Only in
desert regions can the march of these heaps be maintained for great
distances.
Characteristic dunes occur from point to point all along the Atlantic
coast from the State of Maine to the northern coast of Florida. They
also occur along the coasts of our Great Lakes, being particularly
well developed at the southern end of Lake Michigan, where they form,
perhaps, the most notable accumulations within the limits of the
United States.
When blown sands invade a forest and the deposit is rapidly
accumulated, the trees are often buried in an undecayed condition. In
this state, with certain chemical reactions which may take place in
the mass, the woody matter is apt to become replaced by silex
dissolved from the sand, which penetrates the tissues of the plants.
In this way salicified forests are produced, such as are found in the
region of the Rocky Mountains, where the trunks of the trees, now very
hard stone, so perfectly preserve their original structure that when
cut and polished they may be used for decorative purposes. Conspicuous
as is this work of the dunes, it is in a geological way much less
important than that accomplished by the finer dust which drifts from
one region of land to another or into the sea. Because of their
weight, the sand grains journey over the surface of the earth, except,
indeed, where they are uplifted by whirl storms. They thus can not
travel very fast or far. Dust, however, rises into the air, and
journeys for indefinite distances. We thus see how slight differences
in the weight of substances may profoundly affect the conditions of
their deportation.
THE SYSTEM OF WATERS.
The envelope of air wraps the earth completely about, and, though
varying in thickness, is everywhere present over its surface. That of
the waters is much less equally distributed. Because of its weight, it
is mainly gathered in the depths of the earth, where it lies in the
interstices of the rocks and in the great realm of the seas. Only a
very small portion of the fluid is in the atmosphere or on the land.
Perhaps less than a ten thousandth part of the whole is at any one
time on this round from the seas through the air to the land and back
to the great reservoir.
The great water store of the earth is contained in two distinct
realms--in the oceans, where the fluid is concentrated in a quantity
which fills something like nine tenths of the hollows formed by the
corrugations of the earth's surface; and in the rocks, where it is
stored in a finely divided form, partly between the grains of the
stony matter and partly in the substance of its crystals, where it
exists in a combination, the precise nature of which is not well
known, but is called water of crystallization. On the average, it
seems likely that the materials of the earth, whether under the sea or
on the land, have several per cent of their mass of the fluid.
It is not yet known to what depth the water-bearing section of the
earth extends; but, as we shall see more particularly hereafter when
we come to consider volcanoes, the lavas which they send up to the
surface are full of contained water, which passes from them in the
form of steam. The very high temperature of these volcanic ejections
makes it necessary for us to suppose that they come from a great
depth. It is difficult to believe that they originate at less than a
hundred miles below the earth's surface. If, then, the rocks contain
an average of even five per cent of water to the depth of one hundred
miles, the quantity of the fluid stored within the earth is greater
than that which is contained in the reservoir of the ocean. The
oceans, on the average, are not more than three miles deep; spread
evenly over the surface of the whole earth, their depth would be less
than two miles, while the water in the rocks, if it could be added to
the seas, would make the total depth seven miles or more. As we shall
note hereafter, the processes of formation of strata tend to imprison
water in the beds, which in time is returned to the earth's surface by
the forces which operate within the crust.
Although the water in the seas is, as we have seen, probably less than
one half of the store which the earth possesses, the part it plays in
the economy of the planet is in the highest measure important. The
underground water operates solely to promote certain changes which
take place in the mineral realm. Its effect, except in volcanic
processes, are brought about but slowly, and are limited in their
action. The movements of this buried water are exceedingly gradual;
the forces which impel it about and which bring it to do its work
originate in the earth. In the seas the fluid has an exceeding freedom
of motion; it can obey the varied impulses which the solar energy
imposes upon it. The rôle of these wonderful actions which we are
about to trace includes almost everything which goes on upon the
surface of the planet--that which relates to the development of animal
and vegetable life, as well as to the vast geological changes which
the earth is undergoing.
If the surface of the earth were uniformly covered with water to the
depth of ten thousand feet or more, every particle of fluid would, in
a measure, obey the attraction of the sun, of the moon, and,
theoretically, also of all the other bodies in space, on the principle
that every particle of matter in the universe exercises a gravitative
effect on every other. As it is, owing to the divided condition of the
water on the earth's surface, only that which is in the ocean and
larger seas exhibits any measurable influence from these distant
attractions. In fact, only the tides produced by the moon and sun are
of determinable magnitude, and of these the lunar is of greater
importance, the reason being the near position of our satellite to our
own sphere. The solar tide is four tenths as great as the lunar. The
water doubtless obeys in a slight way the attraction of the other
celestial bodies, but the motions thus imparted are too small to be
discerned; they are lost in the great variety of influences which
affect all the matter on the earth.
Although the tides are due to the attraction of the solar bodies,
mainly to that of the moon, the mode in which the result is brought
about is somewhat complicated. It may briefly and somewhat
incompletely be stated as follows: Owing to the fact that the
attracting power of the earth is about eighty times greater than that
of the moon, the centre of gravity of the two bodies lies within the
earth. About this centre the spheres revolve, each in a way swinging
around the other. At this point there is no centrifugal motion arising
from the revolution of the pair of spheres, but on the side of the
earth opposite the moon, some six thousand miles away, the centrifugal
force is considerable, becoming constantly greater as we pass away
from the turning point. At the same time the attraction of the moon on
the water becomes less. Thus the tide opposite the satellite is
formed. On the side toward the moon the same centrifugal action
operates, though less effectively than in the other case, for the
reason that the turning point is nearer the surface; but this action
is re-enforced by the greater attraction of the moon, due to the fact
that the water is much nearer that body.
In the existing conditions of the earth, what we may call the normal
run of the tides is greatly interrupted. Only in the southern ocean
can the waters obey the lunar and solar attraction in anything like a
normal way. In that part of the earth two sets of tides are
discernible, the one and greater due to the moon, the other, much
smaller, to the sun. As these tides travel round at different rates,
the movements which they produce are sometimes added to each other
and sometimes subtracted--that is, at times they come together, while
again the elevation of one falls in the hollow of the other. Once
again supposing the earth to be all ocean covered, computation shows
that the tides in such a sea would be very broad waves, having,
indeed, a diameter of half the earth's circumference. Those produced
by the moon would have an altitude of about one foot, and those by the
sun of about three inches. The geological effects of these swayings
would be very slight; the water would pass over the bottom to and fro
twice each day, with a maximum journey of a hundred or two feet each
way from a fixed point. This movement would be so slow that it could
not stir the fine sediment; its only influence would perhaps be to
help feed the animals which were fixed upon the bottom by drawing the
nurture-bringing water by their mouths.
Although the divided condition of the ocean perturbs the action of the
tides, so that except by chance their waves are rarely with their
centres where the attracting bodies tend to make them, the influence
of these divisions is greatly to increase the geological or
change-bringing influences arising from these movements. When from the
southern ocean the tides start to the northward up the bays of the
Atlantic, the Pacific, or the Indian Ocean, they have, as before
noted, a height of perhaps less than two feet. As they pass up the
narrowing spaces the waves become compressed--that is, an equal volume
of moving water has less horizontal room for its passage, and is
forced to rise higher. We see a tolerably good illustration of the
same principle when we observe a wind-made wave enter a small recess
of the shore, the sides of which converge in the direction of the
motion. With the diminished room, the wave gains in height. It thus
comes about that the tide throughout the Atlantic basin is much higher
than in the southern ocean. On the same principle, when the tide rolls
in against the shores every embayment of a distinct kind, whose sides
converge toward the head, packs up the tidal wave, often increasing
its height in a remarkable way. When these bays are wide-mouthed and
of elongate triangular form, with deep bottoms, the tides which on
their outer parts have a height of ten or fifteen feet may attain an
altitude of forty or fifty feet at the apex of the triangle.
We have already noted the fact that the tide, such as runs in the
southern ocean, exercises little or no influence upon the bottom of
the sea over which it moves. As the height of the confined waters
increases, the range of their journey over the bottom as the wave
comes and goes rapidly increases. When they have an elevation of ten
feet they can probably stir the finer mud on the ocean floor, and in
shallow water move yet heavier particles. In the embayments of the
land, where a great body of water journeys like an alternating river
into extensive basins, the tidal action becomes intense; the current
may be able to sweep along large stones quite as effectively as a
mountain torrent. Thus near Eastport, Me., where the tides have a
maximum rise and fall of over twenty feet, the waters rush in places
so swiftly that at certain stages of the movement they are as much
troubled as those at the rapids of the St. Lawrence. In such portions
of the shore the tides do important work in carving channels into the
lands.
Along the shores of the continents about the North Atlantic, where the
tides act in a vigorous manner, we almost everywhere find an
underwater shelf extending from the shore with a declivity of only
five to ten feet to the mile toward the centre of the sea, until the
depth of about five hundred feet is attained; from this point the
bottom descends more steeply into the ocean's depth. It is probable
that the larger part of the material composing these continental
shelves has been brought to its position by tidal action. Each time
the tidal wave sweeps in toward the shore it urges the finer particles
of sediment along with it. When it moves out it drags them on the
return journey toward the depths of the sea. If this shelf were
perfectly horizontal, the two journeys of the sand and mud grains
would be of the same length; but as the movement takes place up and
down a slope, the bits will travel farther under the impulse which
leads them downward than under that which impels them up. The result
will be that the particles will travel a little farther out from the
shore each time it is swung to and fro in the alternating movement of
the tide.
The effect of tidal movement in nurturing marine life is very great.
It aids the animals fixed on the bottoms of the deep seas to obtain
their provision of food and their share of oxygen by drawing the water
by their bodies. All regions which are visited by strong tides
commonly have in the shallows near the shores a thick growth of
seaweed which furnishes an ample provision of food for the fishes and
other forms of animal life.
A peculiar effect arising from tidal action is believed by students of
the phenomena to be found in the slowing of the earth's rotation on
its axis. The tides rotate around the earth from east to west, or
rather, we should say, the solid mass of the earth rubs against them
as it spins from west to east. As they move over the bottom and as
they strike against the shores this push of the great waves tends in a
slight measure to use up the original spinning impulse which causes
the earth's rotation. Computation shows that the amount of this action
should be great enough gradually to lengthen the day, or the time
occupied by the earth in making a complete revolution on the polar
axis. The effect ought to be great enough to be measurable by
astronomers in the course of a thousand years. On the other hand, the
records of ancient eclipses appear pretty clearly to show that the
length of the day has not changed by as much as a second in the course
of three thousand years. This evidence does not require us to abandon
the supposition that the tides tend to diminish the earth's rate of
rotation. It is more likely that the effect of the reduction in the
earth's diameter due to the loss of heat which is continually going on
counterbalances the influence of the tidal friction. As the diameter
of a rotating body diminishes, the tendency is for the mass to spin
more rapidly; if it expands, to turn more slowly, provided in each
case the amount of the impulse which leads to the turning remains the
same. This can be directly observed by whirling a small weight
attached to a string in such a manner that the cord winds around the
finger with each revolution; it will be noted that as the line
shortens the revolution is more quickly accomplished. We can readily
conceive that the earth is made up of weights essentially like that
used in the experiment, each being drawn toward the centre by the
gravitative stress, which is like that applied to the weight by the
cord.
The fact that the days remain of the same length through vast periods
of time is probably due to this balance between the effects of tidal
action and those arising from the loss of heat--in other words, we
have here one of those delicate arrangements in the way of
counterpoise which serve to maintain the balanced conditions of the
earth's surface amid the great conflicts of diverse energies which are
at work in and upon the sphere.
It should be understood that the effects of the attraction which
produces tides are much more extensive than they are seen to be in the
movements of the sea. So long as the solar and planetary spheres
remain fluid, the whole of their masses partake of the movement. It is
a consequence of this action, as the computations of Prof. George
Darwin has shown, that the moon, once nearer the earth than it is at
present, has by a curious action of the tidal force been pushed away
from the centre of our sphere, or rather the two bodies have repelled
each other. An American student of the problem, Mr. T.J.J. See, has
shown that the same action has served to give to the double stars the
exceeding eccentricity of their orbits.
Although these recent studies of tidal action in the celestial sphere
are of the utmost importance to the theory of the universe, for they
may lead to changes in the nebular hypotheses, they are as yet too
incomplete and are, moreover, too mathematical to be presented in an
elementary treatise such as this.
* * * * *
We now turn to another class of waves which are of even more
importance than those of the tides--to the undulations which are
produced by the action of the wind on the surface of the water. While
the tide waves are limited to the open ocean, and to the seas and bays
which afford them free entrance, wind waves are produced everywhere
where water is subjected to the friction of air which flows over it.
While tidal waves come upon the shores but twice each day, the wind
waves of ordinary size which roll in from the ocean deliver their
blows at intervals of from three to ten seconds. Although the tidal
waves sometimes, by a packing-up process, attain the height of fifty
feet, their average altitude where they come in contact with the shore
probably does not much exceed four feet; usually they come in gently.
It is likely that in a general way the ocean surges which beat against
the coast are of greater altitude.
Wind waves are produced and perform their work in a manner which we
shall now describe. When the air blows over any resisting surface, it
tends, in a way which we can hardly afford here to describe, to
produce motions. If the particle is free to move under the impulse
which it communicates, it bears it along; if it is linked together in
the manner of large masses, which the wind can not transport, it tends
to set it in motion in an alternating way. The sounds of our musical
instruments which act by wind are due to these alternating vibrations,
such as all air currents tend to produce. An Æolian harp illustrates
the action which we are considering. Moving over matter which has the
qualities that we denote by the term fluid, the swayings which the air
produces are of a peculiar sort, though they much resemble those of
the fiddle string. The surface of the liquid rises and falls in what
we term waves, the size of which is determined by the measure of
fluidity, and by the energy of the wind. Thus, because fresh water is
considerably lighter than salt, a given wind will produce in a given
distance for the run of the waves heavier surges in a lake than it
will in the sea. For this reason the surges in a great storm which
roll on the ocean shore, because of the wide water over which they
have gathered their impetus, are in size very much greater than those
of the largest lakes, which do not afford room for the development of
great undulations.
To the eye, a wave in the water appears to indicate that the fluid is
borne on before the wind. Examination, however, shows that the amount
of motion in the direction in which the wind is blowing is very
slight. We may say, indeed, that the essential feature of a wave is
found in the transmission of impulse rather than in the movement of
the fluid matter. A strip of carpet when shaken sends through its
length undulations which are almost exactly like water waves. If we
imagine ourselves placed in a particle of water, moving in the
swayings of a wave in the open and deep sea, we may conceive ourselves
carried around in an ellipse, in each revolution returning through
nearly the same orbit. Now and then, when the particle came to the
surface, it would experience the slight drift which the continual
friction of the wind imposes on the water. If the wave in which the
journey was made lay in the trade winds, where the long-continued,
steadfast blowing had set the water in motion to great depths, the
orbit traversed would be moving forward with some rapidity; where also
the wind was strong enough to blow the tops of the waves over, forming
white-caps, the advance of the particle very near the surface would be
speedy. Notwithstanding these corrections, waves are to be regarded
each as a store of energy, urging the water to sway much in the manner
of a carpet strip, and by the swaying conveying the energy in the
direction of the wave movement.
The rate of movement of wind waves increases with their height.
Slight undulations go forward at the rate of less than half a mile an
hour. The greater surges of the deeps when swept by the strongest
winds move with the speed which, though not accurately determined, has
been estimated by the present writer as exceeding forty miles an hour.
As these surges often have a length transverse to the wind of a mile
or more, a width of about an eighth of a mile, and a height of from
thirty-five to forty-five feet, the amount of energy which they
transmit is very great. If it could be effectively applied to the
shores in the manner in which the energy of exploding gunpowder is
applied by cannon shot, it is doubtful whether the lands could have
maintained their position against the assaults of the sea. But there
are reasons stated below why the ocean waves can use only a very small
part of their energy in rending the rocks against which they strike on
the coast line.
In the first place, we should note that wind waves have very little
influence on the bottom of the deep sea. If an observer could stand on
the sea floor at the depth of a mile below a point over which the
greatest waves were rolling, he could not with his unaided senses
discern that the water was troubled. He would, indeed, require
instruments of some delicacy to find out that it moved at all. Making
the same observations at the depth of a thousand feet, it is possible
that he would note a slight swaying motion in the water, enough
sensibly to affect his body. At five hundred feet in depth the
movement would probably be sufficient to disturb fine mud. At two
hundred feet, the rasping of the surge on the bottom would doubtless
be sufficient to push particles of coarse sand to and fro. At one
hundred feet in depth, the passage of the surge would be strong enough
to urge considerable pebbles before it. Thence up the slope the
driving action would become more and more intense until we attained
the point where the wave broke. It should furthermore be noted that,
while the movement of the water on the floor of the deep sea as the
wave passes overhead would be to and fro, with every advance in the
shallowing and consequent increased friction on the bottom, the
forward element in the movement would rapidly increase. Near the coast
line the effect of the waves is continually to shove the detritus up
the slopes of the continental shelf. Here we should note the fact that
on this shelf the waves play a part exactly the opposite of that
effected by the tides. The tides, as we have noted, tend to drag the
particles down the slope, while the waves operate to roll them up the
declivity.
As the wave in advancing toward the shore ordinarily comes into
continually shallowing water, the friction on the bottom is
ever-increasing, and serves to diminish the energy the surge contains,
and therefore to reduce its proportions. If this action operated
alone, the subtraction which the friction makes would cause the surf
waves which roll in over a continental shelf to be very low, probably
in height less than half that which they now attain. In fact, however,
there is an influence at work to increase the height of the waves at
the expense of its width. Noting that the friction rapidly increases
with the shallowing, it is easy to see that this resistance is
greatest on the advancing front of the wave, and least on its seaward
side. The result is that the front moves more slowly than the rear, so
that the wave is forced to gain in height; but for the fact that the
total friction which the wave encounters takes away most of its
impetus, we might have combers a hundred feet high rolling upon the
shelving shores which almost everywhere face the seas.
As the wave shortens its width and gains in relative height, though
not in actual elevation, another action is introduced which has
momentous consequences. The water in the bottom of the wave is greatly
retarded in its ongoing by its friction over the sea floor, while the
upper part of the surge is much less affected in this way. The result
is that at a certain point in the advance, the place of which is
determined by the depth, the size, and the speed of the undulation,
the front swiftly steepens until it is vertical, and the top shoots
forward to a point where it is no longer supported by underlying
water, when it plunges down in what is called the surf or breaker. In
this part of the wave's work the application of the energy which it
transmits differs strikingly from the work previously done. Before the
wave breaks, the only geological task which it accomplishes is
effected by forcing materials up the slope, in which movement they are
slightly ground over each other until they come within the battering
zone of the shore, where they may be further divided by the action of
the mill which is there in operation.
When the wave breaks on the shore it operates in the following manner:
First, the overturning of its crest sends a great mass of water, it
may be from the height of ten or more feet, down upon the shore. Thus
falling water has not only the force due to its drop from the summit
of the wave, but it has a share of the impulse due to the velocity
with which the surge moved against the shore. It acts, in a word, like
a hammer swung down by a strong arm, where the blow represents not
only the force with which the weight would fall of itself, but the
impelling power of the man's muscles. Any one who will expose his body
to this blow of the surf will recognise how violent it is; he may, if
the beach be pebbly, note how it drives the stones about; fragments
the size of a man's head may be hurled by the stroke to the distance
of twenty feet or more; those as large as the fist may be thrown clear
beyond the limits of the wave. So vigorous is this stroke that the
sound of it may sometimes be heard ten miles inland from the coast
where it is delivered.
Moving forward up the slope of a gently inclined beach, the fragments
of the wave are likely to be of sufficient volume to permit them to
regather into a secondary surge, which, like the first, though much
smaller, again rises into a wall, forming another breaker. Under
favourable conditions as many as four or five of these successive
diminishing surf lines may be seen. The present writer has seen in
certain cases as many as a dozen in the great procession, the lowest
and innermost only a few inches high, the outer of all with a height
of perhaps twenty feet.
Along with the direct bearing action of the surf goes a to-and-fro
movement, due to the rushing up and down of the water on the beach.
This swashing affects not only the broken part of the waves, but all
the water between the outer breaker and the shore. These swayings in
the surf belt often swing the _débris_ on the inner margin over a
range of a hundred feet or more, the movement taking place with great
swiftness, affecting the pebbles to the depth of several inches, and
grinding the bits together in a violent way. Listening to the turmoil
of a storm, we can on a pebbly beach distinctly hear the sound of the
downward stroke, a crashing tone, and the roar of the rolling stones.
As waves are among the interesting things in the world, partly on
account of their living quality and partly because of their immediate
and often exceeding interest to man, we may here note one or two
peculiar features in their action. In the first place, as the reader
who has gained a sense of the changes in form of action may readily
perceive, the beating of waves on the shore converts the energy which
they possess into heat. This probably warms the water during great
storms, so that by the hand we may note the difference in temperature
next the coast line and in the open waters. This relative warmth of
the surf water is perhaps a matter of some importance in limiting the
development of ice along the shore line; it may also favour the
protection of the coast life against the severe cold of the winter
season.
The waves which successively come against the shore in any given time,
particularly if a violent wind is blowing on to the coast, are usually
of about the same size. When, however, in times of calm an old sea, as
it is called, is rolling in, the surges may occasionally undergo very
great variations in magnitude. Not infrequently these occasional waves
are great enough to overwhelm persons who are upon the rocks next the
shore. These greater surges are probably to be accounted for by the
fact that in the open sea waves produced by winds blowing in different
directions may run on with their diverse courses and varied intervals
until they come near the shore. Running in together, it very well
happens that two of the surges belonging to different sets may combine
their forces, thus doubling the swell. The danger which these
conjoined waves bring is obviously greatest on cliff shores, where, on
account of the depth of water, the waves do not break until they
strike the steep.
* * * * *
Having considered in a general way the action of waves as they roll in
to the shore, bearing with them the solar energy which was contributed
to them by the winds, we shall now take up in some detail the work
which goes on along the coast line--work which is mainly accomplished
by wave action.
On most coast lines the observer readily notes that the shore is
divided into two different kinds of faces--those where the inner
margin of the wave-swept belt comes against rocky steeps, and those
bordered by a strand altogether composed of materials which the surges
have thrown up. These may be termed for convenience cliff shores and
wall-beach shores. We shall begin our inquiry with cliff shores, for
in those sections of the coast line the sea is doing its most
characteristic and important work of assaulting the land. If the
student has an opportunity to approach a set of cliffs of hard rock in
time of heavy storm, when the waves have somewhere their maximum
height, he should seek some headland which may offer him safe foothold
whence he can behold the movements which are taking place. If he is so
fortunate as to have in view, as well may be the case, cliffs which
extend down into deep water, and others which are bordered by rude
and generally steeply sloping beaches covered with large stones, he
may perceive that the waves come in against the cliffs which plunge
into deep water without taking on the breaker form. In this case the
undulation strikes but a moderate blow; the wave is not greatly
broken. The part next the rock may shoot up as a thin sheet to a
considerable height; it is evident that while the ongoing wave applies
a good deal of pressure to the steep, it does not deliver its energy
in the effective form of a blow as when the wave overturns, or in the
consequent rush of the water up a beach slope. It is easy to perceive
that firm-set rock cliffs, with no beaches at their bases, can almost
indefinitely withstand the assaults. On the steep and stony beach,
because of its relatively great declivity, the breaker or surf forms
far in, and even in its first plunge often attains the base of the
precipice. The blow of the overfalling as well as that of the inrush
moves about stones of great size; those three feet or more in diameter
are often hurled by the action against the base of the steep, striking
blows, the sharp note of which can often be heard above the general
roar which the commotion produces. The needlelike crags forming isles
standing at a distance from the shore, such as are often found along
hard rock coasts, are singularly protected from the action of
effective waves. The surges which strike against them are unarmed with
stones, and the water at their bases is so deep that it does not sway
with the motion with sufficient energy to move them on the bottom.
Where a cliff is in this condition, it may endure until an elevation
of the coast line brings its base near the level of the sea, or until
the process of decay has detached a sufficient quantity of stone to
form a talus or inclined plane reaching near to the water level.
As before noted, it is the presence of a sloping beach reaching to
about the base of the cliff which makes it possible for the waves to
strike at with a hammer instead of with a soft hand. Battering at the
base of the cliff, the surges cut a crease along the strip on which
they strike, which gradually enters so far that the overhanging rock
falls of its own weight. The fragments thus delivered to the sea are
in turn broken up and used as battering instruments until they are
worn to pieces. We may note that in a few months of heavy weather the
stones of such a fall have all been reduced to rudely spherical forms.
Observations made on the eastern face of Cape Ann, Mass., where the
seas are only moderately heavy, show that the storms of a single
winter reduce the fragments thrown into the sea from the granite
quarries to spheroidal shapes, more than half of their weight commonly
being removed in the form of sand and small pebbles which have been
worn from their surfaces.
We can best perceive the effect of battering action which the sea
applies to the cliffs by noting the points where, owing to some chance
features in the structure in the rock, it has proved most effective.
Where a joint or a dike, or perhaps a softer layer, if the rocks be
bedded, causes the wear to go on more rapidly, the waves soon excavate
a recess in which the pebbles are retained, except in stormy weather,
in an unmoved condition. When the surges are heavy, these stones are
kept in continuous motion, receding as the wave goes back, and rushing
forward with its impulse until they strike against the firm-set rock
at the end of the chasm. In this way they may drive in a cut having
the length of a hundred feet or more from the face of the precipice.
In most cases the roofs over these sea caves fall in, so that the
structure is known as a chasm. Occasionally these roofs remain, in
which case, for the reason that the floor of the cutting inclines
upward, an opening is made to the surface at their upper end, forming
what is called in New England a "spouting horn"; from the inland end
of the tunnel the spray may be thrown far into the air. As long as the
cave is closed at this inner end, and is not so high but that it may
be buried beneath a heavy wave, the inrushing water compresses the
air in the rear parts of the opening. When the wave begins to retreat
this air blows out, sending a gust of spray before it, the action
resembling the discharge of a great gun from the face of a
fortification. It often happens that two chasms converging separate a
rock from the cliff. Then a lowering of the coast may bring the mass
to the state of a columnar island, such as abound in the Hebrides and
along various other shores.
If a cliff shore retreats rapidly, it may be driven back into the
shore, and its face assumes the curve of a small bay. With every step
in this change the bottom is sure to become shallower, so that the
waves lose more and more of their energy in friction over the bottom.
Moreover, in entering a bay the friction which the waves encounter in
running along the sides is greater than that which they meet in
coming in upon a headland or a straight shore. The result is, with the
inward retreat of the steep it enters on conditions which diminish the
effectiveness of the wave stroke. The embayment also is apt to hold
detritus, and so forms in time a beach at the foot of the cliff, over
which the waves rarely are able to mount with such energy as will
enable them to strike the wall in an effective manner. With this
sketch of the conditions of a cliff shore, we will now consider the
fate of the broken-tip rock which the waves have produced on that
section of the coast land.
By observation of sea-beaten cliffs the student readily perceives that
a great amount of rocky matter has been removed from most cliff-faced
shores. Not uncommonly it can be shown that such sea faces have
retreated for several miles. The question now arises, What becomes of
the matter which has been broken up by the wave action? In some part
the rock, when pulverized by the pounding to which it is subjected,
has dissolved in the water. Probably ninety per cent of it, however,
retains the visible state, and has a fate determined by the size of
the fragments of which it is composed. If these be as fine as mud, so
that they may float in the water, they are readily borne away by the
currents which are always created along a storm-swept shore,
particularly by the undertow or bottom outcurrent--the "sea-puss," as
it is sometimes called--that sweeps along the bottom from every shore,
against which the waves form a surf. If as coarse as sand grains, or
even very small pebbles, they are likely to be drawn out, rolling over
the bottom to an indefinite distance from the sea margin. The coarser
stones, however, either remain at the foot of the cliff until they are
beaten to pieces, or are driven along the shore until they find some
embayment into which they enter. The journey of such fragments may,
when the wind strikes obliquely to the shore, continue for many miles;
the waves, running with the wind, drive the fragments in oscillating
journeys up and down the beach, sometimes at the rate of a mile or
more a day. The effect of this action can often be seen where a vessel
loaded with brick or coal is wrecked on the coast. In a month
fragments of the materials may be stretched along for the distance of
many miles on either side of the point where the cargo came ashore.
Entering an embayment deep enough to restrain their further journey,
the fragments of rock form a boulder beach, where the bits roll to and
fro whenever they are struck by heavy surges. The greater portion of
them remain in this mill until they are ground to the state of sand
and mud. Now and then one of the fragments is tossed up beyond the
reach of the waves, and is contributed to the wall of the beach. In
very heavy storms these pebbles which are thrown inland may amount in
weight to many tons for each mile of shore.
The study of a pebbly beach, drawn from crest to the deep water
outside, will give an idea as to the history of its work. On either
horn of the crescent by which the pebbles are imported into the pocket
we find the largest fragments. If the shore of the bay be long, the
innermost part of the recess may show even only very small pebbles, or
perhaps only fine sand, the coarser material having been worn out in
the journey. On the bottom of the bay, near low tide, we begin to find
some sand produced by the grinding action. Yet farther out, below
high-tide mark, there is commonly a layer of mud which represents the
finer products of the mill.
Boulder beaches are so quick in answering to every slight change in
the conditions which affect them that they seem almost alive. If by
any chance the supply of detritus is increased, they fill in between
the horns, diminish the incurve of the bay, and so cause its beach to
be more exposed to heavy waves. If, on the other hand, the supply of
grist to the mill is diminished, the beach becomes more deeply
incurved, and the wave action is proportionately reduced. We may say,
in general, that the curve of these beaches represents a balance
between the consumption and supply of the pebbles which they grind up.
The supply of pebbles brought along the shore by the waves is in many
cases greatly added to by a curious action of seaweeds. If the bottom
of the water off the coast is covered by these fragments, as is the
case along many coast lines within the old glaciated districts, the
spores of algæ are prone to take root upon them. Fastening themselves
in those positions, and growing upward, the seaweeds may attain
considerable size. Being provided with floats, the plant exercises a
certain lifting power on the stone, and finally the tugging action of
the waves on the fronds may detach the fragments from the bottom,
making them free to journey toward the shore. Observing from near at
hand the straight wall of the wave in times of heavy storm, the
present writer has seen in one view as many as a dozen of these
plant-borne stones, sometimes six inches in diameter, hanging in the
walls of water as it was about to topple over. As soon as they strike
the wave-beaten part of the shore these stones are apt to become
separated from the plants, though we can often notice the remains or
prints of the attachments adhering to the surface of the rock. Where
the pebbles off the shore are plenty, a rocky beach may be produced
by this process of importation through the agency of seaweeds without
any supply being brought by the waves along the coast line.
Returning to sand beaches, we enter the most interesting field of
contact between seas and lands. Probably nine tenths of all the coast
lines of the open ocean are formed of arenaceous material. In general,
sand consists of finely broken crystals of silica or quartz. These
bits are commonly distinctly faceted; they rarely have a spherical
form. Not only do accumulations of sand border most of the shore line,
but they protect the land against the assaults of the sea, and this in
the following curious manner: When shore waves beat pebbles against
each other, they rapidly wear to bits; we can hear the sound of the
wearing action as the wave goes to and fro. We can often see that the
water is discoloured by the mud or powdered rock. When, however, the
waves tumble on a sandy coast, they make but a muffled sound, and
produce no mud. In fact, the particles of sand do not touch each other
when they receive the blow. Between them there lies a thin film of
water, drawn in by the attraction known as capillarity, which sucks
the fluid into a sponge or between plates of glass placed near
together. The stroke of the waves slightly compresses this capillary
water, but the faces of the grains are kept apart as sheets of glass
may be observed to be restrained from contact when water is between
them. If the reader would convince himself as to the condition of the
sand grains and the water which is between them, he may do so by
pressing his foot on the wet beach which the wave has just left. He
will observe that it whitens and sinks a little under the pressure,
but returns in good part to its original form when the foot is lifted.
In the experiment he has pushed a part of the contained water aside,
but he has not brought the grains together; they do not make the sound
which he will often hear when the sand is dry. The result is that the
sand on the seashore may wear more in going the distance of a mile in
the dry sand dune than in travelling for hundreds along the wet shore.
If the rock matter in the state of sand wore as rapidly under the
heating of the waves as it does in the state of pebbles, the
continents would doubtless be much smaller than they are. Those coasts
which have no other protection than is afforded by a low sand beach
are often better guarded against the inroads of the sea than the
rock-girt parts of the continents. It is on account of this remarkable
endurance of sand of the action of the waves that the stratified rocks
which make up the crust of the earth are so thick and are to such an
extent composed of sand grains.
The tendency of the _débris_-making influences along the coast line is
to fill in the irregularities which normally exist there; to batter
off the headlands, close up the bays and harbours, and generally to
reduce the shores to straight lines. Where the tide has access to
these inlets, it is constantly at work in dragging out the detritus
which the waves make and thrust into the recesses. These two actions
contend with each other, and determine the conditions of the coast
line, whether they afford ports for commerce or are sealed in by sand
bars, as are many coast lines which are not tide-swept, as that of
northern Africa, which faces the Mediterranean, a nearly tideless sea.
The same is the case with the fresh-water lakes; even the greater of
them are often singularly destitute of shelters which can serve the
use of ships, and this because there are no tides to keep the bays and
harbours open.
THE OCEAN CURRENTS.
The system of ocean currents, though it exhibits much complication in
detail, is in the main and primarily dependent on the action of the
constant air streams known as the trade winds. With the breath from
the lips over a basin of water we can readily make an experiment which
shows in a general way the method in which the winds operate in
producing the circulation of the sea. Blowing upon the surface of the
water in the basin, we find that even this slight impulse at once sets
the upper part in motion, the movement being of two kinds--pulsating
movements or waves are produced, and at the same time the friction of
the air on the surface causes its upper part to slide over the under.
With little floats we can shortly note that the stream which forms
passes to the farther side of the vessel, there divides, and returns
to the point of beginning, forming a double circle, or rather two
ellipses, the longer sides of which are parallel with the line of the
air current. Watching more closely, aiding the sight by the particles
which float at various distances below the surface, we note the fact
that the motion which was at first imparted to the surface gradually
extends downward until it affects the water to the depth of some
inches.
In the trade-wind belt the ocean waters to the depth of some hundreds
of feet acquire a continuous movement in the direction in which they
are impelled by those winds. This motion is most rapid at the surface
and near the tropics. It diminishes downwardly in the water, and also
toward the polar sides of the trade-wind districts. Thus the trades
produce in the sea two broad, slow-moving, deep currents, flowing in
the northern hemisphere toward the southwest, and in the southern
hemisphere toward the northwest. Coming down upon each other
obliquely, these broad streams meet about the middle of the tropical
belt. Here, as before noted, the air of the trade winds leaves the
surface and rises upward. The waters being retained on their level,
form a current which moves toward the west. If the earth within the
tropics were covered by a universal sea, the result of this movement
would be the institution of a current which, flowing under the
equator, would girdle the sphere.
With a girdling equatorial current, because of the intense heat of the
tropics and the extreme cold of the parallels beyond the fortieth
degree of latitude, the earth would be essentially uninhabitable to
man, and hardly so to any forms of life. Its surface would be visited
by fierce winds induced by the very great differences of temperature
which would then prevail. Owing, however, to the barriers which the
continents interpose to the motions of these windward-setting tropical
currents, all the water which they bear, when it strikes the opposing
shores, is diverted to the right and left, as was the stream in the
experiment with the basin and the breath, the divided currents seeking
ways toward high latitudes, conveying their store of heat to the
circumpolar lands. So effective is this transfer of temperature that a
very large part of the heat which enters the waters in the tropical
region is taken out of that division of the earth's surface and
distributed over the realms of sea and land which lie beyond the
limits of the vertical sun. Thus the Gulf Stream, the northern branch
of the Atlantic tropical current, by flowing into the North Atlantic,
contributes to the temperature of the region within the Arctic Circle
more heat than actually comes to that district by the direct influx
from the sun.
The above statements as to the climatal effect of the ocean streams
show us how important it is to obtain a sufficient conception as to
the way in which these currents now move and what we can of their
history during the geologic ages. This task can not yet be adequately
done. The fields of the sea are yet too imperfectly explored to afford
us all the facts required to make out the whole story. Only in the
case of our Gulf Stream can we form a full conception as to the
journey which the waters undergo and the consequence of their motion.
In the case of this current, observations clearly show that it arises
from the junction near the equatorial line of the broad stream created
by the two trade-wind belts. Uniting at the equator, these produce a
westerly setting current, having the width of some hundred miles and a
depth of several hundred feet. Its velocity is somewhat greater than a
mile an hour. The centre of the current, because of the greater
strength of the northern as compared with the southern trades, is
considerably south of the equator. When this great slow-moving stream
comes against the coast of South America, it encounters the projecting
shoulder of that land which terminates at Cape St. Roque. There it
divides, as does a current on the bows of an anchored ship, a
part--rather more than one half--of the stream turning to the
northward, the remainder passing toward the southern pole; this
northerly portion becomes what is afterward known as the Gulf Stream,
the history of which we shall now briefly follow.
Flowing by the northwesterly coast of South America, the northern
share of the tropical current, being pressed in against the land by
the trade winds, is narrowed, and therefore acquires at once a swifter
flow, the increased speed being due to conditions like those which add
to the velocity of the water flowing through a hose when it comes to
the constriction of the nozzle. Attaining the line of the southeastern
or Lesser Antilles, often known as the Windward Islands, a part of
this current slips through the interspaces between these isles and
enters the Gulf of Mexico. Another portion, failing to find sufficient
room through these passages, skirts the Antilles on their eastern and
northern sides, passes by and among the Bahama Islands, there to
rejoin the part of the stream which entered the Caribbean. This
Caribbean portion of the tide spreads widely in that broad sea, is
constricted again between Cuba and Yucatan, again expands in the Gulf
of Mexico, and is finally poured forth through the Straits of Florida
as a stream having the width of forty or fifty miles, a depth of a
thousand feet or more, and a speed of from three to five miles an
hour, exceeding in its rate of flow the average of the greatest
rivers, and conveying more water than do all the land streams of the
earth. In this part of its course the deep and swift stream from the
Gulf of Mexico, afterward to be named the Gulf Stream, receives the
contribution of slower moving and shallower currents which skirted the
Antilles on their eastern verge. The conjoined waters then move
northward, veering toward the east, at first as a swift river of the
sea having a width of less than a hundred miles and of great depth;
with each step toward the pole this stream widens, diminishing
proportionately in depth; the speed of its current decreases as the
original impetus is lost, and the baffling winds set its surface
waters to and fro in an irregular way. Where it passes Cape Hatteras
it has already lost a large share of its momentum and much of its
heat, and is greatly widened.
Although the current of the Gulf Stream becomes more languid as we go
northward, it for a very long time retains its distinction from the
waters of the sea through which it flows. Sailing eastward from the
mouth of the Chesapeake, the navigator can often observe the moment
when he enters the waters of this current. This is notable not only in
the temperature, but in the hue of the sea. North of that line the
sharpness of the parting wall becomes less distinct, the stream
spreads out broadly over the surface of the Atlantic, yet its
thermometric effects are distinctly traceable to Iceland and Nova
Zembla, and the tropical driftwood which it carries affords the
principal timber supply of the inhabitants of the first-named isle.
Attaining this circumpolar realm, and finally losing the impulse which
bore it on, the water of the Gulf Stream partly returns to the
southward in a relatively slight current which bears the fluid along
the coast of Europe until it re-enters the system of tropical winds
and the currents which they produce. A larger portion stagnates in the
circumpolar region, in time slowly to return to the tropical district
in a manner afterward to be described. Although the Gulf Stream in the
region north of Cape Hatteras is so indistinct that its presence was
not distinctly recognised until the facts were subjected to the keen
eye of Benjamin Franklin, its effects in the way of climate are so
great that we must attribute the fitness of northern Europe for the
uses of civilized man to its action. But for the heat which this
stream brings to the realm of the North Atlantic, Great Britain would
be as sterile as Labrador, and the Scandinavian region, the
cradle-land of our race, as uninhabitable as the bleakest parts of
Siberia.
It is a noteworthy fact that when the equatorial current divides on
the continents against which it flows, the separate streams, although
they may follow the shores for a certain distance toward the poles,
soon diverge from them, just as the Gulf Stream passes to the seaward
from the eastern coast of the United States. The reason for this
movement is readily found in the same principle which explains the
oblique flow of the trades and counter trades in their passage to and
from the equatorial belt. The particle of water under the equator,
though it flows to the west, has, by virtue of the earth's rotation,
an eastward-setting velocity of a thousand miles an hour. Starting
toward the poles, the particle is ever coming into regions of the sea
where the fluid has a less easterly movement, due to the earth's
rotation on its axis. Consequently the journeying water by its
momentum tends to move off in an easterly course. Attaining high
latitudes and losing its momentum, it abides in the realm long enough
to become cooled.
We have already noted the fact that only a portion of the waters sent
northward in the Gulf Stream and the other currents which flow from
the equator to the poles is returned by the surface flow which sets
toward the equator along the eastern side of the basins. The largest
share of the tide effects its return journey in other ways. Some
portion of this remainder sets equatorward in local cold streams, such
as that which pours forth through Davis Strait into Baffin Bay,
flowing under the Gulf Stream waters for an unknown distance toward
the tropics. There are several of these local as yet little known
streams, which doubtless bring about a certain amount of circulation
between the polar regions and the tropical districts. Their effect is,
however, probably small as compared with that massive drift which we
have now to note.
The tropical waters when they attain high latitudes are constantly
cooled, and are overlaid by the warmer contributions of that tide, and
are thus brought lower and lower in the sea. When they start downward
they have, as observations show, a temperature not much above the
freezing point of salt water. They do not congeal for the reason that
the salt of the ocean lowers the point at which the water solidifies
to near 28° Fahr. The effect of this action is gradually to press down
the surface cold water until it attains the very bottom in all the
circumpolar regions. At the same time this descending water drifts
along the bottom of the ocean troughs toward the equatorial realm. As
this cold water is heavier than that which is of higher temperature
and nearer the surface, it has no tendency to rise. Being below the
disturbing influences of any current save its own, it does not tend,
except in a very small measure, to mingle with the warmer overlying
fluid. The result is that it continues its journey until it may come
within the tropics without having gained a temperature of more than
35° Fahr., the increase in heat being due in small measure to that
which it receives from the earth's interior and that which it acquires
from the overlying warmer water. Attaining the region of the tropical
current, this drift water from the poles gradually rises, to take the
place of that which goes poleward, becomes warm, and again starts on
its surface journey toward the arctic and antarctic regions.
Nothing is known as to the rate of this bottom drift from the polar
districts toward the equator, but, from some computation which he has
made, the writer is of the opinion that several centuries is doubtless
required for the journey from the Arctic Circle to the tropics. The
speed of the movement probably varies; it may at times require some
thousand years for its accomplishment. The effect of the bottom drift
is to withdraw from seas in high latitudes the very cold water which
there forms, and to convey it beneath the seas of middle latitudes to
a realm where it is well placed for the reheating process. If all the
cold water of circumpolar regions had to journey over the surface to
the equator, the perturbing effect of its flow on the climates of
various lands would be far greater than it is at present. Where such
cold currents exist the effect is to chill the air without adding much
to the rainfall; while the currents setting northward not only warm
the regions near which they flow, but by so doing send from the water
surfaces large quantities of moisture which fall as snow or rain. Thus
the Gulf Stream, directly and indirectly, probably contributes more
than half the rainfall about the Atlantic basin. The lack of this
influence on the northern part of North America and Asia causes those
lands to be sterilized by cold, although destitute of permanent ice
and snow upon their surfaces.
We readily perceive that the effect of the oceanic circulation upon
the temperatures of different regions is not only great but widely
contrasted. By taking from the equatorial belt a large part of the
heat which falls within that realm, it lowers the temperature to the
point which makes the district fit for the occupancy of man, perhaps,
indeed, tenable to all the higher forms of life. This same heat
removed to high latitudes tempers the winter's cold, and thus makes a
vast realm inhabitable which otherwise would be locked in almost
enduring frosts. Furthermore, this distribution of temperatures tends
to reduce the total wind energy by diminishing the trades and counter
trades which are due to the variations of heat which are encountered
in passing polarward from the equator. Still further, but for this
circulation of water in the sea, the oceans about the poles would be
frozen to their very bottom, and this vast sheet of ice might be
extended southward to within the parallels of fifty degrees north and
south latitude, although the waters under the equator might at the
same time be unendurably hot and unfit for the occupancy of living
beings.
A large part of the difficulties which geologists encounter in
endeavouring to account for the changes of the past arise from the
evidences of great climatal revolutions which the earth has undergone.
In some chapters of the great stone book, whose leaves are the strata
of the earth, we find it plainly written in the impressions made by
fossils that all the lands beyond the equatorial belt have undergone
changes which can only be explained by the supposition that the heat
and moisture of the countries have been subjected to sudden and
remarkable changes. Thus in relatively recent times thick-leaved
plants which retained their vegetation in a rather tender state
throughout the year have flourished near to the poles, while shortly
afterward an ice sheet, such as now covers the greater part of
Greenland, extended down to the line of the Ohio River at Cincinnati.
Although these changes of climate are, as we shall hereafter note,
probably due to entangled causes, we must look upon the modifications
of the ocean streams as one of the most important elements in the
causation. We can the more readily imagine such changes to be due to
the alterations in the course and volume of the ocean current when we
note how trifling peculiarities in the geography of the
shores--features which are likely to be altered by the endless changes
which occur in the form of a continent--affect the run of these
currents. Thus the growth of coral reefs in southern Florida, and, in
general, the formation of that peninsula, by narrowing the exit of the
great current from the Gulf of Mexico, has probably increased its
velocity. If Florida should again sink down, that current would go
forth into the North Atlantic with the speed of about a mile an hour,
and would not have momentum enough to carry its waters over half the
vast region which they now traverse. If the lands about the western
border of the Caribbean Sea, particularly the Isthmus of Darien,
should be depressed to a considerable depth below the ocean level,
the tropical current would enter the Pacific Ocean, adding to the
temperature of its waters all the precious heat which now vitalizes
the North Atlantic region. Such a geographic accident would not only
profoundly alter the life conditions of that part of the world, but it
would make an end of European civilization.
In the chapter on climatal changes further attention will be given to
the action of ocean currents from the point of view of their influence
on the heat and moisture of different parts of the world. We now have
to consider the last important influence of ocean currents--that which
they directly exercise on the development of organic life. The most
striking effect of this nature which the sea streams bring about is
caused by the ceaseless transportation to which they subject the eggs
and seeds of animals and plants, as well as the bodies of the mature
form which are moved about by the flowing waters. But for the
existence of these north and south flowing currents, due to the
presence of the continental barriers, the living tenants of the seas
would be borne along around the earth, always in the same latitude,
and therefore exposed to the same conditions of temperature. In this
state of affairs the influences which now make for change in organic
species would be far less than they are. Journeying in the great
whirlpools which the continental barriers make out of the westward
setting tropical currents, these organic species are ever being
exposed to alterations in their temperature conditions which we know
to be favourable to the creation of those variations on which the
advance of organic life so intimately depends. Thus the ocean currents
not only help to vary the earth by producing changes in the climate of
both sea and land, breaking up the uniformity which would otherwise
characterize regions at the same distance from the equator, but they
induce, by the consequences of the migrations which they enforce,
changes in the organic tenants of the sea.
Another immediate effect of ocean streams arises where their currents
of warm water come against shores or shallows of the sea. At these
points, if the water have a tropical temperature, we invariably find a
vast and rapid development of marine animals and plants, of which the
coral-making polyps are the most important. In such positions the
growth of forms which secrete solid skeletons is so rapid that great
walls of their remains accumulate next the shore, the mass being built
outwardly by successive growths until the realm of the land may be
extended for scores of miles into the deep. In other cases vast mounds
of this organic _débris_ may be accumulated in mid ocean until its
surface is interspersed with myriads of islands, all of which mark the
work due to the combined action of currents and the marine life which
they nourish. Probably more than four fifths of all the islands in the
tropical belt are due in this way to the life-sustaining action of the
currents which the trade winds create.
There are many secondary influences of a less important nature which
are due to the ocean streams. The reader will find on most wall-maps
of the world certain areas in the central part of the oceans which are
noted as Sargassum seas, of which that of the North Atlantic, west and
south of the Azore Islands, is one of the most conspicuous. In these
tracts, which in extent may almost be compared with the continents, we
find great quantities of floating seaweed, the entangled fronds of
which often form a mass sufficiently dense to slightly restrain the
speed of ships. When the men on the caravels of Columbus entered this
tangle, they were alarmed lest they should be unable to escape from
its toils. It is a curious fact that these weeds of the sea while
floating do not reproduce by spores the structures which answer to the
seeds of higher plants, but grow only by budding. It seems certain
that they could not maintain their place in the ocean but for the
action of the currents which convey the bits rent off from the shores
where the plant is truly at home. This vast growth of plant life in
the Sargassum basins doubtless contributed considerable and important
deposits of sediment to the sea floors beneath the waters which it
inhabits. Certain ancient strata, known as the Devonian black shale,
occupying the Ohio valley and the neighbouring parts of North America
to the east and north of that basin, appear to be accumulations which
were made beneath an ancient Sargassum sea.
The ocean currents have greatly favoured and in many instances
determined the migrations not only of marine forms, but of land
creatures as well. Floating timber may bear the eggs and seeds of many
forms of life to great distances until the rafts are cast ashore in a
realm where, if the conditions favour, the creatures may find a new
seat for their life. Seeds of plants incased in their often dense
envelopes may, because they float, be independently carried great
distances. So it comes about that no sooner does a coral or other
island rise above the waters of the sea than it becomes occupied by a
varied array of plants. The migrations of people, even down to the
time of the voyages which discovered America, have in large measure
been controlled by the run of the ocean streams. The tropical set of
the waters to the westward helped Columbus on his way, and enabled him
to make a journey which but for their assistance could hardly have
been accomplished. This same current in the northern part of the Gulf
Stream opposed the passage of ships from northern Europe to the
westward, and to this day affects the speed with which their voyages
are made.
THE CIRCUIT OF THE RAIN.
We have now to consider those movements of the water which depend upon
the fact that at ordinary temperatures the sea yields to the air a
continued and large supply of vapour, a contribution which is made in
lessened proportion by water in all stages of coldness, and even by
ice when it is exposed to dry air. This evaporation of the sea water
is proportional to the temperature and to the dryness of the air where
it rests upon the ocean. It probably amounts on the average to
somewhere about three feet per annum; in regions favourably situated
for the process, as on the west coast of northern Africa, it may be
three or four times as much, while in the cold and humid air about the
poles it may be as little as one foot. When contributed to the air,
the water enters on the state of vapour, in which state it tends to
diffuse itself freely through the atmosphere by virtue of the motion
which is developed in particles when in the vaporous or gaseous state.
The greater part of the water evaporated from the seas probably finds
its way as rain at once back into the deep, yet a considerable portion
is borne away horizontally until it encounters the land. The
precipitation of the water from the air is primarily due to the
cooling to which it is subjected as it rises in the atmosphere. Over
the sea the ascent is accomplished by the simple diffusion of the
vapour or by the uprise through the aërial shaft, such as that near
the equator or over the centres of the whirling storms. It is when the
air strikes the slopes of the land that we find it brought into a
condition which most decidedly tends to precipitate its moisture.
Lifted upward, the air as it ascends the slopes is brought into cooler
and more rarefied conditions. Losing temperature and expanding, it
parts with its water for the same reason that it does in the ascending
current in the equatorial belt or in the chimneys of the whirl storms.
A general consequence of this is that wherever moisture-laden winds
from the sea impinge upon a continent they lay down a considerable
part of the water which they contain.
If all the lands were of the same height, the rain would generally
come in largest proportion upon their coastal belt, or those portions
of the shore-line districts over which the sea winds swept. But as
these winds vary in the amount of the watery vapour which they
contain, and as the surface of the land is very irregular, the
rainfall is the most variable feature in the climatal conditions of
our sphere. Near the coasts it ranges from two or three inches in arid
regions--such as the western part of the Sahara and portions of the
coast regions of Chili and Peru--to eight hundred inches about the
head waters of the Brahmapootra River in northern India, where the
high mountains are swept over by the moisture-laden airs from the
neighbouring sea. Here and there detached mountainous masses produce a
singular local increase in the amount of the rainfall. Thus in the
lake district in northwestern England the rainfall on the seaward side
of mountains, not over four thousand feet high, is very much greater
than it is on the other slope, less than a score of miles away. These
local variations are common all over the world, though they are but
little observed.
In general, the central parts of continents are likely to receive much
less rainfall than their peripheral portions. Thus the central
districts of North America, Asia, and Australia--three out of the five
continental masses--have what we may call interior deserts. Africa has
one such, though it is north of the centre, and extends to the shores
of the Mediterranean and the Atlantic. The only continent without this
central nearly rainless field is South America, where the sole
characteristic arid district is situated on the western slope of the
Cordilleran range. In this case the peculiarity is due to the fact
that the strong westerly setting winds which sweep over the country
encounter no high mountains until they strike the Andean chain. They
journey up a long and rather gradual slope, where the precipitation is
gradually induced, the process being completed when they strike the
mountain wall. Passing over its summit, they appear as dry winds on
the Pacific coast.
Even while the winds frequently blow in from the sea, as along the
western coast of the Americas, they may come over water which is
prevailingly colder than the land. This is characteristically the case
on the western faces of the American continent, where the sea is
cooled by the currents setting toward the equator from high latitudes.
Such cool sea air encountering the warm land has its temperature
raised, and therefore does not tend to lay down its burden of
moisture, but seeks to take up more. On this account the rainfall in
countries placed under such conditions is commonly small.
By no means all the moisture which comes upon the earth from the
atmosphere descends in the form of rain or snow. A variable, large,
though yet undetermined amount falls in the form of dew. Dew is a
precipitation of moisture which has not entered the peculiar state
which we term fog or cloud, but has remained invisible in the air. It
is brought to the earth through the radiation of heat which
continually takes place, but which is most effective during the
darkened half of the day, when the action is not counterbalanced by
the sun's rays. While the sun is high and the air is warm there is a
constant absorption of moisture in large part from the ground or from
the neighbouring water areas, probably in some part from those
suspended stores of water, the clouds, if such there be in the
neighbourhood. We can readily notice how clouds drifting in from the
sea often melt into the dry air which they encounter. Late in the
afternoon, even before the sun has sunk, the radiation of heat from
the earth, which has been going on all the while, but has been less
considerable than the incurrent of temperature, in a way overtakes
that influx. The air next the surface becomes cooled from its contact
with the refrigerating earth, and parts with its moisture, forming a
coating of water over everything it touches. At the same time the
moisture escaping from the warmed under earth likewise drops back upon
its cooled surface almost as soon as it has escaped. The thin sheet of
water precipitated by this method is quickly returned to the air when
it becomes warmed by the morning sunshine, but during the night
quantities of it are absorbed by the plants; very often, indeed, with
the lowlier vegetation it trickles down the leaves and enters the
earth about the base of the stem, so that the roots may appropriate
it. Our maize, or Indian corn, affords an excellent example of a plant
which, having developed in a land of droughts, is well contrived,
through its capacities for gathering dew, to protect itself against
arid conditions. In an ordinary dew-making night the leaves of a
single stem may gather as much as half a pint of water, which flows
down their surfaces to the roots. So efficient is this dew supply,
this nocturnal cloudless rain, that on the western coast of South
America and elsewhere, where the ordinary supply of moisture is almost
wanting, many important plants are able to obtain from it much of the
water which they need. The effect is particularly striking along
seashores, where the air, although it may not have the humidity
necessary for the formation of rain, still contains enough to form
dew.
It is interesting to note that the quantity of dew which falls upon an
area is generally proportioned to the amount of living vegetation
which it bears. The surfaces of leaves are very efficient agents of
radiation, and the tangle which they make offers an amount of
heat-radiating area many times as great as that afforded by a surface
of bared earth. Moreover, the ground itself can not well cool down to
the point where it will wring the moisture out of the air, while the
thin membranes of the plants readily become so cooled. Thus vegetation
by its own structure provides itself with means whereby it may be in a
measure independent of the accidental rainfall. We should also note
the fact that the dewfall is a concomitant of cloudless skies. The
quantity which is precipitated in a cloudy night is very small, and
this for the reason that when the heavens are covered the heat from
the earth can not readily fly off into space. Under these conditions
the temperature of the air rarely descends low enough to favour the
precipitation of dew.
Having noted the process by which in the rain circuit the water
leaves the sea and the conditions of distribution when it returns to
the earth, we may now trace in more detail the steps in this great
round. First, we should take note of the fact that the water after it
enters the air may come back to the surface of the earth in either of
two ways--directly in the manner of dewfall, or in a longer circuit
which leads it through the state of clouds. As yet we are not very
well informed as to the law of the cloud-making, but certain features
in this picturesque and most important process have been tolerably
well ascertained.
Rising upward from the sea, the vapour of water commonly remains
transparent and invisible until it attains a considerable height above
the surface, where the cooling tends to make it assume again the
visible state of cloud particles. The formation of these cloud
particles is now believed to depend on the fact that the air is full
of small dust motes, exceedingly small bits of matter derived from the
many actions which tend to bring comminuted solid matter into the air,
as, for instance, the combustion of meteoric stones, which are greatly
heated by friction in their swift course through the air, the
ejections of volcanoes, the smoke of forest and other fires, etc.
These tiny bits, floating in the air, because of their solid nature
radiate their heat, cool the air which lies against them, and thereby
precipitate the water in the manner of dew, exactly as do the leaves
and other structures on the surface of the earth. In fact, dew
formation is essentially like cloud formation, except that in the one
case the water is gathered on fixed bodies, and in the other on
floating objects. Each little dust raft with its cargo of condensed
water tends, of course, to fall downward toward the earth's surface,
and, except for the winds which may blow upward, does so fall, though
with exceeding slowness. Its rate of descent may be only a few feet a
day. It was falling before it took on the load of water; it will fall
a little more rapidly with the added burden, but even in a still air
it might be months or years before it would come to the ground. The
reason for this slow descent may not at first sight be plain, though a
little consideration will make it so.
If we take a shot of small size and a feather of the same weight, we
readily note that their rate of falling through the air may vary in
the proportion of ten to one or more. It is easy to conceive that this
difference is due to the very much less friction which the smaller
body encounters in its motion by the particles of air. With this point
in mind, the student should observe that the surface presented by
solid bodies in relation to their solid contents is the greater the
smaller the diameter. A rough, though not very satisfactory, instance
of this principle may be had by comparing the surface and interior
contents of two boxes, one ten feet square and the other one foot
square. The larger has six hundred feet of surface to one thousand
cubic feet of interior, or about half a square foot of outer surface
to the cubic foot of contents; while the smaller box has six feet of
surface for the single cubic foot of interior, or about ten times the
proportion of exterior to contents. The result is that the smaller
particles encounter more friction in moving toward the earth, until,
in the case of finely divided matter, such as the particles of carbon
in the smoke from an ordinary fire, the rate of down-falling may be so
small as to have little effect in the turbulent conditions of
atmospheric motion.
[Illustration: _Pocket Creek, Cape Ann, Massachusetts. Note the
relatively even size of the pebbles, and the splash wave which sets
them in motion._]
The little drops of water which gather round dust motes, falling but
slowly toward the earth, are free to obey the attractions which they
exercise upon each other--impulses which are partly gravitative and
partly electrical. We have no precise knowledge concerning these
movements, further than that they serve to aggregate the myriad little
floats into cloud forms, in which the rafts are brought near together,
but do not actually touch each other. They are possibly kept apart by
electrical repulsion. In this state of association without union the
divided water may undergo the curiously modified aggregations which
give us the varied forms of clouds. As yet we know little as to the
cause of cloud shapes. We remark the fact that in the higher of these
agglomerations of condensed vapour, the clouds which float at an
elevation of from twenty to thirty thousand feet or more, the masses
are generally thin, and arranged more or less in a leaflike form,
though even here a tendency to produce spherical clouds is apparent.
In this high realm floating water is probably in the frozen state,
answering to the form of dew, which we call hoar frost. The lower
clouds, gathering in the still air, show very plainly the tendency to
agglomerate into spheres, which appears to be characteristic of all
vaporous material which is free to move by its own impulses. It is
probable that the spherical shape of clouds is more or less due to the
same conditions as gathered the stellar matter from the ancient
nebular chaos into the celestial spheres. Upon these spherical
aggregations of the clouds the winds act in extremely varied ways. The
cloud may be rubbed between opposite currents, and so flattened out
into a long streamer; it may take the same form by being carried off
by a current in the manner of smoke from a fire; the spheres may be
kept together, so as to form the patchwork which we call "mackerel"
sky; or they may be actually confounded with each other in a vast
common cloud-heap. In general, where the process of aggregation of two
cloud bodies occurs, changes of temperature are induced in the masses
which are mixed together. If the temperature resulting from this
association of cloud masses is an average increase, the cloud may
become lighter, and in the manner of a balloon move upward. Each of
the motes in the cloud with its charge of vapour may be compared with
the ballast of the balloon; if they are warmed, they send forth a part
of their load of condensed water again to the state of invisible
vapour. Rising to a point where it cools, the vapour gathers back on
the rafts and tends again to weight the cloud downward. The ballast of
an ordinary balloon has to be thrown away from its car; but if some
arrangement for condensing the moisture from the air could be
contrived, a balloon might be brought into the adjustable state of a
cloud, going up or down according as it was heated or cooled.
When the formation of the drop of water or snowflake begins, the mass
is very small. If in descending it encounters great thickness of
cloud, the bit may grow by further condensation until it becomes
relatively large. Generally in this way we may account for the
diversities in the size of raindrops or snowflakes. It often happens
that the particles after taking on the form of snowflakes encounter in
their descent air so warm that they melt into raindrops, or, if only
partly melted, reach the surface as sleet. Or, starting as raindrops,
they may freeze, and in this simple state may reach the earth, or
after freezing they may gather other frozen water about them, so that
the hailstone has a complicated structure which, from the point of
view of classification, is between a raindrop and a snowflake.
In the process of condensation--indeed, in the steps which precede the
formation of rain and snow--there is often more or less trace of
electrical action; in fact, a part of the energy which was involved in
the vapourization of water, on its condensation, even on the dust
motes appears to be converted into electrical action, which probably
operates in part to keep the little aggregates of water asunder. When
they coalesce in drops or flakes, this electricity often assumes the
form of lightning, which represents the swift passage of the electric
store from a region where it is most abundant to one where it is less
so. The variations in this process of conveying the electricity are
probably great. In general, it probably passes, much as an electric
current is conveyed, through a wire from the battery which produces
the force. In other cases, where the tension is high, or, in other
words, where the discharge has to be hastened, we have the phenomena
of lightning in which the current burns its way along its path, as it
may traverse a slender wire, vapourizing it as it goes. In general,
the lightning flash expends its force on the air conductors, or lines
of the moist atmosphere along which it breaks its path, its energy
returning into the vapour which it forms or the heat which it produces
in the other parts of the air. In some cases, probably not one in the
thousand of the flashes, the charge is so heavy that it is not used up
in its descent toward the earth, and so electrifies, or, as we say,
strikes, some object attached to the earth, through which it passes to
the underlying moisture, where it finds a convenient place to take on
a quiet form. Almost all these hurried movements of electrical energy
which intensely heat and light the air which they traverse fly from
one part of a cloud to another, or cross from cloud sphere to cloud
sphere; of those which start toward the earth, many are exhausted
before they reach its surface, and even those that strike convey but a
portion of their original impulse to the ground.
The wearing-out effect of lightning in its journey along the air
conductors in its flaming passages is well illustrated by what happens
when the charge strikes a wire which is not large enough freely to
convey it. The wire is heated, generally made white hot, often melted,
and perhaps scattered in the form of vapour. In doing this work the
electricity may, and often is, utterly dissipated--that is, changed
into heat. It has been proposed to take advantage of this principle in
protecting buildings from lightning by placing in them many thin
wires, along which the current will try to make its way, being
exhausted in melting or vaporizing the metal through which it passes.
There are certain other forms of lightning, or at least of electrical
discharges, which produce light and which may best be described in
this connection. It occasionally happens that the earth becomes so
charged that the current proceeds from its surface to the clouds. More
rarely, and under conditions which we do not understand, the electric
energy is gathered into a ball-like form, which may move slowly along
the surface until it suddenly explodes. It is a common feature of all
these forms of lightning which we have noted that they ordinarily make
in their movement considerable noise. This is due to the sudden
displacement of the air which they traverse--displacement due to the
action of heat in separating the particles. It is in all essential
regards similar to the sounds made by projectiles, such as meteors or
swift cannon shots, as they fly through the air. It is even more
comparable to the sound produced by exploding gunpowder. The first
sound effect from the lightning stroke is a single rending note, which
endures no longer--indeed, not as long--as the explosion of a cannon.
Heard near by, this note is very sharp, reminding one of the sound
made by the breaking of glass. The rolling, continuous sound which we
commonly hear in thunder is, as in the case of the noise produced by
cannon, due to echo from the clouds and the earth. Thunder is
ordinarily much more prolonged and impressive in a mountainous country
than in a region of plains, because the steeps about the hearer
reverberate the original single crash.
The distribution of thunderstorms is as yet not well understood, but
it appears in many cases that they are attendants on the advancing
face of cyclones and hurricanes, the area in front of these great
whirlstorms being subjected to the condensation and irregular air
movements which lead to the development of much electrical energy.
There are, however, certain parts of the earth which are particularly
subjected to lightning flashes. They are common in the region near the
equator, where the ascending currents bring about heavy rains, which
mean a rapid condensation and consequent liberation of electrical
energy. They diminish in frequency toward the arctic regions. An
observer at the pole would probably fail ever to perceive strong
flashes. For the same reason thunderstorms are more frequent in
summer, the time when the difference in temperature between the
surface and the upper air is greatest, when, therefore, the uprushes
of air are likely to be most violent. They appear to be more common in
the night than in the daytime, for the reason that condensation is
favoured by the cooling which occurs in the dark half of the day. It
is rare, indeed, that a thunderstorm occurs near midday, a period when
the air is in most cases taking up moisture on account of the swiftly
increasing heat.
There are other forms of electrical discharges not distinctly
connected with the then existing condensation of moisture. What the
sailors call St. Elmo's fire--a brush of electric light from the mast
tops and other projections of the ship--indicates the passage of
electrical energy between the vessel and the atmosphere. Similar
lights are said sometimes to be seen rising from the surface of the
water. Such phenomena are at present not satisfactorily explained.
Perhaps in the same group of actions comes the so-called
"Jack-o'-lantern" or "Will-o'-the-wisp" fires flashing from the earth
in marshy places, which are often described by the common people, but
have never been observed by a naturalist. If this class of
illuminations really exists, we have to afford them some other
explanation than that they are emanations of self-inflamed
phosphoretted hydrogen, a method of accounting for them which
illogically finds a place in many treatises on atmospheric phenomena.
A gas of any kind would disperse itself in the air; it could not dance
about as these lights are said to do, and there is no chemical means
known whereby it could be produced in sufficient purity and quantity
from the earth to produce the effects which are described.[3]
[Footnote 3: The present writer has made an extended and careful study
of marsh and swamp phenomena, and is very familiar with the aspect of
these fields in the nighttime. He has never been able to see any sign of
the Jack-o'-lantern light. Looking fixedly into any darkness, such as is
afforded by the depths of a wood, the eye is apt to imagine the
appearance of faint lights. Those who have had to do with outpost duty
in an army know how the anxious sentry, particularly if he is new to the
soldier's trade, will often imagine that he sees lights before him.
Sometimes the pickets will be so convinced of the fact that they see
lights that they will fire upon the fiction of the imaginations. These
facts make it seem probable that the Jack-o'-lantern and his companion,
the Will-o'-the-wisp, are stories of the overcredulous.]
In the upper air, or perhaps even beyond the limits of the field
which deserves the name, in the regions extending from the poles to
near the tropics, there occur electric glowings commonly known as the
aurora borealis. This phenomenon occurs in both hemispheres. These
illuminations, though in some way akin to those of lightning, and
though doubtless due to some form of electrical action, are peculiar
in that they are often attended by glows as if from clouds, and by
pulsations which indicate movements not at electric speed. As yet but
little is known as to the precise nature of these curious storms. It
has been claimed, however, that they are related to the sun spots;
those periods when the solar spots are plenty, at intervals of about
eleven years, are the times of auroral discharges. Still further, it
seems probable that the magnetic currents of the earth, that circling
energy which encompasses the sphere, moving round in a general way
parallel to the equator, are intensified during these illuminations of
the circumpolar skies.
GEOLOGICAL WORK OF WATER.
We turn now to the geological work which is performed by falling
water. Where the rain or snow returns from the clouds to the sea, the
energy of position given to the water by its elevation above the earth
through the heat which it acquired from the sun is returned to the air
through which it falls or to the ocean surface on which it strikes. In
this case the circuit of the rain is short and without geological
consequence which it is worth while to consider, except to note that
the heat thus returned is likely to be delivered in another realm than
that in which the falling water acquired the store, thus in a small
way modifying the climate. When, however, the precipitation occurs on
the surface of the land, the drops of frozen or fluid water apply a
part of their energy in important geological work, the like of which
is not done where they return at once to the sea.
[Illustration: Fig. 10.--Showing the diverse action of rain on wooded
and cleared fields, _a_, wooded area; _b_, tilled ground.]
We shall first consider what takes place when the water in the form of
drops of rain comes to the surface of the land. Descending as they do
with a considerable speed, these raindrops apply a certain amount of
energy to the surface on which they fall. Although the beat of a
raindrop is proverbially light, the stroke is not ineffective.
Observing what happens where the action takes place on the surface of
bare rock, we may notice that the grains of sand or small pebbles
which generally abound on such surfaces, if they be not too steeply
inclined, dance about under the blows which they receive. If we could
cover hard plate glass, a much firmer material than ordinary stone,
with such bits, we should soon find that its surface would become
scratched all over by the friction. Moreover, the raindrops
perceptibly urge the small detached bits of stone down the slopes
toward the streams.
If all the earth's surface were bare rocks, the blow of the raindrops
would deserve to be reckoned among the important influences which lead
to the wearing of land. As it is, when a country is in a state of
Nature, only a small part of its surface is exposed to this kind of
wearing. Where there is rain enough to effect any damage, there is
sure to be sufficient vegetation to interpose a living and
self-renewed covering between the rocks and the rain. Even the lichens
which coat what at first sight often seems to be bare rock afford an
ample covering for this purpose. It is only where man bares the field
by stripping away and overturning this protecting vegetation that the
raindrops cut away the earth. The effect of their action can often be
noted by observing how on ploughed ground a flat stone or a potsherd
comes after a rain to cap a little column. The geologist sometimes
finds in soft sandstones that the same action is repeated in a larger
way where a thin fragment of hard rock has protected a column many
feet in height against the rain work which has shorn down the
surrounding rock.
When water strikes the moistened surface it at once loses the droplike
form which all fluids assume when they fall through the air.[4]
[Footnote 4: This principle of the spheroidal form in falling fluids is
used in making ordinary bird shot. The melted lead drops through
sievelike openings, the resulting spheres of the metal being allowed to
fall into water which chills them. Iron shot, used in cutting stone,
where they are placed between the saw and the surface of the rock, are
also made in the same manner. The descending fluid divides into drops
because it is drawn out by the ever-increasing speed of the falling
particles, which soon make the stream so thin that it can not hold
together.]
When the raindrops coalesce on the surface of the earth, the rôle of
what we may call land water begins. Thenceforward until the fluid
arrives at the surface of the sea it is continually at work in
effecting a great range of geological changes, only a few of which can
well be traced by the general student. The work of land water is due
to three classes of properties--to the energy with which it is endowed
by virtue of its height above the sea, a power due to the heat of the
sun; to the capacity it has for taking substances into solution; and
to its property of giving some part of its own substance to other
materials with which it comes in contact. The first of these groups of
properties may be called dynamical; the others, chemical.
The dynamic value of water when it falls upon the land is the amount
of energy it can apply in going down the slope which separates it from
the sea. A ton of the fluid, such as may gather in an ordinary rain on
a thousand square feet of ground in the highlands of a country--say at
an elevation of a thousand feet above the sea--expends before it comes
to rest in the great reservoir as much energy as would be required to
lift that weight from the ocean's surface to the same height. The ways
in which this energy may be expended we shall now proceed in a general
way to trace.
As soon as the water has been gathered, from its drop to its sheet
state--a process which takes place as soon as it falls--the fluid
begins its downward journey. On this way it is at once parted into two
distinct divisions, the surface water and the ground water: the former
courses more or less swiftly, generally at the rate of a mile or more
an hour, in the light of day; the latter enters the interstices of the
earth, slowly descends therein to a greater or less depth, and
finally, journeying perhaps at the rate of a mile a year, rejoins the
surface water, escaping through the springs. The proportion of these
two classes, the surface and the ground water, varies greatly, and an
intermixture of them is continually going on. Thus on the surface of
bare rock or frozen earth all the rain may go away without entering
the ground. On very sandy fields the heaviest rainfall may be taken
up by the porous earth, so that no streams are found. On such surfaces
the present writer has observed that a rainfall amounting to six
inches in depth in two hours produced no streams whatever. We shall
first follow the history of the surface water, afterward considering
the work which the underground movements effect.
If the student will observe what takes place on a level ploughed
field--which, after all, will not be perfectly level, for all fields
are more or less undulating--he will note that, though the surface may
have been smoothed by a roller until it appears like a floor, the
first rain, where the fall takes place rapidly enough to produce
surface streams, will create a series of little channels which grow
larger as they conjoin, the whole appearing to the eye like a very
detailed map, or rather model, of a river system; it is, indeed, such
a system in miniature. If he will watch the process by which these
streamlet beds are carved, he will obtain a tolerably clear idea as to
that most important work which the greater streams do in carving the
face of the lands. The water is no sooner gathered into a sheet than,
guided by the slightest irregularities which it encounters, it begins
to flow. At first the motion is so slow that it does not disturb its
bed, but at some points in the bottom of the sheet the movement soon
becomes swift enough to drag the grains of sand and clay from their
adhesions, bearing them onward. As soon as this beginning of a channel
is formed the water moves more swiftly in the clearer way; it
therefore cuts more rapidly, deepening and enlarging its channel, and
making its motion yet more free. The tiny rills join the greater, all
their channels sway to and fro as directed this way and that by chance
irregularities, until something like river basins are carved out,
those gentle slopes which form broad valleys where the carving has
been due to the wanderings of many streams. If the field be large,
considerable though temporary brooks may be created, which cut
channels perhaps a foot in depth. At the end of this miniature stream
system we always find some part of the waste which has been carved
out. If the streamlet discharges into a pool, we find the tiny
representative of deltas, which form such an important feature on the
coast line where large rivers enter seas or lakes. Along the lines of
the stream we may observe here and there little benches, which are the
equivalent in all save size of the terraces that are generally to be
observed along the greater streams. In fact, these accidents of an
acre help in a most effective way the student to understand the
greater and more complicated processes of continental erosion.
A normal river--in fact, all the greater streams of the
earth--originates in high country, generally in a region of mountains.
Here, because of the elevation of the region, the streams have cut
deep gorges or extensive valleys, all of which have slopes leading
steeply downward to torrent beds. Down these inclined surfaces the
particles worn off from the hard rock by frost and by chemical decay
gradually work their way until they attain the bed of the stream. The
agents which assist gravitation in bearing this detritus downward are
many, but they all work together for the same end. The stroke of the
raindrop accomplishes something, though but little; the direct washing
action of the brooklets which form during times of heavy rain, but dry
out at the close of the storm, do a good deal of the work; thawing and
freezing of the water contained in the mass of detritus help the
movement, for, although the thrust is in both directions, it is most
effective downhill; the wedges of tree roots, which often penetrate
between and under the stones, and there expand in their process of
growth, likewise assist the downward motion. The result is that on
ordinary mountain slopes the layer of fragments constituting the rude
soil is often creeping at the rate of from some inches to some feet a
year toward the torrent bed. If there be cliffs at the top of the
slope, as is often the case, very extensive falls of rock may take
place from it, the masses descending with such speed that they
directly attain the stream. If the steeps be low and the rock divided
into vertical joints, especially where there is a soft layer at the
base of the steep, detached masses from the precipice may move slowly
and steadfastly down the slope, so little disturbed in their journey
that trees growing upon their summits may continue to develop for the
thousands of years before the mass enters the stream bed.
Although the fall of rocks from precipices does not often take place
in a conspicuously large way, all great mountain regions which have
long been inhabited by man abound in traditions and histories of such
accidents. Within a century or two there have been a dozen or more
catastrophes of this nature in the inhabited valleys of the Alps. As
these accidents are at once instructive and picturesque, it is well to
note certain of them in some detail. At Yvorgne, a little parish on
the north shore of the Rhône, just above the lake of Geneva, tradition
tells that an ancient village of the name was overwhelmed by the fall
of a great cliff. The vast _débris_ forming the steep slope which was
thus produced now bears famous vineyards, but the vintners fancy that
they from time to time hear deep in the earth the ringing of the bells
which belonged to the overwhelmed church. In 1806 the district of
Goldau, just north of Lake Lucerne, was buried beneath the ruins of a
peak which, resting upon a layer of clay, slipped away like a
launching ship on the surface of the soft material. The _débris_
overwhelmed a village and many detached houses, and partly filled a
considerable lake. The wind produced by this vast rush of falling rock
was so great that people were blown away by it; some, indeed, were
killed in this singular manner.
The most interesting field of these Swiss mountain falls is a high
mountain valley of amphitheatrical form, known as the Diablerets, or
the devil's own district. This great circus, which lies at the height
of about four thousand feet above the sea, is walled around on its
northern side by a precipice, above which rest, or rather once
rested, a number of mountain peaks of great bulk. The region has long
been valued for the excellent pasturage which the head of the valley
affords. Two costly roads, indeed, have been built into it to afford
footpaths for the flocks and herds and their keepers in the summer
season. Through this human experience with the valley, we have a
record of what has gone on in this part of the mountain wilderness.
Within the period of history and tradition, three very great mountain
falls have occurred in this field, each having made its memory good by
widespread disaster which it brought to the people of the _chalets_.
The last of these was brought about by the fall of a great peak which
spread itself out in a vast field of ruins in the valley below. The
belt of destruction was about half a mile wide and three miles long.
When the present writer last saw it, a quarter of a century ago, it
was still a wilderness of great rocks, but here and there the process
of their decay was giving a foothold for herbage, and in a few
centuries the field will doubtless be so verdure-clad that its story
will not be told on its face. It is likely, however, to be preserved
in the memory of the people, and this through a singular and pathetic
tradition which has grown up about the place, one which, if not true,
comes at least among the legends which we should like to believe.
As told the present writer by a native of the district, it happened
when, in the nighttime the mountain came down, the herdsmen and their
cows gathered in the _chalets_--stout buildings which are prepared to
resist avalanches of snow. In one of these, which was protected from
crushing by the position of the stones which covered it, a solitary
herdsman found himself alive in his unharmed dwelling. With him in the
darkness were the cows, a store of food and water, and his provisions
for the long summer season. With nothing but hope to animate him, he
set to work burrowing upward among the rocks, storing the _débris_ in
the room of the _chalet_. He toiled for some months, but finally
emerged to the light of day, blanched by his long imprisonment in the
darkness, but with the strength to bear him to his home. In place of
the expected warm welcome, the unhappy man found himself received as a
ghost. He was exorcised by the priest and driven away to the distance.
It was only when long afterward his path of escape was discovered that
his history became known.
Returning to the account of the _débris_ which descends at varied
speed into the torrents, we find that when the detritus encounters the
action of these vigorous streams it is rapidly ground to pieces while
it is pushed down the steep channels to the lower country. Where the
stones are of such size that the stream can urge them on, they move
rapidly; at least in times when the torrent is raging. They beat over
each other and against the firm-set rocks; the more they wear, the
smaller they become, and the more readily they are urged forward.
Where the masses are too large to be stirred by the violent current,
they lie unmoved until the pounding of the rolling stones reduces them
to the proportions where they may join the great procession.
Ordinarily those who visit mountains behold their torrents only in
their shrunken state, when the waters stir no stones, and fail even to
bear a charge of mud, all detachable materials having been swept away
when the streams course with more vigour. In storm seasons the
conditions are quite otherwise; then the swollen torrents, their
waters filled with clay and sand, bear with them great quantities of
boulders, the collisions of which are audible above the muffled roar
of the waters, attesting the very great energy of the action.
When the waste on a mountain slope lies at a steep angle, particularly
where the accumulation is due to the action of ancient glaciers, it
not infrequently happens that when the ground is softened with frost
great masses of the material rush down the slope in the manner of
landslides. The observer readily notes that in many mountain regions,
as, for instance, in the White Mountains of New Hampshire, the steep
slopes are often seamed by the paths of these great landslides. Their
movement, indeed, is often begun by sliding snow, which gives an
impulse to the rocks and earth which it encounters in its descent. At
a place known as the Wylie Notch, in the White Mountains, in the early
part of this century, a family of that name was buried beneath a mass
of glacial waste which had hung on the mountain slope from the ancient
days until a heavy rain, following on a period of thaw, impelled the
mass down the slope. Although there have been few such catastrophes
noted in this country, it is because our mountains have not been much
dwelt in. As they become thickly inhabited as the Alps are, men are
sure to suffer from these accidents.
As the volume of a mountain torrent increases through the junction of
many tributaries, the energy of its moving waters becomes sufficient
to sweep away the fragments which come to its bed. Before this stage
is attained the stream rarely touches the solid under rock of the
mountain, the base of the current resting upon the larger loose stones
which it was unable to stir. In this pebble-paved section, because the
stream could not attack the foundation rock, we find no gorges--in
fact, the whole of this upper section of the torrent system is
peculiarly conditioned by the fact that the streams are dealing not
with bed-rock, but with boulders or smaller loose fragments. If they
cut a little channel, the materials from either side slip the faster,
and soon repave the bed. But when the streams have by a junction
gained strength, and can keep their beds clear, they soon carve down a
gorge through which they descend from the upper mountain realm to the
larger valleys, where their conjoined waters take on a riverlike
aspect. It should be noted here that the cutting power of the water
moving in the torrent or in the wave, the capacity it has for abrading
rock, resides altogether in the bits of stone or cutting tools with
which it is armed. Pure water, because of its fluidity, may move over
or against firm-set stones for ages without wearing them; but in
proportion as it moves rocky particles of any size, the larger they
are, the more effective the work, it wears the rock over which it
flows. A capital instance of this may be found where a stream from a
hose is used in washing windows. If the water be pure, there is no
effect upon the glass; but if it be turbid, containing bits of sand,
in a little while the surface will appear cloudy from the multitude of
line scratches which the hard bits impelled by the water have
inflicted upon it. A somewhat similar case occurs where the wind bears
sand against window panes or a bottle which has long lain on the
shore. The glass will soon be deeply carved by the action, assuming
the appearance which we term "ground." This principle is made use of
in the arts. Glass vessels or sheets are prepared for carving by
pasting paper cut into figures on their surfaces. The material is then
exposed to a jet of air or steam-impelling sand grains; in a short
time all the surface which has not been protected by paper has its
polish destroyed and is no longer translucent.
The passage from the torrent to the river, though not in a
geographical way distinct, is indicated to the observant eye by a
simple feature--namely, the appearance of alluvial terraces, those
more or less level heaps of water-borne _débris_ which accumulate
along the banks of rivers, which, indeed, constitute the difference
between those streams and torrents. Where the mountain waters move
swiftly, they manage to bear onward the waste which they receive. Even
where the blocks of stone cling in the bed, it is only a short time
before they are again set in motion or ground to pieces. If by chance
the detritus accumulates rapidly, the slope is steepened and the work
of the torrent made more efficient. As the torrent comes toward the
base of the mountains, where it neither finds nor can create steep
slopes over which to flow, its speed necessarily diminishes. With each
reduction in this feature its carrying power very rapidly diminishes.
Thus water flowing at the rate of ten miles an hour can urge stones
four times the mass that it can move when its speed is reduced to half
that rate. The result is that on the lowlands, with their relatively
gentle slopes, the combined torrents, despite the increase in the
volume of the stream arising from their confluence, have to lay down a
large part of their load of detritus.
If we watch where a torrent enters a mountain river, we observe that
the main stream in a way sorts over the waste contributed to it,
bearing on only those portions which its rate of flow will permit it
to carry, leaving the remainder to be built into the bank in the form
of a rude terrace. This accumulation may not extend far below the
point where the torrent which imported the _débris_ joins the main
stream; a little farther down, however, we are sure to find another
such junction and a second accumulation of terrace material. As these
contributions increase, the terrace accumulations soon become
continuous, lying on one side or the other of the river, sometimes
bordering both banks of the stream. In general, it can be said that so
long as the rate of fall of the torrent exceeds one hundred feet to
the mile it does not usually exhibit these shelves of detritus. Below
that rate of descent they are apt to be formed. Much, however, depends
upon the amount of detritus which the stream bears and the coarseness
of it; moreover, where the water goes through a gorge in the manner of
a flume with steep rocky sides, it can urge a larger amount before it
than when it traverses a wide valley, through which it passes, it may
be, in a winding way.
At first sight it may seem rather a fine distinction to separate
torrents from rivers by the presence or absence of terraces. As we
follow down the stream, however, and study its action in relation to
these terraces, and the peculiar history of the detritus of which they
are composed, we perceive that these latter accumulations are very
important features. Beginning at first with small and imperfect
alluvial plains, the river, as it descends toward the sea, gaining in
store of water and in the amount of _débris_ which comes with that
water from the hills, while the rate of fall and consequent speed of
the current are diminished, soon comes to a stage where it is engaged
in an endless struggle with the terrace materials. In times of flood,
the walls of the terraces compel the tide to flow over the tops of
these accumulations. Owing to the relative thinness of the water
beyond the bed, and to the growth of vegetation there, the current
moves more slowly, and therefore lays down a considerable deposit of
the silt and sand which it contains. This may result during a single
flood in lifting the level of the terrace by some inches in height,
still further serving to restrict the channel. Along the banks of the
Mississippi and other large rivers the most of this detritus falls
near the stream; a little of it penetrates to the farther side of the
plains, which often have a width of ten miles or more. The result is
that a broad elevation is constructed, a sort of natural mole or
levee, in a measure damming the flood waters, which can now only enter
the "back swamps" through the channels of the tributary streams. Each
of these back swamps normally discharges into the main stream through
a little river of its own, along the banks of which the natural levees
do not develop.
We have now to note a curious swinging movement of rivers which was
first well observed by the skilful engineers of British India. This
movement can best be illustrated by its effects. If on any river which
winds through alluvial plains a jetty is so constructed as to deflect
the stream at any point, the course which it follows will be altered
during its subsequent flow, it may be, for the distance of hundreds of
miles. It will be perceived that in its movements a river normally
strikes first against one shore and then against the other. Its water
in a general way moves as does a billiard ball when it flies from one
cushion to another. It is true that in a torrent we have the same
conditions of motion; but there the banks are either of hard rock or,
if of detritus, they are continually moving into the stream in the
manner before described. In the case of the river, however, its points
of collision are often on soft banks, which are readily undermined by
the washing action of the stream. In the ordinary course of events,
the river beginning, we may imagine, with a straight channel, had its
current deflected by some obstacle, it may be even by the slight
pressure of a tributary stream, is driven against one bank; thence it
rebounds and strikes the other. At each point of impinge it cuts the
alluvium away. It can bear on only a small portion of that which it
thus obtains; the greater part of the material is deposited on the
opposite side of the stream, but a little lower down, where it makes a
shallow. On these shallows water-loving plants and even certain trees,
such as the willows and poplars, find a foothold. When the stream
rises, the sediment settles in this tangle, and soon extends the
alluvial plain from the neighbouring bank, or in rarer cases the river
comes to flow on either side of an island of its own construction. The
natural result of this billiard-ball movement of the waters is that
the path of the stream is sinuous. The less its rate of fall and the
greater the amount of silt it obtains from its tributaries, the more
winding its course becomes. This gain in those parts of the river's
curvings where deposition tends to take place may be accelerated by
tree-planting. Thus a skilful owner of a tract of land on the south
bank of the Ohio River, by assiduously planting willow trees on the
front of his property, gained in the course of thirty years more than
an acre in the width of his arable land. When told by the present
writer that he was robbing his neighbours on the other side of the
stream, he claimed that their ignorance of the laws of river motion
was sufficient evidence that they did not deserve to own land.
In the primitive state of a country the water-loving plants,
particularly the trees which flourish in excessively humid conditions,
generally make a certain defence against these incursions of the
streams. But when a river has gained an opening in the bank it can,
during a flood, extend its width often to the distance of hundreds of
feet. During the inundations of the Mississippi the river may at times
be seen to eat away acres of land in a single day along one of the
outcurves of its banks. The undermined forests falling into the flood
join the great procession of drift timber, composed of trees which
have been similarly uprooted, which occupies the middle part of the
stream. This driftwood belt often has a width of three or four hundred
feet, the entangled stems and branches making it difficult for a boat
to pass from one side of the river to the other.
[Illustration: Fig. 11.--Oxbows and cut-off. Showing the changes in
the course of a river in its alluvial plain.]
When the curves of a river have been developed to a certain point (see
Fig. 11), when they have attained what is called the "oxbow" form, it
often happens that the stream breaks through the isthmus which
connects one of the peninsulas with the mainland. Where, as is not
infrequently the case, the bend has a length of ten miles or more, the
water just above and below the new-made opening is apt to differ in
height by some feet. Plunging down the declivity, the stream, flowing
with great velocity, soon enlarges the channel so that its whole tide
may take the easier way. When this result is accomplished, the old
curve is deserted, sand bars are formed across their mouths, which may
gradually grow to broad alluvial plains, so that the long-surviving,
crescent-shaped lake, the remnant of the river bed, may be seen far
from the present course of the ever-changing stream. Gradually the
accumulations of vegetable matter and the silt brought in by floods
efface this moat or oxbow cut-off, as it is so commonly termed.
As soon as the river breaks through the neck of a peninsula in the
manner above described, the current of the stream becomes much swifter
for many miles below and above the opening. Slowly, however, the
slopes are rearranged throughout its whole course, yet for a time the
stream near the seat of the change becomes straighter than before, and
this for the reason that its swifter current is better able to dispose
of the _débris_ which is supplied to it. The effect of a change in the
current produced by such new channels as we have described as forming
across the isthmuses of bends is to perturb the course of the stream
in all its subsequent downward length. Thus an oxbow cut-off formed
near the junction of the Ohio and Mississippi may tend more or less to
alter the swings of the Mississippi all the way to the Gulf of Mexico.
Although the swayings of the streams to and fro in their alluvial
plains will give the reader some idea as to the struggle which the
greater rivers have with the _débris_ which is committed to them, the
full measure of the work and its consequences can only be appreciated
by those who have studied the phenomena on the ground. A river such
as the Mississippi is endlessly endeavouring to bear its burden to the
sea. If its slope were a uniform inclined plane, the task might
readily be accomplished; but in this, as in almost all other large
water ways, the slope of the bed is ever diminishing with its onward
course. The same water which in the mountain torrent of the
Appalachians or Cordilleras rolled along stones several feet in
diameter down slopes of a hundred feet or more to the mile can in the
lower reaches of the stream move no pebbles which are more than one
fourth of an inch in diameter over slopes which descend on the average
about half a foot in a mile. Thus at every stage from the torrent to
the sea the detritus has from time to time to rest within the alluvial
banks, there awaiting the decay which slowly comes, and which may
bring it to the state where it may be dissolved in the water, or
divided into fragments so small that the stream may bear them on. A
computation which the present writer has made shows that, on the
average, it requires about forty thousand years for a particle of
stone to make its way down the Mississippi to the sea after it has
been detached from its original bed. Of course, some bits may make the
journey straightforwardly; others may require a far greater time to
accomplish the course which the water itself makes at most in a few
weeks. This long delay in the journey of the detritus--a delay caused
by its frequent rests in the alluvial plain--brings about important
consequences which we will now consider.
As an alluvial plain is constructed, we generally find at the base
pebbly material which fell to the bottom in the current of the main
stream as the shores grew outward. Above this level we find the
deposits laid down by the flood waters containing no pebbles, and this
for the reason that those weightier bits remained in the stream bed
when the tide flowed over the plain. As the alluvial deposit is laid
down, a good deal of vegetable matter was built into it. Generally
this has decayed and disappeared. On the surface of the plain there
has always been growing abundant vegetation, the remains of which
decayed on the surface in the manner which we may observe at the
present day. This decomposing vegetable matter within and upon the
porous alluvial material produces large quantities of carbonic acid, a
gas which readily enters the rain water, and gives it a peculiar power
of breaking up rock matter. Acting on the _débris_, this gas-charged
water rapidly brings about a decay of the fragments. Much of the
material passes at once into solution in this water, and drains away
through the multitudinous springs which border the river. As this
matter is completely dissolved, as is sugar in water, it goes straight
away to the sea without ever again entering the alluvium. In many, if
not most, cases this dissolving work which is going on in alluvial
terraces is sufficient to render a large part of the materials which
they contain into the state where it disappears in an unseen manner;
thus while the annual floods are constantly laying down accumulations
on the surface of these plains, the springs are bearing it away from
below.
In this way, through the decomposition which takes place in them, all
those river terraces where much vegetable matter is mingled with the
mineral substances, become laboratories in which substances are
brought into solution and committed to the seas. We find in the water
of the ocean a great array of dissolved mineral substances; it,
indeed, seems probable that the sea water contains some share, though
usually small, of all the materials which rivers encounter in their
journey over and under the lands. As the waters of the sea obtain but
little of this dissolved matter along the coast, it seems likely that
the greater share of it is brought into the state of solution in the
natural laboratories of the alluvial plains.
Here and there along the sides of the valleys in which the rivers flow
we commonly find the remains of ancient plains lying at more or less
considerable heights above the level of the streams. Generally these
deposits, which from their form are called terraces, represent the
stages of down-wearing by which the stream has carved out its way
through the rocks. The greater part of these ancient alluvial plains
has been removed through the ceaseless swinging of the stream to and
fro in the valley which it has excavated.
In all the states of alluvial plains, whether they be the fertile
deposits near the level of the streams which built them, or the poorer
and ruder surfaced higher terraces, they have a great value to
mankind. Men early learned that these lands were of singularly uniform
goodness for agricultural use. They are so light that they were easily
delved with the ancient pointed sticks or stone hoes, or turned by the
olden, wooden plough. They not only give a rich return when first
subjugated, but, owing to the depth of the soil and the frequency with
which they are visited by fertilizing inundations, they yield rich
harvests without fertilizing for thousands of years. It is therefore
not surprising that we find the peoples who depended upon tillage for
subsistence first developed on the great river plains. There, indeed,
were laid the foundations of our higher civilization; there alone
could the state which demands of its citizens fixed abodes and
continuous labour take rise. In the conditions which these fields of
abundance afforded, dense populations were possible, and all the arts
which lead toward culture were greatly favoured. Thus it is that the
civilization of China, India, Persia, and Egypt, the beginnings of
man's higher development, began near the mouths of the great river
valleys. These fields were, moreover, most favourably placed for the
institution of commerce, in that the arts of navigation, originating
in the sheltered reaches of the streams, readily found its way through
the estuaries to the open sea.
Passing down the reaches of a great river as it approaches the sea, we
find that the alluvial plains usually widen and become lower. At
length we attain a point where the flood waters cover the surface for
so large a part of the year that the ground is swampy and untillable
unless it is artificially and at great expense of labour won to
agriculture in the manner in which this task has been effected in the
lower portion of the Rhine Valley. Still farther toward the sea, the
plain gradually dips downward until it passes below the level of the
waters. Through this mud-flat section the stream continues to cut
channels, but with the ever-progressive slowing of its motion the
burden of fine mud which it carries drops to the bottom, and
constantly closes the paths through which the water escapes. Every few
years they tend to break a new way on one side or the other of their
former path. Some of the greatest engineering work done in modern
times has been accomplished by the engineers engaged in controlling
the exits of large rivers to the sea. The outbreak of the Yellow River
in 1887, in which the stream, hindered by its own accumulations,
forced a new path across its alluvial plains, destroyed a vast deal of
life and property, and made the new exit seventy miles from the path
which it abandoned.
Below the surface of the open water the alluvial deposits spread out
into a broad fan, which slopes gradually to a point where, in the
manner of the continental shelf, the bottom descends steeply into deep
water.
It is the custom of naturalists to divide the lower section of river
deposits--that part of the accumulation which is near the sea--from
the other alluvial plains, terming the lower portion the delta. The
word originally came into use to describe that part of the alluvium
accumulated by the Nile near its mouth, which forms a fertile
territory shaped somewhat like the fourth letter of the Greek
alphabet. Although the definition is good in the Egyptian instance,
and has a certain use elsewhere, we best regard all the detritus in a
river valley which is in the state of repose along the stream to its
utmost branches as forming one great whole. It is, indeed, one of the
most united of the large features which the earth exhibits. The
student should consider it as a continuous inclined plane of
diminishing slope, extending from the base of the torrents to the
sea, and of course ramifying into the several branches of the river
system. He should further bear in mind the fact that it is a vast
laboratory where rock material is brought into the soluble state for
delivery to the seas.
The diversity in the form of river valleys is exceedingly great.
Almost all the variety of the landscape is due to this impress of
water action which has operated on the surface in past ages. When
first elevated above the sea, the surface of the land is but little
varied; at this stage in the development the rivers have but shallow
valleys, which generally cut rather straight away over the plain
toward the sea. It is when the surface has been uplifted to a
considerable height, and especially when, as is usually the case, this
uplifting action has been associated with mountain-building, that
valleys take on their accented and picturesque form. The reason for
this is easily perceived: it lies in the fact that the rocks over
which the stream flows are guided in the cutting which they effect by
the diversities of hardness in the strata that they encounter. The
work which it does is performed by the hard substances that are
impelled by the current, principally by the sand and pebbles. These
materials, driven along by the stream, become eroding tools of very
considerable energy. As will be seen when we shortly come to describe
waterfalls, the potholes formed at those points afford excellent
evidence as to the capacity of stream-impelled bits of stone to cut
away the firmest bed rocks. Naturally the ease with which this carving
work is done is proportionate to the energy of the currents, and also
to the relative hardness of the moving bits and the rocks over which
they are driven.
So long as the rocks lie horizontally in their natural construction
attitude the course of the stream is not much influenced by the
variations in hardness which the bed exhibits. Where the strata are
very firm there is likely to be a narrow gorge, the steeps of which
rise on either side with but slight alluvial plains; where the beds
are soft the valley widens, perhaps again to contract where in the
course of its descent it encounters another hard layer. Where,
however, the beds have been subjected to mountain-building, and have
been thrown into very varied attitudes by folding and faulting, the
stream now here and now there encounters beds which either restrain
its flow or give it freedom. The stream is then forced to cut its way
according to the positions of the various underlying strata. This
effect upon its course is not only due to the peculiarities of
uplifted rocks, but to manifold accidents of other nature: veins and
dikes, which often interlace the beds with harder or softer partitions
than the country rock; local hardenings in the materials, due to
crystallization and other chemical processes, often create
indescribable variations which are more or less completely expressed
in the path of the stream.
When a land has been newly elevated above the sea there is often--we
may say, indeed, generally--a very great difference between the height
of its head waters and the ocean level. In this condition of a country
the rivers have what we may call a new aspect; their valleys are
commonly narrow and rather steep, waterfalls are apt to abound, and
the alluvial terraces are relatively small in extent. Stage by stage
the torrents cut deeper; the waste which they make embarrasses the
course of the lower waters, where no great amount of down-cutting is
possible for the reason that the bed of the stream is near sea level.
At the same time the alluvial materials, building out to sea, thus
diminish the slope of the stream. In the extreme old age of the river
system the mountains are eaten down so that the torrent section
disappears, and the stream becomes of something like a uniform slope;
the higher alluvial plains gradually waste away, until in the end the
valley has no salient features. At this stage in the process, or even
before it is attained, the valley is likely to be submerged beneath
the sea, where it is buried beneath the deposits formed on the floor;
or a further uplift of the land may occur with the result that the
stream is rejuvenated; or once more endowed with the power to create
torrents, build alluvial plains, and do the other interesting work of
a normal river.
It rarely, if ever, happens that a river valley attains old age before
it has sunk beneath the sea or been refreshed by further upliftings.
In the unstable conditions of the continents, one or the other of
these processes, sometimes in different places both together, is apt
to be going on. Thus if we take the case of the Mississippi and its
principal tributaries, the Ohio and Missouri, we find that for many
geological ages the mountains about their sources have frequently, if
not constantly, grown upward, so that their torrent sections, though
they have worn down tens of thousands of feet, are still high above
the sea level, perhaps on the average as high as they have ever been.
At the same time the slight up-and-down swayings of the shore lands,
amounting in general to less than five hundred feet, have greatly
affected the channels of the main river and its tributaries in their
lower parts. Not long ago the Mississippi between Cairo and the Gulf
flowed in a rather steep-sided valley probably some hundreds of feet
in depth, which had a width of many miles. Then at the close of the
last Glacial period the region sank down so that the sea flooded the
valley to a point above the present junction of the Ohio River with
the main stream. Since then alluvial plains have filled this estuary
to even beyond the original mouth. In many other of our Southern
rivers, as along the shore from the Mississippi to the Hudson, the
streams have not brought in enough detritus to fill their drowned
valleys, which have now the name of bays, of which the Delaware and
Chesapeake on the Atlantic coast, and Mobile Bay on the Gulf of
Mexico, are good examples. The failure of Chesapeake and Delaware Bays
to fill with _débris_ in the measure exhibited by the more southern
valleys is due to the fact that the streams which flow into them to a
great extent drain from a region thickly covered with glacial waste, a
mass which holds the flood waters, yielding the supply but slowly to
the torrents, which there have but a slight cutting power.
In our sketch of river valleys no attention has been given to the
phenomena of waterfalls, those accidents of the flow which, as we have
noted, are particularly apt to characterize rivers which have not yet
cut down to near the sea level. Where the normal uniform descent which
is characteristic of a river's bed is interrupted by a sudden steep,
the fact always indicates the occurrence of one of a number of
geological actions. The commonest cause of waterfalls is due to a
sudden change in the character of horizontal or at least nearly level
beds over which the stream may flow. Where after coursing for a
distance over a hard layer the stream comes to its edge and drops on a
soft or easily eroded stratum, it will cut this latter bed away, and
create a more or less characteristic waterfall. Tumbling down the face
of the hard layer, the stream acquires velocity; the _débris_ which it
conveys is hurled against the bottom, and therefore cuts powerfully,
while before, being only rubbed over the stone as it moved along, it
cut but slightly. Masses of ice have the same effect as stones. Bits
dropping from the ledge are often swept round and round by the eddies,
so that they excavate an opening which prevents their chance escape.
In these confined spaces they work like augers, boring a deep,
well-like cavity. As the bits of stone wear out they are replaced by
others, which fall in from above. Working in this way, the fragments
often develop regular well-like depressions, the cavities of which
work back under the cliffs, and by the undermining process deprive the
face of the wall of its support, so that it tumbles in ruin to the
base, there to supply more material for the potholing action.
Waterfalls of the type above described are by far the commonest of
those which occur out of the torrent districts of a great river
system. That of Niagara is an excellent specimen of the type, which,
though rarely manifested in anything like the dignity of the great
fall, is plentifully shown throughout the Mississippi Valley and the
basin of the Great Lakes. Within a hundred miles of Niagara there are
at least a hundred small waterfalls of the same type. Probably three
quarters of all the larger accidents of this nature are due to the
conditions of a hard bed overlying softer strata.
Falls are also produced in very many instances by dikes which cross
the stream. So, too, though rarely, only one striking instance being
known, an ancient coral reef which has become buried in strata may
afford rock of such hardness that when the river comes to cross it it
forms a cascade, as at the Falls of the Ohio, at Louisville, Ky. It is
a characteristic of all other falls, except those first mentioned,
that they rarely plunge with a clean downward leap over the face of a
precipice which recedes at its base, but move downward over an
irregular sloping surface.
In the torrent district of rivers waterfalls are commonly very
numerous, and are generally due to the varying hardness in the rocks
which the streams encounter. Here, where the cutting action is going
on with great rapidity, slight differences in the resistance which the
rocks make to the work will lead to great variations in the form of
the bed over which they flow, while on the more gently sloping bottoms
of the rivers, where the _débris_ moves slowly, such variations would
be unimportant in their effect. When the torrents escape into the main
river valleys, in regions where the great streams have cut deep
gorges, they often descend from a great vertical height, forming
wonderful waterfalls, such as those which occur in the famous
Lauterbrunnen Valley of Switzerland or in that of the Yosemite in
California. This group of cascades is peculiar in that the steep of
the fall is made not by the stream itself, but by the action of a
greater river or of a glacier which may have some time taken its
place.
Waterfalls have an economic as well as a picturesque interest in that
they afford sources of power which may be a very great advantage to
manufacturers. Thus along the Atlantic coast the streams which come
from the Appalachian highlands, and which have hardly escaped from
their torrent section before they attain the sea, afford numerous
cataracts which have been developed so that they afford a vast amount
of power. Between the James on the south and the Ste. Croix on the
north more than a hundred of these Appalachian rivers have been turned
to economic use. The industrial arts of this part of the country
depend much upon them for the power which drives their machinery. The
whole of the United States, because of the considerable size of its
rivers and their relatively rapid fall, is richly endowed with this
source of energy, which, originating in the sun's heat and conveyed
through the rain, may be made to serve the needs of man. In view of
the fact that recent inventions have made it possible to convert this
energy of falling water into the form of electricity, which may be
conveyed to great distances, it seems likely that our rivers will in
the future be a great source of national wealth.
We must turn again to river valleys, there to trace certain actions
less evident than those already noted, but of great importance in
determining these features of the land. First, we have to note that in
the valley or region drained by a river there is another degrading or
down-wearing action than that which is accomplished by the direct work
of the visible stream. All over such a valley the underground waters,
soaking through the soil and penetrating through the underlying rock,
are constantly removing a portion of the mineral matter which they
take into solution and bear away to the sea. In this way, deprived of
a part of their substance, the rocks are continually settling down by
underwear throughout the whole basin, while they are locally being cut
down by the action of the stream. Hence in part it comes about that in
a river basin we find two contrasted features--the general and often
slight slope of a country toward the main stream and its greater
tributaries, and the sharp indentation of the gorge in which the
streams flow, these latter caused by the immediate and recent action
of the streams.
If now the reader will conceive himself standing at any point in a
river basin, preferably beyond the realms of the torrents, he may with
the guidance of the facts previously noted, with a little use of the
imagination, behold the vast perceptive which the history of the river
valley may unfold to him. He stands on the surface of the soil, that
_débris_ of the rocks which is just entering on its way to the ocean.
In the same region ten thousand years ago he would have stood upon a
surface from one to ten feet higher than the present soil covering. A
million years ago his station would have been perhaps five hundred
feet higher than the surface. Ten million years in the past, a period
less than the lifetime of certain rivers, such as the French Broad
River in North Carolina, the soil was probably five thousand feet or
more above its present plane. There are, indeed, cases where river
valleys appear to have worked down without interruption from the
subsidence of the land beneath the sea to the depth of at least two
miles. Looking upward through the space which the rocks once occupied,
we can conceive the action of the forces in their harmonious
co-operation which have brought the surface slowly downward. We can
imagine the ceaseless corrosion due to the ground water, bringing
about a constant though slow descent of the whole surface. Again and
again the streams, swinging to and fro under the guidance of the
underlying rock, or from the obstacles which the _débris_ they carried
imposed upon them, have crossed the surface. Now and then perhaps the
wearing was intensified by glacial action, for an ice sheet often cuts
with a speed many times as great as that which fluid water can
accomplish. On the whole, this exercise of the constructive
imagination in conceiving the history of a river valley is one of the
most enlarging tasks which the geologist can undertake.
Where in a river valley there are many lateral streams, and especially
where the process of solution carried on by the underground waters is
most effective, as compared with erosive work done in the bed of the
main river, we commonly find the valley sloping gently toward its
centre, the rivers having but slight steeps near their banks. On the
other hand, where, as occasionally happens, a considerable stream fed
by the rain and snow fall in its torrent section courses for a great
distance over high, arid plains, on which the ground water and the
tributaries do but little work, the basin may slope with very slight
declivity to the river margins, and there descend to great depths,
forming very deep gorges, of which the Colorado Cañon is the most
perfect type. As instances of these contrasted conditions, we may
take, on the one hand, the upper Mississippi, where the grades toward
the main stream are gentle and the valley gorge but slightly
exhibited; on the other, the above-mentioned Colorado, which bears a
great tide of waters drawn from the high and relatively rainy region
of the Rocky Mountains across the vast plateau lying in an almost
rainless country. In this section nearly all the down-wearing has been
brought about in the direct path of the stream, which has worn the
elevated plain into a deep gorge during the slow uprising of the
table-land to its present height. In this way a defile nearly a mile
in depth has been created in a prevailingly rather flat country. This
gorge has embranchments where the few great tributaries have done like
work, but, on the whole, this river flows in an almost unbroken
channel, the excavation of which has been due to its swift,
pebble-bearing waters.
The tendency of a newly formed river is to cut a more or less distinct
cañon. As the basin becomes ancient, this element of the gorge tends
to disappear, the reason for this being that, while the river bed is
high above the sea, the current is swift and the down-cutting rapid,
while the slow subsidence of the country on either side--a process
which goes on at a uniform rate--causes the surface of that region to
be left behind in the race for the sea level. As the stream bed comes
nearer the sea level its rate of descent is diminished, and so the
outlying country gradually overtakes it.
In regions where the winters are very cold the effect of ice on the
development of the stream beds both in the torrent and river sections
of the valley is important. This work is accomplished in several
diverse ways. In the first place, where the stream is clear and the
current does not flow too swiftly, the stones on the bottom radiate
their heat through the water, and thus form ice on their surfaces,
which may attain considerable thickness. As ice is considerably
lighter than water, the effect is often to lift up the stones of the
bed if they be not too large; when thus detached from the bottom, they
are easily floated down stream until the ice melts away. The ice which
forms on the surface of the water likewise imprisons the pebbles along
the banks, and during the subsequent thaw may carry them hundreds of
miles toward the sea. It seems likely, from certain observations made
by the writer, that considerable stones may thus be carried from the
Alleghany River to the main Mississippi.
Perhaps the most important effect of ice on river channels is
accomplished when in a time of flood the ice field which covered the
stream, perhaps to the depth of some feet, is broken up into vast
floes, which drift downward with the current. When, as on the Ohio,
these fields sometimes have the area of several hundred acres, they
often collide with the shores, especially where the stream makes a
sharp bend. Urged by their momentum, these ice floes pack into the
semblance of a dam, which may have a thickness of twenty, thirty, or
even fifty feet. Beginning on the shore, where the collision takes
place, the dam may swiftly develop clear across the stream, so that in
a few minutes the way of the waters is completely blocked. The
on-coming ice shoots up upon the accumulation, increases its height,
and extends it up stream, so that in an hour the mass completely bars
the current. The waters then heap up until they break their way over
the obstacle, washing its top away, until the whole is light enough
to be forced down the stream, where, by the friction it encounters on
the bottom and sides of the channel, it is broken to pieces. It is
easy to see that such moving dams of ice may sweep the bed of a river
as with a great broom.
Sometimes where the gorges do not form a stationary dam large cakes of
ice become turned on edge and pack together so that they roll down the
stream like great wheels, grinding the bed rock as they go.
In high northern countries, as in Siberia, the rivers, even the
deepest, often become so far frozen that their channels are entirely
obstructed. Where, as in the case of these Siberian rivers, the flow
is from south to north, it often happens that the spring thaw sets in
before the more northern beds of the main stream are released from
their bondage of frost. In this case the inundations have to find new
paths on either side of the obstructed way. The result is a type of
valleys characterized by very irregular and changeable stream beds,
the rivers having no chance to organize themselves into the shapely
curves which they ordinarily follow.
The supply which finds its way to a river is composed, as has been
already incidentally noted, in part of the water which courses
underground for a greater or less distance before it emerges to the
surface, and in part of that which moves directly over the ground.
These two shares of water have somewhat different histories. On the
share of these two depends the stability of the flow. Where, as in New
England and other glaciated countries, the surface of the earth is
covered with a thick layer of sand and gravel, which, except when
frozen, readily admits the water; the rainfall is to a very great
extent absorbed by the earth, and only yielded slowly to the streams.
In these cases floods are rare and of no great destructive power.
Again, where also the river basin is covered by a dense mantle of
forests, the ground beneath which is coated, as is the case in
primeval woods, with a layer of decomposing vegetation a foot or more
in depth, this spongy mass retains the water even more effectively
than the open-textured glacial deposits above referred to. When the
woods, however, are removed from such an area, the rain may descend to
the streams almost as speedily as it finds its way to the gutters from
the house roofs. It thus comes about that all regions, when reduced to
tillage, and where the rainfall is enough to maintain a good
agriculture, are, except when they have a coating of glacial waste,
exceedingly liable to destructive inundations.
Unhappily, the risk of river floods is peculiarly great in all the
regions of the United States lying much to the east of the Rocky
Mountains, except in the basin of the Great Lakes and in the district
of New England, where the prevalence of glacial sands and gravels
affords the protection which we have noted. Throughout this region the
rainfall is heavy, and the larger part of it is apt to come after the
ground has become deeply snow-covered. The result is a succession of
devastating floods which already are very damaging to the works of
man, and promise to become more destructive as time goes on. More than
in any other country, we need the protection which forests can give us
against these disastrous outgoings of our streams.
LAKES.
In considering the journey of water from the hilltops to the sea, we
should take some account of those pauses which it makes on its way
when for a time it falls into the basin of a lake. These arrests in
the downward motion of water, which we term lakes, are exceedingly
numerous; their proper discussion would, indeed, require a
considerable volume. We shall here note only the more important of
their features, those which are of interest to the general student.
The first and most noteworthy difference in lakes is that which
separates the group of dead seas from the living basins of fresh
water. When a stream attains a place where its waters have to expand
into the lakelike form, the current moves in a slow manner, and the
broad surface exposed to the air permits a large amount of
evaporation. If the basin be large in proportion to the amount of the
incurrent water, this evaporation may exceed the supply, and produce a
sea with no outlet, such as we find in the Dead Sea of Judea, in that
at Salt Lake, Utah, and in a host of other less important basins. If
the rate of evaporation be yet greater in proportion to the flow, the
lake may altogether dry away, and the river be evaporated before it
attains the basin where it might accumulate. In that case the river is
said to sink, but, in place of sinking into the earth, its waters
really rise into the air. Many such sinks occur in the central portion
of the Rocky Mountain district. It is important to note that the
process of evaporation we are describing takes place in the case of
all lakes, though only here and there is the air so dry that the
evaporation prevents the basin from overflowing at the lowest point on
its rim, forming a river which goes thence to the sea. Even in the
case of the Great Lakes of North America a considerable part of the
water which flows into them does not go to the St. Lawrence and thence
to the sea. As long as the lake finds an outlet to the sea its waters
contain but little more dissolved mineral matter than that we find in
the rivers. But because all water which has been in contact with the
earth has some dissolved mineral substances, while that which goes
away by evaporation is pure water, a lake without an outlet gradually
becomes so charged with these materials that it can hold no more in
solution, but proceeds to lay them down in deposits of that compound
substance which from its principal ingredient we name salt. The water
of dead seas, because of the additional weight of the substances which
it holds, is extraordinarily buoyant. The swimmer notes a difference
in this regard in the waters of rivers and fresh-water lakes and those
of the sea, due to this same cause. But in those of dead seas,
saturated with saline materials, the human body can not sink as it
does in the ordinary conditions of immersion. It is easy to understand
how the salt deposits which are mined in many parts of the world have
generally, if not in all cases, been formed in such dead seas.[5]
[Footnote 5: In some relatively rare cases salt deposits are formed in
lagoons along the shores of arid lands, where the sea occasionally
breaks over the beach into the basin, affording waters which are
evaporated, leaving their salt behind them.]
It is an interesting fact that almost all the known dead seas have in
recent geological times been living lakes--that is, they poured over
their brims. In the Cordilleras from the line between Canada and the
United States to central Mexico there are several of these basins. All
of those which have been studied show by their old shore lines that
they were once brimful, and have only shrunk away in modern times.
These conditions point to the conclusion that the rainfall in
different regions varies greatly in the course of the geologic ages.
Further confirmation of this is found in the fact that very great salt
deposits exist on the coast of Louisiana and in northern
Europe--regions in which the rainfall is now so great in proportion to
the evaporation that dead seas are impossible.
Turning now to the question of how lake basins are formed, we note a
great variety in the conditions which may bring about their
construction. The greatest agent, or at least that which operates in
the construction of the largest basins, are the irregular movements of
the earth, due to the mountain-building forces. Where this work goes
on on a large scale, basin-shaped depressions are inevitably formed.
If all those which have existed remained, the large part of the lands
would be covered by them. In most cases, however, the cutting action
of the streams has been sufficient to bring the drainage channels down
to the bottom of the trough, while the influx of sediments has served
to further the work by filling up the cavities. Thus at the close of
the Cretaceous period there was a chain of lakes extending along the
eastern base of the Rocky Mountains, constituting fresh-water seas
probably as large as the so-called Great Lakes of North America. But
the rivers, by cutting down and tilling up, have long since
obliterated these water areas. In other cases the tiltings of the
continent, which sometimes oppose the flow of the streams, may for a
time convert the upper part of a river basin which originally sloped
gently toward the sea into a cavity. Several cases of this description
occurred in New England in the closing stages of the Glacial period,
when the ground rose up to the northward.
We have already noted the fact that the basin of a dead sea becomes in
course of time the seat of extensive salt deposits. These may, indeed,
attain a thickness of many hundred feet. If now in the later history
of the country the tract of land with the salt beneath it were
traversed by a stream, its underground waters may dissolve out the
salt and in a way restore the basin to its original unfilled
condition, though in the second state that of a living lake. It seems
very probable that a portion at least of the areas of Lakes Ontario,
Erie, and Huron may be due to this removal of ancient salt deposits,
remains of which lie buried in the earth in the region bordering these
basins.
By far the commonest cause of lake basins is found in the
irregularities of the surface which are produced by the occupation of
the country by glaciers. When these great sheets of ice lie over a
land, they are in motion down the slopes on which they rest; they wear
the bed rocks in a vigorous manner, cutting them down in proportion to
their hardness. As these rocks generally vary in the resistance which
they oppose to the ice, the result is that when the glacier passes
away the surface no longer exhibits the continued down slope which the
rivers develop, but is warped in a very complicated way. These
depressions afford natural basins in which lakes gather; they may vary
in extent from a few square feet to many square miles. When a glacier
occupies a country, the melting ice deposits on the surface of the
earth a vast quantity of rocky _débris_, which was contained in its
mass. This detritus is irregularly accumulated; in part it is disposed
in the form of moraines or rude mounds made at the margin of the
glacier, in part as an irregular sheet, now thick, now thin, which
covers the whole of the field over which the ice lay. The result of
this action is the formation of innumerable pools, which continue to
exist until the streams have cut channels through which their waters
may drain away, or the basins have become filled with detritus
imported from the surrounding country or by peat accumulations which
the plants form in such places.
Doubtless more than nine tenths of all the lake basins, especially
those of small size, which exist in the world are due to
irregularities of the land surface which are brought about by glacial
action. Although the greater part of these small basins have been
obliterated since the ice left this country, the number still
remaining of sufficient size to be marked on a good map is
inconceivably great. In North America alone there are probably over a
hundred and fifty thousand of these glacial lakes, although by far the
greater part of those which existed when the glacial sheet disappeared
have been obliterated.
Yet another interesting group of fresh-water lakes, or rather we
should call them lakelets from their small size, owes its origin to
the curious underground excavations or caverns which are formed in
limestone countries. The water enters these caverns through what are
termed "sink holes"--basins in the surface which slope gently toward a
central opening through which the water flows into the depths below.
The cups of the sink holes rarely exceed half a mile in diameter, and
are usually much smaller. Their basins have been excavated by the
solvent and cutting actions of the rain water which gathers in them to
be discharged into the cavern below. It often happens that after a
sink hole is formed some slight accident closes the downward-leading
shaft, so that the basin holds water; thus in parts of the United
States there are thousands of these nearly circular pools, which in
certain districts, as in southern Kentucky, serve to vary the
landscape in much the same manner as the glacial lakes of more
northern countries.
Some of the most beautiful lakes in the world, though none more than a
few miles in diameter, occupy the craters of extinct volcanoes. When
for a time, or permanently, a volcano ceases to do its appointed work
of pouring forth steam and molten rock from the depths of the earth,
the pit in the centre of the cone gathers the rain water, forming a
deep circular lake, which is walled round by the precipitous faces of
the crater. If the volcano reawakens, the water which blocks its
passage may be blown out in a moment, the discharge spreading in some
cases to a great distance from the cone, to be accumulated again when
the vent ceases to be open. The most beautiful of these volcanic lakes
are to be found in the region to the north and south of Rome. The
original seat of the Latin state was on the shores of one of these
crater pools, south of the Eternal City. Lago Bolsena, which lies to
the northward, and is one of the largest known basins of this nature,
having a diameter of about eight miles, is a crater lake. The volcanic
cone to which it belongs, though low, is of great size, showing that
in its time of activity, which did not endure very long, this crater
was the seat of mighty ejections. The noblest specimen of this group
of basins is found in Crater Lake, Oregon, now contained in one of the
national parks of the United States.
Inclosed bodies of water are formed in other ways than those
described; the list above given includes all the important classes of
action which produce these interesting features. We should now note
the fact that, unlike the seas, the lakes are to be regarded as
temporary features in the physiography of the land. One and all, they
endure for but brief geologic time, for the reason that the streams
work to destroy them by filling them with sediment and by carving out
channels through which their waters drain away. The nature of this
action can well be conceived by considering what will take place in
the course of time in the Great Lakes of North America. As Niagara
Falls cut back at the average rate of several feet a year, it will be
but a brief geologic period before they begin to lower the waters of
Lake Erie. It is very probable, indeed, that in twenty thousand years
the waters of that basin will be to a great extent drained away. When
this occurs, another fall or rapid will be produced in the channel
which leads from Lake Huron to Lake Erie. This in turn will go through
its process of retreat until the former expanse of waters disappears.
The action will then be continued at the outlets of Lakes Michigan and
Superior, and in time, but for the interposition of some actions which
recreate these basins, their floors will be converted into dry land.
It is interesting to note that lakes owe in a manner the preservation
of their basins to an action which they bring about on the waters that
flow into them. These rivers or torrents commonly convey great
quantities of sediment, which serve to rasp their beds and thus to
lower their channels. In all but the smaller lakelets these turbid
waters lay down all their sediment before they attain the outlet of
the basin. Thus they flow away over the rim rock in a perfectly pure
state--a state in which, as we have noted before, water has no
capacity for abrading firm rock. Thus where the Niagara River passes
from Lake Erie its clean water hardly affects the stone over which it
flows. It only begins to do cutting work where it plunges down the
precipice of the Falls and sets in motion the fragments which are
constantly falling from that rocky face. These Falls could not have
begun as they did on the margin of Lake Ontario except for the fact
that when the Niagara River began to flow, as in relatively modern
times, it found an old precipice on the margin of Lake Ontario, formed
by the waves of the lake, down which the waters fell, and where they
obtained cutting tools with which to undermine the steep which forms
the Falls.
Many great lakes, particularly those which we have just been
considering, have repeatedly changed their outlets, according as the
surface of the land on which they lie has swayed up and down in
various directions, or as glacial sheets have barred or unbarred the
original outlets of the basins. Thus in the Laurentian Lakes above
Ontario the geologist finds evidence that the drainage lines have
again and again been changed. For a time during the Glacial period,
when Lake Ontario and the valley of the St. Lawrence was possessed by
the ice, the discharge was southward into the upper Mississippi or the
Ohio. At a later stage channels were formed leading from Georgian Bay
to the eastern part of Ontario. Yet later, when the last-named lake
was bared, an ice dam appears to have remained in the St. Lawrence,
which held back the waters to such a height that they discharged
through the valley of the Mohawk into the Hudson. Furthermore, at some
time before the Glacial period, we do not know just when, there
appears to have been an old Niagara River, now filled with drift,
which ran from Lake Erie to Ontario, a different channel from that
occupied by the present stream.
The effects of lakes on the river systems with which they are
connected is in many ways most important. Where they are of
considerable extent, or where even small they are very numerous, they
serve to retain the flood waters, delivering them slowly to the
excurrent streams. In rising one foot a lake may store away more water
than the river by its consequent rise at the point of outflow will
carry away in many months, and this for the simple reason that the
lake may be many hundred or even thousand times as wide as the stream.
Moreover, as before noted, the sediment gathered by the stream above
the level of the lake is deposited in its basin, and does not affect
the lower reaches of the river. The result is that great rivers, such
as drain from the Laurentian Lakes, flow clear water, are exempt from
floods, are essentially without alluvial plains or terraces, and form
no delta deposits. In all these features the St. Lawrence River
affords a wonderful contrast to the Mississippi. Moreover, owing to
the clear waters, though it has flowed for a long time, it has never
been able to cut away the slight obstructions which form its rapids,
barriers which probably would have been removed if its waters had been
charged with sediment.
[Illustration: _Muir Glacier, Alaska, showing crevasses and dust
layer on surface of ice._]
CHAPTER VI.
GLACIERS.
We have already noted the fact that the water in the clouds is very
commonly in the frozen state; a large part of that fluid which is
evaporated from the sea attains the solid form before it returns to
the earth. Nevertheless, in descending, at least nine tenths of the
precipitation returns to the fluid state, and does the kind of work
which we have noted in our account of water. Where, however, the water
arrives on the earth in the frozen condition, it enters on a rôle
totally different from that followed by the fluid material.
Beginning its descent to the earth in a snowflake, the little mass
falls slowly, so that when it comes against the earth the blow which
it strikes is so slight that it does no effective work. In the state
of snow, even in the separate flakes, the frozen water contains a
relatively large amount of air. It is this air indeed, which, by
dividing the ice into many flakes that reflect the light, gives it the
white colour. This important point can be demonstrated by breaking
transparent ice into small bits, when we perceive that it has the hue
of snow. Much the same effect is given where glass is powdered, and
for the same reason.
As the snowflakes accumulate layer on layer they imbed air between
them, so that when the material falls in a feathery shape--say to the
depth of a foot--more than nine tenths of the mass is taken up by the
air-containing spaces. As these cells are very small, the circulation
in them is slight, and so the layer becomes an admirable
non-conductor, having this quality for the same reason that feathers
have it--i.e., because the cells are small enough to prevent the
circulation of the air, so that the heat which passes has to go by
conduction, and all gases are very poor conductors. The result is that
a snow coating is in effect an admirable blanket. When the sun shines
upon it, much of the heat is reflected, and as the temperature does
not penetrate it to any depth, only the superficial part is melted.
This molten water takes up in the process of melting a great deal of
heat, so that when it trickles down into the mass it readily
refreezes. On the other hand, the heat going out from the earth, the
store accumulated in its superficial parts in the last warm season,
together with the small share which flows out from the earth's
interior, is held in by this blanket, which it melts but slowly. Thus
it comes about that in regions of long-enduring snowfall the ground,
though frozen to the depth of a foot or more at the time when the
accumulation took place, may be thawed out and so far warmed that the
vegetation begins to grow before the protecting envelope of snow has
melted away. Certain of the early flowers of high latitudes, indeed,
begin to blossom beneath the mantle of finely divided ice.
In those parts of the earth which for the most part receive only a
temporary coating of snow the effect of this covering is
inconsiderable. The snow water is yielded to the earth, from which it
has helped to withdraw the frost, so that in the springtime, the
growing season of plants, the ground contains an ample store of
moisture for their development. Where the snowfall accumulates to a
great thickness, especially where it lodges in forests, the influence
of the icy covering is somewhat to protract the winter and thus to
abbreviate the growing season.
Where snow rests upon a steep slope, and gathers to the depth of
several feet, it begins to creep slowly down the declivity in a manner
which we may often note on house roofs. This motion is favoured by the
gradual though incomplete melting of the flakes as the heat
penetrates the mass. Making a section through a mass of snow which has
accumulated in many successive falls, we note that the top may still
have the flaky character, but that as we go down the flakes are
replaced by adherent shotlike bodies, which have arisen from the
partial melting and gathering to their centres of the original
expanded crystalline bits. In this process of change the mass can move
particle by particle in the direction in which gravity impels it. The
energy of its motion, however, is slight, yet it can urge loose stones
and forest waste down hill. Sometimes, as in the cemetery at Augusta,
Me., where stone monuments or other structures, such as iron railings,
are entangled in the moving mass, it may break them off and convey
them a little distance down the slope.
So long as the summer sun melts the winter's snow, even if the ground
be bare but for a day, the rôle of action accomplished by the snowfall
is of little geological consequence. When it happens that a portion of
the deposit holds through the summer, the region enters on the glacial
state, and its conditions undergo a great revolution, the consequences
of which are so momentous that we shall have to trace them in some
detail. Fortunately, the considerations which are necessary are not
recondite, and all the facts are of an extremely picturesque nature.
Taking such a region as New England, where all the earth is
life-bearing in the summer season, and where the glacial period of the
winter continues but for a short time, we find that here and there on
the high mountains the snow endures throughout most of the summer, but
that all parts of the surface have a season when life springs into
activity. On the top of Mount Washington, in the White Mountains of
New Hampshire, in a cleft known as Tuckerman's Ravine, where the
deposit accumulates to a great depth, the snow-ice remains until
midsummer. It is, indeed, evident that a very slight change in the
climatal conditions of this locality would establish a permanent
accumulation of frozen water upon the summit of the mountain. If the
crest were lifted a thousand feet higher, without any general change
in the heat or rainfall of the district, this effect would be
produced. If with the same amount of rainfall as now comes to the
earth in that region more of it fell as snow, a like condition would
be established. Furthermore, with an increase of rainfall to something
like double that which now descends the snow bore the same proportion
to the precipitation which it does at present, we should almost
certainly have the peak above the permanent snow line, that level
below which all the winter's fall melts away. These propositions are
stated with some care, for the reason that the student should perceive
how delicate may be--indeed, commonly is--the balance of forces which
make the difference between a seasonal and a perennial snow covering.
As soon as the snow outlasts the summer, the region which it occupies
is sterilized to life. From the time the snow begins to hold over the
warm period until it finally disappears, that field has to be reckoned
out of the habitable earth, not only to man, but to the lowliest
organisms.[6]
[Footnote 6: In certain fields of permanent snow, particularly near their
boundaries, some very lowly forms of vegetable life may develop on a
frozen surface, drawing their sustenance from the air, and supplied with
water by the melting which takes place during the summertime. These
forms include the rare phenomenon termed red snow.]
If the snow in a glaciated region lay where it fell, the result would
be a constant elevation of the deposit year by year in proportion to
the annual excess of deposition over the melting or evaporation of the
material. But no sooner does the deposit attain any considerable
thickness than it begins to move in the directions of least
resistance, in accordance with laws which the students of glaciers are
just beginning to discern. In small part this motion is accomplished
by avalanches or snow slides, phenomena which are in a way important,
and therefore merit description. Immediately after a heavy snowfall,
in regions where the slopes are steep, it often happens that the
deposit which at first clung to the surface on which it lay becomes so
heavy that it tends to slide down the slope; a trifling action, the
slipping, indeed, of a single flake, may begin the movement, which at
first is gradual and only involves a little of the snow. Gathering
velocity, and with the materials heaped together from the junction of
that already in motion with that about to be moved, the avalanche in
sliding a few hundred feet down the slope may become a deep stream of
snow-ice, moving with great celerity. At this stage it begins to break
off masses of ice from the glaciers over which it may flow, or even to
move large stones. Armed with these, it rends the underlying earth.
After it has flowed a mile it may have taken up so much earth and
material that it appears like a river of mud. Owing to the fact that
the energy which bears it downward is through friction converted into
heat, a partial melting of the mass may take place, which converts it
into what we call slush, or a mixture of snow and water. Finally, the
torrent is precipitated into the bottom of a valley, where in time the
frozen water melts away, leaving only the stony matter which it bore
as a monument to show the termination of its flow.
It was the good fortune of the writer to see in the Swiss Oberland one
very great avalanche, which came from the high country through a
descent of several thousand feet to the surface of the Upper
Grindelwald Glacier. The first sign of the action was a vague tremor
of the air, like that of a great organ pipe when it begins to vibrate,
but before the pulsations come swiftly enough to make an audible note.
It was impossible to tell when this tremor came, but the wary guide,
noting it before his charge could perceive anything unusual, made
haste for the middle of the glacier. The vibration swelled to a roar,
but the seat of the sound amid the echoing cliffs was indeterminable.
Finally, from a valley high up on the southern face of the glacier,
there leaped forth first a great stone, which sprang with successive
rebounds to the floor of ice. Then in succession other stones and
masses of ice which had outrun the flood came thicker and thicker,
until at the end of about thirty seconds the steep front of the
avalanche appeared like a swift-moving wall. Attaining the cliffs, it
shot forth as a great cataract, which during the continuance of the
flow--which lasted for several minutes--heaped a great mound of
commingled stones and ice upon the surface of the glacier. The mass
thus brought down the steep was estimated at about three thousand
cubic yards, of which probably the fiftieth part was rock material. An
avalanche of this volume is unusual, and the proportion of stony
matter borne down exceptionally great; but by these sudden motions of
the frozen water a large part of the snow deposited above the zone of
complete melting is taken to the lower valleys, where it may disappear
in the summer season, and much of the erosion accomplished in the
mountains is brought about by these falls.
In all Alpine regions avalanches are among the most dreaded accidents.
Their occurrence, however, being dependent upon the shape of the
surface, it is generally possible to determine in an accurate way the
liability of their happening in any particular field. The Swiss take
precaution to protect themselves from their ravages as other folk do
to procure immunity from floods. Thus the authorities of many of the
mountain hamlets maintain extensive forests on the sides of the
villages whence the downfall may be expected, experience having shown
that there is no other means so well calculated to break the blow
which these great snowfalls can deliver, as thick-set trees which,
though they are broken down for some distance, gradually arrest the
stream.
As long as the region occupied by permanent snow is limited to sharp
mountain peaks, relief by the precipitation of large masses to the
level below the snow line is easily accomplished, but manifestly this
kind of a discharge can only be effective from a very small field.
Where the relief is not brought about by these tumbles of snow,
another mode of gravitative action accomplishes the result, though in
a more roundabout way, through the mechanism of glaciers.
We have already noted the fact that the winter's snow upon our
hillsides undergoes a movement in the direction of the slope. What we
have now to describe in a rather long story concerning glaciers rests
upon movements of the same nature, though they are in certain features
peculiarly dependent on the continuity of the action from year to
year. It is desirable, however, that the student should see that there
is at the foundation no more mystery in glacial motion than there is
in the gradual descent of the snow after it has lain a week on a
hillside. It is only in the scale and continuity of the action that
the greatest glacial envelope exceeds those of our temporary
winters--in fact, whenever the snow falls the earth it covers enters
upon an ice period which differs only in degree from that from which
our hemisphere is just escaping.
Where the reader is so fortunate as to be able to visit a region of
glaciers, he had best begin his study of their majestic phenomena by
ascending to those upper realms where the snow accumulates from year
to year. He will there find the natural irregularities of the rock
surface in a measure evened over by a vast sheet of snow, from which
only the summits of the greater mountains rise. He may soon satisfy
himself that this sheet is of great depth, for here and there it is
intersected by profound crevices. If the visit is made in the season
when snow falls, which is commonly during most of the year, he may
observe, as before noted in our winter's snow, that the deposit,
though at first flaky, attains at a short distance below the surface a
somewhat granular character, though the shotlike grains fall apart
when disturbed. Yet deeper, ordinarily a few feet below the surface,
these granules are more or less cemented together; the mass thus loses
the quality of snow, and begins to appear like a whitish ice. Looking
down one of the crevices, where the light penetrates to the depth of a
hundred feet or more, he may see that the bluish hue somewhat
increases with the depth. A trace of this colour is often visible even
in the surface snow on the glacier, and sometimes also in our ordinary
winter fields. In a hole made with a stick a foot or more in depth a
faint cerulean glimmer may generally be discerned; but the increased
blueness of the ice as we go down is conspicuous, and readily leads us
to the conclusion that the air, to which, as we before noted, the
whiteness of the snow is due, is working out of the mass as the
process of compaction goes on. In a glacial district this snow mass
above the melting line is called the _névé_.
Remembering that the excess of snow beyond the melting in a _névé_
district amounts, it may be, to some feet of material each year, we
easily come to the conclusion that the mass works down the slope in
the manner which it does even where the coating is impermanent. This
supposition is easily confirmed: by observing the field we find that
the sheet is everywhere drawing away from the cliffs, leaving a deep
fissure between the _névé_ and the precipices. This crevice is called
by the German-Swiss guides the _Bergschrund_. Passage over it is
often one of the most difficult feats to accomplish which the Alpine
explorer has to undertake. In fact, the very appearance of the
surface, which is that of a river with continuous down slopes, is
sufficient evidence that the mass is slowly flowing toward the
valleys. Following it down, we almost always come to a place where it
passes from the upper valleys to the deeper gorges which pierce the
skirts of the mountain. In going over this projection the mass of
snow-ice breaks to pieces, forming a crowd of blocks which march down
the slope with much more speed than they journeyed when united in the
higher-lying fields. In this condition and in this part of the
movement the snow-ice forms what are called the _seracs_, or curds, as
the word means in the French-Swiss dialect. Slipping and tumbling
down the steep slope on which the _seracs_ develop, the ice becomes
broken into bits, often of small size. These fragments are quickly
reknit into the body of ice, which we shall hereafter term the
glacier, and in this process the expulsion of the air goes on more
rapidly than before, and the mass assumes a more transparent icelike
quality.
The action of the ice in the pressures and strains to which it is
subjected in joining the main glacier and in the further part of its
course demand for their understanding a revision of those notions as
to rigidity and plasticity which we derive from our common experience
with objects. It is hard to believe that ice can be moulded by
pressure into any shape without fracturing, provided the motion is
slowly effected, while at the same time it is as brittle as ice to a
sudden blow. We see, however, a similar instance of contrasted
properties in the confection known as molasses candy, a stick of which
may be indefinitely bent if the flexure is slowly made, but will fly
to pieces like glass if sharply struck. Ice differs from the sugary
substance in many ways; especially we should note that while it may be
squeezed into any form, it can not be drawn out, but fractures on the
application of a very slight tension. The conditions of its movement
we will inquire into further on, when we have seen more of its action.
Entering on the lower part of its course, that where it flows into the
region below the snow line, the ice stream is now confined between the
walls of the valley, a channel which in most cases has been shaped
before the ice time, by a mountain torrent, or perhaps by a slower
flowing river. In this part of its course the likeness of a glacial
stream to one of fluid water is manifest. We see that it twists with
the turn of the gorge, widens where the confining walls are far apart,
and narrows where the space is constricted. Although the surface is
here and there broken by fractures, it is evident that the movement of
the frozen current, though slow, is tolerably free. By placing stakes
in a row across the axis of a glacier, and observing their movement
from day to day, or even from hour to hour if a good theodolite is
used for the purpose, we note that the movement of the stream is
fastest in the middle parts, as in the case of a river, and that it
slows toward either shore, though it often happens, as in a stream of
molten water, that the speediest part of the current is near one side.
Further observations have indicated that the movement is most rapid on
the surface and least at the bottom, in which the stream is also
riverlike. It is evident, in a word, that though the ice is not fluid
in strict sense, the bits of which it is made up move in substantially
the manner of fluids--that is, they freely slip over each other. We
will now turn our attention to some important features of a detailed
sort which glaciers exhibit.
If we visit a glacier during the part of the year when the winter
snows are upon it, it may appear to have a very uninterrupted surface.
But as the summer heat advances, the mask of the winter coating goes
away, and we may then see the structure of the ice. First of all we
note in all valley glaciers such as we are observing that the stream
is overlaid by a quantity of rocky waste, the greater part of which
has come down with the avalanches in the manner before described,
though a small part may have been worn from the bed over which the ice
flows. In many glaciers, particularly as we approach their
termination, this sheet of earth and rock materials often covers the
ice so completely that the novice in such regions finds it difficult
to believe that the ice is under his feet. If the explorer is minded
to take the rough scramble, he can often walk for miles on these
masses of stone without seeing, much less setting foot on any frozen
water. In some of the Alaskan glaciers this coating may bear a forest
growth. In general, this material, which is called moraine, is
distributed in bands parallel to the sides of the glaciers, and the
strips may amount to a half dozen or more. Those on the sides of the
ice have evidently been derived from the precipices which they have
passed. Those in the middle have arisen from the union of the moraines
formed in two or more tributary valleys.
[Illustration: Fig. 12.--Map of glaciers and moraines near Mont Blanc.]
Where the avalanches fall most plentifully, the stones lie buried with
the snow, and only melt out when the stream attains the region where
the annual waste of its surface exceeds the snowfall. In this section
we can see how the progressive melting gradually brings the rocky
_débris_ into plain view. Here and there we will find a boulder
perched on a pedestal of ice, which indicates a recent down-wearing of
the field. A frequent sound in these regions arises from the tumble of
the stones from their pedestals or the slipping of the masses from the
sharp ridge which is formed by the protection given to the ice through
the thick coating of detritus on its surface. These movements of the
moraines often distribute their waste over the glacier, so that in its
lower part we can no longer trace the contributions from the several
valleys, the whole area being covered by the _débris_. At the end of
the ice stream, where its forward motion is finally overcome by the
warmth which it encounters, it leaves in a rude heap, extending often
like a wall across the valley, all the coarse fragments which it
conveys. This accumulation, composed of all the lateral moraines which
have gathered on the ice by the fall of avalanches, is called the
terminal moraine. As the ice stream itself shrinks, a portion of the
detritus next the boundary wall is apt to be left clinging against
those slopes. It is from the presence of these heaps in valleys now
abandoned by glaciers that we obtain some information as to the former
greater extent of glacial action.
The next most noticeable feature is the crevasse. These fractures
often exist in very great numbers, and constitute a formidable barrier
in the explorer's way. The greater part of these ruptures below the
_serac_ zone run from the sides of the stream toward the centre
without attaining that region. These are commonly pointed up stream;
their formation is due to the fact that, owing to the swifter motion
in the central parts of the stream, the ice in that section draws away
from the material which is moving more slowly next the shore. As
before noted, these ice fractures when drawn out naturally form
fissures at right angles to the direction of the strain. In the middle
portions of the ice other fissures form, though more rarely, which
appear to depend on local strains brought about through the
irregularity of the surface over which the ice is flowing.
If the observer is fortunate, he may in his journey over the glacier
have a chance to see and hear what goes on when crevasses are formed.
First he will hear a deep, booming sound beneath his feet, which
merges into a more splintering note as the crevice, which begins at
the bottom or in the distance, comes upward or toward him. When the
sound is over, he may not be able to see a trace of the fracture,
which at first is very narrow. But if the break intersect any of the
numerous shallow pools which in a warm summer's day are apt to cover a
large part of the surface, he may note a line of bubbles rushing up
through the water, marking the escape of the air from the glacier,
some remnant of that which is imprisoned in the original snow. Even
where this indication is wanting, he can sometimes trace the crevice
by the hissing sound of the air streams where they issue from the ice.
If he will take time to note what goes on, he can usually in an hour
or two behold the first invisible crack widen until it may be half an
inch across. He may see how the surface water hastens down the
opening, a little river system being developed on the surface of the
ice as the streams make their way to one or more points of descent. In
doing this work they excavate a shaft which often becomes many feet in
diameter, down which their waters thunder to the base of the glacier.
This well-like opening is called a _moulin_, or mill, a name which, as
we shall see, is well deserved from the work which falling waters
accomplish. Although the institution of the _moulin_ shaft depends
upon the formation of a crevice, it often happens that as the ice
moves farther on its journey its walls are again thrust together,
soldered in the manner peculiar to ice, so that no trace of the
rupture remains except the shaft which it permitted to form. Like
everything else in the glacier, the _moulin_ slowly moves down the
slope, and remains open as long as it is the seat of descending waters
produced by the summer melting. When it ceases to be kept open from
the summer, its walls are squeezed together in the fashion that the
crevices are closed.
Forming here and there, and generally in considerable numbers, the
crevices of a glacier entrap a good deal of the morainal _débris_,
which falls through them to the bottom of the glacier. Smaller bits
are washed into the _moulin_, by the streams arising from the melting
ice, which is brought about by the warm sun of the summer, and
particularly by the warm rains of that season. On those glaciers
where, owing to the irregularity of the bottom over which the ice
flows, these fractures are very numerous, it may happen that all the
detritus brought upon the surface of the glacier by avalanches finds
its way to the floor of the ice.
Although it is difficult to learn what is going on at the under
surface of the glacier, it is possible directly and indirectly to
ascertain much concerning the peculiar and important work which is
there done. The intrepid explorer may work his way in through the
lateral fissures, and even with care safely descend some of the
fissures which penetrate the central parts of a shallow ice stream.
There, it may be at the depth of a hundred feet or more, he will find
a quantity of stones, some of which may be in size like to a small
house held in the body of the ice, but with one side resting upon the
bed rock. He may be so fortunate as to see the stone actually in
process of cutting a groove in the bed rock as it is urged forward by
the motion of the glacier. The cutting is not altogether in the fixed
material, for the boulder itself is also worn and scored in the work.
Smaller pebbles are caught in the space between the erratic and the
motionless rock and ground to bits. If in his explorations the student
finds his way to the part of the floor on which the waters of a
_moulin_ fall, he may have a chance to observe how the stones set in
motion serve to cut the bed rock, forming elongated potholes much as
in the case of ordinary waterfalls, or at the base of those shafts
which afford the beginnings of limestone caverns.
The best way to penetrate beneath the glacier is through the arch of
the stream which always flows from the terminal face of the ice river.
Even in winter time every large glacier discharges at its end a
considerable brook, the waters of which have been melted from the ice
in small part by the outflow of the earth's heat; mainly, however, by
the warmth produced in the friction of the ice on itself and on its
bottom--in other words, by the conversion of that energy of position,
of which we have often to speak, into heat. In the summer time this
subglacial stream is swollen by the surface waters descending through
the crevices and the _moulins_ which come from them, so that the
outflow often forms a considerable river, and thus excavates in the
ice a large or at least a long cavern, the base of which is the bed
rock. In the autumn, when the superficial melting ceases, this gallery
can often be penetrated for a considerable distance, and affords an
excellent way to the secrets of the under ice. The observer may here
see quantities of the rock material held in the grip of the ice, and
forced to a rude journey over the bare foundation stones. Now and then
he may find the glacial mass in large measure made up of stones, the
admixture extending many feet above the bottom of the cavern, perhaps
to the very top of the arch. He may perchance find that these stones
are crushing each other where they are in contact. The result will be
brought about by the difference in the rate of advance of the ice,
which moves the faster the higher it is above the surface over which
it drags, and thus forces the stones on one level over those below.
Where the waters of the subglacial stream have swept the bed rock
clean of _débris_ its surface is scored, grooved, and here and there
polished in a manner which is accomplished only by ice action, though
some likeness to it is afforded where stones have been swept over for
ages by blowing sand. Here and there, often in a way which interrupts
the cavern journey, the shrunken stream, unable to carry forward the
_débris_, deposits the material in the chamber, sometimes filling the
arch so completely that the waters are forced to make a detour. This
action is particularly interesting, for the reason that in regions
whence glaciers have disappeared the deposits formed in the old ice
arches often afford singularly perfect moulds of those caverns which
were produced by the ancient subglacial streams. These moulds are
termed _eskers_.
If the observer be attentive, he will note the fact that the waters
emerging from beneath the considerable glacier are very much charged
with mud. If he will take a glass of the water at the point of escape,
he will often find, on permitting it to settle, that the sediment
amounts to as much as one twentieth of the volume. While the greater
part of this detritus will descend to the bottom of the vessel in the
course of a day, a portion of it does not thus fall. He may also note
that this mud is not of the yellowish hue which he is accustomed to
behold in the materials laid down by ordinary rivers, but has a
whitish colour. Further study will reveal the fact that the difference
is due to the lack of oxidation in the case of the glacial detritus.
River muds forming slowly and during long-continued exposure to the
action of the air have their contained iron much oxidized, which gives
them a part of their darkened appearance. Moreover, they are somewhat
coloured with decayed vegetable matter. The waste from beneath the
glacier has been quickly separated from the bed rock, all the faces of
the grains are freshly fractured, and there is no admixture of organic
matter. The faces of the particles thus reflect light in substantially
the same way as powdered glass or pulverized ice, and consequently
appear white.
A little observation will show the student that this very muddy
character of waters emerging from beneath the glacier is essentially
peculiar to such streams as we have described. Ascending any of the
principal valleys of Switzerland, he may note that some of the streams
flow waters which carry little sediment even in times when they are
much swollen, while others at all seasons have the whitish colour. A
little further exploration, or the use of a good map, will show him
that the pellucid streams receive no contributions of glacial water,
while those which look as if they were charged with milk come, in part
at least, from the ice arches. From some studies which the writer has
made in Swiss valleys, it appears that the amount of erosion
accomplished on equal areas of similar rock by the descent of the
waters in the form of a glacier or in that of ordinary torrents
differs greatly. Moving in the form of ice, or in the state of
ice-confined streams, the mass of water applies very many times as
much of its energy of position to grinding and bearing away the rocks
as is accomplished where the water descends in its fluid state.
The effect of the intense ice action above noted is rapidly to wear
away the rocks of the valley in which the glacier is situated. This
work is done not only in a larger measure but in a different way from
that accomplished by torrents. In the case of the latter, the stream
bed is embarrassed by the rubbish which comes into it; only here and
there can it attack the bed rock by forcing the stones over its
surface. Only in a few days of heavy rain each year is its work at all
effective; the greater part of the energy of position of its waters is
expended in the endless twistings and turnings of its stream, which
result only in the development of heat which flies away into the
atmosphere. In the ice stream, owing to its slow movement and to the
detritus which it forces along the bottom, a vastly greater part of
the energy which impels it down the slope is applied to rock cutting.
None of the boulders, even if they are yards in diameter, obstruct its
motion; small and great alike are to it good instruments wherewith to
attack the bed rocks. The fragments are never left to waste by
atmospheric decay, but are to a very great extent used up in
mechanical work, while the most of the detritus which comes to a
torrent is left in a coarse state when it is delivered to the stream;
the larger part of that which the glacier transports is worn out in
its journey. To a great extent it is used up in attacking the bed
rock. In most cases the _débris_ in the terminal moraine is evidently
but a small part of what entered the ice during its journey from the
uplands; the greater part has been worn out in the rude experiences to
which it has been subjected.
It is evident that even in the regions now most extensively occupied
by glaciers the drainage systems have been shaped by the movement of
ordinary streams--in other words, ice action is almost everywhere,
even in the regions about the poles, an incidental feature in the work
of water, coming in only to modify the topography, which is mainly
moulded by the action of fluid water. When, owing to climatal changes,
a valley such as those of the Alps is occupied by a glacial stream,
the new current proceeds at once, according to its evident needs, to
modify the shape of its channel. An ordinary torrent, because of the
swiftness of its motion, which may, in general, be estimated at from
three to five miles an hour, can convey away the precipitation over a
very narrow bed. Therefore its channel is usually not a hundredth part
as wide as the gorge or valley in which it lies. But when the
discharge takes place by a glacier, the speed of which rarely exceeds
four or five feet a day, the ice stream because of its slow motion has
to fill the trough from side to side, it has to be some thousand times
as deep and wide as the torrent. The result is that as soon as the
glacial condition arises in a country the ice streams proceed to
change the old V-shaped torrent beds into those which have a broad
U-like form. The practised eye can in a way judge how long a valley
has been subjected to glacial action by the extent to which it has
been widened by this process.
In the valleys of Switzerland and other mountain districts which have
been attentively studied it is evident that glacial action has played
a considerable part in determining their forms. But the work has been
limited to that part of the basin in which the ice is abundantly
provided with cutting tools in the stone which have found their way to
the base of the stream. In the region of the _névé_, where the
contributions of rocky matter to the surface of the deposit made from
the few bare cliffs which rise above the sheet of snow is small, the
snow-ice does no cutting of any consequence. Where it passes over the
steep at the head of the deep valley into which it drains, and is
riven into the _seracs_, such stony matter as it may have gathered is
allowed to fall to the bottom, and so comes into a position where it
may do effective work. From this _serac_ section downward the now
distinct ice river, being in general below the snow line, has
everywhere cliffs, on either side from which the contributions of rock
material are abundant. Hence this part of the glacier, though it is
the wasting portion of its length, does all the cutting work of any
consequence which is performed. It is there that the underrunning
streams become charged with sediment, which, as we have noted, they
bear in surprising quantities, and it is therefore in this section of
the valley that the impress of the ice work is the strongest. Its
effect is not only to widen the valley and deepen it, but also to
advance the deep section farther up the stream and its tributaries.
The step in the stream beds which we find at the _seracs_ appears to
mark the point in the course of the glacier where, owing to the
falling of stones to its base, as well as to its swifter movements and
the firmer state of the ice, it does effective wearing.
There are many other features connected with glaciers which richly
repay the study of those who have a mind to explore in the manner of
the physicist interested in ice actions the difficult problems which
they afford; but as these matters are not important from the point of
view of this work, no mention of them will here be made. We will now
turn our attention to that other group of glaciers commonly termed
continental, which now exist about either pole, and which at various
times in the earth's history have extended far toward the equator,
mantling over vast extents of land and shallow sea. The difference
between the ice streams of the mountains and those which we term
continental depends solely on the areas of the fields and the depth of
the accumulation. In an ordinary Alpine region the _névé_ districts,
where the snow gathers, are relatively small. Owing to the rather
steep slopes, the frozen water is rapidly discharged into the lower
valleys, where it melts away. Both in the _névé_ and in the distinct
glacier of the lower grounds there are, particularly in the latter,
projecting peaks, from which quantities of stone are brought down by
avalanches or in ordinary rock falls, so that the ice is abundantly
supplied with cutting tools, which work from its surface down to its
depths.
As the glacial accumulation grows in depth there are fewer peaks
emerging from it, and the streams which it feeds rise the higher until
they mantle over the divides between the valleys. Thus by
imperceptible stages valley glaciers pass to the larger form, usually
but incorrectly termed continental. We can, indeed, in going from the
mountains in the tropics to the poles, note every step in this
transition, until in Greenland we attain the greatest ice mass in the
world, unless that about the southern pole be more extensive. In the
Greenland glacier the ice sheet covers a vast extent of what is
probably a mountain country, which is certainly of this nature in the
southern part of the island, where alone we find portions of the earth
not completely covered by the deep envelope. Thanks to the labours of
certain hardy explorers, among whom Nansen deserves the foremost
place, we now know something as to the conditions of this vast ice
field, for it has been crossed from shore to shore. The results of
these studies are most interesting, for they afford us a clew as to
the conditions which prevail over a large part of the earth during the
Glacial period from which the planet is just escaping, and in the
earlier ages when glaciation was likewise extensive. We shall
therefore consider in a somewhat detailed way the features which the
Greenland glacier presents.
Starting from the eastern shore of that land, if we may thus term a
region which presents itself mainly in the form of ice, we find next
the shore a coast line not completely covered with ice and snow, but
here and there exhibiting peaks which indicate that if the frozen
mantle were removed the country would appear deeply intersected with
fiords in the manner exhibited in the regions to the south of
Greenland or the Scandinavian peninsula. The ice comes down to the
sea through the valleys, often facing the ocean for great distances
with its frozen cliffs. Entering on this seaward portion of the
glacier, the observer finds that for some distance from the coast line
the ice is more or less rifted with crevices, the formation of which
is doubtless due to irregularities of the rock bottom over which it
moves. These ruptures are so frequent that for some miles back it is
very difficult to find a safe way. Finally, however, a point is
attained where these breaks rather suddenly disappear, and thence
inward the ice rises at the rate of upward slope of a few feet to the
mile in a broad, nearly smooth incline. In the central portion of the
region for a considerable part of the territory the ice has very
little slope. Thence it declines toward the other shore, exhibiting
the same features as were found on the eastern versant until near the
coast, when again the surface is beset with crevices which continue to
the margin of the sea.
Although the explorations of the central field of Greenland are as yet
incomplete, several of these excursions into or across the interior
have been made, and the identity of the observations is such that we
can safely assume the whole region to be of one type. We can
furthermore run no risk in assuming that what we find in Greenland, at
least so far as the unbroken nature of the central ice field is
concerned, is what must exist in every land where the glacial envelope
becomes very deep. In Greenland it seems likely that the depth of the
ice is on the average more than half a mile, and in the central part
of the realm the sheet may well have a much greater profundity; it may
be nearly a mile deep. The most striking feature--that of a vast
unbroken expanse, bordered by a region where the ice is ruptured--is
traceable wherever very extensive and presumably deep deposits of ice
have been examined. As we shall see hereafter, these features teach us
much as to the conditions of glacial action--a matter which we shall
have to examine after we have completed our general survey as to the
changes which occur during glacial periods.
In the present state of that wonderful complex of actions which we
term climate, glaciers are everywhere, so far as our observations
enable us to judge, generally in process of decrease. In Switzerland,
although the ancients even in Roman days were in contact with the ice,
they were so unobservant that they did not even remark that the ice
was in motion. Only during the last two centuries have we any
observations of a historic sort which are of value to the geologist.
Fortunately, however, the signs written on the rock tell the story,
except for its measurement in terms of years, as clearly as any
records could give it. From this testimony of the rocks we perceive
that in the geological yesterday, though it may have been some tens of
thousands of years ago, the Swiss glaciers, vastly thickened, and with
their horizontal area immensely expanded, stretched over the Alpine
country, so that only here and there did any of the sharper peaks rise
above the surface. These vast glaciers, almost continually united on
their margins, extended so far that every portion of what is now the
Swiss Republic was covered by them. Their front lay on the southern
lowlands of Germany, on the Jura district of France; on the south, it
stretched across the valley of the Po as far as near Milan. We know
this old ice front by the accumulations of rock _débris_ which were
brought to it from the interior of the mountain realm. We can
recognise the peculiar kinds of stone, and with perfect certainty
trace them to the bed rock whence they were riven. Moreover, we can
follow back through the same evidence the stages of retreat of the
glaciers, until they lost their broad continental character and
assumed something like their present valley form. Up the valley of any
of the great rivers, as, for instance, that of the Rhône above the
lake of Geneva, we note successive terminal moraines which clearly
indicate stages in the retreat of the ice when for a time it ceased to
go backward, or even made a slight temporary readvance. It is easily
seen that on such occasions the stones carried to the ice front would
be accumulated in a heap, while during the time when day by day the
glacier was retreating the rock waste would be left broadcast over the
valley.
As we go up from the course of the glacial streams we note that the
successive moraines have their materials in a progressively less
decayed state. Far away from the heap now forming, and in proportion
to the distance, the stones have in a measure rotted, and the heaps
which they compose are often covered with soil and occupied by
forests. Within a few miles of the ice front the stones still have a
fresh aspect. When we arrive within, say, half a mile of the moraine
now building, we come to the part of the glacial retreat of which we
have some written or traditional account. This is in general to the
effect that the wasting of the glaciers is going on in this century as
it went on in the past. Occasionally periods of heavy snow would
refresh the ice streams, so that for a little time they pushed their
fronts farther down the valley. The writer has seen during one of
these temporary advances the interesting spectacle of ice destroying
and overturning the soil of a small field which had been planted in
grain.
It should be noted that these temporary advances of the ice are not
due to the snowfall of the winter or winters immediately preceding the
forward movement. So slow is the journey of the ice from the _névé_
field to the end of a long glacier that it may require centuries for
the store accumulated in the uplands to affect the terminal portion of
the stream. We know that the bodies of the unhappy men who have been
lost in the crevices of the glacier are borne forward at a uniform and
tolerably computable rate until they emerge at the front, where the
ice melts away. In at least one case the remains have appeared after
many years in the _débris_ which is contributed to the moraine. On
account of this slow feeding of the glacial stream, we naturally may
expect to find, as we do, in fact, that a great snowfall of many
years ago, and likewise a period when the winter's contribution has
been slight, would influence the position of the terminal point of the
ice stream at different times, according to its length. If the length
of the flow be five miles, it may require twenty or thirty years for
the effect to be evident; while if the stream be ten miles long, the
influence may not be noted in less than threescore years. Thus it
comes about that at the present time in the same glacial district some
streams may be advancing while others are receding, though, on the
whole, the ice is generally in process of shrinkage. If the present
rate of retreat should be maintained, it seems certain that at the end
of three centuries the Swiss glaciers as a whole will not have
anything like their present area, and many of the smaller streams will
entirely disappear.
Following the method of the illustrious Louis Agassiz, who first
attentively traced the evidence which shows the geologically recent
great extension of glaciers by studying the evidence of the action in
fields they no longer occupy, geologists have now inspected a large
part of the land areas with a view to finding the proofs of such ice
work. So far as these indications are concerned, the indications which
they have had to trace are generally of a very unmistakable character.
Rarely, indeed, does a skilled student of such phenomena have to
search in any region for more than a day before he obtains indubitable
evidence which will enable him to determine whether or not the field
has recently been occupied by an enduring ice sheet--one which
survives the summer season and therefore deserves the name of glacier.
The indications which he has to consider consist in the direction and
manner in which the surface materials have been carried, the physical
conditions of these materials, the shape of the surface of the
underlying rock as regards its general contour, and the presence or
absence of scratches and groovings on its surface. As these records of
ice action are of first importance in dealing with this problem, and
as they afford excellent subjects for the study of those who dwell in
glaciated regions, we shall note them in some detail.
The geologist recognises several ways in which materials may be
transported on the surface of the earth. They may be cast forth by
volcanoes, making their journey by being shot through the air, or by
flowing in lava streams; it is always easy at a glance, save in very
rare instances, to determine whether fragments have thus been
conveyed. Again, the detritus may be moved by the wind; this action is
limited; it only affects dust, sand, and very small pebbles, and is
easily discriminated. The carriage may be effected by river or marine
currents; here, again, the size of the fragments moved is small, and
the order of their arrangement distinctly traceable. The fragments may
be conveyed by ice rafts; here, too, the observer can usually limit
the probabilities he has to consider by ascertaining, as he can
generally do, whether the region which he is observing has been below
a sea or lake. In a word, the before-mentioned agents of
transportation are of somewhat exceptional influence, and in most
cases can, as explanations of rock transportation, be readily
excluded. When, therefore, the geologist finds a country abundantly
covered with sand, pebbles, and boulders arranged in an irregular way,
he has generally only to inquire whether the material has been carried
by rivers or by glaciers. This discrimination can be quickly and
critically effected. In the first place, he notes that rivers only in
their torrent sections can carry large fragments of rock, and that in
all cases the fragments move down hill. Further, that where deposits
are formed, they have more or less the form of alluvial deposits. If
now the observations show that the rock waste occupying the surface of
any region has been carried up hill and down, across the valleys,
particularly if there are here and there traces of frontal moraines,
the geologist is entitled to suppose--he may, indeed, be sure--that
the carriage has been effected by a glacial sheet.
Important corroborative evidence of ice action is generally to be
found by inspecting the bed rock below the detritus, which indicates
glacial action. Even if it be somewhat decayed, as is apt to be the
case where the ice sheet long since passed away, the bed rock is
likely to have a warped surface; it is cast into ridges and furrows of
a broad, flowing aspect, such as liquid water never produces, which,
indeed, can only be created by an ice sheet moving over the surface,
cutting its bed in proportion to the hardness of the material.
Furthermore, if the bed rock have a firm texture, and be not too much
decayed, we almost always find upon it grooves or scratches, channels
carved by the stones embedded in the body of the ice, and drawn by its
motion over the fixed material. Thus the proof of glacial extension in
the last ice epoch is made so clear that accurate maps can be prepared
showing the realm of its action. This task is as yet incomplete,
although it is already far advanced.
While the study of glaciers began in Europe, inquiries concerning
their ancient extension have been carried further and with more
accuracy in North America than in any other part of the world. We may
therefore well begin our description of the limits of the ice sheets
with this continent. Imagining a seafarer to have approached America
by the North Atlantic, as did the Scandinavians, and that his voyage
came perhaps a hundred thousand years or more before that of Leif
Ericsson, he would have found an ice front long before he attained the
present shores of the land. This front may have extended from south of
Greenland, off the shores of the present Grand Banks of Newfoundland,
thence and westward to central or southern New Jersey. This cliff of
ice was formed by a sheet which lay on the bottom of the sea. On the
New Jersey coast the ice wall left the sea and entered on the body of
the continent. We will now suppose that the explorer, animated with
the valiant scientific spirit which leads the men of our day to seek
the poles, undertook a land journey along the ice front across the
continent. From the New Jersey coast the traveller would have passed
through central Pennsylvania, where, although there probably detached
outlying glaciers lying to the southward as far as central Virginia,
the main front extended westward into the Ohio Valley. In southern
Ohio a tongue of the ice projected southwardly until it crossed the
Ohio River, where Cincinnati now lies, extending a few miles to the
southward of the stream. Thence it deflected northwardly, crossing the
Mississippi, and again the Missouri, with a tongue or lobe which went
far southward in that State. Then again turning to the northwest, it
followed in general the northern part of the Missouri basin until it
came to within sight of the Rocky Mountains. There the ice front of
the main glacier followed the trend of the mountains at some distance
from their face for an unknown extent to the northward. In the
Cordilleras, as far south as southern Colorado, and probably in the
Sierra Nevada to south of San Francisco, the mountain centres
developed local glaciers, which in some places were of very great
size, perhaps exceeding any of those which now exist in Switzerland.
It will thus be seen that nearly one half of the present land area of
North America was beneath a glacial covering, though, as before noted,
the region about the Gulf of Mexico may have swayed upward when the
northern portion of the land was borne down by the vast load of ice
which rested upon it. Notwithstanding this possible addition to the
land, our imaginary explorer would have found the portion of the
continent fit for the occupancy of life not more than half as great as
it is at present.
In the Eurasian continent there was no such continuous ice sheet as in
North America, but the glaciers developed from a number of different
centres, each moving out upon the lowlands, or, if its position was
southern, being limited to a particular mountain field. One of these
centres included Scandinavia, northern Germany, Great Britain about as
far south as London, and a large part of Ireland, the ice covering the
intermediate seas and extending to the westward, so that the passage
of the North Atlantic was greatly restricted between this ice front
and that of North America. Another centre, before noted, was formed in
the Alps; yet another, of considerable area, in the Pyrenees; other
less studied fields existed in the Apennines, in the Caucasus, the
Ural, and the other mountains of northern Asia. Curiously enough,
however, the great region of plains in Siberia does not appear to have
been occupied by a continuous ice sheet, though the similar region in
North America was deeply embedded in a glacier. Coincident with this
development of ice in the eastern part of the continent, the ice
streams of the Himalayan Mountains, some of which are among the
greatest of our upland glaciers, appear to have undergone but a
moderate extension. Many other of the Eurasian highlands were probably
ice-bound during the last Glacial period, but our knowledge concerning
these local fields is as yet imperfect.
In the southern hemisphere the lands are of less extent and, on the
whole, less studied than in the northern realm. Here and there where
glaciers exist, as in New Zealand and in the southern part of South
America, observant travellers have noticed that these ice fields have
recently shrunk away. Whether the time of greatest extension and of
retreat coincided with that of the ice sheets in the north is not yet
determined; the problem, indeed, is one of some difficulty, and may
long remain undecided. It seems, however, probable that the glaciers
of the southern hemisphere, like those in the north, are in process of
retreat. If this be true, then their time of greatest extension was
probably the same as that of the ice sheets about the southern pole.
From certain imperfect reports which we have concerning evidences of
glaciation in Central America and in the Andean district in the
northern part of South America, it seems possible that at one time the
upland ice along the Cordilleran chain existed from point to point
along that system of elevations, so that the widest interval between
the fields of permanent snow with their attendant glaciers did not
much exceed a thousand miles.
Observing the present gradual retreat of those ice remnants which
remain mere shreds and patches of the ancient fields, it seems at
first sight likely that the extension and recession of the great
glaciers took place with exceeding slowness. Measured in terms of
human life, in the manner in which we gauge matters of man's history,
this process was doubtless slow. There are reasons, however, to
believe that the coming and going were, in a geological sense, swift;
they may have, indeed, been for a part of the time of startling
rapidity. Going back to the time of geological yesterday, before the
ice began its development in the northern hemisphere, all the evidence
we can find appears to indicate a temperate climate extending far
toward the north pole. The Miocene deposits found within twelve
degrees, or a little more than seven hundred miles, of the north pole,
and fairly within the realm of lowest temperature which now exists on
the earth, show by the plant remains which they contain that the
conditions permitted the growth of forests, the plants having a
tolerably close resemblance to those which now freely develop in the
southern portion of the Mississippi Valley. Among them there are
species which had the habit of retaining their broad, rather soft
leaves throughout the winter season. The climate appears, in a word,
to have been one where the mean annual temperature must have been
thirty degrees or more higher than the present average of that realm.
Although such conditions near the sea level are not inconsistent with
the supposition that glaciers existed in the higher mountains of the
north, they clearly deny the possibility of the realm being occupied
by continental glaciers.
Although the Pliocene deposits formed in high latitudes have to a
great extent been swept away by the subsequent glacial wearing, they
indicate by their fossils a climatal change in the direction of
greater cold. We trace this change, though obscurely, in a
progressive manner to a point where the records are interrupted, and
the next interpretable indication we have is that the ice sheet had
extended to somewhere near the limits which we have noted. We are then
driven to seek what we can concerning the sojourn of the ice on the
land by the amount of wearing which it has inflicted upon the areas
which it occupied. This evidence has a certain, though, as we shall
see, a limited value.
When the students of glacial action first began the great task of
interpreting these records, they were led to suppose that the amount
of rock cutting which was done by the ice was very great. Observing
what goes on, in the manner we have noted, beneath a valley glacier
such as those of Switzerland, they saw that the ice work went on
rapidly, and concluded that if the ice remained long at work in a
region it must do a vast deal of erosion. They were right in a part of
their premises, but, as we shall see, probably in another part wrong.
Looking carefully over the field where the ice has operated, we note
that, though at first sight the area appears to have lost all trace of
its preglacial river topography, this aspect is due mainly to the
irregular way in which the glacial waste is laid down. Close study
shows us that we may generally trace the old stream valleys down to
those which were no larger than brooks. It is true that these channels
are generally and in many places almost altogether filled in with
rubbish, but a close study of the question has convinced the writer,
and this against a previous view, that the amount of erosion in New
England and Canada, where the work was probably as great as anywhere,
has not on the average exceeded a hundred feet, and probably was much
less than that amount.
Even in the region north of Lake Ontario, over which the ice was deep
and remained for a long time, the amount of erosion is singularly
small. Thus north of Kingston the little valleys in the limestone
rocks which were cut by the preglacial streams, though somewhat
encumbered with drift, remain almost as distinct as they are on
similar strata in central Kentucky, well south of the field which the
ice occupied. In fact, the ice sheet appears to have done the greatest
part of its work and to have affected the surface most in the belt of
country a few hundred miles in width around the edges of the sheet. It
was to be expected that in a continental glacier, as in those of
mountain valleys, the most of the _débris_ should be accumulated about
the margin where the materials dropped from the ice. But why the
cutting action should be greatest in that marginal field is not at
first sight clear. To explain this and other features as best we may,
we shall now consider the probable history of the great ice march in
advance and retreat, and then take up the conditions which brought
about its development and its disappearance.
Ice is in many ways the most remarkable substance with which the
physicist has to deal, and among its eminent peculiarities is that it
expands in freezing, while the rule is that substances contract in
passing from the fluid to the solid state. On this account frozen
water acts in a unique manner when subjected to pressure. For each
additional atmosphere of pressure--a weight amounting to about fifteen
pounds to the square inch--the temperature at which the ice will melt
is lowered to the amount of sixteen thousandths of a degree
centigrade. If we take a piece of ice at the temperature of freezing
and put upon it a sufficient weight, we inevitably bring about a small
amount of melting. Where we can examine the mass under favourable
conditions, we can see the fluid gather along the lines of the
crystals or other bits of which the ice is composed. We readily note
this action by bringing two pieces of ice together with a slight
pressure; when the pressure is removed, they will adhere. The adhesion
is brought about not by any stickiness of the materials, for the
substance has no such property. It is accomplished by melting along
the line of contact, which forms a film of water, that at once
refreezes when the pressure is withdrawn. When a firm snowball is
made by even pressing snow, innumerable similar adhesions grow up in
the manner described. The fact is that, given ice at the temperature
at which it ordinarily forms, pressure upon it will necessarily
develop melting.
The consequences of pressure melting as above described are in
glaciers extremely complicated. Because the ice is built into the
glacier at a temperature considerably below the freezing point, it
requires a great thickness of the mass before the superincumbent
weight is sufficient to bring about melting in its lower parts. If we
knew the height at which a thermometer would have stood in the surface
ice of the ancient glacier which covered the northern part of North
America, we could with some accuracy compute how thick it must have
been before the effect of pressure alone would have brought about
melting; but even then we should have to reckon the temperature
derived from the grinding of the ice over the floor and the crushing
of rocks there effected, as well as the heat which is constantly
though slowly coming forth from the earth's interior. The result is
that we can only say that at some depth, probably less than a mile,
the slowly accumulating ice would acquire such a temperature that,
subjected to the weight above it, the material next the bottom would
become molten, or at least converted into a sludgelike state, in which
it could not rub against the bottom, or move stones in the manner of
ordinary glaciers.
As fast as the ice assumed this liquid or softened state, it would be
squeezed out toward the region where, because of the thinning of the
glacier, it would enter a field where pressure melting did not occur.
It would then resume the solid state, and thence journey to the margin
of the ice in the ordinary manner. We thus can imagine how such a
glacier as occupied the northern part of this continent could have
moved from the central parts toward its periphery, as we can not do if
we assume that the glacier everywhere lay upon the bed rock. There is
no slope from Lake Erie to the Ohio River at Cincinnati. Knowing that
the ice moved down this line, there are but two methods of accounting
for its motion: either the slope of the upper surface to the northward
was so steep that the mass would have been thus urged down, the upper
parts dragging the bottom along with them, or the ice sheet for the
greater part of its extent rested upon pressure-molten water, or
sludge ice, which was easily squeezed out toward the front. The first
supposition appears inadmissible, for the reason that the ice would
have to be many miles deep at Hudson Bay in order that its upper
surface should have slope enough to overcome the rigidity of the
material and bring about the movement. We know that any such depth is
not supposable.
The recent studies in Greenland supply us with strong corroborative
evidence for the support of the view which is here urged. The wide
central field of that area, where the ice has an exceeding slight
declivity, and is unruptured by crevices, can not be explained except
on the supposition that it rests on pressure-molten water. The thinner
section next the shore, where the glacier is broken up by those
irregular movements which its wrestle with the bottom inevitably
induces, shows that there it is in contact with the bed rock, for it
behaves exactly as do the valley glaciers of like thickness.
The view above suggested as to the condition of continental glaciers
enables us to explain not only their movements, but the relatively
slight amount of wearing which they brought about on the lands they
occupied. Beginning to develop in mountain regions, or near the poles
on the lowlands, these sheets, as soon as they attained the thickness
where the ice at their bottom became molten, would rapidly advance for
great distances until they attained districts where the melting
exceeded the supply of frozen material. In this excursion only the
marginal portion of the glacier would do erosive work. This would
evidently be continued for the greatest amount of time near the front
or outer rim of the ice field, for there, we may presume, that for
the longest time the cutting rim would rest upon the bed rock of the
country. As the ice receded, this rim would fall back; thus in the
retreat as in the advance the whole of the field would be subjected to
a certain amount of erosion. On this supposition we should expect to
find that the front of a continental glacier, fed with pressure-molten
water from all its interior district, which became converted into ice,
would attain much warmer regions than the valley streams, where all
the flow took place in the state of ice, and, furthermore, that the
speed of the going on the margin would be much more rapid than in the
Alpine streams. These suppositions are well borne out by the study of
existing continental ice sheets, which move with singular rapidity at
their fronts, and by the ancient glaciers, which evidently extended
into rather warm fields. Thus, when the ice front lay at the site of
Cincinnati, at six hundred feet above the sea, there were no glaciers
in the mountains of North Carolina, though those rise more than five
thousand feet higher in the air, and are less than two hundred miles
farther south. It is therefore evident that the continental glacier at
this time pushed southward into a comparatively warm country in a way
that no stream moving in the manner of a valley glacier could possibly
have done.
The continental glaciers manage in many cases to convey detritus from
a great distance. Thus, when the ice sheet advanced southwardly from
the regions north of the Great Lakes, they conveyed quantities of the
_débris_ from that section as far south as the Ohio River. In part
this rubbish was dragged forward by the ice as the sheet advanced; in
part it was urged onward by the streams of liquid water formed by the
ordinary process of ice melting. Such subglacial rivers appear to have
been formed along the margins of all the great glaciers. We can
sometimes trace their course by the excavation which they have made,
but more commonly by the long ridges of stratified sand and gravel
which were packed into the caverns excavated by these subglacial
rivers, which are known to glacialists as _eskers_, or as serpent
kames. In many cases we can trace where these streams flowed up stream
in the old river valleys until they discharged over their head waters.
Thus in the valley of the Genesee, which now flows from Pennsylvania,
where it heads against the tributaries of the Ohio and Susquehanna, to
Lake Ontario, there was during the Glacial epoch a considerable river
which discharged its waters into those of the Ohio and the Susquehanna
over the falls at the head of its course.
[Illustration: _Front of Muir Glacier, showing ice entering the sea;
also small icebergs._]
The effect of widespread glacial action on a country such as North
America appears to have been, in the first place, to disturb the
attitude of the land by bearing down portions of its surface, a
process which led to the uprising of other parts which lay beyond the
realm of the ice. Within the field of glaciation, so far as the ice
rested bodily on the surface, the rocks were rapidly worn away. A
great deal of the _débris_ was ground to fine powder, and went far
with the waters of the under-running streams. A large part was
entangled in the ice, and moved forward toward the front of the
glacier, where it was either dropped at the margin or, during the
recession of the glacier, was laid upon the surface as the ice melted
away. The result of this erosion and transportation has been to change
the conditions of the surface both as regards soil and drainage. As
the reader has doubtless perceived, ordinary soil is, outside of the
river valleys, derived from the rock beneath where it lies. In
glaciated districts the material is commonly brought from a
considerable distance, often from miles away. These ice-made soils are
rarely very fertile, but they commonly have a great endurance for
tillage, and this for the reason that the earth is refreshed by the
decay of the pebbles which they contain. Moreover, while the tillable
earth of other regions usually has a limited depth, verging downward
into the semisoil or subsoil which represent the little changed bed
rocks, glacial deposits can generally be ploughed as deeply as may
prove desirable.
The drainage of a country recently affected by glaciers is always
imperfect. Owing to the irregular erosion of the bed rocks, and to the
yet more irregular deposition of the detritus, there are very numerous
lakes which are only slowly filled up or by erosion provided with
drainage channels. Though several thousand years have passed by since
the ice disappeared from North America, the greater part of the area
of these fresh-water basins remains, the greater number of them,
mostly those of small size, have become closed.
Where an ice stream descends into the sea or into a large lake, the
depth of which is about as great as the ice is thick, the relative
lightness of the ice tends to make it float, and it shortly breaks off
from the parent mass, forming an iceberg. Where, as is generally the
case in those glaciers which enter the ocean, a current sweeps by the
place where the berg is formed, it may enter upon a journey which may
carry the mass thousands of miles from its origin. The bergs separated
from the Greenland glaciers, and from those about the south pole, are
often of very great size; sometimes, indeed, they are some thousand
feet in thickness, and have a length of several miles. It often
happens that these bergs are formed of ice, which contains in its
lower part a large amount of rock _débris_. As the submerged portion
of the glacier melts in the sea water, these stones are gradually
dropped to the bottom, so that the cargo of one berg may be strewed
along a line many hundred miles in length. It occasionally happens
that the ice mass melts more slowly in those parts which are in the
air than in its under-water portions. It thus becomes top-heavy and
overturns, in which case such stony matter as remains attains a
position where it may be conveyed for a greater distance than if the
glacier were not capsized. It is likely, indeed, that now and then
fragments of rock from Greenland are dropped on the ocean floor in the
part of the Atlantic which is traversed by steamers between our
Atlantic ports and Great Britain.
Except for the risks which they bring to navigators, icebergs have no
considerable importance. It is true they somewhat affect the
temperature of sea and air, and they also serve to convey fragments of
stone far out to sea in a way that no other agent can effect; but, on
the whole, their influence on the conditions of the earth is
inconsiderable.
Icebergs in certain cases afford interesting indices as to the motion
of oceanic currents, which, though moving swiftly at a depth below the
surface, do not manifest themselves on the plain of the sea. Thus in
the region about Greenland, particularly in Davis Strait, bergs have
been seen forcing their way southward at considerable speed through
ordinary surface ice, which was either at rest or moving in the
opposite direction. The train of these bergs, which moves upward from
the south polar continent, west of Patagonia, indicates also in a very
emphatic way the existence of a very strong northward-setting current
in that part of the ocean.
* * * * *
We have now to consider the causes which could bring about such great
extensions of the ice sheet as occurred in the last Glacial period.
Here again we are upon the confines of geological knowledge, and in a
field where there are no well-cleared ways for the understanding. In
facing this problem, we should first note that those who are of the
opinion that a Glacial period means a very cold climate in the regions
where the ice attained its extension are probably in error. Natural as
it may seem to look for exceeding cold as the cause of glaciation, the
facts show us that we can not hold this view. In Siberia and in the
parts of North America bordering on the Arctic Sea the average cold is
so intense that the ground is permanently frozen--as it is, for
instance, in the Klondike district--to the depth of hundreds of feet,
only the surface thawing out during the warm summers. All this region
is cold enough for glaciers, but there is not sufficient snowfall to
maintain them. On the other hand, in Greenland, and in a less though
conspicuous degree in Scandinavia, where the waters of the North
Atlantic somewhat diminish the rigour of the cold, and at the same
time bring about a more abundant snowfall, the two actions being
intimately related, we have very extensive glaciers. Such facts, which
could be very much extended, make it clear that the climate of glacial
periods must have been characterized by a great snowfall, and not by
the most intense cold.
It is evident that what would be necessary again to envelop the boreal
parts of North America with a glacial sheet would not be a
considerable decrease of heat, but an increase in the winter's
contribution of frozen water. Even if the heat released by this
snowfall elevated the average temperature of the winter, as it
doubtless would in a considerable measure, it would not melt off the
snow. That snowfall tends to warm the air by setting free the heat
which was engaged in keeping the water in a state of vapour is
familiarly shown by the warming which attends an ordinary snowstorm.
Even if the fall begin with a temperature of about 0° Fahr., the air
is pretty sure to rise to near the freezing point.
It is evident that no great change of temperature is required in order
to bring about a very considerable increase in the amount of snowfall.
In the ordinary succession of seasons we often note the occurrence of
winters during which the precipitation of snow is much above the
average, though it can not be explained by a considerable climatal
change. We have to account for these departures from the normal
weather by supposing that the atmospheric currents bring in more than
the usual amount of moisture from the sea during the period when great
falls of snow occur. In fact, in explaining variations in the humidity
of the land, whether those of a constant nature or those that are to
be termed accidental, we have always to look to those features which
determine the importation of vapour from the great field of the ocean
where it enters the air. We should furthermore note that these
peculiarities of climate are dependent upon rather slight geographic
accidents. Thus the snowfall of northern Europe, which serves to
maintain the glaciation of that region, and, curiously enough, in some
measure its general warmth, depends upon the movement of the Gulf
Stream from the tropics to high latitudes. If by any geographical
change, such as would occur if Central America were lowered so as to
make a free passage for its waters to the westward, the glaciers of
Greenland and of Scandinavia would disappear, and at the same time the
temperature of those would be greatly lowered. Thus the most evident
cause of glaciation must be sought in those alterations of the land
which affect the movement of the oceanic currents.
Applying this principle to the northern hemisphere, we can in a way
imagine a change which would probably bring about a return of such an
ice period as that from which the boreal realm is now escaping. Let us
suppose that the region of not very high land about Bering Strait
should sink down so as to afford the Kuro Siwo, or North Pacific
equivalent of our Gulf Stream, an opportunity to enter the Arctic Sea
with something like the freedom with which the North Atlantic current
is allowed to penetrate to high latitudes. It seems likely that this
Pacific current, which in volume and warmth is comparable to that of
the Atlantic, would so far elevate the temperature of the arctic
waters that their wide field would be the seat of a great evaporation.
Noting once again the fact that the Greenland glaciers, as well as
those of Norway, are supplied from seas warmed by the Gulf Stream, we
should expect the result of this change would be to develop similar
ice fields on all the lands near that ocean.
Applying the data gathered by Dr. Croll for the Gulf Stream, it seems
likely that the average annual temperature induced in the Arctic Sea
by the free entrance of the Japan current would be between 20° and 30°
Fahr. This would convert this wide realm of waters into a field of
great evaporation, vastly increasing the annual precipitation. It
seems also certain that the greater part of this precipitation would
be in the form of snow. It appears to the writer that this cause alone
may be sufficient to account for the last Glacial period in the
northern hemisphere. As to the probability that the region about
Bering Strait may have been lowered in the manner required by this
view, it may be said that recent studies on the region about Mount St.
Elias show that during or just after the ice epoch the shores in that
portion of Alaska were at least four thousand feet lower than at
present. As this is but a little way from the land which we should
have to suppose to be lowered in order to admit the Japan current, we
could fairly conclude that the required change occurred. As for the
cause of the land movement, geologists are still in doubt. They know,
however, that the attitudes of the land are exceedingly unstable, and
that the shores rarely for any considerable time maintain their
position. It is probable that these swayings of the earth's surface
are due to ever-changing combinations of the weight in different parts
of the crust and the strains arising from the contraction of its inner
parts.
In the larger operations of Nature the effects which we behold,
however simple, are rarely the products of a single cause. In fact,
there are few actions so limited that they can fairly be referred to
one influence. It is therefore proper to state that there are many
other actions besides those above noted which probably enter into
those complicated equations which determine the climatal conditions of
the earth. To have these would carry us into difficult and speculative
inquiries.
As before remarked, all the regions which have been subjected to
glaciation are still each year brought temporarily into the glacial
state. This fact serves to show us that the changes necessary to
produce great ice sheets are not necessarily of a startling nature,
however great the consequences may be. Assuming, then, that relatively
slight alterations of climate may cause the ice sheet to come and go,
we may say that all the influences which have been suggested by the
students of glaciation, and various other slighter causes which can
not be here noted, may have co-operated to produce the peculiar
result. In this equation geographic change has affected the course of
the ocean currents, and has probably been the most influential, or at
least the commonest, cause to which we must attribute the extension of
ice sheets. Next, alterations of the solar heat may be looked to as a
change-bringing action; unfortunately, however, we have no direct
evidence that this is an efficient cause. Thirdly, the variations in
the eccentricity of the earth's orbit, combined with the precession of
the equinoxes and the rotation of the apsides, may be regarded as
operative. The last of all, changes in the constitution of the
atmosphere, have to be taken into account. To these must be added, as
before remarked, many less important actions which influence this
marvellously delicate machine, the work of which is expressed in the
phenomena assembled under the name of climate.
Evidence is slowly accumulating which serves to show that glacial
periods of greater or less importance have been of frequent occurrence
at all stages in the history of the earth of which we have a distinct
record. As these accidents write their history upon the ground alone,
and in a way impermanently, it is difficult to trace the ice times of
ancient geological periods. The scratches on the bed rocks, and the
accumulations of detritus formed as the ice disappeared, have alike
been worn away by the agents of decay. Nevertheless, we can trace here
and there in the older strata accumulations of pebbly matter often
containing large boulders, which clearly were shaped and brought
together by glacial action. These are found in some instances far
south of the region occupied by the glaciers during the last ice
epoch. They occur in rocks of the Cambrian or Silurian age in eastern
Tennessee and western North Carolina; they are also found in India
beyond the limits to which glaciers have attained in modern times.
In closing this inadequate account of glacial action, a story which
for its complete telling would require many volumes, it is well for
the reader to consider once again how slight are the changes of
climate which may alternately withdraw large parts of the land from
the uses of life, and again quickly restore the fields to the service
of plants and animals. He may well imagine that these changes, by
driving living creatures to and fro, profoundly affect the history of
their development. This matter will be dealt with in the volume
concerning the history of organic beings.
When the ice went off from the northern part of this continent, the
surface of the country, which had been borne down by the weight of the
glacier, still remained depressed to a considerable depth below the
level of the sea, the depression varying from somewhere about one
hundred feet in southern New England to a thousand feet or more in
high latitudes. Over this region, which lay beneath the level of the
sea, the glacier, when it became thin enough to float, was doubtless
broken up into icebergs, in the manner which we now behold along the
coast of Greenland. Where the shore was swept by a strong current,
these bergs doubtless drifted away; but along the most of the coast
line they appear to have lain thickly grouped next the shores,
gradually delivering their loads of stones and finer _débris_ to the
bottom. These masses of floating ice in many cases seem to have
prevented the sea waves from attaining the shore, and thus hindered
the formation of those beaches which in their present elevated
condition enable us to interpret the old position of the sea along
coast lines which have been recently elevated. Here and there,
however, from New Jersey to Greenland, we find bits of these ancient
shores which clearly tell the story of that down-sinking of the land
beneath the burden of the ice which is such an instructive feature in
the history of that period.
CHAPTER VII.
THE WORK OF UNDERGROUND WATER.
We have already noted two means by which water finds its way
underground. The simplest and largest method by which this action is
effected is by building in the fluid as the grains of the rock are
laid down on the floors of seas or lakes. The water thus imprisoned is
firmly inclosed in the interstices of the stone, it in time takes up
into its mass a certain amount of the mineral materials which are
contained in the deep-buried rocks. The other portion of the ground
water--that with which we are now to be specially concerned--arises
from the rain which descends into the crevices of the earth; it is
therefore peculiar to the lands. For convenience we shall term the
original embedded fluid _rock water_, and that which originates from
the rain _crevice water_, the two forming the mass of the earth water.
The crevice water of the earth, although forming at no time more than
a very small fraction of the hidden fluid, is an exceedingly potent
geological agent, doing work which, though unseen, yet affords the
very foundations on which rest the life alike of land and sea. When
this water enters the earth, though it is purified of all mineral
materials, it has already begun to acquire a share of a gaseous
substance, carbonic acid, or, as chemists now term it, carbon dioxide,
which enables the fluid to begin its rôle of marvellous activities. In
its descent as rain, probably even before it was gathered in drops in
the cloud realm, the water absorbs a certain portion of this gas from
the atmosphere. Entering the realm of the soil, where the decaying
organic matter plentifully gives forth carbon dioxide, a further store
of the gas is acquired. At the ordinary pressure of the air, water may
take in many times its bulk of the gas.
The immediate effect of carbonic acid when it is absorbed by water is
greatly to increase the capacity which that fluid has for taking
mineral matters into solution. When charged with this gas, in the
measure in which it may be in the soil, water is able to dissolve
about fifty times as much limestone as it can in its perfectly pure
form take up. A familiar instance of this peculiar capacity which the
gas gives may often be seen where the water from a soda-water fountain
drips upon the marble slab beneath. In a few years this slab will be
considerably corroded, though pure water would in the same time have
had no effect upon it.
The first and by far the most important effect of crevice water is
exercised upon the soil, which is at once the product of this action,
and the laboratory where the larger part of the work is done.
Penetrating between the grains of the detrital covering, held in large
quantities in the coating, and continually in slow motion, the
gas-charged water takes a host of substances into solution, and brings
them into a condition where they may react upon each other in the
chemical manner. These materials are constantly being offered to the
roots of plants and brought in contact with the underlying rock which
has not passed into the state of soil. The changes induced in this
stony matter lead to its breaking up, or at least to its softening to
the point where the roots can penetrate it and complete its
destruction. Thus it comes about that the water which to a great
extent divides the rocks into the state of soil, which is continually
wearing away the material on the surface, or leaching it out through
the springs, is also at work in restoring the layer from beneath.
The greater part of the water which enters the soil does not
penetrate to any great depth in the underlying rocks, but finds its
way to the surface after no long journey in the form of small springs.
Generally those superficial springs do not emerge through distinct
channels, but move, though slowly, in a massive way down the slopes
until they enter a water course. Along the banks of any river, however
small, or along the shores of the sea, a pit a few inches deep just
above the level of the water will be quickly filled by a flow from
this sheet which underlies the earth. At a distance from the stream
this sheet spring is in contact with the bed rocks, and may be many
feet below the surface, but it comes to the level of the river or the
sea near their margins. Here and there the shape of the bed rocks,
being like converging house roofs, causes the superficial springs to
form small pipelike channels for the escape of their gathered waters,
and the flow emerges at a definite point. Almost all these sources of
considerable flow are due to the action of the water on the underlying
rock, where we shall now follow that portion of the crevice water
which penetrates deeply into the earth.
Almost all rocks, however firm they may appear to be, are divided by
crevices which extend from the soil level it may be to the depths of
thousands of feet. These rents are in part due to the strains of
mountain-building, which tend to disrupt the firmest stone, leaving
open fractures. They are also formed in other ways, as by the
imperfectly understood agencies which produce joint planes. It often
happens that where rocks are highly tilted water finds its way
downward between the layers, which are imperfectly soldered together,
or a bed of coarse material, such as sandstone or conglomerate, may
afford an easy way by which the water may descend for miles beneath
the surface. Passing through rocks which are not readily soluble, the
water, already to a great extent supplied with mineral matter by its
journey through the soil, may not do much excavating work, and even
after a long time may only slightly enlarge the spaces in which it
may be stored or the channels by which it discharges to the surface.
Hence it comes about that in many countries, even where the waters
penetrate deeply, they do not afford large springs. It is otherwise
where the crevice waters enter limestones composed of materials which
are readily dissolved. In such places we find the rain so readily
entering the underlying rock that no part of the fall goes at once to
the brooks, but all has a long underground journey.
In any limestone district where the beds of the material are thick and
tolerably pure--as, for instance, in the cavern district of southern
Kentucky--the traveller who enters the region notes at once that the
usual small streams which in every region of considerable rainfall he
is accustomed to see intersecting the surface of the country are
entirely absent. In their place he notes everywhere pitlike
depressions of bowl-shaped form, the sink holes to which we have
already adverted. Through the openings in the bottom of these the rain
waters descend into the depths of the earth. Although the most of
these depressions have but small openings in their bottom, now and
then one occurs with a vertical shaft sufficiently large to permit the
explorer to descend into it, though he needs to be lowered down in the
manner of a miner who is entering a shaft. In fact, the journey is
nearly always one of some hazard; it should not be undertaken save
with many precautions to insure safety.
When one is lowered away through an open sink hole, though the descent
may at first be somewhat tortuous, the explorer soon finds himself
swinging freely in the air, it may be at a point some hundred feet
above the base of the bottle-shaped shaft or dome into which he has
entered. Commonly the neck of the bottle is formed where the water has
worked its way through a rather sandy limestone, a rock which was not
readily dissolved by the water. In the pure and therefore easily cut
limestone layers the cavity rapidly expands until the light of the
lantern may not disclose its walls. Farther down there is apt to be a
shelf composed of another impure limestone, which extends off near the
middle of the shaft. If the explorer can land upon this shelf, he is
sure to find that from this imperfect floor the cavern extends off in
one or more horizontal galleries, which he may follow for a great
distance until he comes to the point where there is again a well-like
opening through the hard layer, with another dome-shaped base beneath.
Returning to the main shaft, the explorer may continue his descent
until he attains the base of this vertical section of the cave, where
he is likely to find himself delivered in a pool of water of no great
depth, the bottom of which is occupied by a quantity of small, hard
stones of a flinty nature, which have evidently come from the upper
parts of the cavern. The close observer will have noted that here and
there in the limestone there are flinty bits, such as those which he
finds in the pool. From the bottom of the dome a determined inquirer
can often make his way along the galleries which lead from that level,
though it may be after a journey of miles to the point where he
emerges from the cavern on the banks of an open-air river.
Although a journey by way of the sink holes through a cavern system is
to be commended for the reason that it is the course of the caverning
waters, it is, on the whole, best to approach the cave through their
exits along the banks of a stream or through the chance openings which
are here and there made by the falling in of their roofs. One
advantage of this cavity of entrance is that we can thus approach the
cavern in times of heavy rain when the processes which lead to their
construction are in full activity. Coming in this way to one of the
domes formed beneath a sink hole, we may observe in rainy weather that
the water falling down the deep shaft strikes the bottom with great
force; in many of the Kentucky caves it falls from a greater height
than Niagara. At such times the stones in the basin at the bottom of
the shaft are vigorously whirled about, and in their motion they cut
the rocks in the bottom of the basin--in fact, this cavity is a great
pot hole, like those at the base of open-air cascades. It is now easy
to interpret the general principles which determine the architecture
of the cavern realm.
When it first enters the earth all the work which the water does in
the initial steps of cavern formation is effected by solution. As the
crevice enlarges and deepens, the stream acquires velocity, and begins
to use the bits of hard rock in boring. It works downward in this way
by the mixed mechanical and chemical action until it encounters a hard
layer. Then the water creeps horizontally through the soft stratum,
doing most of its work by solution, until it finds a crevice in the
floor through which it can excavate farther in the downward direction;
so it goes on in the manner of steps until it burrows channels to the
open stream. In time the vertical fall under the sink hole will cut
through the hard layer, when the water, abandoning the first line of
exit, will develop another at a lower level, and so in time it comes
about that there may be several stories of the cave, the lowest being
the last to be excavated. Of the total work thus done, only a small
part is accomplished by the falling of the water, acting through the
boring action of its tools, the bits of stone before mentioned; the
principal part of the task is done by the solvent action of the
carbonated waters on the limestone. In the system of caverns known as
the Mammoth Cave, in Kentucky, the writer has estimated that at least
nine tenths of the stone was removed in the state of solution.
When first excavated, the chambers of a limestone cavern have little
beauty to attract the eye. The curves of the walls are sometimes
graceful, but the aspect of the chambers, though in a measure grand,
is never charming. When, however, the waters have ceased to carve the
openings, when they have been drained away by the formation of
channels on a lower level, there commonly sets in a process known as
stalactitization, which transforms the scene into one of singular
beauty. We have already noted the fact that everywhere in ordinary
rocks there are crevices through which water, moving under the
pressure of the fluid which is above, may find its way slowly
downward. In the limestone roofs of caverns, particularly in those of
the upper story, this ooze of water passes through myriads of unseen
fissures at a rate so slow that it often evaporates in the dry air
without dropping to the floor. When it comes out of the rocks the
water is charged with various salts of lime; when it evaporates it
leaves the material behind on the roof. Where the outflow is so slight
that the fluid does not gather into drops, it forms an incrustation of
limy matter, which often gathers in beautiful flowerlike forms, or
perhaps in the shape of a sheet of alabaster. Where drops are formed,
a small, pendent cone grows downward from the ceiling, over which the
water flows, and on which it evaporates. This cone grows slowly
downward until it may attain the floor of the chamber, which has a
height of thirty feet or more. If all the water does not evaporate,
that which trickles off the apex of the cone, striking on the floor,
is splashed out into a thin sheet, so that it evaporates in a speedy
manner, lays down its limestone, and thus builds another and ruder
cone, which grows upward toward that which is pendent above it.
Finally, they grow together, enlarged by the process which constructed
them, until a mighty column may be formed, sculptured as if by the
hands of a fantastic architect.
[Illustration: Fig. 13.--Stalactites and stalagmites on roof and floor
of a cavern. The arrows show the direction of the moving water.]
All the while that subterranean streams are cutting the caverns
downward the open-air rivers into which they discharge are deepening
their beds, and thereby preparing for the construction of yet lower
stories of caves. These open-air streams commonly flow in steep-sided,
narrow valleys, which themselves were caves until the galleries became
so wide that they could no longer support the roof. Thus we often find
that for a certain distance the roof over a large stream has fallen
in, so that the water flows in the open air. Then it will plunge
under an arch and course, it may be, for some miles, before it again
arrives at a place where the roof has disappeared, or perhaps attains
a field occupied by rocks of another character, in which caverns were
not formed. At places these old river caverns are abandoned by the
streams, which find other courses. They form natural tunnels, which
are not infrequently of considerable length. One such in southwestern
Virginia has been made useful for a railway passing from one valley to
another, thus sparing the expense of a costly excavation. Where the
remnant of the arch is small, it is commonly known as a natural
bridge, of which that in Rockbridge County, in Virginia, is a very
noble example. Arches of this sort are not uncommon in many cavern
countries; five such exist in Carter County, Kentucky, a district in
the eastern part of that State which abounds in caverns, though none
of them are of conspicuous height or beauty.[7]
[Footnote 7: It is reported that one of these natural bridges of Carter
County has recently fallen down. This is the natural end of these
features. As before remarked, they are but the remnants of much more
extensive roofs which the processes of decay have brought to ruin.]
At this stage of his studies on cavern work the student will readily
conceive that, as the surface of the country overlying the cave is
incessantly wearing down, the upper stories of the system are
continually disappearing, while new ones are forming at the present
drainage level of the country. In fact, the attentive eye can in such
a district find here and there evidences of this progressive
destruction. Not only do the caves wear out from above, but their
roofs are constantly falling to their floors, a process which is
greatly aided by the growth of stalactites. Forming in the crevices or
joints between the stones, these rock growths sometimes prize off
great blocks. In other cases the weight of the pendent stalactite
drags the ill-supported masses of the roof to the floor. In this way a
gallery originally a hundred feet below the surface may work its way
upward to the light of day. The entrance by which the Mammoth Cave is
approached appears to have been formed in this manner, and at several
points in that system of caverns the effect of this action may be
distinctly observed.
We must now go a step further on the way of subterranean water, and
trace its action in the depths below the plane of ordinary caves,
which, as we have noted, do not extend below the level of the main
streams of the cavern district. The first group of facts to be
attended to is that exhibited by artesian wells. These occur where
rocks have been folded down into a basinlike form. It often happens
that in such a basin the rocks of which it is composed are some of
them porous, and others impervious to water, and that the porous
layers outcrop on the high margins of the depression and have
water-tight layers over them. These conditions can be well represented
by supposing that we have two saucers, one within the other, with an
intervening layer of sand which is full of water. If now we bore an
opening in the bottom of the uppermost saucer, we readily conceive
that the water will flow up through it. In Nature we often find these
basins with the equivalent of the sandy layer in the model just
described rising hundreds of feet above the valley, so that the
artesian well, so named from the village of Artois, near Paris, where
the first opening of this nature was made, may yield a stream which
will mount upward, especially where piped, to a great height. At many
places in the world it is possible by such wells to obtain a large
supply of tolerably pure water, but in general it is found to contain
too large a supply of dissolved mineral matter or sulphuretted gases
to be satisfactory for domestic purposes. It may be well to note the
fact that the greater part of the so-called artesian wells, or borings
which deliver water to a height above the surface, are not true
artesian sources, in that they do not send up the water by the action
of gravitation, but under the influence of gaseous pressure.
Where, as in the case of upturned porous beds, the crevice water
penetrates far below the earth's surface or the open-air streams which
drain the water away, the fluid acquires a considerable increase of
temperature, on the average about one degree Fahrenheit for each
eighty feet of descent. It may, indeed, become so heated that if it
were at the earth's surface it would not only burst into steam with a
vast explosive energy, but would actually shine in the manner of
heated solids. As the temperature of water rises, and as the pressure
on it increases, it acquires a solvent power, and takes in rocky
matter in a measure unapproached at the earth's surface. At the depth
of ten miles water beginning as inert rain would acquire the
properties which we are accustomed to associate with strong acids.
Passing downward through fissures or porous strata in the manner
indicated in the diagram, the water would take up, by virtue of its
heat and the gases it contained, a share of many mineral substances
which we commonly regard as insoluble. Gold and even platinum--the
latter a material which resists all acids at ordinary
temperatures--enters into the solution. If now the water thus charged
with mineral stores finds in the depths a shorter way to the surface
than that which it descended, which may well happen by way of a deep
rift in the rocks, it will in its ascent reverse the process which it
followed on going down. It will deposit the several minerals in the
order of their solubilities--that is, the last to be taken in will be
the first to be crystallized on the walls of the fissure through which
the upflow is taking place. The result will be the formation of a vein
belonging to the variety known as fissure veins.
[Illustration: Fig. 14.--Diagram of vein. The different shadings show
the variations in the nature of the deposits.]
A vein deposit such as we are considering may, though rarely, be
composed of a single mineral. Most commonly we find the deposit
arranged in a banded form in the manner indicated in the figure (see
diagram 14). Sometimes one material will abound in the lower portions
of the fissure and another in its higher parts, a feature which is
accounted for by the progressive cooling and relinquishment of
pressure to which the water is subjected on its way to the surface.
With each decrement of those properties some particular substance goes
out of the fluid, which may in the end emerge in the form of a warm or
hot spring, the water of which contains but little mineral matter.
Where, however, the temperature is high, some part of the deposit,
even a little gold, may be laid down just about the spring in the
deposits known as sinter, which are often formed at such places.
In many cases the ore deposits are formed not only in the main channel
of the fissure, but in all the crevices on either side of that way. In
this manner, much as in the case of the growth of stalactitic matter
between the blocks of stone in the roofs of a cavern, large fragments
of rock, known as "horses," are often pushed out into the body of the
vein. In some instances the growth of the vein appears to enlarge the
fissure or place of the deposit as the accumulation goes on, the
process being analogous to that by which a growing root widens the
crevice into which it has penetrated. In other instances the fissure
formed by the force has remained wide open, or at most has been but
partly filled by the action of the water.
It not infrequently happens that the ascending waters of hot springs
entering limestones have excavated extensive caves far below the
surface of the earth, these caverns being afterward in part filled by
the ores of various metals. We can readily imagine that the water at
one temperature would excavate the cavern, and long afterward, when at
a lower heat, they might proceed to fill it in. At a yet later stage,
when the surface of the country had worn down many thousands of feet
below the original level, the mineral stores of the caverns may be
brought near the surface of the earth. Some of the most important
metalliferous deposits of the Cordilleras are found in this group of
hot-water caverns. These caverns are essentially like those produced
by cold water, with the exception of the temperature of the fluid
which does the work and the opposite direction of the flow.
In following crevice water which is free to obey the impulses of
gravitation far down into the earth, we enter on a realm where the
rock or construction water, that which was built into the stone at
the time of its formation, is plentiful. Where these two groups of
waters come in contact an admixture occurs, a certain portion of the
rock water joining that in the crevices. Near the surface of the
ground we commonly find that all the construction water has been
washed out by this action. Yet if the rocks be compact, or if they
have layers of a soft and clayey nature, we may find the construction
water, even in very old deposits, remaining near the surface of the
ground. Thus in the ancient Silurian beds of the Ohio Valley a boring
carried a hundred feet below the level of the main rivers commonly
discovers water which is clearly that laid down in the crevices of the
material at the time when the rocks were formed in the sea. In all
cases this water contains a certain amount of gases derived from the
decomposition of various substances, but principally from the
alteration of iron pyrite, which affords sulphuretted hydrogen. Thus
the water is forced to the surface with considerable energy, and the
well is often named artesian, though it flows by gas pressure on the
principle of the soda-water fountain, and not by gravity, as in the
case of true artesian wells.
The passage between the work done by the deeply penetrating surface
water and that due to the fluid intimately blended with the rock built
into the mass at the time of its formation is obscure. We are,
however, quite sure that at great depths beneath the earth the
construction water acts alone not only in making veins, but in
bringing about many other momentous changes. At a great depth this
water becomes intensely heated, and therefore tends to move in any
direction where a chance fissure or other accident may lessen the
pressure. Creeping through the rocks, and moving from zones of one
temperature to another, these waters bring about in the fine
interstices chemical changes which lead to great alterations in the
constitution of the rock material. It is probably in part to these
slow driftings of rock water that beds originally made up of small,
shapeless fragments, such as compose clay slates, sandstones, and
limestones, may in time be altered into crystalline rocks, where there
is no longer a trace of the original bits, all the matter having been
taken to pieces by the process of dissolving, and reformed in the
regular crystalline order. In many cases we may note how a crystal
after being made has been in part dissolved away and replaced by
another mineral. In fact, many of our rocks appear to have been again
and again made over by the slow-drifting waters, each particular state
in their construction being due to some peculiarity of temperature or
of mineral contents which the fluid held. These metamorphic phenomena,
though important, are obscure, and their elucidation demands some
knowledge of petrographic science, that branch of geology which
considers the principles of rock formation. They will therefore not be
further considered in this work.
VOLCANOES.
Of old it was believed that volcanoes represented the outpouring of
fluid rock which came forth from the central realm of the earth, a
region which was supposed still to retain the liquid state through
which the whole mass of our earth has doubtless passed. Recent
studies, however, have brought about a change in the views of
geologists which is represented by the fact that we shall treat
volcanic phenomena in connection with the history of rock water.
In endeavouring to understand the phenomena of volcanoes it is very
desirable that the student should understand what goes on in a normal
eruption. The writer may, therefore, be warranted in describing some
observations which he had an opportunity to make at an eruption of
Vesuvius in 1883, when it was possible to behold far more than can
ordinarily be discerned in such outbreaks--in fact, the opportunity of
a like nature has probably not been enjoyed by any other person
interested in volcanic action. In the winter of 1882-'83 Vesuvius was
subjected to a succession of slight outbreaks. At the time of the
observations about to be noted the crater had been reduced to a cup
about three hundred feet in diameter and about a hundred feet deep.
The vertical shaft at the bottom, through which the outbursts were
taking place, was about a hundred feet across. Taking advantage of a
heavy gale from the northwest, it was practicable, notwithstanding the
explosions, to climb to the edge of the crater wall. Looking down into
the throat of the volcano, although the pit was full of whirling
vapours and the heat was so great that the protection of a mask was
necessary, it was possible to see something of what was going on at
the moment of an explosion.
The pipe of the volcano was full of white-hot lava. Even in a day of
sunshine, which was only partly obscured by the vapours which hung
about the opening, the heat of the lava made it very brilliant. This
mass of fluid rock was in continuous motion, swaying violently up and
down the tube. From four to six times a minute, at the moment of its
upswaying, it would burst as by the explosion of a gigantic bubble.
The upper portion of the mass was blown upward in fragments, the
discharge being like that of shot from a fowling piece; the fragments,
varying in size from small, shotlike bits to masses larger than a
man's head, were shot up sometimes to the height of fifteen hundred
feet above the point of ejection. The wind, blowing at the rate of
about forty miles an hour, drove the falling bits of rock to the
leeward, so that there was no considerable danger to be apprehended
from them. Some seconds after the explosion they could be heard
rattling down on the farther slope of the cone. Observations on the
interval between the discharge and the fall of the fragments made it
easy to compute the height to which they were thrown.
At the moment when the lava in the pipe opened for the passage of the
vapour which created the explosion the movement, though performed in
a fraction of a second, was clearly visible. At first the vapour was
colourless; a few score feet up it began to assume a faint, bluish
hue; yet higher, when it was more expanded, the tint changed to that
of steam, which soon became of the ordinary aspect, and gathered in
swift-revolving clouds. The watery nature of the vapour was perfectly
evident by its odour. Though commingled with sulphurous-acid gas, it
still had the characteristic smell of steam. For a half hour it was
possible to watch the successive explosions, and even to make rough
sketches of the scene. Occasionally the explosions would come in quick
succession, so that the lava was blown out of the tube; again, the
pool would merely sway up and down in a manner which could be
explained only by supposing that great bubbles of vapour were working
their way upward toward the point where they could burst. Each of
these bubbles probably filled a large part of the diameter of the
pipe. In general, the phenomena recalled the escape of the jet from a
geyser, or, to take a familiar instance, that of steam from the pipe
of a high-pressure engine. When the heat is great, steam may often be
seen at the mouth of the pipe with the same transparent appearance
which was observed in the throat of the crater. In the cold air of the
mountain the vapour was rapidly condensed, giving a rainbow hue in the
clouds when they were viewed at the right angle. The observations were
interrupted by the fact that the wind so far died away that large
balls of the ejected lava began to fall on the windward side of the
cone. These fragments, though cooled and blackened on their outside by
their considerable journey up and down through the air, were still so
soft that they splashed when they struck the surface of cinders.
Watching the cone from a distance, one could note that from time to
time the explosions, increasing in frequency, finally attained a point
where the action appeared to be continuous. The transition was
comparable to that which we may observe in a locomotive which, when it
first gets under way, gives forth occasional jets of steam, but,
slowly gaining speed, finally pours forth what to eye and ear alike
seem to be a continuous outrush. All the evidence that we have
concerning volcanic outbreaks corroborates that just cited, and is to
the effect that the essence of the action consists in the outbreak of
water vapour at a high temperature, and therefore endowed with very
great expansive force. Along with this steam there are many other
gases, which always appear to be but a very small part of the whole
escape of a vaporous nature--in fact, the volcanic steam, so far as
its chemical composition has been ascertained, has the composition
which we should expect to find in rock water which had been forced out
from the rock by the tensions that high temperature creates.
Because of its conspicuous nature, the lava which flows from most
volcanoes, or is blown out from them in the form of finely divided
ash, is commonly regarded as the primary feature in a volcanic
outbreak. Such is not really the case. Volcanic explosions may occur
with very little output of fluid rock, and that which comes forth may
consist altogether of the finely divided bits of rock to which we give
the name of ash. In fact, in all very powerful explosions we may
expect to find no lava flow, but great quantities of this finely
divided rock, which when it started from the depths of the earth was
in a fluid state, but was blown to pieces by the contained vapour as
it approached the surface.
If the student is so fortunate as to behold a flood of lava coming
forth from the flanks of a volcano, he will observe that even at the
very points of issue, where the material is white-hot and appears to
be as fluid as water, the whole surface gives forth steam. On a still
day, viewed from a distance, the path of a lava flow is marked by a
dense cloud of this vapour which comes forth from it. Even after the
lava has cooled so that it is safe to walk upon it, every crevice
continues to pour forth steam. Years after the flowing has ceased, and
when the rock surface has become cool enough for the growth of
certain plants upon it, these crevices still yield steam. It is
evident, in a word, that a considerable part of a lava mass, even
after it escapes from the volcanic pipes, is water which is intimately
commingled with the rock, probably lying between the very finest
grains of the heated substance. Yet this lava which has come forth
from the volcano has only a portion of the water which it originally
contained; a large, perhaps the greater part, has gone forth in the
explosive way through the crater. It is reasonably believed that the
fluidity of lava is in considerable measure due to the water which it
contains, and which serves to give the mass the consistence of paste,
the partial fluidity of flour and rock grains being alike brought
about in the same manner.
So much of the phenomena of volcanoes as has been above noted is
intended to show the large part which interstitial water plays in
volcanic action. We shall now turn our attention again to the state of
the deeply buried rock water, to see how far we may be able by it to
account for these strange explosive actions. When sediments are laid
down on the sea floor the materials consist of small, irregularly
shaped fragments, which lie tumbled together in the manner of a mass
of bricks which have been shot out of a cart. Water is buried in the
plentiful interspaces between these bits of stone; as before remarked,
the amount of this construction water varies. In general, it is at
first not far from one tenth part of the materials. Besides the fluid
contained in the distinct spaces, there is a share which is held as
combined water in the intimate structure of the crystals, if such
there be in the mass. When this water is built into the stone it has
the ordinary temperature of the sea bottom. As the depositing actions
continue to work, other beds are formed on the top of that which we
are considering, and in time the layer may be buried to the depth of
many thousand feet. There are reasons to believe that on the floors of
the oceans this burial of beds containing water may have brought great
quantities of fluid to the depth of twenty miles or more below the
outer surface of the rocks.
[Illustration: Fig. 15.--Flow of lava invading a forest. A tree in the
distance is not completely burned, showing that the molten rock had
lost much of its original heat.]
The effect of deep burial is to increase the heat of strata. This
result is accomplished in two different ways. The direct effect
arising from the imposition of weight, that derived from the mass of
stratified material, is, as we know, to bring about a down-sinking of
the earth's crust. In the measure of this falling, heat is engendered
precisely as it is by the falling of a trip-hammer on the anvil, with
which action, as is well known, we may heat an iron bar to a high
temperature. It is true that this down-sinking of the surface under
weight is in part due to the compression of the rocks, and in part to
the slipping away of the soft underpinning of more or less fluid rock.
Yet further it is in some measure brought about by the wrinkling of
the crust. But all these actions result in the conversion of energy of
position into heat, and so far serve to raise the temperature of the
rocks which are concerned in the movements. By far the largest source
of heat, however, is that which comes forth from the earth's interior,
and which was stored there in the olden day when the matter forming
the earth gathered into the mass of our sphere. This, which we may
term the original heat, is constantly flowing forth into space, but
makes its way slowly, because of the non-conductive, or, as we may
phrase it, the "blanketing" effect of the outer rock. The effect of
the strata is the same as that exercised by the non-conductive
coatings which are put on steam boilers. A more familiar comparison
may be had from the blankets used for bedclothing. If on top of the
first blanket we put a second, we keep warmer because the temperature
of the lower one is elevated by the heat from our body which is held
in. In the crust of the earth each layer of rock resists the outflow
of heat, and each addition lifts the temperature of all the layers
below.
When water-bearing strata have been buried to the depth of ten miles,
the temperature of the mass may be expected to rise to somewhere
between seven hundred and a thousand degrees Fahrenheit. If the depth
attained should be fifty miles, it is likely that the temperature will
be five times as great. At such a heat the water which the rocks
contain tends in a very vigorous way to expand and pass into the state
of vapour. This it can not readily do, because of its close
imprisonment; we may say, however, that the tendency toward explosion
is almost as great as that of ignited gunpowder. Such powder, if held
in small spaces in a mass of cast steel, could be fired without
rending the metal. The gases would be retained in a highly compressed,
possibly in a fluid form. If now it happens that any of the strain in
the rocks such as lead to the production of faults produce fissures
leading from the surface into this zone of heated water, the tendency
of the rocks containing the fluid, impelled by its expansion, will be
to move with great energy toward the point of relief or lessened
pressure which the crevice affords. Where rocks are in any way
softened, pressure alone will force them into a cavity, as is shown by
the fact that beds of tolerably hard clay stones in deep coal mines
may be forced into the spaces by the pressure of the rocks which
overlie them--in fact, the expense of cutting out these in-creeping
rocks is in some British mines a serious item in the cost of the
product.
The expansion of the water contained in the deep-lying heated rocks
probably is by far the most efficient agent in urging them toward the
plane of escape which the fissure affords. When the motion begins it
pervades all parts of the rock at once, so that an actual flow is
induced. So far as the movement is due to the superincumbent weight,
the tendency is at once to increase the temperature of the moving
mass. The result is that it may be urged into the fissure perhaps even
hotter than when it started from the original bed place. In proportion
as the rocky matter wins its way toward the surface, the pressure upon
it diminishes, and the contained vapours are freer to expand. Taking
on the vaporous form, the bubbles gather to each other, and when they
appear at the throat of the volcano they may, if the explosions be
infrequent, assume the character above noted in the little eruption of
Vesuvius. Where, however, the lava ascends rapidly through the
channel, it often attains the open air with so much vapour in it, and
this intimately mingled with the mass, that the explosion rends the
materials into an impalpably fine powder, which may float in the air
for months before it falls to the earth. With a less violent movement
the vapour bubbles expand in the lava, but do not rend it apart, thus
forming the porous, spongy rock known as pumice. With a yet slower
ascent a large part of the steam may go away, so that we may have a
flow of lava welling forth from the vent, still giving forth steam,
but with a vapour whose tension is so lowered that the matter is not
blown apart, though it may boil violently for a time after it escapes
into the air.
Although the foregoing relatively simple explanation of volcanic
action can not be said as yet to be generally accepted by geologists,
the reasons are sufficient which lead us to believe that it accounts
for the main features which we observe in this class of explosions--in
other words, it is a good working hypothesis. We shall now proceed in
the manner which should be followed in all natural inquiry to see if
the facts shown in the distribution of volcanoes in space and time
confirm or deny the view.
The most noteworthy feature in the distribution of volcanoes is that,
at the present time at least, all active vents are limited to the sea
floors or to the shore lands within the narrow range of three hundred
miles from the coast. Wherever we find a coast line destitute of
volcanoes, as is the case with the eastern coast of North and South
America, it appears that the shore has recently been carried into the
land for a considerable distance--in other words, old coast lines are
normally volcanic; that is, here and there have vents of this nature.
Thus the North Atlantic, the coasts of which appear to have gone
inland for a great distance in geologically recent times, is
non-volcanic; while the Pacific coast, which for a long time has
remained in its present position, has a singularly continuous line of
craters near the shore extending from Alaska to Tierra del Fuego. So
uninterrupted is this line of volcanoes that if they were all in
eruption it would very likely be possible to journey down the coast
without ever being out of sight of the columns of vapour which they
would send forth. On the floor of the sea volcanic peaks appear to be
very widely distributed; only a few of them--those which attain the
surface of the water--are really known, but soundings show long lines
of elevations which doubtless represent cones distributed along fault
lines, none of the peaks of sufficient height to break the surface of
the sea. It is likely, indeed, that for one marine volcano which
appears as an island there are scores which do not attain the surface.
Volcanic islands exist and generally abound in the ocean and greater
seas; every now and then we observe a new one forming as a small
island, which is apt to be washed away by the sea shortly after the
eruption ceases, the disappearance being speedy, for the reason that
the volcanic ashes of which these cones are composed drift away like
snow before the movement of the waves.
If the waters of the ocean and seas were drained away so that we could
inspect the portion of the earth's surface which they cover as readily
as we do the dry lands, the most conspicuous feature would be the
innumerable volcanic eminences which lie hidden in these watery
realms. Wherever the observer passed from the centres of the present
lands he would note within the limits of those fields only mountains,
much modified by river action; hills which the rivers had left in
scarfing away the strata; and dales which had been carved out by the
flowing waters. Near the shore lines of the vanished seas he would
begin to find mountains, hills, and vales occasionally commingled with
volcanic peaks, those structures built from the materials ejected from
the vents. Passing the coast line to the seaward, the hills and dales
would quickly disappear, and before long the mountains would vanish
from his way, and he would gradually enter on a region of vast rolling
plains beset by volcanic peaks, generally accumulated in long ranges,
somewhat after the manner of mountains, but differing from those
elevations not only in origin but in aspect, the volcanic set of peaks
being altogether made up of conical, cup-topped elevations.
A little consideration will show us that the fact of volcanoes being
in the limit to the sea floors and to a narrow fringe of shore next
certain ocean borders is reconcilable with the view as to their
formation which we have adopted. We have already noted the fact that
the continents are old, which implies that the parts of the earth
which they occupy have long been the seats of tolerably continuous
erosion. Now and then they have swung down partly beneath the sea, and
during their submersion they received a share of sediments. But, on
the whole, all parts of the lands except strips next the coast may be
reckoned as having been subjected to an excess of wearing action far
exceeding the depositional work. Therefore, as we readily see,
underneath such land areas there has been no blanketing process going
on which has served to increase the heat in the deep underlying rocks.
On the contrary, it would be easy to show, and the reader may see it
himself, that the progressive cooling of the earth has probably
brought about a lowering of the temperature in all the section from
the surface to very great depths, so that not only is the rock water
unaffected by increase of heat, but may be actually losing
temperature. In other words, the conditions which we assume bring
about volcanic action do not exist beneath the old land.
Beneath the seas, except in their very greatest depths, and perhaps
even there, the process of forming strata is continually going on.
Next the shores, sometimes for a hundred or two miles away to seaward,
the principal contribution may be the sediment worn from the lands by
the waves and the rivers. Farther away it is to a large extent made up
of the remains of animals and plants, which when dying give their
skeletons to form the strata. Much of the materials laid down--perhaps
in all more than half--consist of volcanic dust, ashes, and pumice,
which drifts very long times before it finds its way to the bottom. We
have as yet no data of a precise kind for determining the average rate
of accumulation of sediments upon the sea floor, but from what is
known of the wearing of the lands, and the amount of volcanic waste
which finds its way to the seas, it is probably not less than about a
foot in ten thousand years; it is most likely, indeed, much to exceed
this amount. From data afforded by the eruptions in Java and in other
fields where the quantity of volcanic dust contributed to the seas can
be estimated, the writer is disposed to believe that the average rate
of sedimentation on the sea floors is twice as great as the estimate
above given.
Accumulating at the average rate of one foot in ten thousand years, it
would require a million years to produce a hundred feet of sediments;
a hundred million to form ten thousand feet, and five hundred million
to create the thickness of about ten miles of bed. At the rate of two
feet in ten thousand years, the thickness accumulated would be about
twenty miles. When we come to consider the duration of the earth's
geologic history, we shall find reasons for believing that the
formation of sediment may have continued for as much as five hundred
million years.
The foregoing inquiries concerning the origin of volcanoes show that
at the present time they are clearly connected with some process which
goes on beneath the sea. An extension of the inquiry indicates that
this relation has existed in earlier geological times; for, although
the living volcanoes are limited to places within three hundred miles
of the sea, we find lava flows, ashes, and other volcanic
accumulations far in the interior of the continents, though the energy
which brought them forth to the earth's surface has ceased to operate
in those parts of the land. In these cases of continental volcanoes it
generally, if not always, appears that the cessation of the activity
attended the removal of the shore line of the ocean or the
disappearance of great inland seas. Thus the volcanoes of the
Yellowstone district may have owed their activity to the immense
deposits of sediment which were formed in the vast fresh-water lakes
which during the later Cretaceous and early Tertiary times stretched
along the eastern face of the Rocky Mountains, forming a Mediterranean
Sea in North America comparable to that which borders southern Europe.
It thus appears that the arrangement of volcanoes with reference to
sea basins has held for a considerable period in the past. Still
further, when we look backward through the successive formations of
the earth's crust we find here and there evidences in old lava flows,
in volcanic ashes, and sometimes in the ruins of ancient cones which
have been buried in the strata, that igneous activity such as is now
displayed in our volcanoes has been, since the earliest days of which
we have any record, a characteristic feature of the earth. There is no
reason to suppose that this action has in the past been any greater or
any less than in modern days. All these facts point to the conclusion
that volcanic action is due to the escape of rock water which has been
heated to high temperatures, and which drives along with it as it
journeys toward a crevice the rock in which it has been confined.
We will now notice some other explanations of volcanic action which
have obtained a certain credence. First, we may note the view that
these ejections from craters are forced out from a supposed liquid
interior of the earth. One of the difficulties of this view is that we
do not know that the earth's central parts are fluid--in fact, many
considerations indicate that such is not the case. Next, we observe
that we not infrequently find two craters, each containing fluid lava,
with the fluid standing at differences of height of several thousand
feet, although the cones are situated very near each other. If these
lavas came from a common internal reservoir, the principles which
control the action of fluids would cause the lavas to be at the same
elevation. Moreover, this view does not provide any explanation of the
fact that volcanoes are in some way connected with actions which go on
on the floors of great water basins. There is every reason to believe
that the fractures in the rocks under the land are as numerous and
deep-going as those beneath the sea. If it were a mere question of
access to a fluid interior, volcanoes should be equally distributed on
land and sea floors. Last of all, this explanation in no wise accounts
for the intermixture of water with the fluid rock. We can not well
believe that water could have formed a part of the deeper earth in the
old days of original igneous fusion. In that time the water must have
been all above the earth in the vaporous state.
Another supposition somewhat akin to that mentioned is that the water
of the seas finds its way down through crevices beneath the floors of
the ocean, and, there coming in contact with an internal molten mass,
is converted into steam, which, along with the fluid rock, escapes
from the volcanic vent. In addition to the objections urged to the
preceding view, we may say concerning this that the lava, if it came
forth under these circumstances, would emerge by the short way, that
by which the water went down, and not by the longer road, by which it
may be discharged ten thousand feet or more above the level of the
sea.
The foregoing general account of volcanic action should properly be
followed by some account of what takes place in characteristic
eruptions. This history of these matters is so ample that it would
require the space of a great encyclopædia to contain them. We shall
therefore be able to make only certain selections which may serve to
illustrate the more important facts.
By far the best-known volcanic cone is that of Vesuvius, which has
been subjected to tolerably complete record for about twenty-four
hundred years. About 500 B.C. the Greeks, who were ever on the search
for places where they might advantageously plant colonies, settled on
the island of Ischia, which forms the western of what is now termed
the Bay of Naples. This island was well placed for tillage as well as
for commerce, but the enterprising colonists were again and again
disturbed by violent outbreaks of one or more volcanoes which lie in
the interior of this island; at one time it appears that the people
were driven away by these explosions.
In these pre-Christian days Vesuvius, then known as Monte Somma, was
not known to be a volcano, it never having shown any trace of
eruption. It appeared as a regularly shaped mountain, somewhat over
two thousand feet high, with a central depression about three miles in
diameter at the top, and perhaps two miles over at the bottom, which
was plainlike in form, with some lakes of bitter water in the centre.
The most we know of this central cavity is connected with the
insurrection of the slaves led by Spartacus, the army of the revolters
having camped for a time on the plain encircled by the crater walls.
The outer slopes of the mountain afforded then a remarkably fertile
soil; some traces, indeed, of the fertility have withstood the modern
eruptions which have desolated its flanks. This wonderful Bay of
Naples became the seat of the fairest Roman culture, as well as of a
very extended commerce. Toward the close of the first century of our
era the region was perhaps richer, more beautifully cultivated, and
the seat of a more elaborate luxury than any part of the shore line of
Europe at the present day. At the foot of the mountain, on the eastern
border of the bay, the city of Pompeii, with a population of about
fifty thousand souls, was a considerable port, with an extensive
commerce, particularly with Egypt. The charming town was also a place
of great resort for rich Egyptians who cared to dwell in Europe. On
the flanks of the mountain there was at least one large town,
Herculaneum, which appears to have been an association of rich men's
residences. On the eastern side of the bay, at a point now known as
Baiæ, the Roman Government had a naval station, which in the year 79
was under the command of the celebrated Pliny, a most voluminous
though unscientific writer on matters of natural history. With him in
that year there was his nephew, commonly known as the younger Pliny,
then a student of eighteen years, but afterward himself an author.
These facts are stated in some detail, for they are all involved in
the great tragedy which we are now to describe.
For many years there had been no eruption about the Bay of Naples. The
volcanoes on Ischia had been still for a century or more, and the
various circular openings on the mainland had been so far quiet that
they were not recognised as volcanoes. Even the inquisitive Pliny,
with his great learning, was so little of a geologist that he did not
know the signs which indicate the seat of volcanic action, though they
are among the most conspicuous features which can meet the eye. The
Greeks would doubtless have recognised the meaning of these physical
signs. In the year 63 the shores of the Bay of Naples were subjected
to a distinctive earthquake. Others less severe followed in subsequent
years. In an early morning in the year 79, a servant aroused the elder
Pliny at Baiæ with the news that there was a wonderful cloud rising
from Monte Somma. The younger Pliny states that in form it was like a
pine tree, the common species in Italy having a long trunk with a
crown of foliage on its summit, shaped like an umbrella. This crown of
the column grew until it spread over the whole landscape, darkening
the field of view. Shortly after, a despatch boat brought a message to
the admiral, who at once set forth for the seat of the disturbance. He
invited his nephew to accompany him, but the prudent young man relates
in his letters to Tacitus, from whom we know the little concerning the
eruption which has come down to us, that he preferred to do some
reading which he had to attend to. His uncle, however, went straight
forward, intending to land at some point on the shore at the foot of
the cone. He found the sea, however, so high that a landing was
impossible; moreover, the fall of rock fragments menaced the ship. He
therefore cruised along the shore for some distance, landing at a
station probably near the present village of Castellamare. At this
point the fall of ashes and pumice was very great, but the sturdy old
Roman had his dinner and slept after it. There is testimony that he
snored loudly, and was aroused only when his servants began to fear
that the fall of ashes and stones would block the way out of his
bedchamber. When he came forth with his attendants, their heads
protected by planks resting on pillows, he set out toward Pompeii,
which was probably the place where he sought to land. After going some
distance, the brave man fell dead, probably from heart disease; it is
said that he was at the time exceedingly asthmatic. No sooner were his
servants satisfied that the life had passed from his body than they
fled. The remains were recovered after the eruption had ceased. The
younger Pliny further relates that after his uncle left, the cloud
from the mountain became so dense that in midday the darkness was that
of midnight, and the earthquake shocks were so violent that wagons
brought to the courtyard of the dwelling to bear the members of the
household away were rolled this way and that by the quakings of the
earth.
Save for the above-mentioned few and unimportant details concerning
the eruption, we have no other contemporaneous account. We have,
indeed, no more extended story until Dion Cassius, writing long after
the event, tells us that Herculaneum and Pompeii were overwhelmed; but
he mixes his story with fantastic legends concerning the appearance of
gods and demons, as is his fashion in his so-called history. Of all
the Roman writers, he is perhaps the most untrustworthy. Fortunately,
however, we have in the deposits of ashes which were thrown out at the
time of this great eruption some basis for interpreting the events
which took place. It is evident that for many hours the Vesuvian
crater, which had been dormant for at least five hundred years, blew
out with exceeding fury. It poured forth no lava streams; the energy
of the uprushing vapours was too great for that. The molten rock in
their path was blown into fine bits, and all the hard material cast
forth as free dust. In the course of the eruption, which probably did
not endure more than two days, possibly not more than twenty-four
hours, ash enough was poured forth to form a thick layer which spread
far over the neighbouring area of land and sea floor. It covered the
cities of Herculaneum and Pompeii to a depth of more than twenty feet,
and over a circle having a diameter of twenty miles the average
thickness may have been something like this amount. So deep was it
that, although almost all the people of these towns survived, it did
not seem to them worth while to undertake to excavate their dwelling
places. At Pompeii the covering did not overtop the higher of the low
houses. An amount of labour which may be estimated at not over one
thirtieth of the value, or at least the cost which had been incurred
in building the city, would have restored it to a perfectly
inhabitable state. The fact that it was utterly abandoned probably
indicates a certain superstitious view in connection with the
eruption.
The fact that the people had time to flee from Herculaneum and
Pompeii, bearing with them their more valuable effects, is proved by
the excavations at these places which have been made in modern times.
The larger part of Pompeii and a considerable portion of Herculaneum
have been thus explored; only rarely have human remains been found.
Here and there, particularly in the cellars, the labourers engaged in
the work of disinterring the cities note that their picks enter a
cavity; examining the space, they find they have discovered the
remains of a human skeleton. It has recently been learned that by
pouring soft plaster of Paris into these openings a mould may be
obtained which gives in a surprisingly perfect manner the original
form of the body. The explanation of this mould is as follows: Along
with the fall of cinders in an eruption there is always a great
descent of rain, arising from the condensation of the steam which
pours forth from the volcano. This water, mingling with the ashes,
forms a pasty mud, which often flows in vast streams, and is
sometimes known as mud lava. This material has the qualities of
cement--that is, it shortly "sets" in a manner comparable to plaster
of Paris or ordinary mortar. During the eruption of 79 this mud
penetrated all the low places in Pompeii, covering the bodies of the
people, who were suffocated by the fumes of the volcanic emanations.
We know that these people were not drowned by the inundation; their
attitudes show that they were dead before the flowing matter
penetrated to where they lay.
It happened that Pompeii lay beyond the influence of the subsequent
great eruptions of Vesuvius, so that it afterward received only slight
ash showers. Herculaneum, on the other hand, has century by century
been more and more deeply buried until at the present time it is
covered by many sheets of lava. This is particularly to be regretted,
for the reason that, while Pompeii was a seaport town of no great
wealth or culture, Herculaneum was the residence place of the gentry,
people who possessed libraries, the records of which can be in many
cases deciphered, and from which we might hope to obtain some of the
lost treasures of antiquity. The papyrus rolls on which the books of
that day were written, though charred by heat and time, are still
interpretable.
After the great explosion of 79, Vesuvius sank again into repose. It
was not until 1056 that vigorous eruptions again began. From time to
time slight explosions occurred, none of which yielded lava flows; it
was not until the date last mentioned that this accompaniment of the
eruption began to appear. In 1636, after a repose of nearly a century
and a half, there came a very great outbreak, which desolated a wide
extent of country on the northwestern side of the cone. At this stage
in the history of the crater the volcanic flow began to attain the
sea. Washing over the edge of the old original crater of Monte Somma,
and thus lowering its elevation, these streams devastated, during the
eruption just mentioned and in various other outbreaks, a wide field
of cultivated land, overwhelming many villages. The last considerable
eruption which yielded large quantities of lava was that of 1872,
which sent its tide for a distance of about six miles.
Since 1636 the eruptions of Vesuvius have steadily increased in
frequency, and, on the whole, diminished in violence. In the early
years of its history the great outbreaks were usually separated by
intervals of a century or more, and were of such energy that the lava
was mostly blown to dust, forming clouds so vast that on two occasions
at least they caused a midnight darkness at Constantinople, nearly
twelve hundred miles away. This is as if a volcano at Chicago should
completely hide the sun in the city of Boston. In the present state of
Vesuvius, the cone may be said to be in slight, almost continuous
eruption. The old central valley which existed before the eruption of
79, and continued to be distinct for long after that time, has been
filled up by a smaller cone, bearing a relatively tiny crater of vent,
the original wall being visible only on the eastern and northern parts
of its circuit, and here only with much diminished height. On the
western face the slope from the base of the mountain to the summit of
the new cone is almost continuous, though the trained eye can trace
the outline of Monte Somma--its position in a kind of bench, which is
traceable on that side of the long slope leading from the summit of
the new cone to the sea. The fact that the lavas of Vesuvius have
broken out on the southwestern side, while the old wall of the cone
has remained unbroken on the eastern versant, has a curious
explanation. The prevailing wind of Naples is from the southwest,
being the strong counter trades which belong in that latitude. In the
old days when the Monte Somma cone was constructed these winds caused
the larger part of the ashes to fall on the leeward side of the cone,
thus forming a thicker and higher wall around that part of the crater.
From the nature of the recent eruptions of Vesuvius it appears likely
that the mountain is about to enter on a second period of inaction.
The pipes leading through the new cone are small, and the mass of this
elevation constitutes a great plug, closing the old crater mouth. To
give vent to a large discharge of steam, the whole of this great mass,
having a depth of nearly two thousand feet, would have to be blown
away. It seems most likely that when the occasion for such a discharge
comes, the vapours of the eruption will seek a vent through some other
of the many volcanic openings which lie to the westward of this great
cone. The history of these lesser volcanoes points to the conclusion
that when the path by way of Vesuvius is obstructed they may give
relief to the steam which is forcing its course to the surface. Two or
three times since the eruption of Pliny, during periods when Vesuvius
had long been quiet, outbreaks have taken place on Ischia or in the
Phlægræn Fields, a region dotted with small craters which lies to the
west of Naples. The last of these occurred in 1552, and led to the
formation of the beautiful little cone known as Monte Nuovo. This
eruption took place near the town of Puzzuoli, a place which was then
the seat of a university, the people of which have left us records of
the accident.
[Illustration: Fig. 16.--Diagrammatic sections through Mount Vesuvius,
showing changes in the form of the cone. (From Phillips.)]
The outbreak which formed Monte Nuovo was slight but very
characteristic. It occurred in and beside a circular pool known as the
Lucrine Lake, itself an ancient crater. At the beginning of the
disturbance the ground opened in ragged cavities, from which mud and
ashes and great fragments of hard rock were hurled high in the air,
some of the stones ascending to a height of several thousand feet.
With slight intermissions this outbreak continued for some days,
resulting in the formation of a hill about five hundred feet high,
with a crater in its top, the bottom of which lay near the level of
the sea. Although this volcanic elevation, being made altogether of
loose fragments, is rapidly wearing down, while the crater is filling
up, it remains a beautiful object in the landscape, and is also
noteworthy for the fact that it is the only structure of this nature
which we know from its beginning. In the Phlægræn Field there are a
number of other craters of small size, with very low cones about them.
These appear to have been the product of brief, slight eruptions. That
known as the Solfatara, though not in eruption during the historic
period, is interesting for the fact that from the crevices of the
rocks about it there comes forth a continued efflux of carbonic-acid
gas. This substance probably arises from the effect of heat contained
in old lavas which are in contact with limestone in the deep
under-earth. We know such limestones are covered by the lavas of
Vesuvius, for the reason that numerous blocks of the rock are thrown
out during eruptions, and are often found embedded in the lava
streams. It is an interesting fact that these craters of the
Phlægræn Field, lying between the seats of vigorous eruption on
Ischia and at Vesuvius, have never been in vigorous eruption. Their
slight outbreaks seem to indicate that they have no permanent
connection with the sources whence those stronger vents obtain their
supply of heated steam.
The facts disclosed by the study of the Vesuvian system of volcanoes
afford the geologist a basis for many interesting conclusions.
In the first place, he notes that the greater part of the cones, all
those of small size, are made up of finely divided rock, which may
have been more or less cemented by the processes of change which
go on within it. It is thus clear that the lava flows are
unessential--indeed, we may say accidental--contributions to the mass.
In the case of Vesuvius they certainly do not amount to as much as one
tenth of the elevation due to the volcanic action. The share of the
lava in Vesuvius is probably greater than the average, for during the
last six centuries this vent has been remarkably lavigerous.[8]
Observation on the volcanoes of other districts show that the Vesuvian
group is in this regard not peculiar. Of nearly two hundred cones
which the writer has examined, not more than one tenth disclose
distinct lavas.
[Footnote 8: I venture to use this word in place of the phrase
"lava-yielding" for the reason that the term is needed in the
description of volcanoes.]
An inspection of the old inner wall of Monte Somma in that portion
where it is best preserved, on the north side of the Atria del
Cavallo, or Horse Gulch--so called for the reason that those who
ascended Vesuvius were accustomed to leave their saddle animals
there--we perceive that the body of the old cone is to a considerable
extent interlaced with dikes or fissures which have been filled with
molten lava that has cooled in its place. It is evident that during
the throes of an eruption, when the lava stands high in the crater,
these rents are frequently formed, to be filled by the fluid rock. In
fact, lava discharges, though they may afterward course for long
distances in the open air, generally break their way underground
through the cindery cone, and first are disclosed at the distance of a
mile or more from the inner walls of the crater. Their path is
probably formed by riftings in the compacted ashes, such as we trace
on the steep sides of the Atria del Cavallo, as before noted. For the
further history of these fissures, we shall have to refer to facts
which are better exhibited in the cone of Ætna.
The amount of rock matter which has been thrown forth from the
volcanoes about the Bay of Naples is very great. Only a portion of it
remains in the region around these cones; by far the greater part has
been washed or blown away. After each considerable eruption a wide
field is coated with ashes, so that the tilled grounds appear as if
entirely sterilized; but in a short time the matter in good part
disappears, a portion of it decays and is leached away, and the most
of the remainder washes into the sea. Only the showers, which
accumulate a deep layer, are apt to be retained on the surface of the
country. A great deal of this powdered rock drifts away in the wind,
sometimes in great quantities, as in those cases where it darkened the
sky more than a thousand miles from the cone. Moreover, the water of
the steam which brought about the discharges and the other gases which
accompanied the vapour have left no traces of their presence, except
in the deep channels which the rain of the condensing steam have
formed on the hillsides. Nevertheless, after all these subtractions
are made, the quantity of volcanic matter remaining on the surface
about the Bay of Naples would, if evenly distributed, form a layer
several hundred feet in thickness--perhaps, indeed, a thousand feet in
depth--over the territory in which the vents occur. All this matter
has been taken in relatively recent times from the depths of the
earth. The surprising fact is that no considerable and, indeed, no
permanent subsidence of the surface has attended this excavation. We
can not believe that this withdrawal of material from the under-earth
has resulted in the formation of open underground spaces. We know full
well that any such, if it were of considerable size, would quickly be
crushed in by the weight of the overlying rocks. We have, indeed, to
suppose that these steam-impelled lavas, which are driven toward the
vent whence they are to go forth in the state of dust or fluid, come
underground from distances away, probably from beneath the floors of
the sea to the westward.
Although the shores of the Bay of Naples have remained in general with
unchanged elevation for about two thousand years, they have here and
there been subjected to slight oscillations which are most likely
connected with the movement of volcanic matter toward the vents where
it is to find escape. The most interesting evidence of this nature is
afforded by the studies which have been made on the ruins of the
Temple of Serapis at Puzzuoli. This edifice was constructed in
pre-Christian times for the worship of the Egyptian god Serapis, whose
intervention was sought by sick people. The fact that this divinity of
the Nile found a residence in this region shows how intimate was the
relation between Rome and Egypt in this ancient day. The Serapeium was
built on the edge of the sea, just above its level. When in modern
days it began to be studied, its floor was about on its original
level, but the few standing columns of the edifice afford indubitable
evidence that this part of the shore has been lowered to the amount of
twenty feet or more and then re-elevated. The subsidence is proved by
the fact that the upper part of the columns which were not protected
by the _débris_ accumulated about them have been bored by certain
shellfish, known as _Lithodomi_, which have the habit of excavating
shelters in soft stone, such as these marble columns afford. At
present the floor on which the ruin stands appears to be gradually
sinking, though the rate of movement is very slow.
Another evidence that the ejections may travel for a great distance
underground on their way to the vent is afforded by the fact that
Vesuvius and Ætna, though near three hundred miles apart, appear to
exchange activities--that is, their periods of outbreak are not
simultaneous. Although these elements of the chronology of the two
cones may be accidental, taken with similar facts derived from other
fields, they appear to indicate that vents, though far separated from
each other, may, so to speak, be fed from a common subterranean
source. It is a singular fact in this connection that the volcano of
Stromboli, though situated between these two cones, is in a state of
almost incessant activity. This probably indicates that the last-named
vent derives its vapours from another level in the earth than the
greater cones. In this regard volcanoes probably behave like springs,
of which, indeed, they may be regarded as a group. The reader is
doubtless aware that hot and cold springs often escape very near
together, the difference in the temperature being due to the depth
from which their waters come forth.
As the accidents of volcanic explosion are of a nature to be very
damaging to man, as well as to the lower orders of Nature, it is fit
that we should note in general the effect of the Neapolitan eruptions
on the history of civilization in that region. As stated above, the
first Greek settlements in this vicinity--those on the island of
Ischia--were much disturbed by volcanic outbreaks, yet the island
became the seat of a permanent and prosperous colony. The great
eruption of 79 probably cost many hundred lives, and led to the
abandonment of two considerable cities, which, however, could at small
cost have been recovered to use. Since that day various eruptions have
temporarily desolated portions of the territory, but only in very
small fields have the ravages been irremediable. Where the ground was
covered with dust, it has in most places been again tillable, and so
rapid is the decay of the lavas that in a century after their flow has
ceased vines can in most cases be planted on their surfaces. The city
of Naples, which lies amid the vents, though not immediately in
contact with any of them, has steadfastly grown and prospered from the
pre-Christian times. It is doubtful if any lives have ever been lost
in the city in consequence of an eruption, and no great inconvenience
has been experienced from them. Now and then, after a great ash
shower, the volcanic dust has to be removed, but the labour is less
serious than that imposed on many northern cities by a snowstorm.
Through all these convulsions the tillage of the district has been
maintained. It has ever been the seat of as rich and profitable a
husbandry as is afforded by any part of Italy. In fact, the ash
showers, as they import fine divided rock very rich in substances
necessary for the growth of plants, have in a measure served to
maintain the fertility of the soil, and by this action have in some
degree compensated for the injury which they occasionally inflict.
Comparing the ravages of the eruptions with those inflicted by war,
unnecessary disease, or even bad politics, and we see that these
natural accidents have been most merciful to man. Many a tyrant has
caused more suffering and death than has been inflicted by these rude
operations of Nature.
From the point of view of the naturalist, Ætna is vastly more
interesting than Vesuvius. The bulk of the cone is more than twenty
times as great as that of the Neapolitan volcano, and the magnitude of
its explosions, as well as the range of phenomena which they exhibit,
incomparably greater. It happens, however, that while human history of
the recorded kind has been intimately bound up with the tiny Vesuvian
cone, partly because the relatively slight nature of its disturbances
permitted men to dwell beside it, the larger Ætna has expelled culture
from the field near its vent, and has done the greater part of its
work in the vast solitude which it has created.[9]
[Footnote 9: In part the excellent record of Vesuvius is due to the fact
that since the early Christian centuries the priests of St. Januarius,
the patron of Naples, have been accustomed to carry his relics in
procession whenever an eruption began. The cessation of the outbreak has
been written down to the credit of the saint, and thus we are provided
with a long story of the successive outbreaks.]
Ætna has been in frequent eruption for a very much longer time than
Vesuvius. In the odes of Pindar, in the sixth century before Christ,
we find records of eruptions. It is said also that the philosopher
Empedocles sought fame and death by casting himself into the fiery
crater. There has thus in the case of this mountain been no such long
period of repose as occurred in Vesuvius. Though our records of the
outbreaks are exceedingly imperfect, they serve to show that the vent
has maintained its activity much more continuously than is ordinarily
the case with volcanoes. Ætna is characteristically a lava-yielding
cone; though the amount of dust put forth is large, the ratio of the
fluid rock which flows away from the crater is very much greater than
at Vesuvius. Nearly half the cone, indeed, may be composed of this
material. Our space does not permit anything like a consecutive story
of the Ætnean eruptions since the dawn of history, or even a full
account of its majestic cone; we can only note certain features of a
particularly instructive nature which have been remarked by the many
able men who have studied this structure and the effects of its
outbreak.
The most important feature exhibited by Ætna is the vast size of its
cone. At its apex its height, though variable from the frequent
destruction and rebuilding of the crater walls, may be reckoned as
about eleven thousand feet. The base on which the volcanic material
lies is probably less than a thousand feet above the sea, so that the
maximum thickness of the heap of volcanic ejections is probably about
two miles. The average depth of this coating is probably about five
thousand feet, and, as the cone has an average diameter of about
thirty miles, we may conclude that the cone now contains about a
thousand cubic miles of volcanic materials. Great as is this mass,
it is only a small part of the ejected material which has gone forth
from the vent. All the matter which in its vaporous state went forth
with the eruption, the other gases and vapours thus discharged, have
disappeared. So, too, a large part of the ash and much of the lava has
been swept away by the streams which drain the region, and which in
times of eruption are greatly swollen by the accompanying torrential
rains. The writer has estimated that if all the emanations from the
volcano--solid, fluid, and gaseous--could be heaped on the cone, they
would form a mass of between two and three thousand cubic miles in
contents. Yet notwithstanding this enormous outputting of earthy
matter, the earth on which the Ætnean cone has been constructed has
not only failed to sink down, but has been in process of continuous,
slow uprising, which has lifted the surface more than a thousand feet
above the level which it had at the time when volcanic action began in
this field. Here, even more clearly than in the case of Vesuvius, we
see that the materials driven forth from the crater are derived not
from just beneath its foundation, but from a distance, from realms
which in the case of this insular volcano are beneath the sea floors.
It is certain that here the migration of rock matter, impelled by the
expansion of its contained water toward the vent, has so far exceeded
that which has been discharged through the crater that an uprising of
the surface such as we have observed has been brought about.
[Illustration: _Mount Ætna, seen from near Catania. The imperfect
cones on the sky line to the left are those of small secondary
eruptions._]
There are certain peculiarities of Mount Ætna which are due in part to
its great size and in part to the climatal conditions of the region in
which it lies. The upper part of the mountain in winter is deeply
snow-clad; the frozen water often, indeed, forms great drifts in the
gorges near the summit. Here it has occasionally happened that a layer
of ashes has deeply buried the mass, so that it has been preserved for
years, becoming gradually more inclosed by the subsequent eruptions.
At one point where this compact snow--which has, indeed, taken on the
form of ice--has been revealed to view, it has been quarried and
conveyed to the towns upon the seacoast. It is likely that there are
many such masses of ice inclosed between the ash layers in the upper
part of the mountain, where, owing to the height, the climate is very
cold. This curious fact shows how perfect a non-conductor the ash beds
of a volcano are to protect the frozen water from the heat of the
rocks about the crater.
The furious rains which beset the mountain in times of great eruptions
excavate deep channels on its sides. The lava outbreaks which attend
almost every eruption, and which descend from the base of the cinder
cone at the height of from five to eight thousand feet above the sea,
naturally find their way into these channels, where they course in the
manner of rivers until the lower and less valleyed section of the cone
is reached.
Such a lava flow naturally begins to freeze on the surface, the lava
at first becoming viscid, much in the manner of cream on the surface
of milk. Urged along by the more fluid lava underneath, this viscid
coating takes a ropy or corrugated form. As the freezing goes deeper,
a firm stone roof may be formed across the gorge, which, when the
current of lava ceases to flow from the crater, permits the lower part
of the stream to drain away, leaving a long cavern or scries of caves
extending far up the cone. The nature of this action is exactly
comparable to that which we may observe when on a frosty morning after
rain we may find the empty channels which were occupied by rills of
water roofed over with ice; the ice roofs are temporary, while those
of lava may endure for ages. Some of these lava-stream caves have been
disclosed, in the manner of ordinary caverns, by the falling of their
roofs; but the greater part are naturally hidden beneath the
ever-increasing materials of the cone.
The lava-stream caves of Ætna are not only interesting because of
their peculiarities of form, which we shall not undertake to describe,
but also for the reason that they help us to account for a very
peculiar feature in the history of the great cone. On the slopes of
the volcano, below the upper cindery portion, there are several
hundred lesser cones, varying from a few score to seven hundred feet
in height. Each of these has its appropriate crater, and has evidently
been the seat of one or more eruptions. As the greater part of these
cones are ancient, many of them being almost effaced by the rain or
buried beneath the ejections which have surrounded their bases since
the time they were formed, we are led to believe that many thousands
of them have been formed during the history of the volcano. The
history of these subsidiary cones appears to be connected with the
lava caves noted above. These caverns, owing to the irregularities of
their form, contain water. They are, in fact, natural cisterns, where
the abundant rainfall of the mountain finds here and there storage.
When, during the throes of an eruption, dikes such as we know often to
penetrate the mountain, are riven outward from the crater through the
mass of the cone, and filled with lava, the heated rock must often
come in contact with these masses of buried water. The result of this
would inevitably be the local generation of steam at a high
temperature, which would force its way out in a brief but vigorous
eruption, such as has been observed to take place when these
peripheral volcanoes are formed. Sometimes it has happened that after
the explosion the lava has found its way in a stream from the fissure
thus opened. That this explanation is sufficient is in a measure shown
by observations on certain effects of lava flows from Vesuvius. The
writer was informed by a very judicious observer, a resident of
Naples, who had interested himself in the phenomena of that volcano,
that the lava streams when they penetrated a cistern, such as they
often encounter in passing over villages or farmsteads, vaporized the
water, and gave rise, through the action of the steam, to small
temporary cones, which, though generally washed away by the further
flow of the liquid rock, are essentially like those which we find on
Ætna. Such subsidiary, or, as they are sometimes called, parasitic
cones, are known about other volcanoes, but nowhere are they so
characteristic as on the flanks of that wonderful volcano.
A very conspicuous feature in the Ætnean cone consists of a great
valley known as the Val del Bove, or Bull Hollow, which extends from
the base of the modern and ever-changeable cinder cone down the flanks
of the older structure to near its base. This valley has steep sides,
in places a thousand or more feet high, and has evidently been formed
by the down-settling of portions of the cone which were left without
support by the withdrawal from beneath them of materials cast forth in
a time of explosion. In an eruption this remarkable valley was the
seat of a vast water flood, the fluid being cast forth from the crater
at the beginning of the explosion. In the mouths of this and other
volcanoes, after a long period of repose, great quantities of water,
gathering from rains or condensed from the steam which slowly escapes
from these openings, often pours like a flood down the sides of the
mountains. In the great eruption of Galongoon, in Java, such a mass of
water, cast forth by a terrific explosion, mingled with ashes, so that
the mass formed a thick mud, was shot forth with such energy that it
ravaged an area nearly eighty miles in diameter, destroying the
forests and their wild inhabitants, as well as the people who dwelt
within the range of the amazing disaster. So powerfully was this water
driven from the crater that the districts immediately at the base of
the cone were in a manner overshot by the vast stream, and escaped
with relatively little injury.
When it comes forth from the base of the cinder cone, or from one of
the small peripheral craters, the lava stream usually appears to be
white hot, and to flow with almost the ease of water. It does not
really have that measure of fluidity; its condition is rather that of
thin paste; but the great weight of the material--near two and a half
times that of water--causes the movement down the slope to be speedy.
The central portion of the lava stream long retains its high
temperature; but the surface, cooling, is first converted into a tough
sheet, which, though it may bend, can hardly be said to flow. Further
hardening converts these outlying portions of the current into hard,
glassy stone, which is broken into fragments in a way resembling the
ice on the surface of a river. It thus comes about that the advancing
front of the lava stream becomes covered, and its motion hindered by
the frozen rock, until the rate of ongoing may not exceed a few feet
an hour, and the appearance is that of a heap of stone slowly rolling
down a slope. Now and then a crevice is formed, through which a thin
stream of liquid lava pours forth, but the material, having already
parted with much of its heat, rapidly cools, and in turn becomes
covered with the coating of frozen fragments. In this state of the
stream the lava flow stands on all sides high above the slope which it
is traversing; it is, in fact, walled in by its own solidified parts,
though it is urged forward by the contribution which continues to flow
in the under arches. In this state of the movement trifling accidents,
or even human interference, may direct the current this way or that.
Some of the most interesting chapters in the history of Ætna relate to
the efforts of the people to turn these slow-moving streams so that
their torrents might flow into wilderness places rather than over the
fields and towns. In the great flow of 1669, which menaced the city of
Catania, a large place on the seashore to the southeast of the cone, a
public-spirited citizen, Señor Papallardo, protecting himself and his
servants with clothing made of hides, and with large shields, set
forth armed with great hooks with the purpose of diverting the course
of the lava mass. He succeeded in pulling away the stones on the
flank of the stream, so that a flow of the molten rock was turned in
another direction. The expedient would probably have been successful
if he had been allowed to continue his labours; but the inhabitants of
a neighbouring village, which was threatened by the off-shooting
current which Papallardo had created, took up arms and drove him and
his retainers away. The flow continued until it reached Catania. The
people made haste to build the city walls on the side of danger higher
than it was before, but the tide mounted over its summit.
Although the lavas which come forth from the volcano evidently have a
high temperature, their capacity for melting other rocks is relatively
small. They scour these rocks, because of their weight, even more
energetically than do powerful torrents of water, but they are
relatively ineffective in melting stone. On Ætna and elsewhere we may
often observe lavas which have flowed through forests. When the tide
of molten rock has passed by, the trees may be found charred but not
entirely burned away; even stems a few inches in diameter retain
strength enough to uphold considerable fringes and clots of the lava
which has clung to them. These facts bear out the conclusion that the
fluidity of the heated stone depends in considerable measure on the
water which is contained, either in its fluid or vaporous state,
between the particles of the material.
If we consider the Italian volcanoes as a whole, we find that they lie
in a long, discontinuous line extending from the northern part of the
valley of the Po, within sight of the Alps, to Ætna, and in
subterranean cones perhaps to the northern coast of Africa. At the
northern end of the line we have a beautiful group of extinct
volcanoes, known as the Eugean Mountains. Thence southward to southern
Tuscany craters are wanting, but there is evidence of fissures in the
earth which give forth thermal waters. From southern Tuscany southward
through Rome to Naples there are many extinct craters, none of which
have been active in the historic period. From Naples southward the
cones of this system, about a dozen in number, are on islands or close
to the margin of the sea. It is a noteworthy fact that the greater
part of these shore or insular vents have been active since the dawn
of history; several of them frequently and furiously so, while none of
those occupying an inland position have been the seat of explosions.
This is a striking instance going to show the relation of these
processes to conditions which are brought about on the sea bottom.
Ætna is, as we have noticed, a much more powerful volcano than
Vesuvius. Its outbreaks are more vigorous, its emanations vastly
greater in volume, and the mass of its constructions many times as
great as those accumulated in any other European cone. There are,
however, a number of volcanoes in the world which in certain features
surpass Ætna as much as that crater does Vesuvius. Of these we shall
consider but two--Skaptar Jokul, of Iceland, remarkable for the volume
of its lava flow, and Krakatoa, an island volcano between Java and
Sumatra, which was the seat of the greatest explosion of which we have
any record.
The whole of Iceland may be regarded as a volcanic mass composed
mainly of lavas and ashes which have been thrown up by a group of
volcanoes lying near the northern end of the long igneous axis which
extends through the centre of the Atlantic. The island has been the
seat of numerous eruptions; in fact, since its settlement by the
Northmen in 1070 its sturdy inhabitants have been almost as much
distressed by the calamities which have come from the internal heat as
they have been by the enduring external cold. They have, indeed, been
between frost and fire. The greatest recorded eruption of Iceland
occurred in 1783, when the volcano of Skaptar, near the southern
border of the island, poured forth, first, a vast discharge of dust
and ashes, and afterward in the languid state of eruption inundated a
series of valleys with the greatest lava flow of which we have any
written record. The dust poured forth into the upper air, being finely
divided and in enormous quantity, floated in the air for months,
giving a dusky hue to the skies of Europe, which led the common people
and many of the learned to fear that the wrath of God was upon them,
and that the day of judgment was at hand. Even the poet Cowper, a man
of high culture and education, shared in this unreasonable view.
The lava flow in this eruption filled one of the considerable valleys
of the island, drying up the river, and inundating the plains on
either side. Estimates which have been made as to the volume of this
flow appear to indicate that it may have amounted to more than the
bulk of the Mont Blanc.
This great eruption, by the direct effect of the calamity, and by the
famine due to the ravaging of the fields and the frightening of the
fish from the shores which it induced, destroyed nearly one fifth of
the Icelandic people. It is, in fact, to be remembered as one of the
three or four most calamitous eruptions of which we have any account,
and, from the point of view of lava flow, the greatest in history.
Just a hundred years after the great Skaptar eruption, which darkened
the skies of Europe, the island of Krakatoa, an isle formed by a small
volcano in the straits of Java, was the seat of a vapour explosion
which from its intensity is not only unparalleled, but almost
unapproached in all accounts of such disturbances. Krakatoa had long
been recognised as a volcanic isle; it is doubtful, however, if it had
ever been seen in eruption during the three centuries or more since
European ships began to sail by it until the month of May of the year
above mentioned. Then an outbreak of what may be called ordinary
violence took place, which after a few days so far ceased that
observers landed and took account of the changes which the convulsion
had brought about. For about three months there were no further signs
of activity, but on the 29th of August a succession of vast explosions
took place, which blew away a great part of the island, forming in its
place a submarine crater two or three miles in diameter, creating
world-wide disturbances of sea and air. The sounds of the outbreak
were heard at a distance of sixteen hundred miles away. The waves of
the air attendant on the explosion ran round the earth at least once,
as was distinctly indicated by the self-recording barometers; it is
possible, indeed, that, crossing each other in their east and west
courses, these atmospheric tides twice girdled the sphere. In effect,
the air over the crater was heaved up to the height of some tens of
thousands of feet, and thence rolled off in great circular waves, such
as may be observed in a pan of milk when a sharp blow pushes the
bottom upward.
The violent stroke delivered to the waters of the sea created a vast
wave, which in the region where it originated rolled upon the shores
with a surf wall fifty or more feet high. In a few minutes about
thirty thousand people were overwhelmed. The wave rolled on beyond its
destructive limits much in the manner of the tide; its influence was
felt in a sharp rise and fall of the waters as far as the Pacific
coast of North America, and was indicated by the tide gauges in the
Atlantic as far north as the coast of Europe.
Owing to the violence of the eruption, Krakatoa poured forth no lava,
but the dust and ashes which ascended into the air--or, in
other words, the finely divided lava which escaped into the
atmosphere--probably amounted in bulk to more than twenty cubic miles.
The coarser part of this material, including much pumice, fell upon
the seas in the vicinity, where, owing to its lightness, it was free
to drift in the marine currents far and wide throughout the oceanic
realm. The finer particles, thrown high into the air, perhaps to the
height of nearly a hundred thousand feet--certainly to the elevation
of more than half this amount--drifted far and wide in the
atmosphere, so that for years the air of all regions was clouded by
it, the sunrise and sunset having a peculiar red glow, which the dust
particles produce by the light which they reflect. In this period, at
all times when the day was clear, the sun appeared to be surrounded by
a dusky halo. In time the greater part of this dust was drawn down by
gravity, some portion of it probably falling on every square foot of
the earth. Since the disappearance of the characteristic phenomena
which it produced in the atmosphere, European observers have noted the
existence of faint clouds lying in the upper part of the air at the
height of a hundred miles or more above the surface. These clouds,
which were at first distinctly visible in the earliest stage of dawn
and in the latest period of the sunset glow, seemed to be in rapid
motion to the eastward, and to be mounting higher above the earth. It
has been not unreasonably supposed that these shining clouds represent
portions of the finest dust from Krakatoa, which has been thrown so
far above the earth's attraction that it is separating itself from the
sphere. If this view be correct, it seems likely that we may look to
great volcanic explosions as a source whence the dustlike particles
which people the celestial spaces may have come. They may, in a word,
be due to volcanic explosions occurring on this and other celestial
spheres.
The question suggested above as to the possibility of volcanic
ejections throwing matter from the earth beyond the control of its
gravitative energy is one of great scientific interest. Computations
(not altogether trustworthy) show that a body leaving the earth's
surface under the conditions of a cannon ball fired vertically upward
would have to possess a velocity at the start of at least seven miles
a second in order to go free into space. It would at first sight seem
that we should be able to reckon whether volcanoes can propel earth
matter upward with this speed. In fact, however, sufficient data are
not obtainable; we only know in a general way that the column of
vapour rises to the height of thirty or forty thousand feet, and this
in eruptions of no great magnitude. In an accident such as that at
Krakatoa, even if an observer were near enough to see clearly what was
going on, the chance of his surviving the disturbance would be small.
Moreover, the ascending vapours, owing to their expansion of the steam
in the column, begin to fly out sideways on its periphery, so that the
upper part of the central section in the discharge is not visible from
the earth.
It is in the central section of the uprushing mass, if anywhere, that
the dust might attain the height necessary to put it beyond the
earth's attraction, bringing it fairly into the realm of the solar
system, or to the position where its own motion and the attraction of
the other spheres would give it an independent orbital movement about
the sun, or perhaps about the earth. We can only say that observations
on the height of volcanic ejections are extremely desirable; they can
probably only be made from a balloon. An ascension thus made beyond
the cloud disk which the eruption produces might bring the observer
where he could discern enough to determine the matter. Although the
movements of the rocky particles could not be observed, the colour
which they would give to the heavens might tell the story which we
wish to know. There is evidence that large masses of stone hurled up
by volcanic eruption have fallen seven miles from the base of the
cone. Assuming that the masses went straight upward at the beginning
of their ascent, and that they were afterward borne outwardly by the
expansion of the column, computations which have a general but no
absolute value appear to indicate that the masses attained a height of
from thirty to fifty miles, and had an initial velocity which, if
doubled, might have carried them into space.
Last of all, we shall note the conditions which attend the eruptions
of submarine volcanoes. Such explosions have been observed in but a
few instances, and only in those cases where there is reason to
believe that the crater at the time of its explosion had attained to
within a few hundred feet of the sea level. In these cases the
ejections, never as yet observed in the state of lava, but in the
condition of dust and pumice, have occasionally formed a low island,
which has shortly been washed away by the waves. Knowing as we do that
volcanoes abound on the sea floor, the question why we do not oftener
see their explosions disturbing the surface of the waters is very
interesting, but not as yet clearly explicable. It is possible,
however, that a volcanic discharge taking place at the depth of
several thousand feet below the surface of the water would not be able
to blow the fluid aside so as to open a pipe to the surface, but would
expend its energy in a hidden manner near the ocean floor. The vapours
would have to expand gradually, as they do in passing up through the
rock pipe of a volcano, and in their slow upward passage might be
absorbed by the water. The solid materials thrown forth would in this
case necessarily fall close about the vent, and create a very steep
cone, such, indeed, as we find indicated by the soundings off certain
volcanic islands which appear only recently to have overtopped the
level of the waters.
As will be seen, though inadequately from the diagrams of Vesuvius,
volcanic cones have a regularity and symmetry of form far exceeding
that afforded by the outlines of any other of the earth's features.
Where, as is generally the case, the shape of the cone is determined
by the distribution of the falling cinders or divided lava which
constitutes the mass of most cones, the slope is in general that known
as a catenary curve--i.e., the line formed by a chain hanging between
two points at some distance from the vertical. It is interesting to
note that this graceful outline is a reflection or consequence of the
curve described by the uprushing vapour. The expansion in the
ascending column causes it to enlarge at a somewhat steadfast rate,
while the speed of the ascent is ever diminishing. Precisely the same
action can be seen in the like rush of steam and other gases and
vapours from the cannon's mouth; only in the case of the gun, even of
the greatest size, we can not trace the movement for more than a few
hundred feet. In this column of ejection the outward movement from the
centre carries the bits of lava outwardly from the centre of the
shaft, so that when they lose their ascending velocity they are drawn
downward upon the flanks of the cone, the amount falling upon each
part of that surface being in a general way proportional to the
thickness of the vaporous mass from which they descend. The result is,
that the thickest part of the ash heap is formed on the upper part of
the crater, from which point the deposit fades away in depth in every
direction. In a certain measure the concentration toward the centre of
the cone is brought about by the draught of air which moves in toward
the ascending column.
Although, in general, ejections of volcanic matter take place through
cones, that being the inevitable form produced by the escaping steam,
very extensive outpourings of lava, ejections which in mass probably
far exceed those thrown forth through ordinary craters, are
occasionally poured out through fissures in the earth's crust. Thus in
Oregon, Idaho, and Washington, in eastern Europe, in southern India,
and at some other points, vast flows, which apparently took place from
fissures, have inundated great realms with lava ejections. The
conditions which appear to bring about these fissure eruptions of lava
are not yet well understood. A provisional and very probable account
of the action can be had in the hypothesis which will now be set
forth.
Where any region has been for a long time the seat of volcanic action,
it is probable that a large amount of rock in a more or less fluid
condition exists beneath its surface. Although the outrushing steam
ejects much of this molten material, there are reasons to suppose that
a yet greater part lies dormant in the underground spaces. Thus in the
case of Ætna we have seen that, though some thousands of miles of
rock matter have come forth, the base of the cone has been uplifted,
probably by the moving to that region of more or less fluid rock. If
now a region thus underlaid by what we may call incipient lavas is
subjected to the peculiar compressive actions which lead to
mountain-building, we should naturally expect that such soft material
would be poured forth, possibly in vast quantities through fault
fissures, which are so readily formed in all kinds of rock when
subject to irregular and powerful strains, such as are necessarily
brought about when rocks are moved in mountain-making. The great
eruptions which formed the volcanic table-lands on the west coast of
North America appear to have owed the extrusion of their materials to
mountain-building actions. This seems to have been the case also in
some of those smaller areas where fissure flows occur in Europe. It is
likely that this action will explain the greater part of these massive
eruptions.
It need not be supposed that the rock beneath these countries, which
when forced out became lava, was necessarily in the state of perfect
fluidity before it was forced through the fissures. Situated at great
depth in the earth, it was under a pressure so great that its
particles may have been so brought together that the material was
essentially solid, though free to move under the great strains which
affected it, and acquiring temperature along with the fluidity which
heat induces as it was forced along by the mountain-building pressure.
As an illustration of how materials may become highly heated when
forced to move particle on particle, it may be well to cite the case
in which the iron stringpiece on top of a wooden dam near Holyoke,
Mass., was affected when the barrier went away in a flood. The iron
stringer, being very well put together, was, it is said, drawn out by
the strain until it became sensibly reddened by the motion of its
particles, and finally fell hissing into the waters below. A like
heating is observable when metal is drawn out in making wire. Thus a
mass of imperfectly fluid rock might in a forced journey of a few
miles acquire a decided increase of temperature.
Although the most striking volcanic action--all such phenomena,
indeed, as commonly receives the name--is exhibited finally on the
earth's surface, a great deal of work which belongs in the same group
of geological actions is altogether confined to the deep-lying rock,
and leads to the formation of dikes which penetrate the strata, but do
not rise to the open air. We have already noted the fact that dikes
abound in the deeper parts of volcanic cones, though the fissures into
which they find their way are seldom riven up to the surface. In the
same way beneath the ground in non-volcanic countries we may discover
at a great depth in the older, much-changed rock a vast number of
these crevices, varying from a few inches to a hundred feet or more in
width, which have been filled with lavas, the rock once molten having
afterward cooled. In most cases these dikes are disclosed to us
through the down-wearing of the earth that has removed the beds into
which the dikes did not penetrate, thus disclosing the realm in which
the disturbances took place.
Where, as is occasionally the case in deep mines, or on some bare
rocky cliff of great height, we can trace a dike in its upward course
through a long distance, we find that we can never distinctly discover
the lower point of its extension. No one has ever seen in a clear way
the point of origin of such an injection. We can, however, often
follow it upward to the place where there was no longer a rift into
which it could enter. In its upward path the molten matter appears
generally to have followed some previously existing fracture, a joint
plane or a fault, which generally runs through the rocks on those
planes. We can observe evidence that the material was in the state of
igneous fluidity by the fact that it has baked the country rocks on
either side of the fissure, the amount of baking being in proportion
to the width of the dike, and thus to the amount of heat which it
could give forth. A dike six inches in diameter will sometimes barely
sear its walls, while one a hundred feet in width will often alter the
strata for a great distance on either side. In some instances, as in
the coal beds near Richmond, Va., dikes occasionally cut through beds
of bituminous coal. In these cases we find that the coal has been
converted into coke for many feet either side of a considerable
injection. The fact that the dike material was molten is still further
shown by the occurrence in it of fragments which it has taken up from
the walls, and which may have been partly melted, and in most cases
have clearly been much heated.
Where dikes extend up through stratified beds which are separated from
each other by distinct layers, along which the rock is not firmly
bound together, it now and then happens, as noted by Mr. G.K. Gilbert,
of the United States Geological Survey, that the lava has forced its
way horizontally between these layers, gradually uplifting the
overlying mass, which it did not break through, into a dome-shaped
elevation. These side flows from dikes are termed laccolites, a word
which signifies the pool-like nature of the stony mass which they form
between the strata.
In many regions, where the earth has worn down so as to reveal the
zone of dikes which was formed at a great depth, the surface of the
country is fairly laced with these intrusions. Thus on Cape Ann, a
rocky isle on the east coast of Massachusetts, having an area of about
twenty square miles, the writer, with the assistance of his colleague,
Prof. R.S. Tarr, found about four hundred distinct dikes exhibited on
the shore line where the rocks had been swept bare by the waves. If
the census of these intrusions could have been extended over the whole
island, it would probably have appeared that the total number exceeded
five thousand. In other regions square miles can be found where the
dikes intercepted by the surface occupy an aggregate area greater than
that of the rocks into which they have been intruded.
Now and then, but rarely, the student of dikes finds one where the
bordering walls, in place of having the clean-cut appearance which
they usually exhibit, has its sides greatly worn away and much melted,
as if by the long-continued passage of the igneous fluid through the
crevice. Such dikes are usually very wide, and are probably the paths
through which lavas found their way to the surface of the earth,
pouring forth in a volcanic eruption. In some cases we can trace their
relation to ancient volcanic cones which have worn down in all their
part which were made up of incoherent materials, so that there remains
only the central pipe, which has been preserved from decay by the
coherent character of the lava which filled it.
The hypothesis that dikes are driven upward into strata by the
pressure of the beds which overlie materials hot and soft enough to be
put in motion when a fissure enters them, and that their movement
upward through the crevice is accounted for by this pressure, makes
certain features of these intrusions comprehensible. Seeing that very
long, slender dikes are found penetrating the rock, which could not
have had a high temperature, it becomes difficult to understand how
the lava could have maintained its fluidity; but on the supposition
that it was impelled forward by a strong pressure, and that the energy
thus transmitted through it was converted into heat, we discover a
means whereby it could have been retained in the liquid condition,
even when forced for long distances through very narrow channels.
Moreover, this explanation accounts for the fact which has long
remained unexplained that dikes, except those formed about volcanic
craters, rarely, if ever, rise to the surface.
The materials contained in dikes differ exceedingly in their chemical
and mineral character. These variations are due to the differences in
Nature of the deposits whence they come, and also in a measure to
exchanges which take place between their own substance and that of the
rocks between which they are deposited. This process often has
importance of an economic kind, for it not infrequently leads to the
formation of metalliferous veins or other aggregations of ores, either
in the dike itself or in the country rock. The way in which this is
brought about may be easily understood by a familiar example. If flesh
be placed in water which has the same temperature, no exchange of
materials will take place; but if the water be heated, a circulation
will be set up, which in time will bring a large part of the soluble
matter into the surrounding water. This movement is primarily
dependent on differences of temperature, and consequently differences
in the quantity of soluble substances which the water seeks to take
up. When a dike is injected into cooler rocks, such a slow circulation
is induced. The water contained in the interstices of the stone
becomes charged with mineral materials, if such exist in positions
where it can obtain possession of them, and as cooling goes on, these
dissolved materials are deposited in the manner of veins. These veins
are generally laid down on the planes of contact between the two kinds
of stone, but they may be formed in any other cavities which exist in
the neighbourhood. The formation of such veins is often aided by the
considerable shrinkage of the lava in the dike, which, when it cools,
tends to lose about fifteen per cent of its volume, and is thus likely
to leave a crevice next the boundary walls. Ores thus formed afford
some of the commonest and often the richest mineral deposits. At
Leadville, in Colorado, the great silver-bearing lodes probably were
produced in this manner, wherein lavas, either those of dikes or those
which flowed in the open air, have come in contact with limestones.
The mineral materials originally in the once molten rock or in the
limy beds was, we believe, laid down on ancient sea floors in the
remains of organic forms, which for their particular uses took the
materials from the old sea water. The vein-making action has served to
assemble these scattered bits of metal into the aggregation which
constitutes a workable deposit. In time, as the rocks wear down, the
materials of the veins are again taken into solution and returned to
the sea, thence perhaps to tread again the cycle of change.
In certain dikes, and sometimes also, perhaps, in lavas known as
basalts, which have flowed on the surface, the rock when cooling, from
the shrinkage which then occurs, has broken in a very regular way,
forming hexagonal columns which are more or less divided on their
length by joints. When worn away by the agencies of decay, especially
where the material forms steep cliffs, a highly artificial effect is
produced, which is often compared, where cut at right angles to the
columns, to pavements, or, where the division is parallel to the
columns, to the pipes of an organ.
What we know of dikes inclines us to the opinion that as a whole they
represent movements of softened rock where the motion-compelling agent
is not mainly the expansion of the contained water which gives rise to
volcanic ejection, but rather in large part due to the weight of
superincumbent strata setting in motion materials which were somewhat
softened, and which tended to creep, as do the clays in deep coal
mines. It is evident, however; it is, moreover, quite natural, that
dike work is somewhat mingled with that produced by the volcanic
forces; but while the line between the two actions is not sharp, the
discrimination is important, and occurs with a distinctness rather
unusual on the boundary line between two adjacent fields of phenomena.
* * * * *
We have now to consider the general effects of the earth's interior
heat so far as that body of temperature tends to drive materials from
the depths of the earth to the surface. This group of influences is
one of the most important which operates on our sphere; as we shall
shortly see, without such action the earth would in time become an
unfit theatre for the development of organic life. To perceive the
effect of these movements, we must first note that in the great
rock-constructing realm of the seas organic life is constantly
extracting from the water substances, such as lime, potash, soda, and
a host of other substances necessary for the maintenance of
high-grade organisms, depositing these materials in the growing
strata. Into these beds, which are buried as fast as they form, goes
not only these earthy materials, but a great store of the sea water as
well. The result would be in course of time a complete withdrawal into
the depths of the earth of those substances which play a necessary
part in organic development. The earth would become more or less
completely waterless on its surface, and the rocks exposed to view
would be composed mainly of silica, the material which to a great
extent resists solution, and therefore avoids the dissolving which
overtakes most other kinds of rocks. Here comes in the machinery of
the hot springs, the dikes, and the volcanoes. These agents, operating
under the influence of the internal heat of the earth, are constantly
engaged in bearing the earthy matter, particularly its precious more
solvent parts, back to the surface. The hot springs and volcanoes work
swiftly and directly, and return the water, the carbon dioxide, and a
host of other vaporizable and soluble and fusible substances to the
realm of solar activity, to the living surface zone of the earth. The
dikes operate less immediately, but in the end to the same effect.
They lift their materials miles above the level where they were
originally laid, probably from a zone which is rarely if ever exposed
to view, placing them near the surface, where the erosive agents can
readily find access to them.
Of the three agents which serve to export earth materials from its
depths, volcanoes are doubtless the most important. They send forth
the greater part of the water which is expelled from the rocks.
Various computations which the writer has made indicate that an
ordinary volcano, such as Ætna, in times of most intense explosion,
may send forth in the form of steam one fourth of a cubic mile or
more of water during each day of its discharge, and in a single great
eruption may pour forth several times this quantity. In its history
Ætna has probably returned to the atmosphere some hundred cubic miles
of water which but for the process would have remained permanently
locked up in its rock prison.
The ejection of rock material, though probably on the average less in
quantity than the water which escapes, is also of noteworthy
importance. The volcanoes of Java and the adjacent isles have, during
the last hundred and twenty years, delivered to the seas more earth
material than has been carried into those basins by the great rivers.
If we could take account of all the volcanic ejections which have
occurred in this time, we should doubtless find that the sum of the
materials thus cast forth into the oceans was several times as great
as that which was delivered from the lands by all the superficial
agents which wear them away. Moreover, while the material from the
land, except the small part which is in a state of complete solution,
all falls close to the shore, the volcanic waste, because of its fine
division or because of the blebs of air which its masses contain, may
float for many years before it finds its way to the bottom, it may be
at the antipodes of the point at which it came from the earth. While
thus journeying through the sea the rock matter from the volcanoes is
apt to become dissolved in water; it is, indeed, doubtful if any
considerable part of that which enters the ocean goes by gravitation
to its floor. The greater portion probably enters the state of
solution and makes its way thence through the bodies of plants and
animals again into the ponderable state.
If an observer could view the earth from the surface of the moon, he
would probably each day behold one of these storms which the volcanoes
send forth. In the fortnight of darkness, even with the naked eye, it
would probably be possible to discern at any time several eruptions,
some of which would indicate that the earth's surface was ravaged by
great catastrophes. The nearer view of these actions shows us that
although locally and in small measure they are harmful to the life of
the earth, they are in a large way beneficent.
CHAPTER VIII.
THE SOIL.
The frequent mention which it has been necessary to make of soil
phenomena in the preceding chapters shows how intimately this feature
in the structure of the earth is blended with all the elements of its
physical history. It is now necessary for us to take up the phenomena
of soils in a consecutive manner.
The study of any considerable river basin enables us to trace the more
important steps which lead to the destructure and renovation of the
earth's detrital coating. In such an interpretation we note that
everywhere the rocks which were built on the sea bottom, and more or
less made over in the great laboratory of the earth's interior, are at
the surface, when exposed to the conditions of the atmosphere, in
process of being taken to pieces and returned to the sea. This action
goes on everywhere; every drop of rain helps it. It is aided by frost,
or even by the changes of expansion and contraction which occur in the
rocks from variations of heat. The result is that, except where the
slopes are steep, the surface is quickly covered with a layer of
fragments, all of which are in the process of decay, and ready to
afford some food to plants. Even where the rock appears bare, it is
generally covered with lichens, which, adhering to it, obtain a share
of nutriment from the decayed material which they help to hold on the
slope. When they have retained a thin sheet of the _débris_, mosses
and small flowering plants help the work of retaining the detritus.
Soon the strong-rooted bushes and trees win a foothold, and by sending
their rootlets, which are at first small but rapidly enlarge, into the
crevices, they hasten the disruption of the stones.
If the construction of soil goes on upon a steep cliff, the quantity
retained on the slope may be small, but at the base we find a talus,
composed of the fragments not held by the vegetation, which gradually
increases as the cliff wears down, until the original precipice may be
quite obliterated beneath a soil slope. At first this process is
rapid; it becomes gradually slower and slower as the talus mounts up
the cliff and as the cliff loses its steepness, until finally a gentle
slope takes the place of the steep.
From the highest points in any river valley to the sea level the
broken-up rock, which we term soil, is in process of continuous
motion. Everywhere the rain water, flowing over the surface or soaking
through the porous mass, is conveying portions of the material which
is taken into solution in a speedy manner to the sea. Everywhere the
expansion of the soil in freezing, or the movements imposed on it by
the growth of roots, by the overturning of trees, or by the
innumerable borings and burrowings which animals make in the mass, is
through the action of gravitation slowly working down the slope. Every
little disturbance of the grains or fragments of the soil which lifts
them up causes them when they fall to descend a little way farther
toward the sea level. Working toward the streams, the materials of the
soil are in time delivered to those flowing waters, and by them urged
speedily, though in most cases interruptedly, toward the ocean.
There is another element in the movement of the soils which, though
less appreciable, is still of great importance. The agents of decay
which produce and remove the detritus, the chemical changes of the bed
rock, and the mechanical action which roots apply to them, along with
the solutional processes, are constantly lowering the surface of the
mass. In this way we can often prove that a soil continuously
existing has worked downward through many thousand feet of strata. In
this process of downgoing the country on which the layer rests may
have greatly changed its form, but the deposit, under favourable
conditions, may continue to retain some trace of the materials which
it derived from beds which have long since disappeared, their position
having been far up in the spaces now occupied by the air. Where the
slopes are steep and streams abound, we rarely find detritus which
belonged in rock more than a hundred feet above the present surface of
the soil. Where, however, as on those isolated table-lands or buttes
which abound in certain portions of the Mississippi Valley, as well as
in many other countries, we find a patch of soil lying on a nearly
level surface, which for geologic ages has not felt the effect of
streams, we may discover, commingled in the _débris_, the harder
wreckage derived from the decay of a thousand feet or more of vanished
strata.
When we consider the effect of organic life on the processes which go
on in the soil, we first note the large fact that the development of
all land vegetation depends upon the existence of this detritus--in a
word, on the slow movement of the decaying rocky matter from the point
where it is disrupted to its field of rest in the depths of the sea.
The plants take their food from the portion of this rocky waste which
is brought into solution by the waters which penetrate the mass. On
the plants the animals feed, and so this vast assemblage of organisms
is maintained. Not only does the land life maintain itself on the
soil, and give much to the sea, but it serves in various ways to
protect this detrital coating from too rapid destruction, and to
improve its quality. To see the nature of this work we should visit a
region where primeval forests still lie upon the slopes of a hilly
region. In the body of such a wood we find next the surface a coating
of decayed vegetable matter, made up of the falling leaves, bark,
branches, and trunks which are constantly descending to the earth.
Ordinarily, this layer is a foot or more in thickness; at the top it
is almost altogether composed of vegetable matter; at the bottom it
verges into the true soil. An important effect of this decayed
vegetation is to restrain the movement of the surface water. Even in
the heaviest rains, provided the mass be not frozen, the water is
taken into it and delivered in the manner of springs to the larger
streams. We can better note the measure of this effect by observing
the difference in the ground covered by this primeval forest and that
which we find near by which has been converted into tilled fields.
With the same degree of rapidity in the flow, the distinct stream
channels on the tilled ground are likely to be from twenty to a
hundred times in length what they are on the forest bed. The result is
that while the brook which drains the forested area maintains a
tolerably constant flow of clean water, the other from the tilled
ground courses only in times of heavy rain, and then is heavily
charged with mud. In the virgin conditions of the soil the downwear is
very slow; in its artificial state this wearing goes on so rapidly
that the sloping fields are likely to be worn to below the soil level
in a few score years.
Not only does the natural coating of vegetation, such as our forests
impose upon the country, protect the soil from washing away, but the
roots of the larger plants are continually at work in various ways to
increase the fertility and depth of the stratum. In the form of
slender fibrils these underground branches enter the joints and bed
planes of the rock, and there growing they disrupt the materials,
giving them a larger surface on which decay may operate. These bits,
at first of considerable size, are in turn broken up by the same
action. Where the underlying rocks afford nutritious materials, the
branches of our tap-rooted trees sometimes find their way ten feet or
more below the base of the true soil. Not only do they thus break up
the stones, but the nutrition which they obtain in the depths is
brought up and deposited in the parts above the ground, as well as in
the roots which lie in the true soil, so that when the tree dies it
becomes available for other plants. Thus in the forest condition of a
country the amount of rock material contributed to the deposit in
general so far exceeds that which is taken away to the rivers by the
underground water as to insure the deepening of the soil bed to the
point where only the strongest roots--those belonging to our
tap-rooted trees--can penetrate through it to the bed rocks.
Almost all forests are from time to time visited by winds which uproot
the trees. When they are thus rent from the earth, the underground
branches often form a disk containing a thick tangle of stones and
earth, and having a diameter of ten or fifteen feet. The writer has
frequently observed a hundred cubic feet of soil matter, some of it
taken from the depth of a yard or more, thus uplifted into the air. In
the path of a hurricane or tornado we may sometimes find thousands of
acres which have been subjected to this rude overturning--a natural
ploughing. As the roots rot away, the _débris_ which they held falls
outside of the pit, thus forming a little hillock along the side of
the cavity. After a time the thrusting action of other roots and the
slow motion of the soil down the slope restore the surface from its
hillocky character to its original smoothness; but in many cases the
naturalist who has learned to discern with his feet may note these
irregularities long after it has been recovered with the forest.
Great as is the effect of plants on the soil, that influence is almost
equalled by the action of the animals which have the habit of entering
the earth, finding there a temporary abiding place. The number of
these ground forms is surprisingly great. It includes, indeed, a host
of creatures which are efficient agents in enriching the earth. The
species of earthworms, some of which occupy forested districts as well
as the fields, have the habit of passing the soil material through
their bodies, extracting from the mass such nutriment as it may
contain. In this manner the particles of mineral matter become
pulverized, and in a measure affected by chemical changes in the
bodies of the creatures, and are thus better fitted to afford plant
food. Sometimes the amount of the earth which the creatures take in in
moving through their burrows and void upon the surface is sufficient
to form annually a layer on the surface of the ground having a depth
of one twentieth of an inch or more. It thus may well happen that the
soil to the depth of two or three feet is completely overturned in the
course of a few hundred years. As the particles which the creatures
devour are rather small, the tendency is to accumulate the finer
portions of the soil near the surface of the earth, where by solution
they may contribute to the needs of the lowly plants. It is probably
due to the action of these creatures that small relics of ancient men,
such as stone tools, are commonly found buried at a considerable depth
beneath the earth, and rarely appear upon the surface except where it
has been subjected to deep ploughing or to the action of running
streams.
Along with the earthworms, the ants labour to overturn the soil;
frequently they are the more effective of the two agents. The common
species, though they make no permanent hillocks, have been observed by
the writer to lay upon the surface each year as much as a quarter of
an inch of sand and other fine materials which they have brought up
from a considerable depth. In many regions, particularly in those
occupied by glacial drift, and pebbly alluvium along the rivers, the
effect of this action, like that of earthworms, is to bring to the
surface the finer materials, leaving the coarser pebbles in the
depths. In this way they have changed the superficial character of the
soil over great areas; we may say, indeed, over a large part of the
earth, and this in a way which fits it better to serve the needs of
the wild plants as well as the uses of the farmer.
Many thousand species of insects, particularly the larger beetles,
have the habit of passing their larval state in the under earth. Here
they generally excavate burrows, and thus in a way delve the soil. As
many of them die before reaching maturity, their store of organic
matter is contributed to the mass, and serves to nourish the plants.
If the student will carefully examine a section of the earth either in
its natural or in its tilled state, he will be surprised to find how
numerous the grubs are. They may often be found to the number of a
score or more of each cubic foot of material. Many of the species
which develop underground come from eggs which have carefully been
encased in organic matter before their deposition in the earth. Thus
some of the carrion beetles are in the habit of laying their eggs in
the bodies of dead birds or field mice, which they then bury to the
depth of some inches in the earth. In this way nearly all the small
birds and mammals of our woods disappear from view in a few hours
after they are dead. Other species make balls from the dung of cattle
in which they lay their eggs, afterward rolling the little spheres, it
may be for hundreds of feet, to the chambers in the soil which they
have previously prepared. In this way a great deal of animal matter is
introduced into the earth, and contributes to its fertility.
Many of our small mammals have the habit of making their dwelling
places in the soil. Some of them, such as the moles, normally abide in
the subterranean realm for all their lives. Others use the excavations
as places of retreat. In any case, these excavations serve to move the
particles of the soil about, and the materials which the animals drag
into the earth, as well as the excrement of the creatures, act to
enrich it. This habit of taking food underground is not limited to the
mammals; it is common with the ants, and even the earthworms, as noted
by Charles Darwin in his wonderful essay on these creatures, are
accustomed to drag into their burrows bits of grass and the slender
leaves of pines. It is not known what purpose they attain by these
actions, but it is sufficiently common somewhat to affect the
conditions of the soil.
The result of these complicated works done by animals and plants on
the soil is that the material to a considerable depth are constantly
being supplied with organic matter, which, along with the mineral
material, constitutes that part of the earth which can support
vegetation. Experiment will readily show that neither crushed rock nor
pure vegetable mould will of itself serve to maintain any but the
lowliest vegetation. It requires that the two materials be mixed in
order that the earth may yield food for ordinary plants, particularly
for those which are of use to man, as crops. On this account all the
processes above noted whereby the waste of plant and animal life is
carried below the surface are of the utmost importance in the creation
and preservation of the soil. It has been found, indeed, in almost all
cases, necessary for the farmer to maintain the fertility of his
fields to plough-in quantities of such organic waste. By so doing he
imitates the work which is effected in virgin soil by natural action.
As the process is costly in time and material, it is often neglected
or imperfectly done, with the result that the fields rapidly diminish
in fertility.
The way in which the buried organic matter acts upon the soil is not
yet thoroughly understood. In part it accomplishes the results by the
materials which on its decay it contributes to the soil in a state in
which they may readily be dissolved and taken up by the roots into
their sap; in part, however, it is believed that they better the
conditions by affording dwelling places for a host of lowly species,
such as the forms which are known as bacteria. The organisms probably
aid in the decomposition of the mineral matter, and in the conversion
of nitrogen, which abounds in the air or the soil, into nitrates of
potash and soda--substances which have a very great value as
fertilizers. Some effect is produced by the decay of the foreign
matter brought into the soil, which as it passes away leaves channels
through which the soil water can more readily pass.
By far the most general and important effect arising from the decay
of organic matter in the earth is to be found in the carbon dioxide
which is formed as the oxygen of the air combines with the carbon
which all organic material contains. As before noted, water thus
charged has its capacity for taking other substances into solution
vastly increased, and on this solvent action depends in large part the
decay of the bed rocks and the solution of materials which are to be
appropriated by the plants.
Having now sketched the general conditions which lead to the formation
of soils, we must take account of certain important variations in
their conditions due to differences in the ways in which they are
formed and preserved. These matters are not only of interest to the
geologist, but are of the utmost importance to the life of mankind, as
well as all the lower creatures which dwell upon the lands. First, we
should note that soils are divisible into three great groups, which,
though not sharply parted from each other, are sufficiently peculiar
for the purposes of classification. Where the earth material has been
derived from the rocks which nearly or immediately underlie it, we
have a group of soils which may be entitled those of immediate
derivation--that is, derived from rocks near by, or from beds which
once overlaid the level and have since been decayed away. Next, we
have alluvial soils, those composed of materials which have been
transported by streams, commonly from a great distance, and laid down
on their flood plains. Third, the soils the mineral matters of which
have been brought into their position by the action of glaciers; these
in a way resemble those formed by rivers, but the materials are
generally imperfectly sorted, coarse and fine being mingled together.
Last of all, we have the soils due to the accumulation of blown dust
or blown sand, which, unlike the others, occupy but a small part of
the land surface. It would be possible, indeed, to make yet another
division, including those areas which when emerging from the sea were
covered with fine, uncemented detritus ready at once to serve the
purposes of a soil. Only here and there, and but seldom, do we find
soils of this nature.
It is characteristic of soils belonging to the group to which we have
given the title of immediate derivation that they have accumulated
slowly, that they move very gradually down the slopes on which they
lie, and that in all cases they represent, with a part of their mass
at least, levels of rock which have disappeared from the region which
they occupied. The additions made to their mass are from below, and
that mass is constantly shrinking, generally at a pretty rapid rate,
by the mineral matter which is dissolved and goes away with the spring
water. They also are characteristically thin on steep slopes,
thickening toward the base of the incline, where the diminished grade
permits the soil to move slowly, and therefore to accumulate.
In alluvial soils we find accumulations which are characterized by
growth on their upper surfaces, and by the distant transportation of
the materials of which they are composed. In these deposits the
outleaching removes vast amounts of the materials, but so long as the
floods from time to time visit their surfaces the growth of the
deposits is continued. This growth rarely takes place from the waste
of the bed rocks on which the alluvium lies. It is characteristic of
alluvial soils that they are generally made up of _débris_ derived
from fields where the materials have undergone the change which we
have noted in the last paragraph; therefore these latter deposits have
throughout the character which renders the mineral materials easily
dissolved. Moreover, the mass as it is constructed is commonly mingled
with a great deal of organic waste, which serves to promote its
fertility. On these accounts alluvial grounds, though they vary
considerably in fertility, commonly afford the most fruitful fields of
any region. They have, moreover, the signal advantage that they often
may be refreshed by allowing the flood waters to visit them, an
action which but for the interference of man commonly takes place once
each year. Thus in the valley of the Nile there are fields which have
been giving rich grain harvests probably for more than four thousand
years, without any other effective fertilizing than that derived from
the mud of the great river.
The group of glaciated soils differs in many ways from either of those
mentioned. In it we find the mineral matter to have been broken up,
transported, and accumulated without the influence of those conditions
which ordinarily serve to mix rock _débris_ with organic matter during
the process by which it is broken into bits. When vegetation came to
preoccupy the fields made desolate by glacial action, it found in most
places more than sufficient material to form soils, but the greater
part of the matter was in the condition of pebbles of very hard rock
and sand grains, fragments of silex. Fortunately, the broken-up state
of this material, by exposing a great surface of the rocky matter to
decay, has enabled the plants to convert a portion of the mass into
earth fit for the uses of their roots. But as the time which has
elapsed since the disappearance of the glaciers is much less than that
occupied in the formation of ordinary soil, this decay has in most
cases not yet gone very far, so that in a cubic foot of glaciated
waste the amount of material available for plants is often only a
fraction of that held in the soils of immediate derivation.
In the greater portion of the fields occupied by glacial waste the
processes which lead to the introduction of organic matter into the
earth have not gone far enough to set in effective work the great
laboratory which has to operate in order to give fertile soil. The
pebbles hinder the penetration of the roots as well as the movement of
insects and other animals. There has not been time enough for the
overturning of trees to bring about a certain admixture of vegetable
matter with the soil--in a word, the process of soil-making, though
the first condition, that of broken-up rock, has been accomplished,
is as yet very incomplete. It needs, indeed, care in the introduction
of organic matter for its completion.
It is characteristic of glacial soils that they are indefinitely deep.
This often is a disadvantageous feature, for the reason that the soil
water may pass so far down into the earth that the roots are often
deprived of the moisture which they need, and which in ordinary soils
is retained near the surface by the hard underlayer. On the other
hand, where the glacial waste is made up of pebbles formed from rocks
of varied chemical composition, which contain a considerable share of
lime, potash, soda, and other substances which are required by plants,
the very large surface which they expose to decay provides the soil
with a continuous enrichment. In a cubic foot of pebbly glacial earth
we often find that the mass offers several hundred times as much
surface to the action of decay as is afforded by the underlying solid
bed rock from which a soil of immediate derivation has to win its
mineral supply. Where the pebbly glacial waste is provided with a
mixture of vegetable matter, the process of decay commonly goes
forward with considerable rapidity. If the supply of such matter is
large, such as may be produced by ploughing in barnyard manure or
green crops, the nutritive value of the earth may be brought to a very
high point.
It is a familiar experience in regions where glacial soils exist that
the earth beneath the swamps when drained is found to be
extraordinarily well suited for farming purposes. On inspecting the
pebbles from such places, we observe that they are remarkably decayed.
Where the masses contain large quantities of feldspar, as is the case
in the greater part of our granitic and other crystalline rocks, this
material in its decomposition is converted into kaolin or feldspar
clay, and gives the stones a peculiar white appearance, which marks
the decomposition, and indicates the process by which a great variety
of valuable soil ingredients are brought into a state where they may
be available for plants.
In certain parts of the glacial areas, particularly in the region near
the margin of the ice sheet, where the glacier remained in one
position for a considerable time, we find extensive deposits of
silicious sand, formed of the materials which settled from the
under-ice stream, near where they escaped from the glacial cavern.
These kames and sand plains, because of the silicious nature of their
materials and the very porous nature of the soil which they afford,
are commonly sterile, or at most render a profit to the tiller by dint
of exceeding care. Thus in Massachusetts, although the first settlers
seized upon these grounds, and planted their villages upon them
because the forests there were scanty and the ground free from
encumbering boulders, were soon driven to betake themselves to those
areas where the drift was less silicious, and where the pebbles
afforded a share of clay. Very extensive fields of this sandy nature
in southeastern New England have never been brought under tillage.
Thus on the island of Martha's Vineyard there is a connected area
containing about thirty thousand acres which lies in a very favourable
position for tillage, but has been found substantially worthless for
such use. The farmers have found it more advantageous to clear away
the boulders from the coarser drift in order to win soil which would
give them fair returns.
Those areas which are occupied by soil materials which have been
brought into their position by the action of the wind may, as regards
their character, be divided into two very distinct groups--the dunes
and loess deposits. In the former group, where, as we have noted (see
page 123), the coarse sea sands or those from the shores of lakes are
driven forward as a marching hillock, the grains of the material are
almost always silicious. The fragments in the motion are not taken up
into the air, but are blown along the surface. Such dune accumulations
afford an earth which is even more sterile than that of the glacial
sand plains, where there is generally a certain admixture of pebbles
from rocks which by their decomposition may afford some elements of
fertility. Fortunately for the interests of man, these wind-borne
sands occupy but a small area; in North America, in the aggregate,
there probably are not more than one thousand square miles of such
deposits.
Where the rock material drifted by the winds is so fine that it may
rise into the air in the form of dust, the accumulations made of it
generally afford a fertile soil, and this for the reason that they are
composed of various kinds of rock, and not, as in the case of dunes,
of nearly pure silica. In some very rare cases, where the seashore is
bordered by coral reefs, as it is in parts of southern Florida, and
the strand is made up of limestone bits derived from the hard parts
which the polyps secrete, small dunes are made of limy material.
Owing, however, in part to the relatively heavy nature of this
substance, as well as to the rapid manner in which its grains become
cemented together, such limestone dunes never attain great size nor
travel any distance from their point of origin.
As before noted, dust accumulations form the soil in extended areas
which lie to the leeward of great deserts. Thus a considerable part of
western China and much of the United States to the west of the
Mississippi is covered by these wind-blown earths. Wherever the
rainfall is considerable these loess deposits have proved to have a
high agricultural value.
Where a region has an earth which has recently passed from beneath the
sea or a great lake, the surface is commonly covered by incoherent
detritus which has escaped consolidation into hard rock by the fact
that it has not been buried and thus brought into the laboratory of
the earth's crust. When such a region becomes dry land, the materials
are immediately ready to enter into the state of soil. They commonly
contain a good deal of waste derived from the organic life which
dwelt upon the sea bottom and was embedded in the strata as they were
formed. Where these accumulations are made in a lake, the land
vegetation at once possesses the field, even a single year being
sufficient for it to effect its establishment. Where the lands emerge
from the sea, it requires a few years for the salt water to drain away
so that the earth can be fit for the uses of plants. In a general way
these sea-bottom soils resemble those formed in the alluvial plains.
They are, however, commonly more sandy, and their substances less
penetrated by that decay which goes on very freely in the atmosphere
because of the abundant supply of oxygen, and but slowly on the sea
floor. Moreover, the marine deposits are generally made up in large
part of silicious sand, a material which is produced in large
quantities by the disruption of the rocks along the sea coast. The
largest single field of these ocean-bottom soils of North America is
found in the lowland region of the southern United States, a wide belt
of country extending along the coast from the Rio Grande to New York.
Although the streams have channelled shallow valleys in the beds of
this region, the larger part of its surface still has the peculiar
features of form and composition which were impressed upon it when it
lay below the surface of the sea.
Local variations in the character of the soil covering are exceedingly
numerous, and these differences of condition profoundly affect the
estate of man. We shall therefore consider some of the more important
of these conditions, with special reference to their origin.
The most important and distinctly marked variation in the fertility of
soils is that which is produced by differences in the rainfall. No
parts of the earth are entirely lacking in rain, but over considerable
areas the precipitation does not exceed half a foot a year. In such
realms the soil is sterile, and the natural coating of vegetation
limited to those plants which can subsist on dew or which can take on
an occasional growth at such times as moisture may come upon them.
With a slight increase in precipitation, the soil rapidly increases in
productivity, so that we may say that where as much as about ten
inches of water enters the earth during the summer half of the year,
it becomes in a considerable measure fit for agriculture. Observations
indicate that the conditions of fertility are not satisfied where the
rainfall is just sufficient to fill the pores of the soil; there must
be enough water entering the earth to bring about a certain amount of
outflow in the form of springs. The reason of this need becomes
apparent when we study the evident features of those soils which,
though from season to season charged with water, do not yield springs,
but send the moisture away through the atmosphere. Wherever these
conditions occur we observe that the soil in dry seasons becomes
coated with a deposit of mineral matter, which, because of its taste,
has received the name of alkali. The origin of this coating is as
follows: The pores of the soil, charged from year to year with
sufficient water to fill them, become stored with a fluid which
contains a very large amount of dissolved mineral matter--too much,
indeed, to permit the roots of plants, save a few species which have
become accustomed to the conditions, to do their appointed work. In
fact, this water is much like that of the sea, which the roots of only
a few of our higher plants can tolerate. When the dry season comes on,
the heat of the sun evaporates the water at the surface, leaving
behind a coating composed of the substances which the water contains.
The soil below acts in the manner of a lamp-wick to draw up fluid as
rapidly as the heat burns it away. When the soil water is as far as
possible exhausted, the alkali coating may represent a considerable
part of the soluble matter of the soil, and in the next rainy season
it may return in whole or in part to the under-earth, again to be
drawn in the manner before described to the upper level. It is
therefore only when a considerable share of the ground water goes
forth to the streams in each year that the alkaline materials are in
quantity kept down to the point where the roots of our crop-giving
plants can make due use of the soil. Where, in an arid region, the
ground can be watered from the enduring streams or from artificial
reservoirs, the main advantage arising from the process is commonly
found in the control which it gives the farmer in the amount of the
soil water. He can add to the rainfall sufficient to take away the
excess of mineral matter. When such soils are first brought under
tillage it is necessary to use a large amount of water from the
canals, in order to wash away the old store of alkali. After that a
comparatively small contribution will often keep the soil in excellent
condition for agriculture. It has been found, however, in the
irrigated lands beside the Nile that where too much saving is
practised in the irrigation, the alkaline coating will appear where it
has been unknown before, and with it an unfitness of the earth to bear
crops.
Although the crust of mineral matters formed in the manner above
described is characteristic of arid countries, and in general peculiar
to them, a similar deposit may under peculiar conditions be formed in
regions of great rainfall. Thus on the eastern coast of New England,
where the tidal marshes have here and there been diked from the sea
and brought under tillage, the dissolved mineral matters of the soil,
which are excessive in quantity, are drawn to the surface, forming a
coating essentially like that which is so common in arid regions. The
writer has observed this crust on such diked lands, having a thickness
of an eighth of an inch. In fact, this alkali coating represents
merely the extreme operation of a process which is going on in all
soils, and which contributes much to their fertility. When rain falls
and passes downward into the earth, it conveys the soluble matter to a
depth below the surface, often to beyond the point where our ordinary
crop plants, such as the small grains, can have access to it, and
this for the reason that their roots do not penetrate deeply. When dry
weather comes and evaporation takes place from the surface, the fluid
is drawn up to the upper soil layer, and there, in process of
evaporation, deposits the dissolved materials which it contains. Thus
the mineral matter which is fit for plant food is constantly set in
motion, and in its movement passes the rootlets of the plants. It is
probably on this account--at least in part--that very wet weather is
almost as unfavourable to the farmer as exceedingly dry, the normal
alternation in the conditions being, as is well known, best suited to
his needs.
So long as the earth is subjected to conditions in which the rainfall
may bring about a variable amount of water in the superficial detrital
layer, we find normal fruitful soils, though in their more arid
conditions they may be fit for but few species of plants. When, by
increasing aridity, we pass to conditions where there is no tolerably
permanent store of water in the _débris_, the material ceases to have
the qualities of a soil, and becomes mere rock waste. At the other
extreme of the scale we pass to conditions where the water is
steadfastly maintained in the interstices of the detritus, and there
again the characteristic of the soil and its fitness for the uses of
land vegetation likewise disappear. In a word, true soil conditions
demand the presence of moisture, but that in insufficient quantities,
to keep the pores of the earth continually filled; where they are thus
filled, we have the condition of swamps. Between these extremes the
level at which the water stands in the soil in average seasons is
continually varying. In rainy weather it may rise quite to the
surface; in a dry season it may sink far down. As this water rises and
falls, it not only moves, as before noted, the soluble mineral
materials, but it draws the air into and expels it from the earth with
each movement. This atmospheric circulation of the soil, as has been
proved by experiment, is of great importance in maintaining its
fertility; the successive charges of air supply the needs of the
microscopic underground creatures which play a large part in enriching
the soil, and the direct effect of the oxygen in promoting decay is
likewise considerable. A part of the work which is accomplished by
overturning the earth in tillage consists in this introduction of the
air into the pores of the soil, where it serves to advance the actions
which bring mineral matters into solution.
[Illustration: _Mountain gorge, Himalayas, India. Note the difference
in the slope of the eroded rocks and the effect of erosion upon them;
also the talus slopes at the base of the cliffs which the torrent is
cutting away. On the left of the foreground there is a little bench
showing a recent higher line of the water._]
In the original conditions of any country which is the seat of
considerable rainfall, and where the river system is not so far
developed as to provide channels for the ready exit of the waters, we
commonly find very extensive swamps; these conditions of bad drainage
almost invariably exist where a region has recently been elevated
above the level of the sea, and still retains the form of an irregular
rolling plain common to sea floors, and also in regions where the work
done by glaciers has confused the drainage which the antecedent
streams may have developed. In an old, well-elaborated river system
swamps are commonly absent, or, if they occur, are due to local
accidents of an unimportant nature.
For our purpose swamps may be divided into three groups--climbing
bogs, lake bogs, and marine marshes. The first two of these groups
depend on the movements of the rain water over the land; the third on
the action of the tides. Beginning our account with the first and most
exceptional of these groups, we note the following features in their
interesting history:
Wherever in a humid region, on a gentle slope--say with an inclination
not exceeding ten feet to the mile--the soil is possessed by any
species of plants whose stems grow closely together, so that from
their decayed parts a spongelike mass is produced, we have the
conditions which favour the development of climbing bogs. Beginning
usually in the shores of a pool, these plants, necessarily of a
water-loving species, retain so much moisture in the spongy mass
which they form that they gradually extend up the slope. Thus
extending the margin of their field, and at the same time thickening
the deposit which they form, these plants may build a climbing bog
over the surface until steeps are attained where the inclination is so
great that the necessary amount of water can not be held in the spongy
mass, or where, even if so held, the whole coating will in time slip
down in the manner of an avalanche.
The greater part of the climbing bogs of the world are limited to the
moist and cool regions of high latitudes, where species of moss
belonging to the genus _Sphagnum_ plentifully flourish. These plants
can only grow where they are continuously supplied with a bath of
water about their roots. They develop in lake bogs as far south as
Mexico, but in the climbing form they are hardly traceable south of
New England, and are nowhere extensively developed within the limits
of the United States. In more northern parts of this continent, and in
northwestern Europe, particularly in the moist climate of Ireland,
climbing bogs occupy great areas, and hold up their lakes of
interstitially contained water over the slopes of hills, where the
surface rises at the rate of thirty feet or more to the mile. So long
as the deposit of decayed vegetable matter which has accumulated in
this manner is thin, therefore everywhere penetrated by the fibrous
roots of the moss, it may continue to cling to its sloping bed; but
when it attains a considerable thickness, and the roots in the lower
part decay, the pulpy mass, water-laden in some time of heavy rain,
break away in a vast torrent of thick, black mud, which may inundate
the lower lands, causing widespread destruction.
In more southern countries, other water-loving plants lead to the
formation of climbing bogs. Of these, the commonest and most effective
are the species of reeds, of which our Indian cane is a familiar
example. Brakes of this vegetation, plentifully mingled with other
species of aquatic growth, form those remarkable climbing bogs known
as the Dismal and other swamps, which numerously occur along the coast
line of the United States from southern Maryland to eastern Texas.
Climbing bogs are particularly interesting, not only from the fact
that they are eminently peculiar effects of plant growth, but because
they give us a vivid picture of those ancient morasses in which grew
the plants that formed the beds of vegetable matter now appearing in
the state of coal. Each such bed of buried swamp material was, with
rare exceptions, where the accumulation took place in lakes, gathered
in climbing bogs such as we have described.
Lake bogs occur in all parts of the world, but in their best
development are limited to relatively high latitudes, and this for the
reason that the plants which form vegetable matter grow most
luxuriantly in cool climates and in regions where the level of the
basin is subject to less variation than occurs in the alternating wet
and dry seasons which exist in nearly all tropical regions. The
fittest conditions are found in glaciated regions, where, as before
noted, small lakes are usually very abundant. On the shores of one of
these pools, of size not so great that the waves may attain a
considerable height, or in the sheltered bay of a larger lake, various
aquatic plants, especially the species of pond lilies, take root upon
the bottom, and spread their expanded leaves on the surface of the
water. These flexible-leaved and elastic-stemmed plants can endure
waves which attain no more than a foot or two of height, and by the
friction which they afford make the swash on the shore very slight. In
the quiet water, rushes take root, and still further protect the
strand, so that the very delicate vegetation of the mosses, such as
the _Sphagnum_, can fix itself on the shore.
As soon as the _Sphagnum_ mat has begun its growth, the strength given
by its interlaced fibres enables it to extend off from the shore and
float upon the water. In this way it may rapidly enlarge, if not
broken up by the waves, so that its front advances into the lake at
the rate of several inches each year. While growing outwardly it
thickens, so that the bottom of the mass gradually works down toward
the floor of the basin. At the same time the lower part of the sheet,
decaying, contributes a shower of soft peat mud to the floor of the
lake. In this way, growing at its edge, deepening, and contributing to
an upgrowth from the bottom, a few centuries may serve entirely to
fill a deep basin with peaty accumulation. In general, however, the
surface of the bog closes over the lake before the accumulation has
completely filled the shoreward portions of the area. In these
conditions we have what is familiarly known as a quaking bog, which
can be swayed up and down by a person who quickly stoops and rises
while standing on the surface. In this state the tough and thick sheet
of growing plants is sufficient to uphold a considerable weight, but
so elastic that the underlying water can be thrown into waves. Long
before the bog has completely filled the lake with the peaty
accumulations the growth of trees is apt to take place on its surface,
which often reduces the area to the appearance of a very level wet
wood.
[Illustration: Fig. 17.--Diagram showing beginning of peat bog: A,
lake; B, lilies and rushes; C, lake bog; D, climbing bog.]
Climbing and lake bogs in the United States occupy a total area of
more than fifty thousand square miles. In all North America the total
area is probably more than twice as great. Similar deposits are
exceedingly common in the Eurasian continent and in southern
Patagonia. It is probable that the total amount of these fields in
different parts of the world exceeds half a million square miles.
These two groups of fresh-water swamps have an interest, for the
reason that when reduced to cultivation by drainage and by subsequent
removal of the excess of peaty matter, by burning or by natural decay,
afford very rich soil. The fairest fields of northern Europe,
particularly in Great Britain and Ireland, have been thus won to
tillage. In the first centuries of our era a large part of
England--perhaps as much as one tenth of the ground now tilled in that
country--was occupied by these lands, which retained water in such
measure as to make them unfit for tillage, the greater portion of this
area being in the condition of thin climbing bog. For many centuries
much of the energy of the people was devoted to the reclamation of
these valuable lands. This task of winning the swamp lands to
agriculture has been more completely accomplished in England than
elsewhere, but it has gone far on the continent of Europe,
particularly in Germany. In the United States, owing to the fact that
lands have been cheap, little of this work of swamp-draining has as
yet been accomplished. It is likely that the next great field of
improvement to be cultivated by the enterprising people will be found
in these excessively humid lands, from which the food-giving resources
for the support of many million people can be won.
[Illustration: Fig. 18.--Diagram showing development of swamp: A,
remains of lake; B, surface growth; c, peat.]
The group of marine marshes differs in many important regards from
those which are formed in fresh water. Where the tide visits any
coast line, and in sheltered positions along that shore, a number of
plants, mostly belonging to the group of grasses, species which have
become accustomed to having their roots bathed by salt water, begin
the formation of a spongy mat, which resembles that composed of
_Sphagnum_, only it is much more solid. This mat of the marine marshes
soon attains a thickness of a foot or more, the upper or growing
surface lying in a position where it is covered for two or three hours
at each visit of the tide. Growing rapidly outward from the shore, and
having a strength which enables it to resist in a tolerably effective
manner waves not more than two or three feet high, this accumulation
makes head against the sea. To a certain extent the waves undermine
the front of the sheet and break up masses of it, which they
distribute over the shallow bottom below the level at which these
plants can grow. In this deeper water, also, other marine animals and
plants are continually developing, and their remains are added to the
accumulations which are ever shallowing the water, thus permitting a
further extension of the level, higher-lying marsh. This process
continues until the growth has gone as far as the scouring action of
the tidal currents will permit. In the end the bay, originally of
wide-open water, is only such at high tide. For the greater part of
the time it appears as broad savannas, whose brilliant green gives
them the aspect of rare fertility.
Owing to the conditions of their growth, the deposits formed in marine
marshes contain no distinct peat, the nearest approach to that
substance being the tangle of wirelike roots which covers the upper
foot or so of the accumulation. The greater part of the mass is
composed of fine silt, brought in by the streams of land water which
discharge into the basin, and by the remains of animals which dwelt
upon the bottom or between the stalks of the plants that occupy the
surface of the marshes. These interspaces afford admirable shelter to
a host of small marine forms. The result is, that the tidal marshes,
as well as the lower-lying mud flats, which have been occupied by the
mat of vegetation, afford admirable earth for tillage. Unfortunately,
however, there are two disadvantages connected with the redemption of
such lands. In the first place, it is necessary to exclude the sea
from the area, which can only be accomplished by considerable
engineering work; in the second place, the exclusion of the tide
inevitably results in the silting up of the passage by which the water
found its way to the sea. As these openings are often used for
harbours, the effect arising from their destruction is often rather
serious. Nevertheless, in some parts of the world very extensive and
most fertile tracts of land have thus been won from the sea; a large
part of Holland and shore-land districts in northern Europe are made
up of fields which were originally covered by the tide. Near the mouth
of the Rhine, indeed, the people have found these sea-bottom soils so
profitable that they have gone beyond the zone of the marshes, and
have drained considerable seas which of old were permanently covered,
even at the lowest level of the waters.
On the coast of North America marine marshes have an extensive
development, and vary much in character. In the Bay of Fundy, where
the tides have an altitude of fifty feet or more, the energy of their
currents is such that the marsh mat rarely forms. Its place, however,
is taken by vast and ever-changing mud flats, the materials of which
are swept to and fro by the moving waters. The people of this region
have learned an art of a peculiar nature, by which they win broad
fields of excellent land from the sea. Selecting an area of the flats,
the surface of which has been brought to within a few feet of high
tide, they inclose it with a stout barrier or dike, which has openings
for the free admission of the tidal waters. Entering this basin, the
tide, moving with considerable velocity, bears in quantities of
sediment. In the basin, the motion being arrested, this sediment
falls to the bottom, and serves to raise its level. In a few months
the sheet of sediment is brought near the plane of the tidal movement,
then the gates are closed at times when the tide has attained half of
its height, so that the ground within the dike is not visited by the
sea water, and can be cultivated.
[Illustration: Fig. 19.--Map of Ipswich marshes, Massachusetts, formed
behind a barrier beach.]
Along the coast of New England the ordinary marine marshes attain an
extensive development in the form of broad-grassed savannas. With this
aspect, though with a considerable change in the plants which they
bear, the fringe of savannas continues southward along the coast to
northern Florida. In the region about the mouth of the Savannah River,
so named from the vast extent of the tidal marshes, these fields
attain their greatest development. In central and southern Florida,
however, where the seacoast is admirably suited for their development,
these coastal marshes of the grassy type disappear, their place being
taken by the peculiar morasses formed by the growth of the mangrove
tree.
In the mangrove marshes the tree which gives the areas their name
covers all the field which is visited by the tide. This tree grows
with its crown supported on stiltlike roots, at a level above high
tide. From its horizontal branches there grow off roots, which reach
downward into the water, and thence to the bottom. The seeds of the
mangrove are admirably devised so as to enable the plant to obtain a
foothold on the mud flats, even where they are covered at low tide
with a depth of two or three feet of water. They are several inches in
length, and arranged with booklets at their lower ends; floating near
the bottom, they thus catch upon it, and in a few weeks' growth push
the shoot to the level of the water, thus affording a foundation for a
new plantation. In this manner, extending the old forests out into the
shallow water of the bays, and forming new colonies wherever the water
is not too deep, these plants rapidly occupy all the region which
elsewhere would appear in the form of savannas.
[Illustration: Fig. 20.--Diagram showing mode of growth of mangroves.]
The tidal marshes of North America, which may be in time converted to
the uses of man, probably occupy an area exceeding twenty thousand
square miles. If the work of reclaiming such lands from the sea ever
attains the advance in this country that it has done in Holland, the
area added to the dry land by engineering devices may amount to as
much as fifty thousand square miles--a territory rather greater than
the surface of Kentucky, and with a food-yielding power at least five
times as great as is afforded by that fertile State. In fact, these
conquests from the sea are hereafter to be among the great works which
will attract the energies of mankind. In the arid region of the
Cordilleras, as well as in many other countries, the soil, though
destitute of those qualities which make it fit for the uses of man,
because of the absence of water in sufficient amount, is, as regards
its structure and depth, as well as its mineral contents, admirably
suited to the needs of agriculture. The development of soils in desert
regions is in almost all cases to be accounted for by the former
existence in the realms they occupy of a much greater rainfall than
now exists. Thus in the Rocky Mountain country, when the deep soils
of the ample valleys were formed, the lakes, as we have before noted,
were no longer dead seas, as is at present so generally the case, but
poured forth great streams to the sea. Here, as elsewhere, we find
evidence that certain portions of the earth which recently had an
abundant rainfall have now become starved for the lack of that supply.
All the soils of arid regions where the trial has been made have
proved very fertile when subjected to irrigation, which can often be
accomplished by storing the waters of the brief rainy season or by
diverting those of rivers which enter the deserts from well-watered
mountain fields. In fact, the soil of these arid realms yields
peculiarly ample returns to the husbandman, because of certain
conditions due to the exceeding dryness of the air. This leads to an
absence of cloudy weather, so that from the time the seed is planted
the growth is stimulated by uninterrupted and intense sunshine. The
same dryness of the air leads, as we have seen, to a rapid evaporation
from the surface, by which, in a manner before noted, the dissolved
mineral matter is brought near the top of the soil, where it can best
serve the greater part of our crop plants. On these accounts an acre
of irrigated soil can be made to yield a far greater return than can
be obtained from land of like chemical composition in humid regions.
In many parts of the world, particularly in the northern and western
portions of the Mississippi Valley, there are widespread areas, which,
though moderately well watered, were in their virgin state almost
without forests. In the prairie region the early settlers found the
country unwooded, except along the margins of the streams. On the
borders of the true prairies, however, they found considerable areas
of a prevailingly forested land, with here and there a tract of
prairie. There were several of these open fields south of the Ohio,
though the country there is in general forested; one of these prairie
areas, in the Green River district of Kentucky, was several thousand
square miles in extent. At first it was supposed that the absence of
trees in the open country of the Mississippi Valley was due to some
peculiarity of the soil, but experience shows that plantations
luxuriantly develop, and that the timber will spread rapidly in the
natural way. In fact, if the seeds of the trees which have been
planted since the settlement of the country were allowed to develop as
they seek to do, it would only be a few centuries before the region
would be forest-clad as far west as the rainfall would permit the
plants to develop. Probably the woods would attain to near the
hundredth meridian.
In the opinion of the writer, the treeless character of the Western
plains is mainly to be accounted for by the habit which our Indians
had of burning the herbage of a lowly sort each year, so that the
large game might obtain better pasturage. It is a well-known fact to
all those who have had to deal with cattle on fields which are in the
natural state that fire betters the pasturage. Beginning this method
of burning in the arid regions to the west of the original forests,
the natural action of the fire has been gradually to destroy these
woods. Although the older and larger trees, on account of their thick
bark and the height of their foliage above the ground, escaped
destruction, all the smaller and younger members of the species were
constantly swept away. Thus when the old trees died they left no
succession, and the country assumed its prairie character. That the
prairies were formed in this manner seems to be proved by the
testimony which we have concerning the open area before mentioned as
having existed in western Kentucky. It is said that around the
timberless fields there was a wide fringe of old fire-scarred trees,
with no undergrowth beneath their branches, and that as they died no
kind of large vegetation took their place. When the Indians who set
these fires were driven away, as was the case in the last decade of
the last century, the country at once began to resume its timbered
condition. From the margin and from every interior point where the
trees survived, their seeds spread so that before the open land was
all subjugated to the plough it was necessary in many places to clear
away a thick growth of the young forest-building trees.
The soils which develop on the lavas and ashes about an active volcano
afford interesting subjects for study, for the reason that they show
how far the development of the layer which supports vegetation may
depend upon the character of the rocks from which it is derived. Where
the materials ejected from a volcano lie in a rainy district, the
process of decay which converts the rock into soil is commonly very
rapid, a few years of exposure to the weather being sufficient to
bring about the formation of a fertile soil. This is due to the fact
that most lavas, as well as the so-called volcanic ashes, which are of
the same material as the lavas, only blown to pieces, are composed of
varied minerals, the most of which are readily attacked by the agents
of decay. Now and then, however, we find the materials ejected from a
particular volcano, or even the lavas and ashes of a single eruption,
in such a chemical state that soils form upon them with exceeding
slowness.
* * * * *
The foregoing incomplete considerations make it plain that the
soil-covering of the earth is the result of very delicate adjustments,
which determine the rate at which the broken-down rocks find their
path from their original bed places to the sea. The admirable way in
which this movement is controlled is indicated by the fact that almost
everywhere we find a soil-covering deep enough for the use of a varied
vegetation, but rarely averaging more than a dozen feet in depth. Only
here and there are the rocks bare or the earth swathed in a profound
mass of detritus. This indicates how steadfast and measured is the
march of the rock waste from the hills to the sea. Unhappily, man,
when by his needs he is forced to till the soil, is compelled to break
up this ancient and perfect order. He has to strip the living mantle
from the earth, replacing it with growth of those species which serve
his needs. Those plants which are most serviceable--which are, indeed,
indispensable in the higher civilization, the grains--require for
their cultivation that the earth be stripped bare and deeply stirred
during the rainy season, and thus subjected to the most destructive
effect of the rainfall. The result is, that in almost all grain fields
the rate of soil destruction vastly surpasses that at which the
accumulation is being made. We may say, indeed, that, except in
alluvial plains, where the soil grows by flood-made additions to its
upper surface, no field tilled in grain can without exceeding care
remain usable for a century. Even though the agriculturist returns to
the earth all the chemical substances which he takes away in his
crops, the loss of the soil by the washing away of its substance to
the stream will inevitably reduce the region to sterility.
It is not fanciful to say that the greatest misfortune which in a
large way man has had to meet in his agriculture arises from this
peculiar stress which grain crops put upon the soil. If these grains
grew upon perennial plants, in the manner of our larger fruits, the
problem of man's relation to the soil would be much simpler than it is
at present. He might then manage to till the earth without bringing
upon it the inevitable destruction which he now inflicts. As it is, he
should recognise that his needs imperil this ancient and precious
element in the earth's structure, and he should endeavour in every
possible way to minimize the damage which he brings about. This result
he may accomplish in certain simple ways.
First, as regards the fertility of the soil, as distinguished from the
thickness of the coating, it may be said that modern discoveries
enable us to see the ways whereby we may for an indefinite period
avoid the debasement of our great heritage, the food-giving earth. We
now know in various parts of the world extensive and practically
inexhaustible deposits, whence may be obtained the phosphates,
potash, soda, etc., which we take from the soil in our crops. We also
have learned ways in which the materials contained in our sewage may
be kept from the sea and restored to the fields. In fact, the recent
developments of agriculture have made it not only easy, but in most
cases profitable, to avoid this waste of materials which has reduced
so many regions to poverty. We may fairly look forward to the time,
not long distant, when the old progressive degradation in the
fertility of the soil coating will no longer occur. It is otherwise
with the mass of the soil, that body of commingled decayed rock and
vegetable matter which must possess a certain thickness in order to
serve its needs. As yet no considerable arrest has been made in the
processes which lead to the destruction of this earthy mass. In all
countries where tillage is general the rivers are flowing charged with
all they can bear away of soil material. Thus in the valley of the Po,
a region where, if the soil were forest-clad, the down-wearing of the
surface would probably be at no greater rate than one foot in five
thousand years, the river bears away the soil detritus so rapidly that
at the present time the downgoing is at the rate of one foot in eight
hundred years, and each decade sees the soil disappear from hillsides
which were once fertile, but are now reduced to bare rocks. All about
the Mediterranean the traveller notes extensive regions which were
once covered with luxuriant forests, and were afterward the seats of
prosperous agriculture, where the soil has utterly disappeared,
leaving only the bare rocks, which could not recover its natural
covering in thousands of years of the enforced fallow.
Within the limits of the United States the degradation of the soil,
owing to the peculiar conditions of the country, is in many districts
going forward with startling rapidity. It has been the habit of our
people--a habit favoured by the wide extent of fertile and easily
acquired frontier ground--recklessly to till their farms until the
fields were exhausted, and then to abandon them for new ground. By
shallow ploughing on steep hillsides, by neglect in the beginning of
those gulches which form in such places, it is easy in the hill
country of the eastern United States to have the soil washed away
within twenty years after the protecting forests have been destroyed.
The writer has estimated that in the States south of the Ohio and
James Rivers more than eight thousand square miles of originally
fertile ground have by neglect been brought into a condition where it
will no longer bear crops of any kind, and over fifteen hundred miles
of the area have been so worn down to the subsoil or the bed rock that
it may never be profitable to win it again to agricultural uses.
Hitherto, in our American agriculture, our people have been to a great
extent pioneers; they have been compelled to win what they could in
the cheapest possible way and with the rudest implements, and without
much regard to the future of those who were in subsequent generations
to occupy the fields which they were conquering from the wilderness
and the savages. The danger is now that this reckless tillage, in a
way justified of old, may be continued and become habitual with our
people. It is, indeed, already a fixed habit in many parts of the
country, particularly in the South, where a small farmer expects to
wear out two or three plantations in the course of his natural life.
Many of them manage to ruin from one to two hundred acres of land in
the course of half a century of uninterrupted labour. This system
deserves the reprobation of all good citizens; it would be well,
indeed, if it were possible to do so, to stamp it out by the law. The
same principle which makes it illegal for a man to burn his own
dwelling house may fairly be applied in restraining him from
destroying the land which he tills.
There are a few simple principles which, if properly applied, may
serve to correct this misuse of our American soil. The careful tiller
should note that all soils whatever which lie on declivities having a
slope of more than one foot in thirty inevitably and rapidly waste
when subject to plough tillage. This instrument tends to smear and
consolidate the layer of earth over which its heel runs, so that at a
depth of a few inches below the surface a layer tolerably impervious
to water is formed. The result is that the porous portion of the
deposit becomes excessively charged with water in times of heavy rain,
and moves down the hillside in a rapid manner. All such steep slopes
should be left in their wooded state, or, if brought into use, should
be retained as pasture lands.
Where, as is often the case with the farms in hilly countries, all the
fields are steeply inclined, it is an excellent precaution to leave
the upper part of the slope with a forest covering. In this condition
not only is the excessive flow of surface water diminished, but the
moisture which creeps down the slope from the wooded area tends to
keep the lower-lying fields in a better state for tillage, and
promotes the decay of the underlying rocks, and thus adds to the body
and richness of the earth.
On those soils which must be tilled, even where they tend to wash
away, the aim should be to keep the detritus open to such a depth that
it may take in as much as possible of the rainfall, yielding the water
to the streams through the springs. This end can generally be
accomplished by deep ploughing; it can, in almost all cases, be
attained by under-drainage. The effect of allowing the water to
penetrate is not only to diminish the superficial wearing, but to
maintain the process of subsoil and bed-rock decay by which the
detrital covering is naturally renewed. Where, as in many parts of the
country, the washing away of the soil can not otherwise be arrested,
the progress of the destruction can be delayed by forming with the
skilful use of the plough ditches of slight declivity leading along
the hillsides to the natural waterways. One of the most satisfactory
marks of the improvement which is now taking place in the agriculture
of the cotton-yielding States of this country is to be found in the
rapid increase in the use of the ditch system here mentioned. This
system, combined with ploughing in the manner where the earth is with
each overturning thrown uphill, will greatly reduce the destructive
effect of rainfall on steep-lying fields. But the only effective
protection, however, is accomplished by carefully terracing the
slopes, so that the tilled ground lies in level benches. This system
is extensively followed in the thickly settled portions of Europe, but
it may be a century before it will be much used in this country.
The duty of the soil-tiller by the earth with which he deals may be
briefly summed up: He should look upon himself as an agent necessarily
interfering with the operations which naturally form and preserve the
soil. He should see that his work brings two risks; he may impoverish
the accumulation of detrital material by taking out the plant food
more rapidly than it is prepared for use. This injurious result may be
at any time reparable by a proper use of manures. Not so, however,
with the other form of destruction, which results in the actual
removal of the soil materials. Where neglect has brought about this
disaster, it can only be repaired by leaving the area to recover
beneath the slowly formed forest coating. This process in almost all
cases requires many thousands of years for its accomplishment. The man
who has wrought such destruction has harmed the inheritance of life.
CHAPTER IX.
THE ROCKS AND THEIR ORDER.
In the preceding chapters of this book the attention of the student
has been directed mainly to the operations of those natural forces
which act upon the surface of the earth. Incidentally the consequences
arising from the applications of energy to the outer part of the
planet have been attended to, but the main aim has been to set forth
the work which solar energy, operating in the form of heat,
accomplishes upon the lands. We have now to consider one of the great
results of these actions, which is exhibited in the successive strata
that make up the earth's crust.
The most noteworthy effect arising from the action of the solar forces
on the earth and their co-operation with those which originate in our
sphere is found in the destruction of beds or other deposits of rock,
and the removal of the materials to the floors of water basins, where
they are again aggregated in strata, and gradually brought once more
into a stable condition within the earth. This work is accomplished by
water in its various states, the action being directly affected by
gravitation. In the form of steam, water which has been built into
rocks and volcanically expelled by tensions, due to the heat which it
has acquired at great depths below the surface, blows forth great
quantities of lava, which is contributed to the formation of strata,
either directly in the solid form or indirectly, after having been
dissolved in the sea. Acting as waves, water impelled by solar energy
transmitted to it by the winds beats against the shores, wearing away
great quantities of rock, which is dragged off to the neighbouring sea
bottoms, there to resume the bedded form. Moving ice in glaciers,
water again applying solar energy given to it by its elevation above
the sea, most effectively grinds away the elevated parts of the crust,
the _débris_ being delivered to the ocean. In the rain the same work
is done, and even in the wind the power of the sun serves to abrade
the high-lying rocks, making new strata of their fragments.
As gravity enters as an element in all the movements of divided rock,
the tendency of the waste worn from the land is to gather on to the
bottoms of basins which contain water. Rarely, and only in a small
way, this process results in the accumulation of lake deposits; the
greater part of the work is done upon the sea floor. When the beds are
formed in lake basins, they may be accumulated in either of two very
diverse conditions. They may be formed in what are called dead seas,
in which case the detrital materials are commonly small in amount, for
the reason that the inflowing streams are inconsiderable; in such
basins there is normally a large share of saline materials, which are
laid down by the evaporation of the water. In ordinary lakes the
deposits which are formed are mostly due to the sediment that the
rivers import. These materials are usually fine-grained, and the sand
or pebbles which they contain are plentifully mingled with clay. Hence
lake deposits are usually of an argillaceous nature. As organic life,
such as secretes limestone, is rarely developed to any extent in lake
basins, limy beds are very rarely formed beneath those areas of water.
Where they occur, they are generally due to the fact that rivers
charged with limy matter import such quantities of the substance that
it is precipitated on the bottom.
As lake deposits are normally formed in basins above the level of the
sea, and as the drainage channels of the basins are always cutting
down, the effect is to leave such strata at a considerable height
above the sea level, where the erosive agents may readily attack them.
In consequence of this condition, lacustrine beds are rarely found of
great antiquity; they generally disappear soon after they are formed.
Where preserved, their endurance is generally to be attributed to the
fact that the region they occupy has been lowered beneath the sea and
covered by marine strata.
The great laboratory in which the sedimentary deposits are
accumulated, the realm in which at least ninety-nine of the hundred
parts of these materials are laid down, is the oceanic part of the
earth. On the floors of the seas and oceans we have not only the
region where the greater part of the sedimentation is effected, but
that in which the work assumes the greatest variety. The sea bottoms,
as regards the deposits formed upon them, are naturally divided into
two regions--the one in which the _débris_ from the land forms an
important part of the sediment, and the other, where the remoteness
of the shores deprives the sediment of land waste, or at least of
enough of that material in any such share as can affect the character
of the deposits.
What we may term the littoral or shore zone of the sea occupies a belt
of prevailingly shallow water, varying in width from a few score to a
few hundred miles. Where the bottom descends steeply from the coast,
where there are no strong off-shore setting currents, and where the
region is not near the mouth of a large river which bears a great tide
of sediment to the sea, the land waste may not affect the bottom for
more than a mile or two from the shore. Where these conditions are
reversed, the _débris_ from the air-covered region may be found three
or four hundred miles from the coast line. It should also be noted
that the incessant up-and-down goings of the land result in a constant
change in the position of the coast line, and consequently in the
extension of the land sediment, in the course of a few geological
periods over a far wider field of sea bottom than that to which they
would attain if the shores remained steadfast.
It is characteristic of the sediments deposited within the influence
of the continental detritus that they vary very much in their action,
and that this variation takes place not only horizontally along the
shores in the same stratum, but vertically, in the succession of the
beds. It also may be traced down the slope from the coast line to deep
water. Thus where all the _débris_ comes from the action of the waves,
the deposits formed from the shore outwardly will consist of coarse
materials, such as pebbles near the coast, of sand in the deeper and
remoter section, and of finer silt in the part of the deposit which is
farthest out. With each change in the level of the coast line the
position of these belts will necessarily be altered. Where a great
river enters the sea, the changes in the volume of sediment which it
from time to time sends forth, together with the alternations in the
position of its point of discharge, led to great local complexities in
the strata. Moreover, the turbid water sent forth by the stream may,
as in the case of the tide from the Amazon, be drifted for hundreds of
miles along the coast line or into the open sea.
The most important variations which occur in the deposits of the
littoral zone are brought about by the formations of rocks more or
less composed of limestone. Everywhere the sea is, as compared with
lake waters, remarkably rich in organic life. Next the shore, partly
because the water is there shallow, but also because of its relative
warmth and the extent to which it is in motion, organic life, both
that of animals and plants, commonly develops in a very luxuriant way.
Only where the bottom is composed of drifting sands, which do not
afford a foothold for those species which need to rest upon the shore,
do we fail to find that surface thickly tenanted with varied forms.
These are arranged according to the depth of the bottom. The species
of marine plants which are attached to fixed objects are limited to
the depth within which the sunlight effectively penetrates the water;
in general, it may be said that they do not extend below a depth of
one hundred feet. The animal forms are distributed, according to their
kinds, over the floor, but few species having the capacity to endure
any great range in the pressure of the sea water. Only a few forms,
indeed, extend from low tide to the depth of a thousand feet.
The greatest development of organic life, the realm in which the
largest number of species occur, and where their growth is most rapid,
lies within about a hundred feet of the low-tide level. Here sunlight,
warmth, and motion in the water combine to favour organic development.
It is in this region that coral reefs and other great accumulations of
limestone, formed from the skeletons of polyps and mollusks, most
abundantly occur. These deposits of a limy nature depend upon a very
delicate adjustment of the conditions which favour the growth of
certain creatures; very slight geographic changes, by inducing
movements of sand or mud, are apt to interrupt their formation,
bringing about a great and immediate alteration in the character of
the deposits. Thus it is that where geologists find considerable
fields of rock, where limestones are intercalated with sandstones and
deposits of clay, they are justified in assuming that the strata were
laid down near some ancient shore. In general, these coast deposits
become more and more limy as we go toward the tropical realms, and
this for the reason that the species which secrete large amounts of
lime are in those regions most abundant and attain the most rapid
growth. The stony polyps, the most vigorous of the limestone makers,
grow in large quantities only in the tropical realm, or near to it,
where ocean streams of great warmth may provide the creatures with the
conditions of temperature and food which they need.
As we pass from the shore to the deeper sea, the share of land
detritus rapidly diminishes until, as before remarked, at the distance
of five hundred miles from the coast line, very little of that waste,
except that from volcanoes, attains the bottom of the sea. By far the
larger part of the contributions which go to the formation of these
deep-sea strata come from organic remains, which are continually
falling upon the sea floor. In part, this waste is derived from
creatures which dwell upon the bottom; in considerable measure,
however, it is from the dead bodies of those forms which live near the
surface of the sea, and which when dying sink slowly through the
intermediate realm to the bottom.
Owing to the absence of sunlight, the prevailingly cold water of the
deeper seas, and the lack of vegetation in those realms, the growth of
organic forms on the deep-sea floor is relatively slow. Thus it
happens that each shell or other contribution to the sediment lies for
some time on the bottom before it is buried. While in this condition
it is apt to be devoured by some of the many species which dwell on
the bottom and subsist from the remains of animals and plants which
they find there. In all cases the fossilization of any form depends
upon the accumulation of sediment before the processes of destruction
have overtaken them, and among these processes we must give the first
place to the creatures which subsist on shells, bones, or other
substances of like nature which find their way to the ocean floor. In
the absolute darkness, the still water, and the exceeding cold of the
deeper seas, animals find difficult conditions for development.
Moreover, in this deep realm there is no native vegetation, and, in
general, but little material of this nature descends to the bottom
from the surface of the sea. The result is, the animals have to
subsist on the remains of other animals which at some step in the
succession have obtained their provender from the plants which belong
on the surface or in the shallow waters of the sea. This limitation
of the food supply causes the depths of the sea to be a realm of
continual hunger, a region where every particle of organic matter is
apt to be seized upon by some needy creature.
In consequence of the fact that little organic matter on the deeper
sea floors escapes being devoured, the most of the material of this
nature which goes into strata enters that state in a finely divided
condition. In the group of worms alone--forms which in a great
diversity of species inhabit the sea floor--we find creatures which
are specially adapted to digesting the _débris_ which gathers on the
sea bottom. Wandering over this surface, much in the manner of our
ordinary earthworms, these creatures devour the mud, voiding the
matter from their bodies in a yet more perfectly divided form. Hence
it comes about that the limestone beds, so commonly formed beneath the
open seas, are generally composed of materials which show but few and
very imperfect fossils. Studying any series of limestone beds, we
commonly find that each layer, in greater or less degree, is made up
of rather massive materials, which evidently came to their place in
the form of a limy mud. Very often this lime has crystallized, and
thus has lost all trace of its original organic structure.
One of the conspicuous features which may be observed in any
succession of limestone beds is the partings or divisions into layers
which occur with varied frequency. Sometimes at vertical intervals of
not more than one or two inches, again with spacings of a score of
feet, we find divisional planes, which indicate a sudden change in the
process of rock formation. The lime disappears, and in place of it we
have a thin layer of very fine detritus, which takes on the form of a
clay. Examining these partings with care, we observe that on the upper
surface on the limestone the remains of the animal which dwelt on the
ancient sea floor are remarkably well preserved, they having evidently
escaped the effect of the process which reduced their ancestors,
whose remains constitute the layer, to mud. Furthermore, we note that
the shaly layer is not only lacking in lime, but commonly contains no
trace of animals such as might have dwelt on the bottom. The fossils
it bears are usually of species which swam in the overlying water and
came to the bottom after death. Following up through the layer of
shale, we note that the ordinary bottom life gradually reappears, and
shortly becomes so plentiful that the deposit resumes the character
which it had before the interruption began. Often, however, we note
that the assemblage of species which dwelt on the given area of sea
floor has undergone a considerable change. Forms in existence in the
lower layer may be lacking in the upper, their place being taken by
new varieties.
So far the origin of these divisional planes in marine deposits has
received little attention from geologists; they have, indeed, assumed
that each of these alterations indicates some sudden disturbance of
the life of the sea floors. They have, however, generally assumed that
the change was due to alterations in the depth of the sea or in the
run of ocean currents. It seems to the writer, however, that while
these divisions may in certain cases be due to the above-mentioned
and, indeed, to a great variety of causes, they are in general best to
be explained by the action of earthquakes. Water being an exceedingly
elastic substance, an earthquake passes through it with much greater
speed than it traverses the rocks which support the ocean floor. The
result is that, when the fluid and solid oscillate in the repeated
swingings which a shock causes, they do not move together, but rub
over each other, the independent movements having the swing of from a
few inches to a foot or two in shocks of considerable energy.
When the sea bottom and the overlying water, vibrating under the
impulse of an earthquake shock, move past each other, the inevitable
result is the formation of muddy water; the very fine silt of the
bottom is shaken up into the fluid, which afterward descends as a
sheet to its original position. It is a well-known fact that such
muddying of water, in which species accustomed to other conditions
dwell, inevitably leads to their death by covering their breathing
organs and otherwise disturbing the delicately balanced conditions
which enable them to exist. We find, in fact, that most of the tenants
of the water, particularly the forms which dwell upon the bottom, are
provided with an array of contrivances which enable them to clear away
from their bodies such small quantities of silt as may inconvenience
them. Thus, in the case of our common clam, the breathing organs are
covered with vibratory cilia, which, acting like brooms, sweep off any
foreign matter which may come upon their surfaces. Moreover, the
creature has a long, double, spoutlike organ, which it can elevate
some distance above the bottom, through which it draws and discharges
the water from which it obtains food and air. Other forms, such as the
crinoids, or sea lilies, elevate the breathing parts on top of tall
stems of marvellous construction, which brings those vital organs at
the level, it may be, of three or four feet above the zone of mud. In
consequence of the peculiar method of growth, the crinoids often
escape the damage done by the disturbance of the bottom, and thus form
limestone beds of remarkable thickness; sometimes, indeed, we find
these layers composed mainly of crinoidal remains, which exhibit only
slight traces of partings such as we have described, being essentially
united for the depth of ten or twenty feet. Where the layers have been
mainly accumulated by shellfish, their average thickness is less than
half a foot.
When we examine the partitions between the layers of limestone, we
commonly find that, however thin, they generally extend for an
indefinite distance in every direction. The writer has traced some of
these for miles; never, indeed, has he been able to find where they
disappeared. This fact makes it clear that the destruction which took
place at the stage where these partings were formed was widespread; so
far as it was due to earthquake shocks, we may fairly believe that in
many cases it occurred over areas which were to be measured by tens of
thousands of square miles. Indeed, from what we know of earthquake
shocks, it seems likely that the devastation may at times have
affected millions of square miles.
Another class of accidents connected with earthquakes may also
suddenly disturb the mud on the sea bottom. When, as elsewhere noted,
a shock originates beneath the sea, the effect is suddenly to elevate
the water over the seat of the jarring and the regions thereabouts to
the height of some feet. This elevation quickly takes the shape of a
ringlike wave, which rolls off in every direction from its point of
origin. Where the sea is deep, the effect of this wave on the bottom
may be but slight; but as the undulation attains shallower water, and
in proportion to the shoaling, the front of the surge is retarded in
its advance by the friction of the bottom, while the rear part, being
in deeper water, crowds upon the advancing line. The action is
precisely that which has been described as occurring in wind-made
waves as they approach the beach; but in this last-named group of
undulations, because of the great width of the swell, the effect of
the shallowing is evident in much deeper water. It is likely that at
the depth of a thousand feet the passing of one of these vast surges
born of earthquakes may so stir the mud of the sea floor as to bring
about a widespread destruction of life, and thus give rise to many of
the partitions between strata.
If we examine with the microscope the fine-grained silts which make up
the shaly layers between limestones, we find the materials to be
mostly of inorganic origin. It is hard to trace the origin of the
mineral matter which it contains; some of the fragments are likely to
prove of Volcanic origin; others, bits of dust from meteorites; yet
others, dust blown from the land, which may, as we know, be conveyed
for any distance across the seas. Mingled with this sediment of an
inorganic origin we almost invariably find a share of organic waste,
derived not from creatures which dwelt upon the bottom, but from those
which inhabited the higher-lying waters. If, now, we take a portion of
the limestone layer which lies above or below the shale parting, and
carefully dissolve out with acids the limy matter which it contains,
we obtain a residuum which in general character, except so far as the
particles may have been affected by the acid, is exactly like the
material which forms the claylike partition. We are thus readily led
to the conclusion that on the floors of the deeper seas there is
constantly descending, in the form of a very slow shower, a mass of
mineral detritus. Where organic life belonging to the species which
secrete hard shells or skeletons is absent, this accumulation,
proceeding with exceeding slowness, gradually accumulates layers,
which take on a shaly character. Where limestone-making animals
abound, they so increase the rate of deposition that the proportion of
the mineral material in the growing strata is very much reduced; it
may, indeed, become as small as one per cent of the mass. In this case
we may say that the deposit of limestone grew a hundred times as fast
as the intervening beds of shale.
The foregoing considerations make it tolerably clear that the sea
floor is in receipt of two diverse classes of sediment--those of a
mineral and those of an organic origin. The mineral, or inorganic,
materials predominate along the shores. They gradually diminish in
quantity toward the open sea, where the supply is mainly dependent on
the substances thrown forth from volcanoes, on pumice in its massive
or its comminuted form--i.e., volcanic dust, states of lava in which
the material, because of the vesicles which it contains, can float for
ages before it comes to rest on the sea bottom. Variations in the
volcanic waste contributed to the sea floor may somewhat affect the
quantity of the inorganic sediments, but, as a whole, the downfalling
of these fragments is probably at a singularly uniform rate. It is
otherwise with the contributions of sediment arising from organic
forms. This varies in a surprising measure. On the coral reefs, such
as form in the mid oceans, the proportion of matter which has not come
into the accumulation through the bodies of animals and plants may be
as small as one tenth of one per cent, or less. In the deeper seas, it
is doubtful whether the rate of animal growth is such as to permit the
formation of any beds which have less than one half of their mass made
up of materials which fell through the water.
In certain areas of the open seas the upper part of the water is dwelt
in by a host of creatures, mostly foraminifera, which extract
limestone from the water, and, on dying, send their shells to the
bottom. Thus in the North Atlantic, even where the sea floor is of
great depth beneath the surface, there is constantly accumulating a
mass of limy matter, which is forming very massive limestone strata,
somewhat resembling chalk deposits, such as abundantly occur in Great
Britain, in the neighbouring parts of Europe, in Texas, and elsewhere.
Accumulations such as this, where the supply is derived from the
surface of the water, are not affected by the accidents which divide
beds made on the bottom in the manner before described. They may,
therefore, have the singularly continuous character which we note in
the English chalk, where, for the thickness of hundreds of feet, we
may have no evident partitions, except certain divisions, which have
evidently originated long after the beds were formed.
We have already noted the fact that, while the floors of the deeper
seas appear to lack mountainous elevations, those arising from the
folding of strata, they are plentifully scattered over with volcanic
cones. We may therefore suppose that, in general, the deposits formed
on the sea floor are to a great extent affected by the materials which
these vents cast forth. Lava streams and showers represent only a
part of the contributions from volcanoes, which finally find their way
to the bottom. In larger part, the materials thrown forth are probably
first dissolved in the water and then taken up by the organic species;
only after the death of these creatures does the waste go to the
bottom. As hosts of these creatures have no solid skeleton to
contribute to the sea floor, such mineral matter as they may obtain is
after their death at once restored to the sea.
Not only does the contribution of organic sediment diminish in
quantity with the depth which is attained, but the deeper parts of the
ocean bed appear to be in a condition where no accumulations of this
nature are made, and this for the reason that the water dissolves the
organic matter more rapidly than it is laid down. Thus in place of
limestone, which would otherwise form, we have only a claylike
residuum, such as is obtained when we dissolve lime rocks in acids.
This process of solution, by which the limy matter deposited on the
bottom is taken back into the water, goes on everywhere, but at a rate
which increases with the depth. This increase is due in part to the
augmentation of pressure, and in part to the larger share of carbonic
dioxide which the water at great depths holds. The result is, that
explorations with the dredge seem to indicate that on certain parts of
the deeper sea floors the rocks are undergoing a process of
dissolution comparable to that which takes place in limestone caverns.
So considerable is the solvent work that a large part of the inorganic
waste appears to be taken up by the waters, so as to leave the bottom
essentially without sedimentary accumulations. The sea, in a word,
appears to be eating into rocks which it laid down before the
depression attained its present great depth.
We should here note something of the conditions which determine the
supply of food which the marine animals obtain. First of all, we may
recur to the point that the ocean waters appear to contain something
of all the earth materials which do not readily decompose when they
are taken into the state of solution. These mineral substances,
including the metals, are obtained in part from the lands, through the
action of the rain water and the waves, but perhaps in larger share
from the volcanic matter which, in the form of floating lava, pumice,
or dust, is plentifully delivered to the sea. Except doubtfully, and
at most in a very small way, this chemical store of the sea water can
not be directly taken into the structures of animals; it can only be
immediately appropriated by the marine plants. These forms can only
develop in that superficial realm of the seas which is penetrated by
the sunlight, or say within the depth of five hundred feet, mostly
within one hundred feet of the surface, about one thirtieth of the
average, and about one fiftieth of the maximum ocean depth. On this
marine plant life, and in a small measure on the vegetable matter
derived from the land, the marine animals primarily depend for their
provender. Through the conditions which bring about the formation of
_Sargassum_ seas, those areas of the ocean where seaweeds grow afloat,
as well as by the water-logging and weighting down of other vegetable
matter, some part of the plant remains is carried to the sea floor,
even to great depths; but the main dependence of the deep-sea forms of
animals is upon other animal forms, which themselves may have obtained
their store from yet others. In fact, in any deep-sea form we might
find it necessary to trace back the food by thousands of steps before
we found the creature which had access to the vegetable matter. It is
easy to see how such conditions profoundly limit the development of
organic being in the abysm of the ocean.
The sedentary animals, or those which are fixed to the sea bottom--a
group which includes the larger part of the marine species--have to
depend for their sustenance on the movement of the water which passes
their station. If the seas were perfectly still, none of these
creatures except the most minute could be fed; therefore the currents
of the ocean go far by their speed to determine the rate at which life
may flourish. At great depths, as we have seen, these movements are
practically limited to that which is caused by the slow movement which
the tide brings about. The amount of this motion is proportional to
the depth of the sea; in the deeper parts, it carries the water to and
fro twice each day for the distance of about two hundred and fifty
feet. In the shallower water this motion increases in proportion to
the shoaling, and in the regions near the shores the currents of the
sea which, except the massive drift from the poles, do not usually
touch the bottom, begin to have their influence. Where the water is
less than a hundred feet in depth, each wave contributes to the
movement, which attains its maximum near the shore, where every surge
sweeps the water rapidly to and fro. It is in this surge belt, where
the waves are broken, that marine animals are best provided with food,
and it is here that their growth is most rapid. If the student will
obtain a pint of water from the surf, he will find that it is clouded
by fragments of organic matter, the quantity in a pound of the fluid
often amounting to the fiftieth part of its weight. He will thus
perceive that along the shore line, though the provision of victuals
is most abundant, the store is made from the animals and plants which
are ground up in the mill. In a word, while the coast is a place of
rapid growth, it is also a region of rapid destruction; only in the
case of the coral animals, which associate their bodies with a number
of myriads in large and elaborately organized communities, do we find
animals which can make such head against the action of the waves that
they can build great deposits in their realm.
It should be noted that a part of the advantage which is afforded to
organic life by the shore belt is due to the fact that the waters are
there subjected to a constant process of aëration by the whipping into
foam and spray which occurs where the waves overturn.
It will be interesting to the student to note the great number of
mechanical contrivances which have been devised to give security to
animals and plants which face these difficult conditions arising from
successive violent blows of falling water. Among these may be briefly
noted those of the limpets--mollusks which dwell in a conical shell,
which faces the water with a domelike outside, and which at the moment
of the stroke is drawn down upon the rock by the strong muscle which
fastens the creature to its foundation. The barnacles, which with
their wedge-shaped prows cut the water at the moment of the stroke,
but open in the pauses between the waves, so that the creature may
with its branching arms grasp at the food which floats about it; the
nullipores, forms of seaweed which are framed of limestone and cling
firmly to the rock--afford yet other instances of protective
adaptations contrived to insure the safety of creatures which dwell in
the field of abundant food supply.
* * * * *
The facts above presented will show the reader that the marine
sediments are formed under conditions which permit a great variety in
the nature of the materials of which they are composed. As soon as the
deposits are built into rocks and covered by later accumulations,
their materials enter the laboratory of the under earth, where they
are subjected to progressive changes. Even before they have attained a
great depth, through the laying down of later deposits upon them,
changes begin which serve to alter their structure. The fragments of a
soluble kind begin to be dissolved, and are redeposited, so that the
mass commonly becomes much more solid, passing from the state of
detritus to that of more or less solid rock. When yet more deeply
buried, and thereby brought into a realm of greater warmth, or perhaps
when penetrated by dikes and thereby heated, these changes go yet
further. More of the material is commonly rearranged by solution and
redeposition, so that limestone may be converted into crystalline
marble, granular sandstones into firm masses, known as quartzites, and
clays into the harder form of slate. Where the changes go to the
extreme point, rocks originally distinctly bedded probably may be so
taken to pieces and made over that all traces of their stratification
may be destroyed, all fossils obliterated, and the stone transformed
into mica schist, or granite or other crystalline rock. It may be
injected into the overlying strata in the form of dikes, or it may be
blown forth into the air through volcanoes. Involved in
mountain-folding, after being more or less changed in the manner
described, the beds may become tangled together like the rumpled
leaves of a book, or even with the complexity of snarled thread. All
these changes of condition makes it difficult for the geologist to
unravel the succession of strata so that he may know the true order of
the rocks, and read from them the story of the successive geological
periods. This task, though incomplete, has by the labours of many
thousand men been so far advanced that we are now able to divide the
record into chapters, the divisions of the geologic ages, and to give
some account of the succession of events, organic and geographic,
which have occurred since life began to write its records.
EARTHQUAKES.
In ordinary experience we seem to behold the greater part of the earth
which meets our eyes as fixed in its position. A better understanding
shows us that nothing in this world is immovable. In the realm of the
inorganic world the atoms and molecules even in solid bodies have to
be conceived as endowed with ceaseless though ordered motions. Even
when matter is built into the solid rock, it is doubtful whether any
grain of it ever comes really to rest. Under the strains which arise
from the contraction of the earth's interior and the chemical changes
which the rocks undergo, each bit is subject to ever-changing
thrusts, which somewhat affect its position. If we in any way could
bring a grain of sand from any stratum under a microscope, so that we
could perceive its changes of place, we should probably find that it
was endlessly swaying this way and that, with reference to an ideally
fixed point, such as the centre of the earth. But even that centre,
whether of gravity or of figure, is probably never at rest.
Earth movements may be divided into two groups--those which arise from
the bodily shifting of matter, which conveys the particles this way or
that, or, as we say, change their place, and those which merely
produce vibration, in which the particles, after their vibratory
movement, return to their original place. For purposes of illustration
the first, or translatory motion, may be compared to that which takes
place when a bell is carried along upon a locomotive or a ship; and
the second, or vibratory movement, to what takes place when the bell
is by a blow made to ring. It is with these ringing movements, as we
may term them, that we find ourselves concerned when we undertake the
study of earthquakes.
It is desirable that the reader should preface his study of
earthquakes by noting the great and, at the same time, variable
elasticity of rocks. In the extreme form this elasticity is very well
shown when a toy marble, which is made of a close-textured rock, such
as that from which it derives its name, is thrown upon a pavement
composed of like dense material. Experiment will show that the little
sphere can often be made to bounce to the height of twenty feet
without breaking. If, then, with the same energy the marble is thrown
upon a brick floor, the rebound will be very much diminished. It is
well to consider what happens to produce the rebound. When the sphere
strikes the floor it changes its shape, becoming shorter in the axis
at right angles to the point which was struck, and at the same instant
expanded along the equator of that axis. The flattening remains for
only a small fraction of a second; the sphere vibrates so that it
stretches along the line on which it previously shortened, and, as
this movement takes place with great swiftness, it may be said to
propel itself away from the floor. At the same time a similar movement
goes on in the rock of the floor, and, where the rate of vibration is
the same, the two kicks are coincident, and so the sphere is impelled
violently away from the point of contact. Where the marble comes in
contact with brick, in part because of the lesser elasticity of that
material, due to its rather porous structure, and partly because it
does not vibrate at the same rate as the marble, the expelling blow is
much less strong.
All rocks whatever, even those which appear as incoherent sands, are
more or less set into vibratory motion whenever they are struck by a
blow. In the crust of the earth various accidents occur which may
produce that sudden motion which we term a blow. When we have examined
into the origin of these impulses, and the way in which they are
transmitted through the rocks, we obtain a basis for understanding
earthquake shocks. The commonest cause of the jarrings in the earth is
found in the formation of fractures, known as faults. If the reader
has ever been upon a frozen lake at a time when the weather was
growing colder, and the ice, therefore, was shrinking, he may have
noted the rending sound and the slight vibration which comes with the
formation of a crack traversing the sheet of ice. At such a time he
feels a movement which is an earthquake, and which represents the
simpler form of those tremors arising from the sudden rupture of fault
planes. If he has a mind to make the experiment, he may hang a bullet
by a thread from a small frame which rests upon the ice, and note that
as the vibration occurs the little pendulum sways to and fro, thus
indicating the oscillations of the ice. The same instrument will move
in an identical manner when affected by a quaking in the rocks.
Where the rocks are set in vibration by a rent which is formed in
them, the phenomena are more complicated, and often on a vastly larger
scale than in the simple conditions afforded by a sheet of ice. The
rocks on either side of the rupture generally slide over each other,
and the opposing masses are rent in their friction upon one another;
the result is, not only the first jar formed by the initial fracture,
but a great many successive movements from the other breakages which
occur. Again, in the deeper parts of the crust, the fault fissures are
often at the moment of their formation filled by a violent inrush of
liquid rock. This, as it swiftly moves along, tears away masses from
the walls, and when it strikes the end of the opening delivers a blow
which may be of great violence. The nature of this stroke may be
judged by the familiar instance where the relatively slow-flowing
stream from a hydrant pipe is suddenly choked by closing the stopcock.
Unless the plumber provides a cushion of air to diminish the energy of
the blow, it is often strong enough to shake the house. Again, when
steam or other gases are by a sudden diminution of pressure enabled to
expand, they may deliver a blow which is exactly like that caused by
the explosion of gunpowder, which, even when it rushes against the
soft cushion of the air, may cause a jarring that may be felt as well
as heard to a great distance. Such movements very frequently occur in
the eruptions of volcanoes; they cause a quivering of the earth, which
may be felt for a great distance from the immediate seat of the
disturbance.
When by any of the sudden movements which have been above described a
jar is applied to the rocks, the wave flies through the more or less
elastic mass until the energy involved in it is exhausted. This may
not be brought about until the motion has travelled for the distance
of hundreds of miles. In the great earthquake of 1755, known as the
Lisbon shock, the records make it seem probable that the movement was
felt over one eighth part of the earth's surface. Such great
disturbances probably bring about a motion of the rocks near the point
of origin, which may be expressed in oscillations having an amplitude
of one to two feet; but in the greater number of earthquakes the
maximum swing probably does not exceed the tenth of that amount. Very
sensible shaking, even such as may produce considerable damage to
buildings, are caused by shocks in which the earth vibrates with less
than an inch of swing.
When a shock originates, the wave in the rocks due to the compression
which the blow inflicts runs at a speed varying with the elasticity of
the substance, but at the rate of about fifteen hundred feet a second.
The movements of this wave are at right angles to the seat of the
originating disturbance, so that the shock may come to the surface in
a line forming any angle between the vertical and the nearly
horizontal. Where, as in a volcanic eruption, the shock originates
with an explosion, these waves go off in circles. Where, however, as
is generally the case, the shock originates in a fault plane, which
may have a length and depth of many miles, the movement has an
elliptical form.
If the earthquake wave ran through a uniform and highly elastic
substance, such as glass, it would move everywhere with equal speed,
and, in the case of the greater disturbances, the motion might be felt
over the whole surface of the earth. But as the motion takes place
through rocks of varying elasticity, the rate at which it journeys is
very irregular. Moving through materials of one density, and with a
rate of vibration determined by those conditions, the impulse is with
difficulty communicated to strata which naturally vibrate at another
speed. In many cases, as where a shock passing through dense
crystalline strata encounters a mass of soft sandstone, the wave, in
place of going on, is reflected back toward its point of origin. These
earthquake echoes sometimes give rise to very destructive movements.
It often happens that before the original tremors of a shock have
passed away from a point on the surface the reflex movements rush in,
making a very irregular motion, which may be compared to that of the
waves in a cross-sea.
The foregoing account of earthquake action will serve to prepare the
reader for an understanding of those very curious and important
effects which these accidents produce in and on the earth. Below the
surface the sensible action of earthquake shocks is limited. It has
often been observed that people in mines hardly note a swaying which
may be very conspicuous to those on the surface, the reason for this
being that underground, where the rocks are firmly bound together, all
those swingings which are due to the unsupported position of such
objects as buildings, columnar rocks, trees, and the waters of the
earth, are absent. The effect of the movements which earthquakes
impress on the under earth is mainly due to the fact that in almost
every part of the crust tensions or strains of other kinds are
continually forming. These may for ages prove without effect until the
earth is jarred, when motions will suddenly take place which in a
moment may alter the conditions of the rocks throughout a wide field.
In a word, a great earthquake caused by the formation of an extensive
fault is likely to produce any number of slight dislocations, each of
which is in turn shock-making, sending its little wave to complicate
the great oscillation. Nor does the perturbing effect of these jarring
movements cease with the fractures which they set up and the new
strains which are in turn developed by the motions which they induce.
The alterations of the rocks which are involved in chemical changes
are favoured by such motions. It is a familiar experience that a
vessel of water, if kept in the state of repose, may have its
temperature lowered three or four degrees below the freezing point
without becoming frozen. If the side of the vessel is then tapped with
the finger, so as to send a slight quake through the mass, it will
instantly congeal. Molecular rearrangements are thus favoured by
shocks, and the consequences of those which run through the earth are,
from a chemical point of view, probably important.
The reader may help himself to understand something of the complicated
problem of earth tensions, and the corresponding movements of the
rocks, by considering certain homely illustrations. He may observe how
the soil cracks as it shrinks in times of drought, the openings
closing when it rains. In a similar way the frozen earth breaks open,
sometimes with a shock which is often counted as an earthquake. Again,
the ashes in a sifter or the gravel on a sieve show how each shaking
may relieve certain tensions established by gravity, while they create
others which are in turn to be released by the next shock. An ordinary
dwelling house sways and strains with the alternations of temperature
and moisture to which it is subjected in the round of climatal
alterations. Now and then we note the movements in a cracking sound,
but by far the greater part of them escape observation.
With this sketch of the mechanism of earthquake shocks we now turn to
consider their effects upon the surface of the earth. From a
geological point of view, the most important effect of earthquake
shocks is found in the movement of rock masses down steep slopes,
which is induced by the shaking. Everywhere on the land the agents of
decay and erosion tend to bring heavy masses into position where
gravitation naturally leads to their downfall, but where they may
remain long suspended, provided they are not disturbed. Thus, wherever
there are high and steep cliffs, great falls of rock are likely to
occur when the earthquake movements traverse the under earth. In more
than one instance observers, so placed that they commanded a view of
distant mountains, have noticed the downfall of precipices in the path
of the shock before the trembling affected the ground on which they
stood. In the famous earthquake of 1783, which devastated southern
Italy, the Prince of Scylla persuaded his people to take refuge in
their boats, hoping that they might thereby escape the destruction
which threatened them on the land. No sooner were the unhappy folk on
the water than the fall of neighbouring cliffs near the sea produced a
great wave, which overwhelmed the vessels.
Where the soil lies upon steep slopes, in positions in which it has
accumulated during ages of tranquillity, a great shock is likely to
send it down into the valleys in vast landslides. Thus, in the
earthquake of 1692, the Blue Mountains of Jamaica were so violently
shaken that the soil and the forests which stood on it were
precipitated into the river beds, so that many tree-clad summits
became fields of bare rock. The effect of this action is immensely to
increase the amount of detritus which the streams convey to the sea.
After the great Jamaica shock, above noted, the rivers for a while
ceased to flow, their waters being stored in the masses of loose
material. Then for weeks they poured forth torrents of mud and the
_débris_ of vegetation--materials which had to be swept away as the
streams formed new channels.
In all regions where earthquake movements are frequent, and the shock
of considerable violence, the trained observer notes that the surfaces
of bare rock are singularly extensive, the fact being that many of
these areas, where the slope lies at angles of from ten to thirty
degrees, which in an unshaken region would be thickly soil-covered,
are deprived of the coating by the downward movement of the waste
which the disturbances bring about. A familiar example of this action
may be had by watching the workmen engaged in sifting sand, by casting
the material on a sloping grating. The work could not be done but for
an occasional blow applied to the sifter. An arrangement for such a
jarring motion is commonly found in various ore-dressing machines,
where the object is to move fragments of matter over a sloping
surface.
Even where the earth is so level that an earthquake shock does not
cause a sliding motion of the materials, such as above described,
other consequences of the shaking may readily be noted. As the motion
runs through the mass, provided the movement be one of considerable
violence, crevices several feet in width, and sometimes having the
length of miles, are often formed. In most cases these fissures,
opened by one pulsation of the shock, are likely to be closed by the
return movement, which occurs the instant thereafter. The consequences
of this action are often singular, and in cases constitute the most
frightful elements of a shock which the sufferer beholds. In the great
earthquake of 1811, which ravaged the section of the Mississippi
Valley between the mouth of the Ohio and Vicksburg, these crevices
were so numerously formed that the pioneers protected themselves from
the danger of being caught in their jaws by felling trees so that they
lay at right angles to the direction in which the rents extended,
building on these timbers platforms to support their temporary
dwelling places. The records of earthquakes supply many instances in
which people have been caught in these earth fissures, and in a single
case it is recorded that a man who disappeared into the cavity was in
a moment cast forth in the rush of waters which in this, as in many
other cases, spouts forth as the walls of the opening come together.
Sometimes these rents are attended by a dislocation, which brings the
earth on one side much higher than on the other. The step thus
produced may be many miles in length, and may have a height of twenty
feet or more. It needs no argument to show that we have here the top
of a fault such as produced the shock, or it may be one of a secondary
nature, such as any earthquake is likely to bring about in the strata
which it traverses. In certain cases two faults conjoin their action,
so that a portion of the surface disappears beneath the earth,
entombing whatever may have stood on the vanished site. Thus in the
great shock known as that of Lisbon, which occurred in 1755, the stone
quay along the harbour, where many thousand people had sought refuge
from the falling buildings of the city, suddenly sank down with the
multitude, and the waters closed over it; no trace of the people or of
the structure was to be found after the shock was over. There is a
story to the effect that during the same earthquake an Arab village in
northern Africa sank down, the earth on either side closing over it,
so that no trace of the habitations remained. In both these instances
the catastrophes are best explained by the diagram.
[Illustration: Fig. 21.--Diagram showing how a portion of the earth's
surface may be sunk by faulting. Fig. A shows the original position;
B, the position after faulting; b b' and c c' the planes of the
faults; the arrows the direction of the movement.]
In the earthquake of 1811 the alluvial plains on either side of the
Mississippi at many points sank down so that arable land was converted
into lakes; the area of these depressions probably amounted to some
hundred square miles. The writer, on examining these sunken lands,
found that the subsidences had occurred where the old moats or
abandoned channels of the great river had been filled in with a
mixture of decaying timber and river silt. When violently shaken, this
loose-textured _débris_ naturally settled down, so that it formed a
basin occupied by a crescent-shaped lake. The same process of settling
plentifully goes on wherever the rocks are still in an uncemented
state. The result is often the production of changes which lead to the
expulsion of gases. Thus, in the Charleston earthquake of 1883, the
surface over an area of many hundred square miles was pitted with
small craters, formed by the uprush of water impelled by its contained
gases. These little water volcanoes--for such we may call
them--sometimes occur to the number of a dozen or more on each acre of
ground in the violently shaken district. They indicate one result of
the physical and chemical alterations which earthquake shocks bring
about. As earthquakes increase in violence their effect upon the soil
becomes continually greater, until in the most violent shocks all the
loose materials on the surface of the earth may be so shaken about as
to destroy even the boundaries of fields. After the famous earthquake
of Riobamba, which occurred on the west coast of South America in
1797, the people of the district in which the town of that name was
situated were forced to redivide their land, the original boundaries
having disappeared. Fortunately, shocks of this description are
exceedingly rare. They occur in only a few parts of the world.
Certain effects of earthquakes where the shock emerges beneath the sea
have been stated in the account of volcanic eruptions (see page 299).
We may therefore note here only certain of the more general facts.
While passing through the deep seas, this wave may have a height of
not more than two or three feet and a width of some score miles. As it
rolls in upon the shore the front of the undulation is retarded by the
friction of the bottom in such a measure that its speed is diminished,
while the following part of the waves, being less checked, crowds up
toward this forward part. The result is, that the surge mounts ever
higher and higher as it draws near the shore, upon which it may roll
as a vast wave having the height of fifty feet or more and a width
quite unparalleled by any wave produced from wind action. Waves of
this description are most common in the Pacific Ocean. Although but
occasional, the damage which they may inflict is very great. As the
movement approaches the shore, vessels, however well anchored, are
dragged away to seaward by the great back lash of the wave, a
phenomenon which may be perceived even in the case of the ordinary
surf. Thus forced to seaward, the crews of the ships may find their
vessels drawn out for the distance of some miles, until they come near
the face of the advancing billow. This, as it approaches the shore,
straightens up to the wall-fronted form, and then topples upon the
land. Those vessels which are not at once crushed down by the blow are
generally hurled far inland by the rush of waters. In the great
Jamaica earthquake of 1692 a British man-of-war was borne over the
tops of certain warehouses and deposited at a distance from the shore.
Owing to the fact that water is a highly elastic material, the shocks
transmitted to it from the bottom are sent onward with their energy
but little diminished. While the impulse is very violent, these
oscillations may prove damaging to shipping. The log-books of mariners
abound in stories of how vessels were dismasted or otherwise badly
shaken by a sudden blow received in the midst of a quiet sea. The
impression commonly conveyed to the sailors is that the craft has
struck upon a rock. The explanation is that an earthquake jar, in
traversing the water, has delivered its blow to the ship. As the speed
of this jarring movement is very much greater than that of any
ordinary wave, the blow which it may strike may be most destructive.
There seems, indeed, little reason to doubt that a portion of the
vessels which are ever disappearing in the wilderness of the ocean are
lost by the crushing effect of these quakings which pass through the
waters of the deep.
We have already spoken of the earthquake shock as an oscillation. It
is a quality of all bodies which oscillate under the influence of a
blow, such as originates in earthquake shocks, to swing to and fro,
after the manner of the metal in a bell or a tuning fork, in a
succession of movements, each less than the preceding, until the
impulse is worn out, or rather, we should in strict sense say,
changed to other forms of energy. The result is, that even in the
slightest earthquake shock the earth moves not once to and fro, but
very many times. In a considerable shock the successive diminishing
swingings amount to dozens before they become so slight as to elude
perception. Although the first swaying is the strongest, and generally
the most destructive, the quick to-and-fro motions are apt to continue
and to complete the devastation which the first brings about. The
vibrations due to any one shock take place with great rapidity. They
may, indeed, be compared to those movements which we perceive in the
margin of a large bell when it has received a heavy blow from the
clapper. The reader has perhaps seen that for a moment the rim of the
bell vibrates with such rapidity that it has a misty look--that is,
the motions elude the sight. It is easy to see that a shaking of this
kind is particularly calculated to disrupt any bodies which stand free
in the air and are supported only at their base.
In what we may call the natural architecture of the earth, the
pinnacles and obelisks, such as are formed in many high countries, the
effect of these shakings is destructive, and, as we have seen, even
the firmer-placed objects, such as the strong-walled cliffs and steep
slopes of earth, break down under the assaults. It is therefore no
matter of surprise that the buildings which man erects, where they are
composed of masonry, suffer greatly from these tremblings. In almost
all cases human edifices are constructed without regard to other
problems of strength than those which may be measured by their weight
and the resistance to fracture from gravitation alone. They are not
built with expectation of a quaking, but of a firm-set earth.
The damage which earthquakes do to buildings is in most cases due to
the fact that they sway their walls out of plumb, so that they are no
longer in position to support the weight which they have to bear. The
amount of this swaying is naturally very much greater than that which
the earth itself experiences in the movement. A building of any height
with its walls unsupported by neighbouring structures may find its
roof rocked to and fro through an arc which has a length of feet,
while its base moves only through a length of inches. The reader may
see an example of this nature if he will poise a thin book or a bit of
plank a foot long on top of a small table; then jarring the table so
that it swings through a distance of say a quarter of an inch, he will
see that the columnar object swings at its top through a much greater
distance, and is pretty sure to be overturned.
Where a building carries a load in its upper parts, such as may be
afforded by its heavy roof or the stores which it contains, the effect
of an earthquake shock such as carries the earth to and fro becomes
much more destructive than it might otherwise be. This weight lags
behind when the earth slips forward in the first movement of the
oscillation, with the effect that the walls of the building are pretty
sure to be thrust so far beyond the perpendicular that they give way
and are carried down by the weight which they bore. It has often been
remarked in earthquake shocks that tall columns, even where composed
of many blocks, survive a shock which overturns lower buildings where
thin walls support several floors, on each of which is accumulated a
considerable amount of weight. In the case of the column, the strains
are even, and the whole structure may rock to and fro without toppling
over. As the energy of the undulations diminish, it gradually regains
the quiet state without damage. In the ordinary edifice the irregular
disposition of the weight does not permit the uniform movement which
may insure safety. Thus, if the city of Washington should ever be
violently shaken, the great obelisk, notwithstanding that it is five
hundred feet high, may survive a disturbance which would wreck the
lower and more massive edifices which lie about it.
Where, as is fortunately rarely the case, the great shock comes to
the earth in a vertical direction, the effect upon all movable objects
is in the highest measure disastrous. In such a case buildings are
crushed as if by the stroke of a giant's hand. The roofs and floors
are at one stroke thrown to the foundations, and all the parts of the
walls which are not supported by strong masonry continuous from top to
bottom are broken to pieces. In such cases it has been remarked that
the bodies of men are often thrown considerable distances. It is
asserted, indeed, that in the Riobamba shock they were cast upward to
the height of more than ninety feet. It is related that the solo
survivor of a congregation which had hastened at the outset of the
disturbance into a church was thrown by the greatest and most
destructive shock upward and through a window the base of which was at
the height of more than twenty feet from the ground.
It is readily understood that an earthquake shock may enter a building
in any direction between the vertical and the horizontal. As the
movement exhausts itself in passing from the place of its origin, the
horizontal shocks are usually of least energy. Those which are
accurately vertical are only experienced where the edifices are placed
immediately over the point where the motion originates. It follows,
therefore, that the destructive work of earthquakes is mainly
performed in that part of the field where the motion is, as regards
its direction, between the vertical and the horizontal--a position in
which the edifice is likely to receive at once the destructive effect
arising from the sharp upward thrust of the vertical movement and the
oscillating action of that which is in a horizontal direction. Against
strains of this description, where the movements have an amplitude of
more than a few inches, no ordinary masonry edifice can be made
perfectly safe; the only tolerable security is attained where the
building is of well-framed timber, which by its elasticity permits a
good deal of motion without destructive consequences. Even such
buildings, however, those of the strongest type, may be ruined by the
greater earthquakes. Thus, in the Mississippi Valley earthquake of
1811, the log huts of the frontiersmen, which are about as strong as
any buildings can be made, were shaken to pieces by the sharp and
reiterated shocks.
It is by no means surprising to find that the style of architecture
adopted in earthquake countries differs from that which is developed
in regions where the earth is firm-set. The people generally learn
that where their buildings must meet the trials of earthquakes they
have to be low and strong, framed in the manner of fortifications, to
withstand the assault of this enemy. We observe that Gothic
architecture, where a great weight of masonry is carried upon slender
columns and walls divided by tall windows, though it became the
dominant style in the relatively stable lands of northern Europe,
never gained a firm foothold in those regions about the Mediterranean
which are frequently visited by severe convulsions of the earth. There
the Grecian or the Romanesque styles, which are of a much more massive
type, retain their places and are the fashions to the present day.
Even this manner of building, though affording a certain security
against slight tremblings, is not safe in the greater shocks. Again
and again large areas in southern Italy have been almost swept of
their buildings by the destructive movements which occur in that
realm. The only people who have systematically adapted their
architectural methods to earthquake strains are the Japanese, who in
certain districts where such risks are to be encountered construct
their dwellings of wood, and place them upon rollers, so that they may
readily move to and fro as the shock passes beneath them. In a measure
the people of San Francisco have also provided against this danger by
avoiding dangerous weights in the upper parts of their buildings, as
well as the excessive height to which these structures are lifted in
some of our American towns.
Earthquakes of sensible energy appear to be limited to particular
parts of the earth's crust. The regions, indeed, where within the
period of human history shocks of devastating energy have occurred do
not include more than one fifteenth part of the earth's surface. There
is a common notion that these movements are most apt to happen in
volcanic regions. It is, indeed, true that sensible shocks commonly
attend the explosions from great craters, but the records clearly show
that these movements are very rarely of destructive energy. Thus in
the regions about the base of Vesuvius and of Ætna, the two volcanoes
of which most is known, the shocks have never been productive of
extensive disaster. In fact, the reiterated slight jarrings which
attend volcanic action appear to prevent the formation of those great
and slowly accumulated strains which in their discharge produce the
most violent tremblings of the earth. The greatest and most continuous
earthquake disturbances of history--that before noted in the early
days of this century, in the Mississippi Valley, where shocks of
considerable violence continued for two years--came about in a field
very far removed from active volcanoes. So, too, the disturbances
beneath the Atlantic floor which originated the shocks that led to the
destruction of Lisbon, and many other similar though less violent
movements, are developed in a field apparently remote from living
volcanoes. Eastern New England, which has been the seat of several
considerable earthquakes, is about as far away from active vents as
any place on the habitable globe. We may therefore conclude that,
while volcanoes necessarily produce shocks resulting from the
discharge of their gases and the intrusion of lava into the dikes
which are formed about them, the greater part of the important shocks
are in no wise connected with volcanic explosions.
With the exception of the earthquake in the Mississippi Valley, all
the great shocks of which we have a record have occurred in or near
regions where the rocks have been extensively disturbed by
mountain-building forces, and where the indications lead us to
believe that dislocations of strata, such as are competent to rive the
beds asunder, may still be in progress. This, taken in connection with
the fact that many of these shocks are attended by the formation of
fault planes, which appear on the surface, lead us to the conclusion
that earthquakes of the stronger kind are generally formed by the
riving of fissures, which may or may not be developed upward to the
surface. This view is supported by many careful observations on the
effect which certain great earthquakes have exercised on the buildings
which they have ravaged. The distinguished observer, Mr. Charles
Mallet, who visited the seat of the earthquake which, in 1854,
occurred in the province of Calabria in Italy, with great labour and
skill determined the direction in which the shock moved through some
hundreds of edifices on which it left the marks of its passage.
Platting these lines of motion, he found that they were all referred
to a vertical plane lying at the depth of some miles beneath the
surface, and extending for a great distance in a north and south
direction. This method of inquiry has been applied to other fields,
with the result that in the case of all the instances which have been
subjected to this inquiry the seat of the shock has been traced to
such a plane, which can best be accounted for by the supposition of a
fault.
The method pursued by Mr. Mallet in his studies of the origin of
earthquakes, and by those who have continued his inquiry, may be
briefly indicated as follows: Examining disrupted buildings, it is
easy to determine those which have been wrecked by a shock that
emerged from the earth in a vertical direction. In these cases, though
tall walls may remain standing, the roofs and floors are thrown into
the cellars. With a dozen such instances the plane of what is called
the seismic vertical is established (_seismos_ is the Greek for
earthquake). Then on either side of this plane, which indicates the
line but not the depth of the disturbance, other observations may be
made which give the clew to the depth. Thus a building may be found
where the northwest corner at its upper part has been thrown off. Such
a rupture was clearly caused by an upward but oblique movement, which
in the first half of the oscillation heaved the structure upwardly
into the northwest, and then in the second half, or rebound, drew the
mass of the building away from the unsupported corner, allowing that
part of the masonry to fly off and fall to the ground. Constructing a
line at right angles to the plane of the fracture, it will be found to
intersect the plane, the position of which has been in part determined
by finding the line where it intersects the earth, or the seismic
vertical before noted. Multiplying such observations on either side of
the last-mentioned line, the attitude of the underground parts of the
plane, as well as the depth to which it attained, can be approximately
determined.
It is worth while to consider the extent to which earthquake shocks
may affect the general quality of the people who dwell in countries
where these disturbances occur with such frequency and violence as to
influence their lives. There can be no question that wherever
earthquakes occur in such a measure as to produce widespread terror,
where, recurring from time to time, they develop in men a sense of
abiding insecurity, they become potent agents of degradation. All the
best which men do in creating a civilization rests upon a sense of
confidence that their efforts may be accumulated from year to year,
and that even after death the work of each man may remain as a
heritage to his kind. It is likely, indeed, that in certain realms, as
in southern Italy, a part of the failure of the people to advance in
culture is due to their long experience of such calamities, and the
natural expectation that they will from time to time recur. In a
similar way the Spanish settlements in Central and South America,
which lie mostly in lands that are subject to disastrous shocks, may
have been retarded by the despair, as well as the loss of property
and life, which these accidents have so frequently inflicted upon
them. It will not do, however, to attribute too much to such
terrestrial influences. By far the most important element in
determining the destiny of a people is to be found in their native
quality, that which they owe to their ancestors of distant
generations. In this connection it is well to consider the history of
the Icelandic people, where a small folk has for a thousand years been
exposed to a range and severity of trials, such as earthquakes,
volcanic explosions, and dearth of harvests may produce, and all these
in a measure that few if any other countries experience.
Notwithstanding these misfortunes, the Icelanders have developed and
maintained a civilization which in all else, except its material
results, on the average transcends that which has been won by any
other folk in modern times. If a people have the determining spirit
which leads to high living, they can successfully face calamities far
greater than those which earthquakes inflict.
It was long supposed that the regions where earthquakes are not
noticeable by the unaided senses were exempt from all such
disturbances. The observations which seismologists have made in recent
years point to the conclusion that no part of the earth's surface is
quite exempt from movements which, though not readily perceived, can
be made visible by the use of appropriate instruments. With an
apparatus known as the horizontal pendulum it is possible to observe
vibrations which do not exceed in amplitude the hundredth part of an
inch. This mechanism consists essentially of a slender bar supported
near one end by two wires, one from above, the other from below. It
may readily be conceived that any measurable movement will cause the
longer end of the rod to sway through a considerable arc. Wherever
such a pendulum has been carefully observed in any district, it has
been found that it indicates the occurrence of slight tremors. Even
certain changes of the barometer, which alter the weight of the
atmosphere that rests upon the earth to the amount indicated by an
inch in the height of the mercury column, appears in all cases to
create such tremors. Many of these slight shocks may be due to the
effect of more violent quakings, which have run perhaps for thousands
of miles from their point of origin, and have thus been reduced in the
amplitude of their movement. Others are probably due to the slight
motion brought about through the chemical changes of the rocks, which
are continuously going on. The ease with which even small motions are
carried to a great distance may be judged by the fact that when the
ground is frozen the horizontal pendulum will indicate the jarring due
to a railway train at the distance of a mile or more from the track.
In connection with the earth jarring, it would be well to note the
occurrence of another, though physically different, kind of movement,
which we may term earth swayings, or massive movements, which slowly
dislocate the vertical, and doubtless also the horizontal, position of
points upon its surface. It has more than once been remarked that in
mountain countries, where accurate sights have been taken, the heights
of points between the extremities of a long line appear somewhat to
vary in the course of a term of years. Thus at a place in the
Apennines, where two buildings separated by some miles of distance are
commonly intervisible over the crest of a neighbouring peak, it has
happened that a change of level of some one of the points has made it
impossible to see the one edifice from the other. Knowing as we do
that the line of the seacoast is ever-changing, uprising taking place
at some points and down-sinking at others, it seems not unlikely that
these irregular swayings are of very common occurrence. Moreover,
astronomers are beginning to remark the fact that their observatories
appear not to remain permanently in the same position--that is, they
do not have exactly the same latitude and longitude. Certain of these
changes have recently been explained by the discovery of a new and
hitherto unnoted movement of the polar axis. It is not improbable,
however, that the irregular swaying of the earth's crust, due to the
folding of strata and to the alterations in the volume of rocks which
are continually going on, may have some share in bringing about these
dislocations.
Measured by the destruction which was wrought to the interests of man,
earthquakes deserve to be reckoned among the direst calamities of
Nature. Since the dawn of history the records show us that the
destruction of life which is to be attributed to them is to be counted
by the millions. A catalogue of the loss of life in the accidents of
this description which have occurred during the Christian era has led
the writer to suppose that probably over two million persons have
perished from these shocks in the last nineteen centuries.
Nevertheless, as compared with other agents of destruction, such as
preventable disease, war, or famine, the loss which has been inflicted
by earth movements is really trifling, and almost all of it is due to
an obstinate carelessness in the construction of buildings without
reference to the risks which are known to exist in earthquake-ridden
countries.
Although all our exact knowledge concerning the distribution of
earthquakes is limited to the imperfect records of two or three
thousand years, it is commonly possible to measure in a general way
the liability to such accidents which may exist in any country by a
careful study of the details of its topography. In almost every large
area the process of erosion naturally leaves quantities of rock,
either in the form of detached columns or as detrital accumulations
deposited on steep slopes. These features are of relatively slow
formation, and it is often possible to determine that they have been
in their positions for a time which is to be measured by thousands of
years. Thus, on inspecting a country such as North America, where the
historic records cover but a brief time, we may on inquiry determine
which portions of its area have long been exempt from powerful shocks.
Where natural obelisks and steep taluses abound--features which would
have disappeared if the region had been moved by great shocks--we may
be sure that the field under inspection has for a great period been
exempt from powerful shaking. Judged by this standard, we may safely
say that the region occupied by the Appalachian Mountains has been
exempt from serious trouble. So, too, the section of the Cordilleras
lying to the east of what is commonly called the Great Basin, between
the Rocky Mountains and the Sierra Nevada, has also enjoyed a long
reign of peace. In glaciated countries the record is naturally less
clear than in those parts of the world which have been subjected to
long-continued, slow decay of the rocks. Nevertheless, in those fields
boulders are often found poised in position which they could not have
maintained if subjected to violent shaking. Judged by this evidence,
we may say that a large part of the northern section of this
continent, particularly the area about the Great Lakes, has been
exempt from considerable shocks since the glacier passed away.
The shores which are subject to the visitations of the great marine
waves, caused by earthquake shocks occurring beneath the bottom of the
neighbouring ocean, are so swept by those violent inundations that
they lose many features which are often found along coasts that have
been exempted from such visitations. Thus wherever we find extensive
and delicately moulded dunes, poised stones, or slender pinnacled
rocks along a coast, we may be sure that since these features were
formed the district has not been swept by these great waves.
[Illustration: Fig. 22.--Poised rocks indicating a long exemption from
strong earthquakes in the places where such features occur.]
Around the northern Atlantic we almost everywhere find the glacial
waste here and there accumulated near the margin of the sea in the
complicated sculptured outlines which are assumed by kame sands and
gravels. From a study of these features just above the level of high
tide, the writer has become convinced that the North Atlantic district
has long been exempt from the assaults of other waves than those which
are produced during heavy storms. At the present time the waves
formed by earthquakes appear to be of destructive violence only on the
west coast of South America, where they roll in from a region of the
Pacific lying to the south of the equator and a few hundred miles from
the shore of the continent, which appears to be the seat of
exceedingly violent shocks. A similar field occurs in the Atlantic
between the Lesser Antilles and the Spanish peninsula, but no great
waves have come thence since the time of the Lisbon earthquake. The
basin of the Caribbean and the region about Java appear to be also
fields where these disturbances may be expected, though in each but
one wave of this nature has been recorded. Therefore we may regard
these secondary results of a submarine earthquake as seldom phenomena.
DURATION OF GEOLOGICAL TIME.
Although it is beyond the power of man to conceive any such lapses of
time as have taken place in the history of this earth, it is
interesting, and in certain ways profitable, to determine as near as
possible in the measure of years the duration of the events which are
recorded in the rocks. Some astronomers, basing their conclusions on
the heat-containing power of matter, and on the rate at which energy
in this form flows from the sun, have come to the conclusion that our
planet could not have been in independent existence for more than
about twenty million years. The geologist, however, resting his
conclusions on the records which are the subject of his inquiry, comes
on many different lines to an opinion which traverses that entertained
by some distinguished astronomers. The ways in which the student of
the earth arrives at this opinion will now be set forth.
By noting the amount of sediment carried forth to the sea by the
rivers, the geologist finds that the lands of the earth--those, at
least, which are protected by their natural envelopes of
vegetation--are wearing down at a rate which pretty certainly does
not exceed one foot in about five thousand years, or two hundred feet
in a million years. Discovering at many places on the earth's surface
deposits which originally had a thickness of five thousand feet or
more, which have been worn down to the depths of thousands of feet in
a single rather brief section of geological time, the student readily
finds himself prepared to claim that a period of from five to ten
million years has often been required for the accomplishment of but a
very small part of the changes which he knows to have occurred on this
earth.
As the geologist follows down through the sections of the stratified
rocks, and from the remains of strata determines the erosion which has
borne away the greater part of the thick deposits which have been
exposed to erosion, he comes upon one of those breaks in the
succession, or encounters what is called an unconformity, as when
horizontal strata lie against those which are tilted. In many cases he
may observe that at this time there was a great interval unrepresented
by deposits at the place where his observations are made, yet a great
lapse of time is indicated by the fact that a large amount of erosion
took place in the interval between the two sets of beds.
Putting together the bits of record, and assuming that the rate of
erosion accomplished by the agents which operate on the land has
always been about the same, the geologist comes to the conclusion that
the section of the rocks from the present day to the lowest strata of
the Laurentian represents in the time required for their formation not
less than a hundred million years; more likely twice that duration. To
this argument objection is made by some naturalists that the agents of
erosion may have been more active in the past than they are at
present. They suggest that the rainfall may have been much greater or
the tides higher than they now are. Granting all that can be claimed
on this score, we note the fact that the rate of erosion evidently
does not increase in anything like a proportionate way with the
amount of rainfall. Where a country is protected by its natural
coating of vegetation, the rain is delivered to the streams without
making any considerable assault upon the surface of the earth, however
large the fall may be. Moreover, the tides have little direct cutting
power; they can only remove detritus which other agents have brought
into a condition to be borne away. The direct cutting power of the
tidal movement does not seem to be much greater in the Bay of Fundy,
where the maximum height of the waves amounts to fifty feet, than on
the southern coast of Massachusetts, where the range is not more than
five. So far as the observer can judge, the climatal conditions and
the other influences which affect the wear of rocks have not greatly
varied in the past from what they are at the present day. Now and then
there have been periods of excessive erosion; again, ages in which
large fields were in the conditions of exceeding drought. It is,
however, a fair presumption that these periods in a way balance each
other, and that the average state was much like that which we find at
present.
If after studying the erosive phenomena exhibited in the structure of
the earth the student takes up the study of the accumulations of
strata, and endeavours to determine the time required for the laying
down of the sediments, he finds similar evidence of the earth's great
antiquity. Although the process of deposition, which has given us the
rocks visible in the land masses, has been very much interrupted, the
section which is made by grouping the observations made in various
fields shows that something like a maximum thickness of a hundred and
fifty thousand feet of beds has been accumulated in that part of
geologic time during which strata were being laid down in the fields
that are subjected to our study. Although in these rocks there are
many sets of beds which were rapidly formed, the greater part of them
have been accumulated with exceeding slowness. Many fine shales, such
as those which plentifully occur in the Devonian beds of this country,
must have required a thousand years or more for the deposition of the
materials that now occupy an inch in depth. In those sections a single
foot of the rock may well represent a period of ten thousand years. In
many of the limestones the rate of accumulation could hardly have been
more speedy. The reckoning has to be rough, but the impression which
such studies make upon the mind of the unprejudiced observer is to the
effect that the thirty miles or so of sedimentary deposits could not
have been formed in less than a hundred million years. In this
reckoning it should be noted that no account is taken of those great
intervals of unrecorded time, such as elapsed between the close of the
Laurentian and the beginning of the Cambrian periods.
There is a third way in which we may seek an interpretation of
duration from the rocks. In each successive stage of the earth's
history, in different measure in the various ages, mountains were
formed which in time, during their exposure to the conditions of the
land, were worn down to their roots and covered by deposits
accumulated during the succeeding ages. A score or more of these
successively constructed series of elevations may readily be observed.
Of old, it was believed that mountain ranges were suddenly formed, but
there is, however, ample evidence to prove that these disturbed
portions of the strata were very gradually dislocated, the rate of the
mountainous growth having been, in general, no greater in the past
than it is at the present day, when, as we know full well, the
movements are going on so slowly that they escape observation. Only
here and there, as an attendant on earthquake shocks or other related
movements of the crust, do we find any trace of the upward march which
produces these elevations. Although not a subject for exact
measurements, these features of mountain growth indicate a vast lapse
of time, during which the elevations were formed and worn away.
Yet another and very different method by which we may obtain some
gauge of the depths of the past is to be found in the steps which have
led organic life from its lowest and earliest known forms to the
present state of advancement. Taking the changes of species which have
occurred since the beginning of the last ice epoch, we find that the
changes which have been made in the organic life have been very small;
no naturalist who has obtained a clear idea of the facts will question
the statement that they are not a thousandth part of the alterations
which have occurred since the Laurentian time. The writer is of the
opinion that they do not represent the ten thousandth part of those
vast changes. These changes are limited in the main to the
disappearance of a few forms, and to slight modifications in those
previously in existence which have survived to the present day. So far
as we can judge, no considerable step in the organic series has taken
place in this last great period of the earth's history, although it
has been a period when, as before noted, all the conditions have
combined to induce rapid modifications in both animals and plants. If,
then, we can determine the duration of this period, we may obtain a
gauge of some general value.
Although we can not measure in any accurate way the duration of the
events which have taken place since the last Glacial period began to
wane, a study of the facts seems to show that less than a hundred
thousand years can not well be assumed for this interval. Some of the
students who have approached the subject are disposed to allow a
period of at least twice this length as necessary for the perspective
which the train of events exhibits. Reckoning on the lowest estimate,
and counting the organic changes which take place during the age as
amounting to the thousandth part of the organic changes since the
Laurentian age, we find ourselves in face once again of that
inconceivable sum which was indicated by the physical record.
Here, again, the critics assert that there may have been periods in
the history of the earth when the changes of organic life occurred in
a far swifter manner than in this last section of the earth's history.
This supposition is inadmissible, for it rests on no kind of proof; it
is, moreover, contraindicated by the evident fact that the advance in
the organic series has been more rapid in recent time than at any
stage of the past. In a word, all the facts with which the geologist
deals are decidedly against the assumption that terrestrial changes in
the organic or the inorganic world ever proceed in a spasmodic manner.
Here and there, and from time to time, local revolutions of a violent
nature undoubtedly occur, but, so far as we may judge from the aspect
of the present or the records of the past, these accidents are
strictly local; the earth has gone forward in its changes much as it
is now advancing. Its revolutions have been those of order rather than
those of accident.
The first duty of the naturalist is to take Nature as he finds it. He
must avoid supposing any methods of action which are not clearly
indicated in the facts that he observes. The history of his own and of
all other sciences clearly shows that danger is always incurred where
suppositions as to peculiar methods of action are introduced into the
interpretation. It required many centuries of labour before the
students of the earth came to adopt the principle of explaining the
problems with which they had to deal by the evidence that the earth
submitted to them. Wherever they trusted to their imaginations for
guidance, they fell into error. Those who endeavour to abbreviate our
conception of geologic time by supposing that in the olden days the
order of events was other than that we now behold are going counter to
the best traditions of the science.
Although the aspect of the record of life since the beginning of the
Cambrian time indicates a period of at least a hundred million years,
it must not be supposed that this is the limit of the time required
for the development of the organic series. All the important types of
animals were already in existence in that ancient period with the
exception of the vertebrates, the remains of which have apparently now
been traced down to near the Cambrian level. In other words, at the
stage where we first find evidence of living beings the series to
which they belong had already climbed very far above the level of
lifeless matter. Few naturalists will question the statement that half
the work of organic advance had been accomplished at the beginning of
the Cambrian rocks. The writer is of the opinion that the development
which took place before that age must have required a much longer
period than has elapsed from that epoch to the present day. We thus
come to the conclusion that the measurement of duration afforded by
organic life indicates a yet more lengthened claim of events, and
demands more time than appears to be required for the formation of the
stratified rocks.
The index of duration afforded by the organic series is probably more
trustworthy than that which is found in the sedimentary strata, and
this for the reason that the records of those strata have been
subjected to numerous and immeasurable breaks, while the development
of organic life has of necessity been perfectly continuous. The one
record can at any point be broken without interrupting the sequences;
the other does not admit of any breaches in the continuity.
THE MOON.
Set over against the earth--related to, yet contrasted with it in many
ways--the moon offers a most profitable object to the student of
geology. He should often turn to it for those lessons which will be
briefly noted.
In the beginning of their mutual history the materials of earth and
moon doubtless formed one vaporous body which had been parted from the
concentrating mass of the sun in the manner noted in the sketch of
the history of the solar system. After the earth-moon body had
gathered into a nebulous sphere, it is most likely that a ring
resembling that still existing about Saturn was formed about the
earth, which in time consolidated into the satellite. Thenceforth the
two bodies were parted, except for the gravitative attraction which
impelled them to revolve about their common centre of gravity, and
except for the light and heat they might exchange with one another.
The first stages after the parting of the spheres of earth and moon
appear to have been essentially the same in each body. Concentrating
upon their centres, they became in time fluid by heat; further on,
they entered the rigid state--in a word, they froze--at least in their
outer parts. At this point in their existence their histories utterly
diverge; or rather, we may say, the development of the earth continued
in a vast unfolding, while that of the moon appears to have been
absolutely arrested in ways which we will now describe.
With the naked eye we see on the moon a considerable variation in the
light of different parts of its surface; we discern that the darker
patches appear to be rudely circular, and that they run together on
their margins. Seeing this little, the ancients fancied that our
satellite had seas and lands like the earth. The first telescopes did
not dispel their fancies; even down to the early part of this century
there were astronomers who believed the moon to be habitable; indeed,
they thought to find evidence that it was the dwelling place of
intelligent beings who built cities, and who tried to signal their
intellectual kindred of this planet. When, however, strong glasses
were applied to the exploration, these pleasing fancies were rudely
dispelled.
Seen with a telescope of the better sort, the moon reveals itself to
be in large part made up of circular depressions, each surrounded by a
ringlike wall, with nearly level but rough places between. The
largest of these walled areas is some four hundred miles in diameter;
thence they grade down to the smallest pits which the glass can
disclose, which are probably not over as many feet across. The writer,
from a careful study of these pits, has come to the conclusion that
the wider are the older and the smaller the last formed. The rude
elevations about these pits--some of which rise to the height of ten
thousand feet or more--constitute the principal topographic reliefs of
the lunar surface. Besides the pits above mentioned, there are
numerous fractures in the surface of the plains and ringlike ridges;
on the most of these the walls have separated, forming trenches not
unlike what we find in the case of some terrestrial breaks such as
have been noted about volcanoes and elsewhere. It may be that the
so-called canals of Mars are of the same nature.
[Illustration: Fig. 23.--Lunar mountains near the Gulf of Iris.]
The most curious feature on the moon's surface are the bands of
lighter colour, which, radiating from certain of the volcanolike
pits--those of lesser size and probably of latest origin--extend in
some cases for five hundred miles or more across the surface. These
light bands have never been adequately explained. It seems most likely
that they are stains along the sides of cracks, such as are sometimes
observed about volcanoes.
The eminent peculiarity of the moon is that it is destitute of any
kind of gaseous or aqueous envelope. That there is no distinct
atmosphere is clearly shown by the perfectly sharp and sudden way in
which the light of a star disappears when it goes behind the moon and
the clear lines of the edge of the satellite in a solar eclipse. The
same evidence shows that there is no vapour of water; moreover, a
careful search which the writer has made shows that the surface has
none of those continuous down grades which mark the work of water
flowing over the land. Nearly all of the surface consists of shallow
or deep pits, such as could not have been formed by water action. We
therefore have not only to conclude that the moon is waterless, but
that it has been in this condition ever since the part that is turned
toward us was shaped.
As the moon, except for the slight movement termed its "libration,"
always turns the same face to us, so that we see in all only about
four sevenths of its surface, it has naturally been conjectured that
the unseen side, which is probably some miles lower than that turned
toward us, might have a different character from that which we behold.
There are reasons why this is improbable. In the first place, we see
on the extreme border of the moon, when the libration turns one side
the farthest around toward the earth, the edge of a number of the
great walled pits such as are so plenty on the visible area; it is
fair to assume that these rings are completed in the invisible realm.
On this basis we can partly map about a third of the hidden side.
Furthermore, there are certain bands of light which, though appearing
on the visible side, evidently converge to some points on the other.
It is reasonable to suppose that, as all other bands radiate from
walled pits, these also start from such topographic features. In this
way certain likenesses of the hidden area to that which is visible is
established, thus making it probable that the whole surface of the
satellite has the same character.
Clearly as the greater part of the moon is revealed to us--so clearly,
indeed, that it is possible to map any elevation of its surface that
attains the height of five hundred feet--the interpretation of its
features in the light of geology is a matter of very great
difficulty. The main points seem to be tolerably clear; they are as
follows: The surface of the moon as we see it is that which was formed
when that body, passing from the state of fluidity from heat, formed a
solid crust. The pits which we observe on its surface are the
depressions which were formed as the mass gradually ceased to boil.
The later formed of these openings are the smaller, as would be the
case in such a slowing down of a boiling process.
As the diameter of the moon is only about one fourth of that of the
earth, its bulk is only about one sixteenth of that of its planet;
consequently, it must have cooled to the point of solidification ages
before the larger sphere attained that state. It is probable that the
same changeless face that we see looked down for millions of years on
an earth which was still a seething, fiery mass. In a word, all that
vast history which is traceable in the rocks beneath our feet--which
is in progress in the seas and lands and is to endure for an
inconceivable time to come--has been denied our satellite, for the
reason that it had no air with which to entrap the solar heat and no
water to apply the solar energy to evolutionary processes. The heat
which comes upon the moon as large a share for each equal area as it
comes upon the earth flies at once away from the airless surface, at
most giving it a temporary warmth, but instituting no geological work
unless it be a little movement from the expansion and contraction of
the rocks. During the ages in which the moon has remained thus
lifeless the earth, owing to its air and water, has applied a vast
amount of solar energy to geological work in the development and
redevelopment of its geological features and to the processes of
organic life. We thus see the fundamental importance of the volatile
envelopes of our sphere, how absolutely they have determined its
history.
It would be interesting to consider the causes which led to the
absence of air and water on the moon, but this matter is one of the
most debatable of all that relates to that sphere; we shall therefore
have to content ourselves with the above brief statements as to the
vast and far-acting effects which have arisen from the non-existence
of those envelopes on our nearest neighbour of the heavens.
METHODS IN STUDYING GEOLOGY.
So far as possible the preceding pages, by the method adopted in the
presentation of facts, will serve to show the student the ways in
which he may best undertake to trace the order of events exhibited in
the phenomena of the earth. Following the plan pursued, we shall now
consider certain special points which need to be noted by those who
would adopt the methods of the geologist.
At the outset of his studies it may be well for the inquirer to note
the fact that familiarity with the world about him leads the man in
all cases to a certain neglect and contempt of all the familiar
presentations of Nature. We inevitably forget that those points of
light in the firmament are vast suns, and we overlook the fact that
the soil beneath our feet is not mere dirt, but a marvellous
structure, more complicated in its processes than the chemist's
laboratory, from which the sustenance of our own and all other lives
is drawn. We feel our own bodies as dear but commonplace possessions,
though we should understand them as inheritances from the
inconceivable past, which have come to us through tens of thousands of
different species and hundreds of millions of individual ancestors. We
must overlook these things in our common life. If we could take them
into account, each soul would carry the universe as an intellectual
burden.
It is, however, well from time to time to contemplate the truth, and
to force ourselves to see that all this apparently simple and ordinary
medley of the world about us is a part of a vast procession of events,
coming forth from the darkness of the past and moving on beyond the
light of the present day. Even in his professional work the
naturalist of necessity falls into the commonplace way of regarding
the facts with which he deals. If he be an astronomer, he catalogues
the stars with little more sense of the immensities than the man who
keeps a shop takes account of his wares. Nevertheless, the real profit
of all learning is in the largeness of the understanding which it
develops in man. The periods of growth in knowledge are those in which
the mind, enriched by its store, enlarges its conception while it
escapes from commonplace ways of thought. With this brief mention of
what is by far the most important principle of guidance which the
student can follow, we will turn to the questions of method that the
student need follow in his ordinary work.
With almost all students a difficulty is encountered which hinders
them in acquiring any large views as to the world about them. This is
due to the fact that they can not make and retain in memory clear
pictures of the things they see. They remember words rather than
things--in fact, the training in language, which is so large a part of
an education, tends ever to diminish the element of visual memory. The
first task of the student who would become a naturalist is to take his
knowledge from the thing, and to remember it by the mental picture of
the thing. In all education in Nature, whether the student is guided
by his own understanding or that of the teacher, a first and very
continuous aim should be to enforce the habit of recalling very
distinct images of all objects which it is desired to remember. To
this end the student should practise himself by looking intently upon
a landscape or any other object; then, turning away, he should try to
recall what he has beheld. After a moment the impression by the sight
should be repeated, and the study of the memory renewed. The writer
knows by his own experience that even in middle-aged people, where it
is hard to breed new habits, such deliberate training can greatly
increase the capacity of the memory for taking in and reproducing
images which are deemed of importance. Practice of this kind should
form a part of every naturalist's daily routine. After a certain time,
it need not be consciously done. The movements of thought and action
will, indeed, become as automatic as those which the trained fencer
makes with his foil.
Along with the habit of visualizing memories, and of storing them
without the use of words, the student should undertake to enlarge his
powers of conceiving spaces and directions as they exist in the field
about him. Among savages and animals below the grade of man, this
understanding of spacial relations is very clear and strong. It
enables the primitive man to find his way through the trackless
forest, and the carrier pigeon to recover his mate and dwelling place
from the distance of hundreds of miles away. In civilized men,
however, the habit of the home and street and the disuse of the
ancient freedom has dulled, and in some instances almost destroyed,
all sense of this shape of the external world. The best training to
recover this precious capacity will now be set forth.
The student should begin by drawing a map on a true scale, however
roughly the work may be done, of those features of the earth about him
with which he is necessarily most familiar. The task may well be begun
with his own dwelling or his schoolroom. Thence it may be extended so
as to include the plan of the neighbouring streets or fields. At
first, only directions and distances should be platted. After a time
to these indications should be added on the map lines indicating in a
general way contours or the lines formed by horizontal planes
intersecting the area subject to delineation. After attaining certain
rude skill in such work, the student may advantageously make
excursions to districts which he can see only in a hurried way. As he
goes, he should endeavour to note on a sketch map the positions of the
hills and streams and the directions of the roads. A year of holiday
practice in such work will, if the tasks occupy somewhere about a
hundred hours of his time, serve greatly to extend or reawaken what
may be called the topographic sense, and enable him to place in terms
of space the observations of Nature which he may make.
In his more detailed work the student should select some particular
field for his inquiry. If he be specially interested in geologic
phenomena, he will best begin by noting two classes of facts--those
exhibited in the rocks as they actually appear in the state of repose
as shown in the outcrops of his neighbourhood, and those shown in the
active manifestations of geological work, the decay of the rocks and
the transportation of their waste, or, if the conditions favour, the
complicated phenomena of the seashores.
As soon as the student begins to observe, he should begin to make a
record of his studies. To the novice in any science written, and
particularly sketched, notes are of the utmost importance. These,
whether in words or in drawings, should be made in face of the facts;
they should, indeed, be set down at the close of an observation,
though not until the observer feels that the object he is studying has
yielded to him all which it can at that time give. It is well to
remark that where a record is made at the outset of a study the
student is apt to feel that he is in some way pledged to shape all he
may see to fit that which he has first written. In his early
experience as a teacher, the writer was accustomed to have students
compare their work of observation and delineation with that done by
trained men on the same ground. It now seems to him best for the
beginner at first to avoid all such reference of his own work to that
of others. So great is the need of developing independent motive that
it is better at the outset to make many blunders than to secure
accuracy by trust in a leader. The skilful teacher can give fitting
words of caution which may help a student to find the true way, but
any reference of his undertakings to masterpieces is sure to breed a
servile habit. Therefore such comparisons are fitting only after the
habit of free work has been well formed. The student who can afford
the help of a master, or, better, the assistance of many, such as some
of our universities offer, should by all means avail himself of this
resource. More than any other science, geology, because of the
complexity of the considerations with which it has to deal, depends
upon methods of labour which are to a great extent traditional, and
which can not, indeed, be well transmitted except in the personal way.
In the distinctly limited sciences, such as mathematics, physics, or
even those which deal with organic bodies, the methods of work can be
so far set forth in printed directions that the student may to a great
extent acquire sound ways of work without the help of a teacher.
Although there is a vast and important literature concerning geology,
the greater part of it is of a very special nature, and will convey to
the beginner no substantial information whatever. It is not until he
has become familiar with the field with which he is enabled to deal in
the actual way that he can transfer experience thus acquired to other
grounds. Therefore beyond the pleasing views which he may obtain by
reading certain general works on the science, the student should at
the outset of his inquiry limit his work as far as possible to his
field of practice, using a good text-book, such as Dana's Manual of
Geology, as a source of suggestions as to the problems which his field
may afford.
The main aim of the student in this, as in other branches of inquiry,
is to gain practice in following out the natural series of actions. To
the primitive man the phenomenal world presents itself as a mere
phantasmagoria, a vast show in which the things seen are only related
to each other by the fact that they come at once into view. The end of
science is to divine the order of this host, and the ways in which it
is marshalled in its onward movement and the ends to which its march
appears to be directed. So far as the student observes well, and thus
gains a clear notion of separated facts, he is in a fair way to
gather the data of knowledge which may be useful; but the real value
of these discernments is not gained until the observations go
together, so as to make something with a perspective. Until the store
of separate facts is thus arranged, it is merely crude material for
thought; it is not in the true meaning science, any more than a store
of stone and mortar is architecture. When the student has developed an
appetite for the appreciation of order and sources of energy in
phenomena, he has passed his novitiate, and becomes one of that happy
body of men who not only see what is perceived by the mass of their
fellows, but are enabled to look through those chains of action which,
when comprehended, serve to rationalize and ennoble all that the
senses of man, aided by the instruments which he has devised, tell us
concerning the visible world.
INDEX.
Ætna, Mount, 381.
Agriculture,
American, 346;
in England, winning swamp lands for, 335;
recent developments of, 345.
Alaska, changes on the coast of, 96.
Ants taking food underground, 319;
work of the, on the soil, 318.
Apsides, revolution of the, 61, 62.
Arabians, chemical experiments of the, 13.
Arches, natural, in cavern districts, 258.
Artesian wells, 258, 259.
Arts, advance of Italian fine, 19.
Asteroids, 53;
motions of, about their centres and about the sun, 53.
Astronomers, the solar system and the early, 79.
Astronomy, 31-80;
growth of, since the time of Galileo, 33, 34;
the first science, 10.
Atmosphere, 97-206;
along the tropical belt, 102;
as a medium of communication between different regions, 99;
deprived of water, containing little heat, 105;
beginning of the science of the, 117;
counter-trade movements of the, 105;
envelope of the earth, 98;
expansion of, in a hollow wall during the passage of a storm, 114;
heat-carrying power of the, 105;
heights to which it extends, 99;
in water, 99;
movements no direct influence on the surface of the earth, 122;
movements of the, qualified by the condition which
it encounters, 118;
of mountains, 98;
of the seashore, 98;
of the earth, 98;
of the sun, 73;
snow as an evidence of, 65;
supplying needs of underground creatures, 331;
uprushes of, 101, 102;
upward strain of the, next the earth, 107;
weight and motion of the, 120, 121.
Atmospheric circulation of the soil, 330, 331;
envelopes, 97.
Aurora borealis, 168.
Avalanches, 210-213;
dreaded, in the Alpine regions, 212;
great, in the Swiss Oberland, 211, 212;
rocky, 175-177.
Axis,
imaginary changes in the earth's, 59;
of the earth's rotation, 58;
polar, inclined position of, 58;
polar, nodding movement of the axes, 54;
rotations of the planetary spheres on their axes, 56.
Barometer, causes of changes in the, 117, 118.
Basalts, 309.
Beaches, 93, 142, 144;
boulder, 142, 143;
pebbly, 142;
sand, 144.
Beetles, work of, on the soil, 318, 319.
Belief of the early astronomers about the solar system, 79.
_Bergschrund_, the, 214.
Birds and mammals contributing to the fertility of the soil, 319.
"Blanketing," 269.
Bogs,
climbing, 331-334;
lake, 331-333;
peat, 334, 335;
quaking, 334.
Botany, rapid advance in, 14, 15.
Boulders, 217, 220.
Breakers, 135, 137, 139.
Bridges, natural, 257, 258.
Canals of Mars, 67.
Cañon, newly formed river cutting a, 195.
Cataracts, 193.
Caves, 253-258, 261;
architecture of, 255-258;
hot-water, 261;
mammoth cave, 258;
stalactites and stalagmites on the roof and floor of, 257.
Chasms, 140, 141.
Chemistry, 6, 12, 14;
advance of, 12;
modern, evolving from the studies of alchemists, 13, 14.
Chromosphere, 73.
Civilization of the Icelanders, 384.
Cliffs, sea-beaten, 132, 141, 142.
Climate,
changes of, due to modifications of the ocean streams, 153;
effect of the ocean on the, 147;
of the Gulf Stream, 149, 150.
Clouds, 159;
formation of, 162, 163;
shape of, 163;
water of, usually frozen, 207;
cloud-making, laws of, 161, 162.
Coast,
changes on the Scandinavian, 96;
line, effect of tide on the, 145;
of Greenland, 226;
of New Jersey sinking, 95;
marine, changes in, 95.
Cold in Siberia, 243.
Comets, 47, 50;
collisions of, 50;
kinship of meteorites and, 48;
omens of calamity to the ancients, 50;
the great, of 1811, 49, 50.
Cones. See under VOLCANOES.
Conflict between religion and science, 20, 22;
between the Protestant countries and the followers of science, 20.
Continental shelves, 125.
Continents and oceans, 83;
changes in position of, 91;
cyclones of the, 111;
forms of, 90;
proofs that they have endured for many years, 92;
shape of, 84, 96.
Coral reefs, 153, 353.
Corona, realm of the, 73.
Craters. See under VOLCANOES.
Crevasse, a barrier to the explorer, 218.
Crevice water, 250.
Curds, 214.
Currents,
coral reefs in Florida affecting the velocity of, 153;
equatorial, 150;
of the Gulf Stream, 147-149;
hot and cold, of the sea, 102;
ocean, 145;
oceanic action of trade winds on, 145;
effect on migration of, 157;
icebergs indicating, 243;
surface, history of, 172;
uprushing, near the equator, 106.
Cyclones, 111;
cause of, 111;
of North America, 111;
secondary storms of, 112.
Deltas, 173, 187.
Deposits, vein, 260, 261.
Deserts, interior, 158.
Dew, 159, 160;
a concomitant of cloudless skies, 160,
and vegetation, 160;
formation of, 159-161.
Diablerets, 174.
Diagram of a vein, 260;
showing development of swamp, 335;
how a portion of the earth's surface may be sunk by faulting, 374;
growth of mangroves, 340;
the effect of the position of the fulcrum point
in the movement of the land masses, 94.
Diameter of our sphere at the equator, 62;
of the earth, 82.
Dikes, 192, 293; 305-310;
abounding in volcanic cones, 305;
cutting through coal, 306;
driven upward, 307;
formation of, 305, 310;
material of, 307, 308;
representing movements of softened rock, 309;
their relation to volcanic cones, 307;
variations of the materials of, 307, 308;
waterfalls produced by, 192;
zone of, 306.
Dismal Swamp, 95, 333.
Distances,
general idea of, 27;
good way to study, 27, 28;
training soldiers to measure, 28.
Doldrums, 104, 109;
doldrum of the equator, 109;
of the hurricane, 109.
Drainage, imperfect, of a country affected by glaciers, 242.
Dunes, 123, 124, 325, 326, 387;
moulded, 387.
Duration of geological time, 389.
Dust accumulations from wind, in China, 122.
Earth,
a flattened sphere, 82;
air envelope of the, 98;
amount of heat falling from the sun on the, 41;
antiquity of the, 391;
atmosphere of the, 98;
attracting power of the, 127;
axis of the rotation of the, 58;
composition of the atmosphere of the, 98;
crust of the, affected by weight, 93;
deviation of the path of the, varied, 61;
diameter of the, 82;
of the, affected by loss of heat, 131;
difference in altitude of the surface of the, 83;
discovery that it was globular, 31, 32;
effect of imaginary changes in the relations of sun and, 59;
effect of the interior heat of the, 309, 310;
effect of the sun on the, 60, 61;
formerly in a fluid state, 82;
imaginary view of the, from the moon, 81;
important feature of the surface of the, 83;
jarring caused by faults, 367;
surface of the, determined by heat and light from the sun, 57;
most important feature of the surface of the, 83;
motion of the, affecting the direction of trade winds, 103;
movements, 366;
natural architecture of the, 377;
no part of the, exempt from movement, 384;
parting of the moon and, 396;
path of the, around the sun, 55, 56, 59, 60;
revolving from east to west, 103;
shrinking of the, from daily escape of heat, 89;
soil-covering of the, 343;
study of the, 81-96;
swaying, 385;
tensions, problem of, 371;
tremors, caused by chemical changes in the rocks, 385;
tropical belt of the, 74;
viewed from the surface of the moon, 311, 312;
water store of the, 125.
Earthquakes, 277, 278, 280, 356, 358, 370-384, 388-390;
accidents of, 358;
action of, 356;
agents of degradation, 383, 384;
basis of, 367;
certain limitations to, 380, 381;
Charleston, of 1883, 374, 375;
countries, architecture in, 381;
echoes, 369, 370;
damages of, 377, 390;
effect of,
on the soil, 375;
the surface of the earth, 371;
formed by riving of fissures, 382;
great, occurring where rocks have been
disturbed by mountain-building, 381, 382;
Herculaneum and Pompeii destroyed by an, 277, 280;
Italian, in 1783, 371, 372;
important, not connected with volcanic explosions, 381;
Jamaica, in 1692, 372, 376;
Lisbon, in 1755, 368, 369, 373, 374, 381;
maximum swing of, 369;
measuring the liability to, 386, 387;
mechanism of, 370, 371;
method of the study of, followed by Mr. Charles Mallet, 382, 383;
Mississippi, in 1811, 373, 374, 380, 381;
movement of the earth during, 377;
originating from a fault plane, 367, 369, 370;
originating from the seas, 358, 375;
oscillation of, 376;
poised rocks indicating a long exemption from strong, 388;
Riobamba, in 1797, 375;
shocks of, and their effect upon people, 383;
the direct calamities of Nature, 386;
waves of, 389.
Earthworms, 317-319;
taking food underground, 319.
Eclipses, record of ancient, 130.
Electrical action in the formation of rain and snow, 164.
Elevations of seas and lands, 83.
Energy indestructible, 23.
Envelope, lower, of the sun, 74.
Equator,
diameter of our sphere at the, 62;
doldrum of the, 109;
updraught under the, 102;
uprushing current near the, 106.
Equinoxes, precession of the, 61, 62.
_Eskers_, 221.
Expansion of air contained in a hollow wall during the
passage of the storm, 114.
Experiment, illustrating consolidation of disseminated
materials of the sun and planets, 40.
Falls. See WATERFALLS.
Fault planes, 382.
Feldspar, 324.
Floods, 180, 197;
rarity of, in New England, 121;
river, frequent east of Rocky Mountains, 198.
Föhns, 121.
Forests, salicified, 124.
Fossilization, 354-356.
Fulcrum point, 95.
Galactic plane, 45.
Galongoon, eruption of, 294.
Geological work of water, 168-206.
Glacial action in the valleys of Switzerland, 224;
periods, 63, 243, 246;
in the northern hemisphere, 246;
waste, 324.
Glaciation,
effect of,
in North America, 241;
in Central America, 234;
South America, 234.
Glaciers, 207-249;
action of ice in forming, 230-232;
Alaskan, 216;
continental, 225, 239, 240;
discharge of, 220;
exploring, 220;
extensive, in Greenland and Scandinavia, 244;
former, of North America, 232, 234;
map of, and moraines near Mont Blanc, 217;
motions of, 213;
retreat of the, 228, 230, 235;
secrets of the under ice of, 221;
speed of a, 224;
study of, in the Swiss valleys, 222;
testimony of the rocks regarding, 228;
when covered with winter snows, 216;
valley, 216.
Gombridge, 1830, 74.
Gravitation, law of, 4.
Greeks' idea of the heavens, 31;
not mechanically inventive, 22.
Gulf Stream, current of the, 147.
Heat,
amount of, daily escaping from the earth, 89;
amount of, falling from the sun on the earth, 41;
belief of the ancients regarding, 42;
dominating effect on air currents of tropical, 104;
energy with which it leaves the sun, 41;
internal,
of the earth, 88, 89;
of the earth's interior, 309, 310;
sun, effect on the atmosphere of the, 100;
Prof. Newcomb's belief regarding the, of the sun, 52;
radiation of the earth's, causing winds, 101;
solar, 41;
tropical, and air currents, 104.
Hills, sand, 123.
Horizontal pendulum, 384.
Horse latitudes, 104.
"Horses," 261.
Hurricanes, 107, 110, 317;
commencement of, 107;
doldrum of, 109;
felt near the sea, 110;
in the tropics, 110.
Hypothesis,
nebular, 34, 35, 39, 52, 56;
working, 4, 5.
Ice action,
effect of intense, 222, 223;
in forming glaciers, 230, 232;
recent studies in Greenland of, 239;
depth of, in Greenland, 227;
effect of, on river channels, 196;
effect of, on stream beds, 196;
expanding when freezing, 237;
epoch, 92, 93, 246;
floating, 242;
made soils rarely fertile, 241;
mass, greatest, in Greenland, 226, 227;
moulded by pressure, 215;
streams,
continental, 225, 226;
of the mountains, 225;
of the Himalayan Mountains, 234.
Icebergs, 242, 243;
indicating oceanic currents, 243.
Iceland, volcanic eruptions in, 297, 298.
Instruments, first, astronomical, 10, 11.
Inventions, mechanical, aiding science, 22.
Islands, 84, 272;
continental, 84;
in the deeper seas made up of volcanic ejections, 272;
volcanic, 272.
Jack-o'-lantern, 167.
Jupiter,
gaseous wraps of, 97;
path of the earth affected by, 59, 60;
the largest planet of the sun, 69.
Kames, 325.
Kant, Immanuel, and nebular hypothesis, 34.
Kaolin, 324.
Klondike district, cold in, 243, 244.
Krakatoa,
eruption of, 298-300;
effect of, on the sea, 299;
effect of, on the sun, 300.
Lacolites, 306.
Lacustrine beds, 351.
Lagoons, salt deposits found in, 200.
Lake basins,
formation of, 200, 201;
bogs, 331, 333, 334;
deposits, 350, 351.
Lakes, 199-206;
effect of, on the river system, 205;
fresh-water, 145;
formed from caverns, 202;
great, changing their outlets, 205;
of extinct volcanoes, 203;
temporary features of the land, 203;
volcanic, 203.
Lands,
great, relatively unchangeable, 96;
table, 91;
movements resulting in change of coast line, 351, 352;
shape of the seas and, 83, 84;
accounting for the changes in the attitude of the, 95;
and water, divisions of, 84;
dry, surface of, 85;
general statement as to the division of the, 83, 84;
surface, shape of the, 85;
triangular forms of great, 90.
Latitudes, horse, troublesome to mariners, 104.
Laplace and nebular hypothesis, 34.
Lava, 266-268, 270, 271, 292, 293, 295, 296, 303, 304;
flow of, invading a forest, 268;
from Vesuvius, 293;
of 1669, 295, 296;
temperature of, 295, 296;
incipient, 304;
outbreaks of, 292, 303;
stream eaves, 292, 293.
Law, natural,
Aristotle and, 3;
of gravitation, 4;
of the conservation of energy, 23.
Leaves, radiation of, 160.
Length of days affected by tidal action, 131.
Level surfaces, 91.
Life, organic, evolution of, 15, 16.
Light, belief of the ancients regarding, 42.
Lightning, 24, 164-168;
noise from, 166;
proceeding from the earth to the clouds, 165;
protection of buildings from, 165;
stroke, wearing-out effect of, 165.
Limestones, 353, 357, 358, 360, 364;
formation of, 357, 360.
Lisbon, earthquake of, 1755, 368, 369.
Lowell, Mr. Percival, observations on Venus, 64.
Lunar mountains near the Gulf of Iris, 397.
Mackerel sky, 35.
Mallet, Mr. Charles, and the study of earthquakes, 382, 383.
Man as an inventor of tools, 10.
Mangroves, 340;
diagram showing mode of growth, 340;
marshes of, 339.
Map of glaciers and moraines near Mont Blanc, 217;
of Ipswich marshes, 338.
Mapping with contour lines, 27.
Maps,
desirable, for the study of celestial geography, 77;
geographic sketch, 26, 27.
Marching sands jeopardizing agriculture, 123.
Marine animals, sustenance of, 361-363;
deposits, 325-327, 349, 356;
marshes, 336-340;
waves caused by earthquakes, 387.
Mars, 65-67, 84, 97;
belief that it has an atmosphere, 65;
canals of, 67;
gaseous wraps of, 97;
more efficient telescopes required for the study of, 67;
nearer to the earth than other planets, 65.
Marshes,
mangrove, 339;
map of Ipswich, 338;
marine, 336-340;
deposits found in, 336;
of North America, 337;
on the coast of New England, 339;
phenomena of, 167, 168;
tidal, good earth for tillage, 337;
tidal, of North America, 340.
Mercury, 55, 63, 78;
nearest to the sun, 63;
time in which it completes the circle of its year, 55.
Meteorites, 47, 48;
kinship of comets and, 48.
Meteors, 47;
falling, 47;
composition of, 48;
flashing, 39, 40, 47;
speed of, 47;
inflamed by friction with air, 99.
Methods in studying geology, 400.
Milky Way, 45;
voyage along the path of the, 44, 45.
Mineral crusts, 328, 329;
deposits, 308.
Moon, 38, 395-400;
absence of air and water on the, 399;
attended by satellites, 57;
attraction which it exercises on the earth, 62;
curious feature of the, 397;
destitute of gaseous or aqueous envelope, 397;
diameter of the, 399;
imaginary view of the earth from the, 81;
"libration" of the, 398;
made up of circular depressions, 396, 397;
movements of the, 78;
no atmosphere in the, 97;
parting of the earth and, 396;
position of the, in relation to the earth, 62;
tidal action and the, 131;
tides of the, 126, 127;
why does the sun not act in the same manner as the, 78.
Moraines, 216, 218, 229, 230;
map of glaciers and, near Mont Blanc, 217;
movements of the, 216-218;
terminal, 228.
_Moulin_, 219.
Mount Ætna, 288-310;
lava yielding, 290, 293, 294;
lava stream caves of, 292, 293;
more powerful than Vesuvius, 297;
peculiarities of, 291, 292;
size of, 289-291;
turning of the torrents of, 295.
Mountain-building, 90-93, 304;
folding, 86, 87, 90, 365;
attributed to cooling of the earth, 88;
growth, 392;
Swiss falls, 174;
torrents, energy of, 177.
Mountains, 85, 86, 89, 90-93; 174-178;
form and structure of, 86;
partly caused by escape of heat from the earth, 89;
sections of, 87.
Mount Nuova, formation of, 284.
Mount Vesuvius, 263-285, 288, 289, 293, 302, 381;
description of the eruption of, in A.D. 79, 277-280;
diagrammatic sections through, showing changes in the form
of the cone, 283;
eruption of, in 1056, 281;
in 1882-'83, 264, 266;
eruption of, in 1872, 282;
eruptions of, increased since 1636, 282;
flow of lava from, 285;
likely to enter on a period of inaction, 282, 283;
outbreak of, in 1882-'83, 264, 266.
Naples, prosperity of the city, 289.
Nebular hypothesis, 34, 35, 39, 52.
Neptune, 70.
_Névé_, the, 214;
no ice-cutting in the region of the, 224.
Newcomb's (Prof.) belief regarding the heat of the sun, 52.
Niagara Falls, 191, 192, 204;
cutting back of, 204.
North America,
changes in the form of, 91, 92;
triangular form of, 90.
Ocean,
average depth of the, 89;
climatal effect of the, 147;
currents, 145;
effect of, on migration, 156;
effect of, on organic life, 154;
floor, 85, 93;
hot and cold currents of the, 102;
sinking of the, 93, 94;
the laboratory of sedimentary deposits, 351;
depth of the, 89, 126.
Oceanic circulation, effect of, on the temperature, 152.
Oceans and continents, 83.
Orbit,
alterations of the, and the seasons, 60, 61;
changing of the, 59-63;
shape of the, 61-63.
Organic life, 315, 317, 321, 352, 353, 363;
action of, on the soil, 317, 321;
advantages of the shore belt to, 363;
development of in the sea, 352, 353;
effect of ocean currents on, 154;
processes of, in the soil, 315;
decay of, in the earth, 321.
Orion, 46.
Oscillations of the shores of the Bay of Naples, 287.
Oxbow of a river, 182, 183.
Oxbows and cut-off, 182.
Pebbles,
action of seaweeds on, 143;
action of the waves on, 142, 144.
Photosphere, 74.
Plains, 86;
alluvial, 91, 179, 182, 184-186, 325;
history of, 91;
sand, 325.
Planets, 38;
attended by satellites, 57;
comparative sizes of the, 68;
experiments illustrating consolidation of disseminated
materials of the sun and, 40;
gaseous wraps of, 97;
important observations by the ancients of fixed stars
and planets, 43;
movements of, 57-61;
outer, 78;
table of relative masses of sun and, 77.
Plant life in the Sargassum basins, 156.
Plants and animals,
protection of,
by mechanical contrivances, 364;
and trees, work of the roots of, on the soil, 316, 317;
water-loving, 181;
forming climbing bogs, 332.
Polar axes, nodding movement of, 54.
Polar snow cap, 66.
Polyps, 155, 353.
Pools, circular, 203.
Prairies, 340, 342.
Radiation of heat, 159.
Rain, 152, 156, 164, 168, 170, 328, 330;
circuit of the, 156-168;
drops, force of, 169, 170;
spheroidal form of, 170;
electrical action in the formation of snow and, 164;
work of the, 171.
Realm, unseen solar, 75.
Reeds, 332.
Religion,
conflict between science and, 20, 22;
struggle between paganism and, 21.
Rivers and _débris_, 183;
changes in the course of, in alluvial plain, 182;
deposition of, accelerated by tree-planting, 181;
great, always clear, 205;
inundation of the Mississippi, eating away land, 182;
muds, 222;
newly formed, cutting a cañon, 195;
of snow-ice, 211;
origin of a normal, 173;
oxbow of a, 182,183;
sinking of, 199;
swinging movement of, 179-181;
river-valleys, 193, 194;
diversity in the form of 188-191.
Rocks, 145;
accidents from falling, 174;
cut away by sandstones, 188;
divided by crevices, 252;
duration of events recorded in, 389, 390,
ejection of, material, 311;
falling of, 174-176;
formation of, 262, 263;
from the present day to the strata of the Laurentian, 390;
migration of, 291;
poised, indicating a long exemption from strong earthquakes, 388;
rents in, 252, 253;
stratification of, 349, 350, 352, 365, 390;
testimony of the, in regard to glaciers, 228;
under volcanoes, 303;
variable elasticity of, 366;
vibration of, 367, 368;
rock-waste, march of the, 343;
water, 250, 267.
Rotation of the earth affected by tides, 130;
of the planetary spheres on their axes, 56.
Salicified forests, 124.
Salt deposits formed in lagoons, 200;
found in lakes, 199-200.
Sand bars, 183;
endurance of, against the waves, 145;
hills, travelling of, 123;
marching, 123;
silicious stones cutting away rooks, 188.
Satellites, 53, 54;
motions of, about their centres and about the sun, 53, 54.
Saturn, 38, 53, 57, 396;
cloud bands of, 70;
gaseous wraps of, 97;
path of the earth affected by, 59, 60.
Savages, primitive, students of Nature, 1.
Scandinavia, changes on the coasts of, 96.
Science,
advance of, due to mechanical inventions, 22;
astronomy beginning with, 10;
chemical, characteristics of, 14;
conflict between religion and, 20, 22;
conflict between the Roman faith and, 20;
mechanical inventions as aids to, 22, 23;
modern and ancient, 4;
natural, 5, 6;
of botany in Aristotle's time, 14;
of physiology, 15;
of zoölogy in Aristotle's time, 14;
resting practically on sight, 10.
Scientific development,
historic outlines of, 17;
tools used in measuring and weighing, as an aid to vision, 12.
Sea,
battering action of the, 140;
coast ever changing, 385, 386;
effect of volcanic eruptions on the, 299;
floor deposits of the, affected by volcanoes, 360, 361;
in receipt of organic and mineral matter, 359;
hot and cold currents of the, 102;
littoral zone of the, 351, 352;
puss, 142;
rich in organic life, 352, 353;
solvent action of the, 361;
strata, formation of, 354;
water, minerals in, 185;
weeds, 155, 156.
Seas, dead,
originally living lakes, 200;
water of, buoyant, 199;
eventually the seat of salt deposits, 199-201;
general statement as to division of, 83, 84;
shape of the, 83, 84.
Seashore, air of the, 98.
Seasons, changing the character of the, 61, 62.
Sense of hearing, 9,10;
of sight, 10;
of smell, 9, 10;
of taste, 9, 10;
of touch, 9, 10.
_Seracs_, 214.
Shocks, earthquake. See under EARTHQUAKES.
Shore lines, variation of, 83, 84.
Shores, cliff, 138-142.
Sink holes, 202;
in limestone districts, 253, 254.
Skaptar,
eruption of, 297, 298;
lava from the eruption of, 298.
Sky, mackerel, 35.
Snow, 207-225, 244;
as an evidence of atmosphere, 65;
blankets, early flowers beginning to blossom under, 208;
covering, difference between an annual and perennial, 210;
effect of, on plants, 208;
electrical action in the formation of rain and, 164;
flakes, formation of, 164;
red, 210;
slides, 210;
slides, phenomena of, 210, 211.
Soil,
alluvial, 321, 322;
atmospheric circulation of, 330, 331;
conditions leading to formation of, 313, 331;
continuous motion of the, 314;
covering of the earth, 343;
decay of the, 314, 315;
degradation of the, 344-348;
means for correcting, 346-348;
destruction in grain fields greater than the accumulation, 344;
developing on lava and ashes an interesting study, 343;
development of, in desert regions, 340;
effect of animals and plants on the, 317-320;
effect of earthquakes on the, 375;
fertility of the, distinguished from the coating, 344, 345;
fertility of, affected by rain, 327;
formation of, 314-321;
glacial, characteristics of, 324;
glaciated, 323, 324;
irrigation of the, 328-330;
local variation of, 327;
mineral, 321;
of arid regions fertile when subjected to irrigation, 341;
of dust or blown sand, 321;
of immediate derivation, 321, 322;
phenomena, 313;
processes of organic life in the, 315;
variation in, 321-331;
vegetation protecting the, 316, 317;
washing away of the, 346, 347;
winning, from the sea, 337;
work of ants on the, 318;
tiller, duty of the, 348.
Solar bodies,
general conditions of the, 63-71;
forces, action of, on the earth, 349;
system, 52, 56;
independent from the fixed stars system, 43;
original vapour of, 52, 53;
singular features of our, 68;
tide, 127.
Spheres,
difference in magnitude of, 51;
motions of the, 50, 51;
planetary, rotation of, on their axes, 56.
Spots, sun, 72.
Spouting horn, 141.
Springs, formation of small, 252.
Stalactitization, 256.
Stalagmites and stalactites on the roof and floor of a cavern, 257.
Stars as dark bodies in the heavens, 47;
discovery of Fraunhofer and others on, 23, 38;
double, 39;
and tidal action, 131;
earliest study of, 10;
fixed, important observations by the ancients of planets and, 43;
not isolated suns, 38, 39;
variation in the light of, 46;
limit of, seen by the naked eye, 11;
revolution of one star about another, 46, 47;
shooting, 47;
speed of certain, 51;
study of, 31-80;
sudden flashing forth of, due to catastrophe, 46;
voyage through the, 44, 45;
star, wandering, 74.
Stellar realm, 31-80.
Storms,
circular, 111;
desert, 121, 122;
expansion of air contained in a hollow wall during
the passage of, 114;
great principle of, 105, 106;
in the Sahara, 121;
lightning, more frequent in summer, 167;
paths of, 115;
secondary, of cyclones, 112;
spinning, 115;
thunder, 165-167;
whirling, 106, 124;
whirling peculiarity of, 108, 109.
Strabo, writings of, 18.
Sun,
atmosphere of the, 73;
constitution of the, 72;
distance of the earth from the, 29;
effect from changes in the, and earth, 59;
envelope of the, 73, 74, 97;
experiments illustrating consolidation of disseminated
materials of planets and, 40;
finally, dark and cold, 42;
formation of the eight planets of the, 53;
heat leaving the, 41;
heat of the, 76;
imaginary journey from the, into space, 44;
mass of the, 76, 77;
path of the earth around the, 55;
physical condition of the, 71;
Prof. Newcomb's belief regarding the heat of the, 52;
spots, 75;
abundant at certain intervals, 72;
difficulty in revealing cause of, 75;
structure of the, a problem before the use of the telescope, 72;
table of relative masses of, and planets, 77;
three stages in the history of the, 71;
tides, 126;
why does it not act in the same manner as the moon? 78.
Surfaces, level, 90.
Surf belt, swayings of the, 137.
Swamps,
diagram showing remains of, 335;
Dismal Swamp, 95, 333;
drainage of, 334, 335;
fresh-water, 334, 335;
phenomena of, 167, 168.
Table-lands, 91.
Table of relative masses of sun and planets, 77.
Telescopes, 11, 12, 45;
first results of, 72;
power of, 11;
revelations of, 45.
Temperature,
effects of, produced by vibration, 42;
in the doldrum belt, 118;
of North America, 118;
of the Atlantic Ocean, 118.
Tempests, rate of, 99, 100.
Thunder, 166;
more pronounced in the mountains, 166.
Thunderstorms, 165, 166;
distribution of, 166, 167.
Tidal action,
recent studies of, 131, 132;
marshes of North America, 340.
Tides,
carving channels, 129;
effecting the earth's rotation, 130;
effect of, on marine life, 130;
height of, 128, 129;
moon and sun, 126, 127;
normal run of the, 127;
production of, 131;
of the trade winds, 150;
solar, 127;
travelling of, 127, 128.
Tillage introducing air into the pores of the soil, 331.
Tornadoes, 112, 113, 317;
development of, 113;
effect of, on buildings, 113;
fiercest in North America, 113;
length of, 115;
resemblance of, to hurricanes, 115;
upsucking action of, 114, 115.
Torrents, 177-179, 204.
Trade winds. See under WINDS.
Training in language,
diminishing visual memory, 401;
soldiers to measure distances, 28;
to measure intervals of time, 28;
for a naturalist, 25-29.
Tunnels, natural, 257.
Uranus, 70.
Valley of Val del Bove formed from disturbances of Mount Ætna, 294.
Valleys,
diversity in the form of river, 188-191;
river, 193.
Vapour, 156, 157, 159, 163;
gravitative attraction of, 34, 35;
nebular theory of, 52, 53;
original, of the solar system, 52, 53.
Vegetation,
and dew, 160;
in a measure, independent of rain, 160;
protecting the soil, 316, 317.
Vein, diagram of a, 260.
Venus, 64, 78;
recent observations of, by Mr. Percival Lowell, 64.
Vesuvian system, study of the, 285.
Vesuvius. See MOUNT VESUVIUS.
Visualizing memories, 402, 403.
Volcanic action, 268-276.
Volcanic eruption of A.D. 79, 288;
important facts concerning, 276-279;
islands, 272;
lava a primary feature in, 266;
observations of, made from a balloon, 301;
peaks along the floor of the sea, 272, 273;
possibility of throwing matter beyond control of gravitative
energy, 300.
Volcanoes, 125, 203, 263;
abounding on the sea floor, 302;
accidents from eruptions of, 288;
along the Pacific coast, 271;
ash showers of, maintaining fertility of the soil, 289;
distribution of, 271;
eruption of, 286-294, 368;
explosions from, coming from a supposed liquid interior
of the earth, 275;
exporting earth material, 310;
little water, 375;
Italian, considered collectively, 296, 297;
Neapolitan eruptions of and the history of civilization, 288;
subsidence of the earth after eruption of, 287, 291;
origin of, 263-274;
phenomena of, 263-267;
submarine, 301;
travelling of ejections from, 287, 288.
Waters,
crevice, 250;
of the earth, 250, 251;
cutting action of, 117, 192;
drift, from the poles, 151;
journey of, from the Arctic Circle to the tropics, 151, 152;
dynamic value of, 171;
expansion of, in rocks, 270;
geological work of, 168-206;
in air, 99;
of the clouds usually frozen, 207;
pure, no power for cutting rocks, 204;
rock, 250, 263;
sea, minerals in, 185;
store of the earth, 125;
system of, 125, 156;
tropical, 151;
velocity of the, under the equator, 150;
wearing away rocks, 178, 179;
underground, carrying mineral matter to the sea, 193;
chemical changes of, leading to changes in rock material, 262, 263;
effect of carbonic-acid gas on, 251;
operations of the, 126;
wearing away rocks, 178, 179;
work of, 250.
Waterfalls, 189-193;
cause of, 191;
the Yosemite, 192;
Niagara, 191, 192;
numerous in the torrent district of rivers, 192;
produced by dikes, 192;
valuable to manufactures, 192, 193.
Waterspouts, 115, 116;
atmospheric cause of, 116;
firing at, 116;
life of a, 116;
picturesqueness of, 116;
the water of fresh, 117.
Waves, 128, 129, 132, 145;
action of friction on, 135, 136;
break of the, 136;
endurance of sand against the, 145;
force of, 133, 136, 139;
marine, caused by earthquakes, 387;
of earthquakes, 389;
peculiar features in the action of, 137;
size of, 137, 138;
stroke of the, 144;
surf, 135;
tidal height of, 132;
undulations of, 132;
wind, 132;
wind influence of, on the sea, 134, 135;
wind-made, 128.
Ways and means of studying Nature, 9.
Weeds of the sea, 155.
Well, artesian, 258, 259.
Whirling of fluids and gas, 36, 37.
Whirlwinds in Sahara, 121.
Will-o'-the-wisp, 167.
Winds, 101, 110, 122, 317;
effect of sand, 122;
hurricane, 110;
illustration of how they are produced, 101;
in Martha's Vineyard, 120;
of the forests, work of the, 317;
of tornadoes, effect of, 113;
on the island of Jamaica, 119, 120;
regimen of the, 119;
variable falling away in the nighttime, 100;
trade, 102-105; 145, 146, 150;
action of, on ocean currents, 145:
affected by motion of the earth, 103;
belt, motion of the ocean in, 146;
flow and counter-flow of the, 150;
tide of the, 150;
uniform condition of the, 102;
waves, work of, 132, 134, 135.
Witchcraft, belief of, in the early ages, 21.
Zoölogy, rapid advance in, 14, 15.
End of the Project Gutenberg EBook of Outlines of the Earth's History, by
Nathaniel Southgate Shaler
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