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Title: Report on the geology of the Henry Mountains
Author: Grove Karl Gilbert
Release date: January 16, 2025 [eBook #75119]
Language: English
Original publication: Washington: Government Printing Office, 1877
Credits: Richard Tonsing and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK REPORT ON THE GEOLOGY OF THE HENRY MOUNTAINS ***
[Illustration:
FRONTISPIECE.—Half-stereogram of Mount Ellsworth, drawn to illustrate
the form of the displacement and the progress of the erosion.
The base of the figure represents the sea-level. The remote half shows
the result of uplift alone; the near half, the result of uplift and
erosion,
or the actual condition. (See page 95.)
]
DEPARTMENT OF THE INTERIOR.
U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY OF THE ROCKY MOUNTAIN REGION.
J. W. POWELL, IN CHARGE.
REPORT
ON THE
GEOLOGY OF THE HENRY MOUNTAINS.
By G. K. GILBERT.
[Illustration: [Logo]]
WASHINGTON:
GOVERNMENT PRINTING OFFICE.
1877.
DEPARTMENT OF THE INTERIOR,
U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY
OF THE ROCKY MOUNTAIN REGION,
_Washington, D. C., March 5, 1877_.
SIR: I have the honor to transmit herewith a report on the Geology of
the Henry Mountains, by Mr. G. K. Gilbert.
I am, with great respect, your obedient servant,
J. W. POWELL,
_In charge_.
The Hon. SECRETARY OF THE INTERIOR,
_Washington, D. C._
DEPARTMENT OF THE INTERIOR,
U. S. GEOGRAPHICAL AND GEOLOGICAL SURVEY
OF THE ROCKY MOUNTAIN REGION,
_Washington, D. C., March 1, 1877_.
DEAR SIR: I submit herewith my report on the Geology of the Henry
Mountains, prepared from material gathered under your direction in the
years 1875 and 1876.
I am, with great respect, your obedient servant,
G. K. GILBERT.
Prof. J. W. POWELL,
_In charge_.
PREFACE.
If these pages fail to give a correct account of the structure of the
Henry Mountains the fault is mine and I have no excuse. In all the
earlier exploration of the Rocky Mountain Region, as well as in much of
the more recent survey, the geologist has merely accompanied the
geographer and has had no voice in the determination of either the route
or the rate of travel. When the structure of a mountain was in doubt he
was rarely able to visit the points which should resolve the doubt, but
was compelled to turn regretfully away. Not so in the survey of the
Henry Mountains. Geological exploration had shown that they were well
disposed for examination, and that they promised to give the key to a
type of structure which was at best obscurely known; and I was sent by
Professor Powell to make a study of them, without restriction as to my
order or method. I was limited only in time, the snow stopping my work
two months after it was begun. Two months would be far too short a
period in which to survey a thousand square miles in Pennsylvania or
Illinois, but among the Colorado Plateaus it proved sufficient. A few
comprehensive views from mountain tops gave the general distribution of
the formations, and the remainder of the time was spent in the
examination of the localities which best displayed the peculiar features
of the structure. So thorough was the display and so satisfactory the
examination, that in preparing my report I have felt less than ever
before the desire to revisit the field and prove my conclusions by more
extended observation.
In the description of the details of the structure a demand arose for a
greater number of geographic titles than were readily suggested by
natural forms or other accidents, and recourse was had to the names of
geologists. Except that the present members of my own corps are not
included, the names chosen are of those whose cognate studies have given
me most aid. Mr. Steward and Mr. Howell saw the Henry Mountains before I
did, and gleaned something of their structure from a distance; Dr.
Newberry, Mr. Marvine, Dr. Peale, and Mr. Holmes have described allied
phenomena in Colorado and New Mexico; and the works of Messrs. Jukes,
Geikie, Scrope, and Dana have been among my chief sources of information
in regard to igneous mountains in general. If any of these gentlemen
feel offended that their names have been attached to natural features so
insignificant, I can assure them that the affront will never be repeated
by the future denizens of the region. The herders who build their hut at
the base of the Newberry Arch are sure to call it “the Cedar Knoll”; the
Jukes Butte will be dubbed “Pilot Knob”, and the Scrope, “Rocky Point”.
During the preparation of my report every part of the discussion has
been submitted to Professor Powell for criticism, and many of his
suggestions are embodied in the text. Similar and valuable aid was
received from Capt. C. E. Dutton and Mr. William B. Taylor in the study
of the physical problems to which the discussion of the intrusive
phenomena gave rise. Captain Dutton rendered an important service also
by the study of the collection of igneous rocks, and his report,
included in the fourth chapter, testifies to the thoroughness of his
work. The supervision of the publication has fallen in large share upon
Mr. J. C. Pilling, and the text has had the advantage of his literary
criticism, as well as of his watchful care.
G. K. G.
CONTENTS.
Page.
CHAPTER I. INTRODUCTORY 1
The rock series 3
Unconformities 8
The Great Folds 11
Cliffs and plateaus 13
How to reach the Henry Mountains 14
CHAPTER II. STRUCTURE OF THE HENRY MOUNTAINS 18
CHAPTER III. DETAILED DESCRIPTION OF THE MOUNTAINS 22
Mount Ellsworth 22
Mount Holmes 27
Mount Hillers 30
Mount Pennell 35
Mount Ellen 38
Stereogram of the Henry Mountains 49
CHAPTER IV. THE LACCOLITE 51
The Henry Mountains intrusives, by _Captain C. E. Dutton_ 61
The question of cause 72
The stretching of strata 80
The conditions of rock flexure 83
The question of cover and the question of age 84
The history of the laccolite 95
Laccolites of other regions 97
Possible analogues of the laccolite 98
CHAPTER V. LAND SCULPTURE 99
I. Erosion 99
A. Processes of erosion 99
Weathering 100
Transportation 101
Corrasion 101
B. Conditions controlling erosion 102
Rate of erosion and declivity 102
Rate of erosion and rock texture 103
Rate of erosion and climate 103
Transportation and comminution 106
Transportation and declivity 108
Transportation and quantity of water 109
Corrasion and transportation 111
Corrasion and declivity 112
Declivity and quantity of water 113
II. Sculpture 115
Sculpture and declivity 115
The law of structure 115
The law of divides 116
Sculpture and climate 117
Bad-lands 120
Equal action and interdependence 123
III. Systems of drainage 124
The stability of drainage lines 124
The instability of drainage lines 125
The stability of divides 138
The instability of divides 139
Consequent and inconsequent drainage 143
The drainage of the Henry Mountains 144
CHAPTER VI. ECONOMIC 151
CHAPTER I.
INTRODUCTORY.
The Henry Mountains have been visited only by the explorer. Previous to
1869 they were not placed upon any map, nor was mention made of them in
any of the published accounts of exploration or survey in the Rocky
Mountain region. In that year Professor Powell while descending the
Colorado River in boats passed near their foot, and gave to them the
name which they bear in honor of Prof. Joseph Henry, the distinguished
physicist. In 1872 Prof A. H. Thompson, engaged in the continuance of
the survey of the river, led a party across the mountains by the
Penellen Pass, and climbed some of the highest peaks. Frontiersmen in
search of farming and grazing lands or of the precious metals have since
that time paid several visits to the mountains; but no survey was made
of them until the years 1875 and 1876, when Mr. Walter H. Graves and the
writer visited them for that purpose.
They are situated in Southern Utah, and are crossed by the meridian of
110° 45′ and the thirty-eighth parallel. They stand upon the right bank
of the Colorado River of the West, and between its tributaries, the
Dirty Devil and the Escalante.
At the time of their discovery by Professor Powell the mountains were in
the center of the largest unexplored district in the territory of the
United States—a district which by its peculiar ruggedness had turned
aside all previous travelers. Up to that time the greater part of the
knowledge that had been gained of the interior of the continent had been
acquired in the search for routes for transcontinental railways; and the
cañons of the Colorado Basin, opposing more serious obstacles to travel
than the mountain ranges which were met in other latitudes, were by
common consent avoided by the engineers.
The same general causes which have rendered the region so difficult of
access and passage have made it a desert, almost without economic value.
The physical conditions of elevation and aridity which have caused it to
be so deeply carved in cañons, have prevented the streams with which it
is scantily watered from being bordered by tracts of land which can be
irrigated; and agriculture without irrigation being in that climate an
impossibility, there is nothing to attract the farmer. As will be
explained in the sequel, the mountains offer no inducements to the miner
of the precious metals. There is timber upon their flanks and there is
coal near at hand, but both are too far removed from other economic
interests to find the market which would give them value. It is only for
purposes of grazing that they can be said to have a money value, and so
distant are they at present from any market that even that value is
small.
But while the Henry Mountains contribute almost nothing to our direct
material interests, they offer in common with the plateaus which
surround them a field of surpassing interest to the student of
structural geology. The deep carving of the land which renders it so
inhospitable to the traveler and the settler, is to the geologist a
dissection which lays bare the very anatomy of the rocks, and the dry
climate which makes the region a naked desert, soilless and almost
plantless, perfects the preparation for his examination.
The study of the mountains is further facilitated by their isolation.
They mark a limited system of disturbances, which interrupt a region of
geological calm, and structurally, as well as topographically, stand by
themselves.
The Henry Mountains are not a range, and have no trend; they are simply
a group of five individual mountains, separated by low passes and
arranged without discernible system. The highest rise about 5,000 feet
above the plateau at their base and 11,000 feet above the level of the
ocean. Projecting so far above the surface of the desert, they act as
local condensers of moisture, and receive a comparatively generous
supply of rain. Springs abound upon their flanks, and their upper slopes
are clothed with a luxuriant herbage and with groves of timber. The
smaller mountains and the foot-hills of the larger are less generously
watered and but scantily clothed with vegetation. Their extent is small.
From Ellen Peak to Mount Ellsworth, the two summits which are the most
widely separated, the distance is but twenty-eight miles, and a circle
of eighteen miles radius will include the whole group.
Mount Ellen which is the most northerly of the group, has an extreme
altitude of 11,250 feet, and surpasses all its companions in horizontal
extent as well as altitude. Its crest-line is continuous for two miles,
with an elevation varying little from 11,000 feet. From it there radiate
spurs in all directions, descending to a series of foot-hills as
conspicuous in their topography as they are interesting in their
structure. In some places the base of the mountain is guarded by a
continuous, steep ridge, through which a passage must be sought by the
approaching traveler, but within which movement in any direction is
comparatively unimpeded.
Mount Pennell is a single peak rising to an altitude of 11,150 feet. On
one side its slopes join those of Mount Ellen in Pennellen Pass (7,550
feet), and on the other those of Mount Hillers in the Dinah Creek Pass
(7,300 feet). Its profiles are simple, and it lacks the salient spurs
that characterize Mount Ellen. From the west it is difficult of
approach, being guarded by a barrier ridge continuous with that of Mount
Ellen.
Mount Hillers is more rugged in its character, and although compact in
its general form, is carved in deep gorges and massive spurs. Its
rugosity is contrasted by the smoothness of its pedestal, which to the
south and west and north is a sloping plain merging with the surrounding
plateau.
Mount Ellsworth (8,000 feet) and Mount Holmes (7,750 feet) stand close
together, but at a little distance from the others. The pass which
separates them from Mount Hillers has an altitude of 5,250 feet. They
are single peaks, peculiarly rugged in their forms, and unwatered by
springs. They stand almost upon the brink of the Colorado, which here
flows through a cañon 1,500 feet in depth.
THE ROCK SERIES.
The sedimentary rocks which occur in the Henry Mountains and their
immediate vicinity, range from the summit of the Cretaceous to the
summit of the Carboniferous. It is probable that they were covered at
one time by some thousands of feet of Tertiary strata, but from the
immediate banks of the Colorado these have been entirely eroded, and
their nearest vestiges lie thirty miles to the westward, where they have
been protected by the lava-beds of the Aquarius Plateau.
_Cretaceous._—The Cretaceous strata do not reach to the Colorado River,
but they extend to the Henry Mountains, and are well displayed upon the
flanks. They include four principal sandstones, with intervening shales,
in the following (descending) order:
1. The Ma-suk′ Sandstone, yellow, heavy-bedded 500 feet.
2. The Ma-suk′ Shale, gray, argillaceous, and toward the
top slightly arenaceous 500 feet.
3. The Blue Gate Sandstone, yellow and heavy-bedded 500 feet.
4. The Blue Gate Shale, blue-black and argillaceous,
weathering to a fine gray clay (_Inoceramus deformis_
and _I. problematicus_) 1,000 feet.
5. The Tu-nunk′ Sandstone, yellow and heavy-bedded 100 feet.
6. The Tu-nunk′ Shale, blue-black and argillaceous,
weathering to a fine gray clay (_Inoceramus
problematicus_ and _Baculites anceps_) 400 feet.
7. The Henry’s Fork Group, consisting of—
_a._ Friable yellow sandstone with numerous fossils
(_Ostrea prudentia_, _Gryphea Pitcheri_, _Exogyra
læviuscula_, _Exogyra ponderosa_, _Plicatula
hydrotheca_, _Camptonectes platessa_, and _Callista
Deweyi_) 10 feet.
_b._ Arenaceous shales, purple, green, and white, with
local beds of conglomerate 190 feet.
_c._ Coarse sandstone and conglomerate, with many white
grains and pebbles, interleaved with local beds of
purple and red shale, and containing immense
silicified tree-trunks 300 feet.
—————
Total Cretaceous 3,500 feet.
The three upper sandstones, the Masuk, the Blue Gate, and the Tununk,
are so nearly identical in their lithologic characters that I was unable
to discriminate them in localities where their sequence was unknown.
This was especially the case upon the summits of Mounts Ellen and
Pennell where they occur in a somewhat metamorphosed condition. All of
them contain thin beds of coal, none of which are continuous over large
areas, and only one of which was observed of workable thickness. At the
western foot of Mount Ellen, a bed four feet thick lies at the base of
the Blue Gate Sandstone.
There is almost equal difficulty in discriminating the Masuk, the Blue
Gate, and the Tununk shales. The first is usually of a paler color and
is more apt to include arenaceous bands. It has not been found to
contain fossils, while the lower shales rarely fail to afford them when
search is made. The Blue Gate and Tununk shales are typical examples of
fine argillaceous sediments. They are beautifully laminated and are
remarkably homogeneous. It is only in fresh escarpments that the
lamination is seen, the weathered surface presenting a structureless
clay. The fossils of these shales are so numerous, when they have been
sought out and studied, that they will probably serve not merely to
discriminate the two, but also to correlate them with some of the beds
which have been examined elsewhere in the Colorado Basin. For the
present I am unable to refer any of the Cretaceous rocks above the
Henry’s Fork Group to the divisions which have been recognized
elsewhere, and it is for this reason that I have given local, and
perhaps temporary names to such beds as I have need to mention in the
discussion of the structure of the mountains.
The fossils of the Henry’s Fork Group have been more fully collected,
and they have been referred without question by Dr. White to the group
of that name, as recognized in the Green River Basin (Geology of the
Uinta Mountains, pp. 82 and 94). The white grit which lies at the base
of the group is a conspicuous bed of unusual persistence, and is
recognized wherever Cretaceous rocks are found in the upper basin of the
Colorado.
_Jura-Trias._—The rocks which intervene between the base of the
Cretaceous and the summit of the Carboniferous are of doubtful age,
having been referred to the Trias by one geologist and to the Jura and
Trias by others, while the fossils recently discovered by Mr. Howell
(Geology of Uinta Mountains, page 80) lead to the suspicion at least
that they are all Jurassic. It is probable that the uncertainty will
soon be dispelled by the more thorough working of Mr. Howell’s new
localities; but while it remains, it seems best to recognize its
existence in our nomenclature, and I shall include the whole of the
doubtful series under the title of _Jura-Trias_. At the Henry Mountains
it is easily divided into four groups, as follows:
1. Flaming Gorge Group; arenaceous shales or bad-land
sandstones, purple and white at top and red below 1,200 feet.
2. Gray Cliff Group; massive cross-laminated sandstone,
buff to red in color 500 feet.
3. Vermilion Cliff Group; massive cross-laminated
sandstone, red, with a purple band at the top 500 feet.
4. Shin-ar´-ump Group; consisting of—
_a._ Variegated clay shale; purple and white above and
chocolate below, with silicified wood 300 feet.
_b._ Gray conglomerate, with silicified wood; the
“Shinarump Conglomerate” 30 feet.
_c._ Chocolate-colored shale, in part sandy 400 feet.
—————
Total Jura-Trias 2,930 feet.
The rock of the Flaming Gorge Group is of a peculiar character. It is
ordinarily so soft that in its manner of weathering it appears to be a
shale. It is eroded so much more rapidly than the Henry’s Fork
conglomerate above it, that the latter is undermined, and always appears
in the topography as the cap of a cliff. Nevertheless, it is not
strictly speaking a shale. The chief product of its weathering is sand,
and wherever it can be examined in an unweathered condition it is found
to be a fine-grained sandstone, massive and cross-laminated like those
of the Gray and Vermilion Cliffs, but devoid of a firm cement. In a
number of localities it has acquired, locally and accidentally, a
cement, and it is there hardly distinguishable from the firmer
sandstones which underlie it. In the immediate vicinity of the Henry
Mountains it varies little except in color from summit to base, but in
other localities not far distant it is interrupted near the base by
thick beds of gypsum and gypsiferous clays, and by a sectile,
fossiliferous limestone.
The Gray Cliff and Vermilion Cliff sandstones are often difficult to
distinguish, but the latter is usually the firmer, standing in bold
relief in the topography, with level top, and at its edge a precipitous
face. The former is apt to weather into a wilderness of dome-like
pinnacles, so steepsided that they cannot often be scaled by the
experienced mountaineer, and separated by narrow clefts which are
equally impassable.
The colors of the two sandstones are not invariable. The lower, which
although not reddened throughout its mass is usually stained upon its
surface with a uniform deep color, appears in Mount Ellsworth and at
other points of elevation with as pale a tint as that of the Gray Cliff.
The latter sandstone, on the other hand, where it lies low, is often as
deep in color as the Vermilion. Standing upon one of the summits of the
Henry Mountains and looking eastward, I found myself unable to
distinguish the Gray Cliff Sandstone by color either from the lower part
of the Flaming Gorge Group or from the Vermilion Sandstone. The
bleaching of the redder sandstone in Mount Ellsworth is probably a
result of metamorphism; the reddening of the gray sandstone may depend
on the hydration of the iron which it contains.
The thickness of individual strata in these great sandstones is
remarkable, and is one of the elements which must be taken into account
in the discussion of the problem—which to my mind is yet unsolved—of the
manner in which such immense quantities of homogeneous sand were
accumulated. Ordinarily the depth of strata is indefinable, on account
of the impossibility of distinguishing stratification from lamination;
but where, as in this case, the lamination is oblique to the
stratification, the upper and lower limits of each stratum are
definitely marked. I have at several points measured single strata with
thicknesses of about fifty feet, and near Waterpocket Cañon a stratum of
Vermilion Cliff sandstone was found to be 105 feet thick.
One other measurement is worthy of record; the inclination which oblique
lamination bears to the plane of the stratum in which it occurs appears
to have a definite limit. The maximum of a series of measurements made
at points where to the eye the dip seemed to be unusually great, is 24°.
The sandy layers at the base of the Shinarump Group are characterized by
profuse ripple-marks.
_Carboniferous._—Beneath the Jura-Trias is the Carboniferous. A few
hundred feet of its upper member, the Aubrey Sandstone, are exposed near
the summit of Mount Ellsworth. At that point the sandstone is altered to
the condition of a quartzite, but where it is cut by the upper and lower
cañons of the Dirty Devil River it is massive and cross-laminated,
differing from the Gray Cliff sandstone chiefly in the abundance of its
calcareous cement.
UNCONFORMITIES.
From the Masuk Sandstone to the Aubrey Sandstone, inclusive, there is
perfect conformity of dip. The fold system of the region, of which a
description will be found in succeeding pages, was established after the
deposition of all these strata, and the whole series were flexed
together. Nevertheless, the strata do not represent continuous
deposition. There were intervals in which the sea receded and exposed to
erosion the sediments which it had accumulated. Shallow valleys and
waterways were excavated, and when the sea returned it deposited new
sediments upon the somewhat uneven surface of the old.
The first occurrence of this sort was at the close of the Aubrey epoch.
Its evidence was not found in the Henry Mountains; but at the confluence
of the Paria with the Colorado, eighty miles to the southeast, the
surface of the upper member of the Aubrey Group, which is there a cherty
limestone, is unevenly worn, and in its depressions are beds of
conglomerate, the pebbles of which are derived from the chert of the
limestone itself. The shaly, rippled sandstones which succeed this
conglomerate indicate that the water remained shallow for a time, and in
the middle of the Shinarump epoch the region was once more abandoned by
the sea. The chocolate shales and shaly sands were unevenly worn, and
the first deposit that the returning sea spread over them was a
conglomerate. The evidence of this break is found at many points. The
Shinarump conglomerate although remarkably persistent for a conglomerate
thins out and disappears at a number of points, and at the margins of
its areas it is evident to the eye that it occupies depressions of the
surface on which it rests.
The next break is at the base of the Vermilion Cliff Group. In the
region of the Virgin River and Kanab Creek the change from the
variegated shales of the Upper Shinarump to the homogeneous sandstone of
the Vermilion Cliff is gradual, the interval being filled by a series of
alternating shales and sandstones; but further to the east, in the
region of the Henry Mountains and Waterpocket Cañon, the change is
abrupt, and the firm sandstone rests directly upon the soft shale. The
abruptness of the change would suggest that the currents which brought
the sand had swept away all evidence of the intermediate conditions
which are likely to have connected the epochs represented by the two
sediments; but in one locality, at least, there is direct evidence that
the surface of the clay was exposed to the air before it was covered by
the sand. On the northern flank of Mount Ellsworth are the vestiges of a
system of mud-cracks, such as form where wet clays are dried in the sun.
Where the under surface of the Vermilion sandstone is exposed to view,
it is seen to be marked by a network of ridges which once occupied the
sun-cracks of the Shinarump clay; and where the clay is seen in
juxtaposition, tapering fillets of sand can be traced from the ridges
downward ten feet into the clay.
[Illustration:
FIG. 1.—Fossil Suncracks in the Shinarump Shale.
]
From the base of the Vermilion to the summit of the Cretaceous no
evidence of land erosion has been found; but the association of coal
seams with all of the Cretaceous sandstones except the lowest, shows
that the sea-bottom was frequently brought to the surface of the water
if it was not carried above.
Thus it is evident that the strata of the Henry Mountain region do not
represent continuous sedimentation. At the close of the Aubrey epoch, in
the middle of the Shinarump, and again at the close of the Shinarump,
not merely was the accumulation of sediments interrupted, but the
process was reversed, and a portion of the deposits which had already
been formed were excavated by the agency of rains and rivers, and swept
away to some other region. Each break is indefinite, alike as regards
the interval during which the record of the sea was interrupted and as
regards the extent of the record which was at the same time obliterated.
And yet the evidence of these breaks is of such nature that it would
probably elude observation if a single section only were examined, and
in a region masked by the soil and vegetation of a humid climate it
would hardly be discovered except by accident. The parallelism of
contiguous strata is not alone sufficient evidence that they were
consecutive in time.
At the close of the Cretaceous period there came an epoch of
disturbance. The system of strata which has been described was bent into
great waves, and the crests of the waves were lifted so high above the
sea that they lost thousands of feet by erosion.
In the troughs between the waves lakes remained, in which the material
removed from the crests was redeposited, and by a later change the lake
waters rose so as to cover the truncated crests, and deposit upon the
worn edges of the upbent strata a series of unconforming, fresh-water,
Tertiary sediments.
Thus was produced the only _unconformity of dip_ which involves the
Henry Mountain rocks, and even this is not to be observed in the
immediate vicinity of the mountains, for a later erosion has thence
removed all of the Tertiary strata, and has resumed the degradation of
the older beds.
THE GREAT FOLDS.
The disturbances at the close of the Cretaceous period were of the
Kaibab type[1]. It seems as though the crust of the earth had been
divided into great blocks, each many miles in extent, which were moved
from their original positions in various ways. Some were carried up and
others down, and the majority were left higher at one margin than at the
other. But although they moved independently, they were not cleft
asunder. The strata remained continuous, and were flexed instead of
faulted at the margins of the blocks. Subsequent erosion has obliterated
in great part the inequality of the surface, and the higher-lying blocks
do not stand as mountains, but are outlined by zones of tilted strata
which mark the flexures by which the blocks are separated. Along the
zones of flexure it frequently happens that a hard stratum outcropping
between two that are softer will be preserved from erosion and form a
long, continuous ridge. Such ridges, and other forms produced by the
erosion of the flexures, are conspicuous features of the topography, and
the tracing out of the limits of the blocks is a simple matter. Indeed
the flexures are the first elements of the structure to attract
attention, and it is easy in studying them to overlook the fact that
they merely mark the limits between displaced masses of great extent. If
the reader will examine Plate I at the end of the volume, he will
observe that the system of parallel ridges and valleys which follow the
line of the Waterpocket flexure are very conspicuous features; but it is
only by some such generalization as that given in the stereogram of the
same region (Plate II) that the full structural significance of the
flexures can be realized. Each map was obtained by photography from a
model in relief, in which the proportionate heights of the several
features were not exaggerated. The stereogram was produced by the
restoration of the top of the Cretaceous, the Masuk sandstone, in the
form and position it would have, had there been no erosion of the
region, but displacement only.
Footnote 1:
For a definition of the Kaibab structure, see “Geology of the Uinta
Mountains,” pp. 14 and 17, and American Journal of Science for July
and August, 1876, pp. 21 and 85.
I must caution the reader against an implication of rigidity which might
attach to the meaning of the word “block”, as I have used it in speaking
of the great displaced rock masses. To what extent they may be regarded
as rigid is uncertain, but the presence upon their surfaces of numerous
minor flexures, such as appear in the stereogram, would seem to imply
that their rigidity is not of a high order.
In the northwest corner of the area represented by the stereogram are a
few faults belonging to a system which occupies a large area in that
direction. The system of faults and the system of flexures are
independent, the latter having originated at the close of the Cretaceous
period, and the former after the formation of the Tertiary rocks of the
region, which are referred by Professor Powell to the Bitter Creek
epoch. Over a large district the Tertiary strata were covered by a deep
mantle of lava, which has protected them from erosion to such an extent
that the structure of the district is portrayed in its topography. The
district is its own stereogram, each uplifted block constituting a
mountain and each depressed block flooring a valley.
Not all the displacements of the later system are by faulting, but by
far the greater number. Of the earlier system of displacements none are
simple faults, but a few are combinations of fault and flexure.
[Illustration:
FIG. 2.—Cross-section of the Waterpocket Flexure, opposite the Masuk
Plateau. Scale, one inch = 3,500 feet. 1, Masuk Sandstone. 2, Masuk
Shale. 3, Blue Gate Sandstone. 4, Blue Gate Shale. 5, Tununk
Sandstone. 6, Tununk Shale. a, Gryphea Sandstone. 7, Henry’s Fork
Conglomerate. 8, Flaming Gorge Shale. 9, Fossiliferous Limestone.
10, Gray Cliff Sandstone. 11, Vermilion Cliff Sandstone. 12, Upper
Shinarump Shale. 13, Shinarump Conglomerate. 14, Lower Shinarump
Shale. 15, Aubrey Sandstone.
]
[Illustration:
FIG. 3.—View of the Waterpocket Cañon and the Waterpocket Flexure. The
cliff at the left is capped by the Henry’s Fork Conglomerate. The
arched rocks at the right are the Gray and Vermilion Cliff
Sandstones.
]
[Illustration:
FIG. 4.—Waterpocket Flexure, as seen from the south end of Mount
Ellen.
]
The Waterpocket flexure, represented in the stereogram (Plate II), is
better known in detail than any other of the great flexures of Southern
Utah. It is far from following a straight line, but like most lines of
orographic disturbance swerves to the right and left, while maintaining
a general trend. The amount of its “throw”, or the difference in level
between adjacent parts of the two blocks which it divides, is
inconstant, its maximum being 7,000 feet. At some points the flexed
strata are inclined at an angle of 50° while at others their greatest
dip is but 15°. Toward the north the flexure twice divides. One of its
branches, the Blue Gate flexure, has a throw in the same direction, and
by its separation diminishes the throw of the main flexure. The other,
the Red Gate flexure, has a throw in the opposite direction, and by its
separation increases the throw of the main flexure. Or in other words,
the blocks at the west of the main flexure stand higher than those at
the east; and of two blocks which lie at the west, that at the north of
the Red Gate flexure stands higher than that at the south; while of two
blocks at the east, that which lies northwest of the Blue Gate flexure
is higher than the one at the southeast.
CLIFFS AND PLATEAUS.
Let us now return to the topographic map (Plate I) and examine the forms
into which erosion has wrought the disturbed strata. Thanks to the
aridity of the climate, the erosion has been greatly influenced by the
varying texture of the rocks. Every hard stratum, if inclined, stands
forth in a ridge, or if level, caps a plateau. The Masuk Sandstone,
undermined by the weathering of the Masuk Shale, breaks off everywhere
in a cliff which completely encircles the Masuk Plateau. The plateau
stands upon the Blue Gate Sandstone, and this breaking off in a cliff
upon all sides constitutes another plateau. The Blue Gate Plateau, in
turn, rests upon the Tununk, and that again upon the Henry’s Fork.
Passing either to the north or to the south from the Masuk Plateau, one
descends a great geological stairway, of which each step is a hard
sandstone and each riser a soft shale. Toward the Waterpocket flexure
the edge of each plateau is upturned, and if one goes westward from the
Masuk Plateau, he will cross in the first mile the upturned rocks of all
the lower tables.
The preservation of the Masuk Plateau is due in part to the fact that it
lies in a slight synclinal, but chiefly to the arrangement of the
drainage lines. No streams cross the Henry Mountains, but all go around,
and the plateau occupies the divide between those which flow southward
to the Colorado and those which flow northward to the Dirty Devil.
The antithesis of the Masuk Plateau is to be seen in the Circle Cliffs,
on the summit of the Waterpocket fold. At the lowest point of the Masuk
synclinal a circling cliff has been formed, which facing outward
surrounds a plateau. At the highest point of the Waterpocket fold, which
is in a certain sense anticlinal, a circling cliff has been formed
which, facing inward, surrounds a valley. The two phenomena are alike
illustrations of the law that in regions of inclined strata cliffs face
toward districts of elevation and away from districts of depression.
HOW TO REACH THE HENRY MOUNTAINS.
[Illustration:
FIG. 5.—Ways and Means.
]
No one but a geologist will ever profitably seek out the Henry
Mountains, and I will therefore, in marking out a route by which they
may be reached, select whenever there is option those paths which will
give him the best introduction to this wonderful land. There is no
wagon-road to the mountains, and although a wagon might carry his
baggage the greater part of the way, he must provide himself with other
means of transportation. At Salt Lake City he can procure pack-mules and
pack-saddles, or _apparajos_, and everything necessary for a mountain
“outfit”. His route southward follows the line of the Utah Southern
Railway to Juab Valley, and then touches the Mormon towns of Nephi,
Gunnison, and Salina. At Salina he halts his train for a day while he
rides a few miles up the creek to see the unconformity between the
Tertiary above, and the Jura-Trias and Cretaceous below. This is at
present the last settlement on the route, but there are “ranches” as far
as Rabbit Valley, and if he delays a few years he will find a town
there. By way of the “Twist” road and King’s Meadows he goes to Grass
Valley, and thence to Fish Lake. The lake lies between two upheaved
blocks of trachyte, and covers one which is relatively depressed, and
tilted to the north. At the south end of the lake he stands on the
higher end of the depressed block, and if he follows the shore to the
outlet at the north he will find that the water is contained by a
moraine, which has been thrown across the valley by an ancient glacier,
descending from the mountain at the west. From Fish Lake he goes to
Rabbit Valley, and there delays a day or two to climb Thousand Lake
Mountain. Looking west from the summit he sees the lava-capped plateaus
of the faulted district among which he has journeyed since he left the
“Twist”—huge tables of trachyte bounded by cliffs of displacement, of
which only the sharpest edges have been worn away; and when his eye has
become accustomed to the _facies_ of the faults, he perceives that there
is an identity in structure between the great and the small features.
Just as the whole district is divided into blocks, of which the
dimensions are measured by miles, and the displacements by thousands of
feet, so the greater blocks are sometimes divided into smaller, of which
the dimensions are measured by rods and furlongs and the displacements
by tens and hundreds of feet. Looking eastward he sees the region of the
great flexures spread out before him like a map. The Waterpocket flexure
starts from the very mountain beneath him, and curving to the right,
runs far to the south and is lost in the distance. Beyond it are the
Henry Mountains springing abruptly from the desert; and against the
horizon are outlined other island mountains, gray in the distance. To
the left is the San Rafael Fold, the rival of the Waterpocket in
grandeur; and all about are tables and cliffs. The vivid hues of the
naked rocks are obscured only by the desert haze, and the whole
structure is pictured forth by form and color.
[Illustration:
FIG. 6.—The Unconformity of the lower cañon of Salina Creek. The
horizontal strata are Tertiary; the inclined, Cretaceous.
]
To reach the Henry Mountains from Rabbit Valley, he must cross the
Waterpocket flexure; and so continuous and steep are the monoclinal
ridges which follow the line of flexure, that there are but four points
known where he can effect a passage. Except at these points, the barrier
is impassable from Thousand Lake Mountain to the Colorado River, a
distance of eighty miles. The most difficult and circuitous route I will
not describe. The remaining three diverge but slightly from each other.
Starting from Rabbit Valley he follows for a few miles the valley of the
Dirty Devil, which here, through the “Red Gate”, passes from the
trachyte plateaus and enters the land of cañons. He does not follow it
far, but where the river enters a cañon in the Aubrey Sandstone, bears
to the left, and by the aid of a trail which Indians have made finds a
sinuous but easy pathway along a monoclinal valley, following the
outcrop of the lower Shinarump. At his right the Aubrey Sandstone rises
to form the plateau through which the river defiles. At his left the
Vermilion Sandstone stands in a vertical wall. Beneath his feet are the
shaly sandstones of the Shinarump Group, bare of vegetation and
displaying a profusion of ripple-marks, such as is rarely if ever
equaled. A ride of twelve miles brings him once more to the Dirty Devil
River, which emerging from its Carboniferous cañon dives at once into a
still deeper cañon through the Vermilion and Gray Cliff Sandstones. He
can follow the river if he tries, and emerge with it beyond the flexure;
but the way is difficult and the Indian trail he has followed thus far
leads on to another cañon. The monoclinal valley which has opened so
easy a way continues for fifteen miles farther, and in that distance is
crossed by four waterways, each of which leads by a narrow cañon through
the great sandstones. The first and fourth are impassable. The second
carries no permanent stream, and is called the “Capitol Cañon”. The
third affords passage to Temple Creek. The smoothest road lies through
Capitol Cañon, but the Temple Creek Cañon has an advantage in the
presence of water, and is furthermore attractive by reason of the
picture-writings on the walls.
He has now to cross the Blue Gate flexure, and to do this he leaves
Temple Creek a little below the mouth of its cañon. Seeking once more
the guidance of a trail, he journeys southeastward over the gypsum and
sand of the Flaming Gorge Group to a pass which from a distance he has
detected in the monoclinal ridge marking the Henry’s Fork conglomerate.
Through this pass Tantalus Creek sometimes runs on its way to join the
Dirty Devil, and he may find a stream of muddy water; but the bottom is
more likely to be dry with the exception of a few pools. Passing through
the gap he finds before him a similar opening in the Tununk Ridge, and
beyond that a break in the Blue Gate Cliff. From Tantalus Creek he
ascends to the pass in the Blue Gate Cliff, and climbing to the summit
of a sharp divide in the shales descends again to Lewis Creek, which
there follows a cañon through the Blue Gate Plateau. Here he finds
bowlders of the Henry Mountain trachyte—for Lewis Creek rises in the
Henry Mountains—and a few hours’ ride toward the sources of the stream
brings him to the base of Mount Ellen.
_Distances._
From Salt Lake City to Salina 155 miles.
From Salina to Fish Lake 38 miles.
From Fish Lake to Rabbit Valley 27 miles.
From Rabbit Valley to Temple Creek Cañon 27 miles.
From Temple Creek Cañon to Lewis Creek 18 miles.
Thence to the base of Mount Ellen 10 miles.
———
Total from Salt Lake City to the Henry Mountains 275 miles.
CHAPTER II.
THE STRUCTURE OF THE HENRY MOUNTAINS.
The mountains stand within the province of the great flexures, but are
independent of them. Fifteen miles to the westward runs the Waterpocket
flexure. Thirty miles to the north is the San Rafael fold. At the east
the strata rise toward a great uplift, of which the full form is
unknown. But where the Henry Mountains stand the rocks are unaffected by
these disturbances. They have a uniform dip of about 45′ to the
northwest, and form a perfect datum-plane from which to measure the
magnitude of the displacements which have given rise to the mountains.
The mountains are composed of a large number of parts which are in a
certain degree individual and homologous. By the generalization of the
characters of those parts a conception has been obtained of a _type
structure_ to which the entire series of phenomena has been referred.
In laying the material before the reader, the following plan will be
followed:
First. The type of structure will be briefly set forth.
Second. The phenomena by which the type is at once demonstrated and
illustrated will be described in detail.
Third. The type of structure will be discussed.
If the structure of the mountains be as novel to the reader as it was to
the writer, and if it be as strongly opposed to his preconception of the
manner in which igneous mountains are constituted, he may well question
the conclusions in regard to it while they are unsustained by proof. I
can only beg him to suspend his judgment until the whole case shall have
been presented. On some accounts it would have been well to follow in
writing the order of investigation, and develop the general plan of
structure as it was developed in the field, by the addition here of one
element and there of another; or at least to assemble the facts before
announcing my deductions. But such a course would be at the expense of
an important element of convenience and brevity. As will appear in the
sequel, the preliminary explanation of the type structure furnishes a
complement of categories and terms by the aid of which the description
of the details of observation, essentially tedious, is greatly
abbreviated.
[Illustration:
FIG. 7.—Ideal Cross-section of a Mountain of Eruption.
]
[Illustration:
FIG. 8.—Ideal Cross-section of a Laccolite, showing the typical form
and the arching of the overlying strata.
]
It is usual for igneous rocks to ascend to the surface of the earth, and
there issue forth and build up mountains or hills by successive
eruptions. The molten matter starting from some region of unknown depth
passes through all superincumbent rock-beds, and piles itself up on the
uppermost bed. The lava of the Henry Mountains behaved differently.
Instead of rising through all the beds of the earth’s crust, it stopped
at a lower horizon, insinuated itself between two strata, and opened for
itself a chamber by lifting all the superior beds. In this chamber it
congealed, forming a massive body of trap. For this body the name
_laccolite_ (λάκκος, _cistern_, and λίθος, _stone_) will be used. Figure
7 and Figure 8 are ideal sections of a mountain of eruption and of a
laccolite.
The laccolite is the chief element of the type of structure exemplified
in the Henry Mountains.
It is evident that the intrusion of a laccolite will produce upon the
surface as great a hill as the extrusion of the same quantity of matter,
the mass which is carried above the original surface being precisely
equivalent to that which is displaced by the laccolite; and it is
further evident that where the superior rock is horizontally stratified
every stratum above the laccolite will be uplifted, and, unless it is
fractured, will be upbent, and will portray, more or less faithfully, by
its curvature, the form of the body it covers.
Associated with the laccolites of the Henry Mountains are _sheets_ and
_dikes_.
The term _sheet_ will be applied in this report to broad, thin,
stratified bodies of trap, which have been intruded along the partings
between sedimentary strata, and conform with the inclosing strata in
dip. _Dikes_ differ from sheets in that they intersect the sedimentary
strata at greater or less angles, occupying fissures produced by the
rupture of the strata.
The logical distinction between dike and sheet is complete, but in
nature it not unfrequently happens that the same body of trap is a sheet
in one place and a dike in another. Between the sheet and the laccolite
there is a complete gradation. The laccolite is a greatly thickened
sheet, and the sheet is a broad, thin, attenuated laccolite.
[Illustration:
FIG. 9.—Ideal Cross-section of a Laccolite, with accompanying Sheets
and Dikes.
]
In the district under consideration the laccolite is usually, perhaps
always, accompanied by dikes and sheets (see Figure 9). There are sheets
beneath laccolites and sheets above them. The superior sheets have never
been observed to extend beyond the curved portion of the superior
strata. Dikes rise from the upper surfaces of the laccolites. They are
largest and most numerous about the center, but, like the superior
sheets, they often extend nearly to the limit of the flexure of the
uplifted strata. The larger often radiate from the center outward, but
there is no constancy of arrangement. Where they are numerous they
reticulate.
In the accompanying diagrams dikes are represented beneath as well as
above the laccolites. These are purely hypothetical, since they have not
been seen. In a general way, the molten rock must have come from below,
but the channel by which it rose has in no instance been determined by
observation.
The horizontal distribution of the laccolites is as irregular as the
arrangement of volcanic vents. They lie in clusters, and each cluster is
marked by a mountain. In Mount Ellen there are perhaps thirty
laccolites. In Mount Holmes there are two; and in Mount Ellsworth one.
Mount Pennell and Mount Hillers each have one large and several smaller
ones.
[Illustration:
FIG. 10.—Ideal Cross-section of Grouped Laccolites.
]
Their vertical distribution likewise is irregular. Some have intruded
themselves between Cretaceous strata, others between Jura-Triassic, and
others between Carboniferous. From the highest to the lowest the range
is not less than 4,000 feet. Those which are above not unfrequently
overlap those which lie below, as represented in the ideal section,
Figure 10.
The erosion of the mountains has given the utmost variety of exposure to
the laccolites. In one place are seen only arching strata; in another,
arching strata crossed by a few dikes; in another, arching strata filled
with a network of dikes and sheets. Elsewhere a portion of the laccolite
itself is bared, or one side is removed so as to exhibit a natural
section. Here the sedimentary cover has all been removed, and the
laccolite stands free, with its original form; there the hard trachyte
itself has been attacked by the elements and its form is changed.
Somewhere, perhaps, the laccolite has been destroyed and only a dike
remains to mark the fissure through which it was injected.
CHAPTER III.
DETAILED DESCRIPTION OF THE MOUNTAINS.
MOUNT ELLSWORTH.
It has already been stated that the strata about the bases of the Henry
Mountains are nearly level; but the country which is built of them is
far from level. The arrangement of the drainage lines has caused the
degradation of some parts to greatly exceed that of others, so that
while the district at the south, which borders the Colorado River, is
paved with the red sandstones of the Jura-Trias series, the adjacent
region at the north still carries the yellow sandstones and blue shales
of the Cretaceous series. All about Mount Ellsworth are the upper strata
of the Jura-Trias. The lower beds of the same series rise upon its
flanks and arch over its summit.
A description of the structure of the mountain must include, first, the
arch of the strata; second, the faults which modify the arch; third, the
system of trachyte dikes and trachyte sheets; and fourth, the sculpture
of the mountain. In its general proportions the arch is at once simple
and symmetrical. From all sides the strata rise, slowly at first, but
with steadily increasing rate, until the angle of 45° is reached. Then
the dip as steadily diminishes to the center, where it is nothing. A
model to exhibit the form of the dome would resemble a round-topped hat;
only the level rim would join the side by a curve instead of an angle,
and the sides would not be perpendicular, but would flare rapidly
outward (see Figure 11). The base of the arch is not circular, but is
slightly oval, the long diameter being one-third greater than the short.
The length of the uplift is a little more than four miles; the width a
little more than three miles, and the height about 5,000 feet. The
curvature fades away so gradually at its outer limit that it is not easy
to tell where it ends, and the horizontal dimensions assigned to the
dome are no more than rude approximations. But there is another element
which can be given more exactly. The line of maximum dip, which
separates the convex upper portion of the dome from the concave
periphery, is easily traced out in nature, and runs at the foot of the
steep part of the mountain. It surrounds an area two miles in width and
two and two-thirds miles in length.
[Illustration:
FIG. 11A.—View from the west spur of Mount Ellsworth, showing the
trachyte dikes of the north spur and revetments of sandstone and
trachyte.
]
[Illustration:
FIG. 11.—Stereogram of Mount Ellsworth; an ideal restoration of the
form of the overarching strata.
]
The Ellsworth arch is almost but not completely isolated. The Holmes
arch, upon the east side, stands so near that the bases of the two
impinge and coalesce, and the same thing happens, though less notably,
with the Hillers arch at the north.
The simplicity of the arch is further impaired by faults—not great
faults dividing the whole uplift, but a system of small displacements
which are themselves subordinate phenomena of the uplift. They are
restricted to the central portion, and never occur so low down as the
line of maximum dip. The strata of the upper part of the arch are
divided into a number of prismoid blocks which stand at slightly
different levels but are not sufficiently deranged to destroy the
general form of the arch. The greatest throw is only a few hundred feet.
All or nearly all of the fault planes are occupied by dikes of trachyte.
The trachyte injections are not confined to the fault planes, nor is
their area so restricted as the fault area. Dikes and sheets abound from
the crest of the dome down to what might be called its springing
line—the line of maximum dip. At the center, dikes are more numerous;
near the limit, sheets. The central area is crowded so full of dikes,
and the weathering brings them so conspicuously to the surface, that the
softer sedimentaries are half concealed, and from some points of view
the trachyte appears to make the entire mass. The accompanying plan
(Figure 12) shows the arrangement of the dikes in one of the outer
amphitheaters of the mountain, where they are less complicated than in
the central region. The trends of two spurs (_a b_ and _c d_) are
indicated by the hatchings. They join the main crest of the mountain at
_e_, and inclose between them a deep amphitheater which opens to the
west. Upon the steep walls of the amphitheater the dikes outcrop in
lines of crags, dividing rough slopes of yellow and purple and brown
sandstone. The profile of one of the walls of the amphitheater (from _a_
to _b_, Figure 12) is drawn in Figure 13 for the sake of exhibiting the
relation of the dikes to faulted blocks of sandstone. It will be seen
that the throw of the faults is not constantly in one direction.
[Illustration:
FIG. 14.—Profile of the Northern Spur of Mount Ellsworth. 1, 2, 3, 4,
and 5 are Trachyte Dikes. _A_, Aubrey Sandstone. _S_, Shinarump
Shale. _V_, Vermilion Cliff Sandstone. _G_, Gray Cliff Sandstone.
]
The zone of sheets is just inside the line of maximum dip. Usually only
one or two sheets are laid bare by the erosion, but at one point (see
Figure 14) four can be counted. Toward the center of the uplift all of
these are limited by the erosion and exhibit their broken edges.
Downward, or toward the periphery, they dip out of sight. Laterally they
can be traced along the mountain side for varying distances, but they
soon wedge out and are replaced by others _en echelon_. In thickness the
sheets rarely exceed 50 feet, and never 100. They are always thin as
compared to the rock masses which separate them, but, by reason of their
superior ability to resist erosion, monopolize a large share of the
surface, and mask a still greater amount with their _débris_.
[Illustration:
FIG. 12.—Ground plan of Trachyte Dikes on the western flank of Mount
Ellsworth.
]
[Illustration:
FIG. 13.—Profile of a western spur of Mount Ellsworth, showing the
arrangement of Dikes and Faults. The dotted bed is the purple band
at the top of the Vermilion Cliff Sandstone. The dikes of trachyte
are indicated by letters.
]
The sedimentary rocks are not altered beyond the region of trachyte
intrusion. The mere flexure of the strata was not accompanied by a
perceptible change of constitution. In the zone of sheets there is
little change except along the surfaces of the contact. For a few feet,
or perhaps only a few inches, there is a discoloration (usually a
decolorization) and a slight induration, without notable alteration of
minerals. But in the region of reticulated dikes none of the
sedimentaries are unchanged; crystals are developed, colors are
modified, and hardness is increased, so that the physical properties of
familiar strata no longer serve for their identification. Still there is
no crumpling.
The trachyte masses and the altered rocks in contact with them are so
much more durable than the unaltered strata about them that they have
been left by the erosion in protuberances. The outcrop of every dike and
sheet is a crag or a ridge, and the mountain itself survives the general
degradation of the country only in virtue of its firmer rock masses.
Nevertheless, the mountain, because it was higher than its surroundings,
has been exposed to more rapid erosion, and has been deprived of a
greater depth of strata. From the base of the arch there have been worn
3,500 feet of Cretaceous, and from 500 to 1,500 feet of the Jura-Trias
series, which is here about 3,000 feet thick. From the summit of the
arch more than 2,500 feet of the Jura-Trias have been removed.
The strata exposed high up on the mountain being older than those at the
base, and the dip being everywhere directed away from the center, it is
evident that the mountain is surrounded by concentric outcrops of beds
which lift their escarpments toward it. It is usually the case, where
the strata which incline against the flank of a mountain are eroded,
that the softer are excavated the more rapidly, while the harder are
left standing in _ridges_; and an alternation of beds suitable for the
formation of a ridge occurs here. One of the upturned beds is the
massive Vermilion Cliff sandstone, and beneath it are the shales of the
Shinarump Group. By the yielding of the shales the sandstone is left
prominent, and it circles the base of the mountain in a monoclinal
ridge. But the ridge is of a peculiar character, and has really no title
to the name except in the homology of its structure with that of the
typical monoclinal ridge. It lacks the continuity which is implied by
the word “ridge”. The drainage of Mount Ellsworth is from the center of
the dome outward. A half dozen drainage lines originate in the high
crests and pass outward through the zone of upturned strata. Lower down
their interspaces are divided by others, and when they reach the
circling escarpment of the Vermilion sandstone their number is fifteen.
Each of these cuts the ridge to its base, and the effect of the whole is
to reduce it to a row of sandstone points circling about the mountain.
Each point of sandstone lies against the foot of a mountain spur, as
though it had been built for a _retaining wall_ to resist the out-thrust
of the spur. Borrowing a name from the analogy, I shall call these
elementary ridges _revet-crags_, and speak of the spurs which bear them
as being _revetted_. The accompanying sketch is designed to illustrate
the structure, but is not drawn from nature. In the view of Mount
Hillers (Figure 27) the revetments may be seen, and in the bird’s-eye
view of the Henry Mountains (Plate V), as well as in the Frontispiece,
the revet-crags of Mount Ellsworth also are portrayed. The diagram of
the north spur of Mount Ellsworth (Figure 14) shows the revet-crag of
that spur at _V_.
[Illustration:
FIG. 15.—Revet-Crags.
]
The revet-crags of Vermilion sandstone follow, in a general way, the
line of maximum dip about the base of Mount Ellsworth; but a few of them
rise higher, and one—that which joins the northwest spur—climbs until it
is but little lower than the summit of the mountain. Outside the circle
of Vermilion Cliff sandstone lies the Gray Cliff sandstone, and in a few
places it takes the form of a revetment. Inside the same circle there
are many revetments, constituted by trachyte sheets bedded in Shinarump
shales (Figure 14). Conforming perfectly with the strata, the sheets
yield by erosion forms which are identical with those afforded by hard
sedimentary beds, and to the distant eye the impression of the arching
structure of Mount Ellsworth is conveyed less by what can be seen of the
strata than by ascending revetments of trachyte sheets, which simulate
and interpret the strata.
[Illustration:
FIG. 16.—Mount Holmes, from the north.
]
The laccolite of Mount Ellsworth is not exposed to view, but I am
nevertheless confident of its existence—that the visible arching strata
envelop it, that the visible forest of dikes join it, and that the
visible faulted blocks of the upper mountain achieved their displacement
while floated by the still liquid lava. The proof, however, is not in
the mountain itself, but depends on the association of the phenomena of
curvature and dike and sheet with laccolites, in other mountains of the
same group. In the sequel these will be described, but it chances that
the mountain next to be considered is even less developed by erosion
than Mount Ellsworth.
MOUNT HOLMES.
The order of sequence which places Mount Ellsworth before Mount Holmes
is the order of complexity. The former contains one laccolite, the
latter two. Neither of the two is visible, but the strata which envelop
them shadow forth their forms and leave no question of their duality.
They are so closely combined that the lesser seems a mere appendage of
the greater. From the center of the greater there is a descent of strata
in all directions, but from the center of the lesser the rocks incline
toward one-half only of the horizon. Where the two convex arches join
there is a curved groin—a zone of concave curvature uniting the two
convexities. About the compound figure can be obscurely seen a line of
maximum dip, and beyond that the fading of the curves. The curves
throughout are so gentle that it was found exceedingly difficult to
establish their limits. In a general way it may be said that each of the
Holmes arches is as broad as the Ellsworth arch, but the vertical
displacement is less. In the formation of the greater Holmes arch the
amount of uplift was 3,000 feet; for the lesser arch, 1,500 feet.
There is no evidence in the forms of the arches which proves one to be
older than the other. Studying the curves in the field, I could not
discover that either arch asserted itself more strongly than the other
in their common ground. They seem to meet upon equal terms. Still it is
probable, _a priori_, that they were formed successively and not
simultaneously. The coincidence in time of two eruptions of lava from
neighboring vents is no more unlikely than the coincidence of the two
irruptions, and the same principle of least resistance which causes
individual laccolitic arches to assume spheroidal forms, would have
given to the compound arch of two laccolites, coincident in time, a
simple instead of a compound form.
Assuming that the arches were successive in origin I shall in another
and more appropriate chapter discuss the problem of their chronological
order in the light of their somewhat peculiar drainage system.
The lesser arch betrays no dikes nor sheets. The Vermilion Cliff
sandstone covers it to the top. The greater is crowned by a few grand
dikes which govern its topography. From the center a long dike runs to
the south, a short one to the north, two to the east, and one to the
west. The course of each is a mountain spur, and between them are
amphitheaters and gorges. Clinging to the dikes are bodies of altered
sandstone, but the great sandstone masses of the summit were unaltered
and from them have been excavated the gorges. Along the dike-filled
fissures there has been some faulting, but there is no reason to believe
that the displacement is great in amount. Toward the flanks of the
mountain there are a few sheets, the outermost of which is far within
the line of maximum flexure. Their escarpments instead of facing upward
like the revetting sheets of Mount Ellsworth, face downward; their
buried and unknown edges are the edges toward the mountain. Their
thinning toward the periphery of the arch is conspicuous to the eye in
many instances, as is also the thinning of the dikes.
Another peculiarity of dike form, one which has since been noted in a
number of localities, was first detected in Mount Holmes. It consists in
a definite upper limit. The dike so marked is often as even upon its
upper surface as an artificial stone wall. The upper surface may be
level or may incline toward one end of the dike, but in either case it
is sure to be found parallel to the bedding of the strata which inclose
the dike. This fact led to the suspicion, afterward confirmed by more
direct evidence, that the flat top of the dike was molded by an unbroken
stratum of rock bridging across the fissure which the lava filled
(Figure 20). The converse phenomenon can be observed in the ridge which
joins Mounts Ellsworth and Holmes. A great dike there forms the crest of
the ridge for half a mile, its base being buried in sandstone; but at
the end of the ridge the strata are seen to be continuous beneath it
(Figure 21).
[Illustration:
FIG. 17.—Stereogram of the Holmes Arches; an ideal restoration of the
form of the overarching strata.
]
[Illustration:
FIG. 18.—Ideal cross-section of the Laccolites of Mount Holmes.
]
[Illustration:
FIG. 19.—A flat-topped dike.
]
[Illustration:
FIG. 20.—Ideal Cross-section of Flat-topped Dikes; _a_, before
denudation; _b_, after denudation.
]
[Illustration:
FIG. 21.—Ideal Cross-section of a Flat-bottomed Dike.
]
[Illustration:
FIG. 22.—Diagram to illustrate a hypothetical explanation of
Flat-edged Dikes.
]
That a fissure several feet or several scores of feet in width should
end thus abruptly, demands explanation, and the phenomena immediately
concerned offer none. Nevertheless it is easy to make an assumption
which if true renders both cases clear. If we assume that the fissure
instead of ending at the crosshead is merely offset, and resumes its
course beyond, and that the dike contained in it has two bodies
connected by a thin sheet (Figure 22), we shall have no difficulty in
conceiving the erosion which will produce either of the natural
appearances described.
The rocks which constitute Mount Holmes are the same as those about its
base. The Vermilion Cliff and Gray Cliff Sandstones alone appear in the
crests. The underlying Shinarump shales are cut by the erosion at a few
points only, and those are near the base. For this reason the Vermilion
Sandstone is not undermined about the base, and the circle of
revet-crags which surrounds Mount Ellsworth finds no counterpart. There
are, indeed, a few revetments of Gray Cliff sandstone, but they are
scattered and for the most part inconspicuous.
In the general view of Mount Holmes (Figure 16), one of the main dikes
crowns the nearest spur, and another the spur leading to the right. At
the left are minor dikes, and high up is a trap sheet notched on its
lower edge. At the left base of the mountain lies the lesser arch.
Figure 23 gives a section exhibited by one of the northward cañons. It
shows one of the faults of the upper part of the arch and illustrates
the thinning of the sheets as they descend.
[Illustration:
FIG. 23.—Section shown in a northward cañon of Mount Holmes. _a_,
Vestige of Trachyte sheet. _b_ _b_ _b_, Trachyte sheets. _c_,
Trachyte dike. 1, Gray Cliff Sandstone. 2 2, Purple Sandstone. 3,
Vermilion Cliff Sandstone. 4, Shinarump Shale.
]
MOUNT HILLERS.
Next in order to the north is Mount Hillers. Let it not be supposed,
however, that there is discernible system in the geographic arrangement
of the mountains or of the laccolites. A chart of the mountain peaks and
a chart of the laccolites would alike prove intractable in the hands of
those geologists who draw parallel lines through groups of volcanic
vents by way of showing their trend. They are as perfectly heterotactous
as they could be made by an artificial arrangement.
The diagram (Figure 24) shows the relation of the laccolite groups to
each other and to the meridian. The principal mountain summits are
indicated by triangles, and the curved lines inclose areas of
disturbance.
Mount Hillers and its foot-hills are constituted by a group of no less
than eight laccolites, and a ninth, the Howell laccolite, is
conveniently classed with them, although not contiguous.
[Illustration:
FIG. 25.—Cross-section of Mount Hillers.
FIG. 26.—The same, with ideal representation of the underground
structure.
Scale, 1 inch = 4,000 feet. 1. Tununk Sandstone; 2. Henry’s Fork
Conglomerate; 3. Gray
Cliff Sandstone. The full black lines represent trachyte sheets, and
the broad black area the
Hillers Laccolite.
]
[Illustration:
FIG. 27.—Mount Hillers, from the south.
]
_The Hillers laccolite_ is the largest in the Henry Mountains. Its depth
is about 7,000 feet, and its diameters are four miles and three and
three-quarter miles. Its volume is about ten cubic miles. The upper half
constitutes the mountain, the lower half the mountain’s deep-laid
foundation. Of the portion which is above ground, so to speak, and
exposed to atmospheric degradation, less than one-half has been stripped
of its cover of overarching strata. The remainder is still mantled and
shielded by sedimentary beds and by many interleaved sheets of trachyte.
The portion which has been uncovered is not left in its original shape,
but is sculptured into alpine forms and scored so deeply that not less
than 1,000 feet of its mass are shown in section. All about the eroded
(south) face of the mountain the base is revetted by walls of Vermilion
and Gray Cliff sandstone, strengthened by trachyte sheets. At the
extreme south these stand nearly vertical (80°), and their inclination
diminishes gradually in each direction, until at the east and west bases
of the mountain it is not more than 60°. On the north side there are no
revet-crags, and the inclination is comparatively slight. It would
appear that the laccolite was asymmetric, and was so much steeper-sided
on the south that that side suffered most rapid degradation.
[Illustration:
FIG. 24.—Ground Plan of the Henry Mountains. The curved lines show the
limits of the principal displacements; the triangles, the positions
of the main peaks. N, Mount Ellen. P, Mount Pennell. H, Mount
Hillers. M, Mount Holmes. E, Mount Ellsworth.
]
By reference to the section (Figure 25) it will be seen that the
sedimentary strata of the north flank stretch quite to the summit of the
mountain. The same beds which form the revet-crags on the southern base
constitute also some of the highest peaks. Since these rest directly
upon the laccolite, it is assumed that the next lower beds of the
stratigraphic series form its floor; and the base of the laccolite is
drawn in the ideal section on the level which the Shinarump Group holds
where it is unaffected by the displacements of the mountain.
It is noteworthy that wherever the sedimentaries appear upon the
mountain top they are highly metamorphic. But in the revet-crags there
is very little alteration. Massive sandstone, divided by sheets and
dikes several hundred feet in thickness, is discolored and indurated at
the contact surface only, and ten feet away betrays no change.
The engraving of Mount Hillers (Figure 27) exhibits the south face with
its revet-crags and bold spurs of trap. In nature the effect is
heightened by the contrast of color, the bright red revetments being
strongly relieved against the dark gray of the laccolite. The strata of
the summits cannot be discriminated at a distance. They are too near the
laccolite in hardness to differ from it in the style of their sculpture.
Of the minor laccolites of the cluster there are three so closely joined
to the chief that they merge topographically with the mountain. They are
not well exposed for study. The smallest, which overlooks the pass
between Mount Hillers and Mount Pennell (A, Figure 28) has probably lost
the whole of its cover and with it so much of its substance that the
original form and surface cannot be seen. Its floor is probably the
Tununk sandstone. East of it and lying a little deeper in the Cretaceous
series is a second laccolite (B), broader, lower, and less eroded. The
third (C) joins the great one on the northeast, and is so closely united
that it was at first supposed to be the same body. Later examinations
have shown, however, that its immediate roof is the Henry’s Fork
conglomerate, and its horizon is thus established as more than two
thousand feet above that of its great companion.
[Illustration:
FIG. 30.—The Steward Laccolite.
]
[Illustration:
FIG. 28.—Diagram of the Hillers Cluster of Laccolites; Ground plan.
]
[Illustration:
FIG. 29.—Diagram of the Hillers Cluster of Laccolites; Elevation.
The upper horizontal line marks the base of the Cretaceous; the
middle, the base of
the Jura-Trias; the lower, the level of the sea.
]
The _Steward laccolite_ (Figure 30) is better exposed for study. It was
buried in the soft bad-land sandstone of the Flaming Gorge Group, and
its matrix has been so far washed away that nearly the whole body of
trap is revealed. It is weathered out, like a chert-nodule on the face
of a block of limestone. At one end it is bared quite to the base, and
the sandstone floor on which it rests is brought in sight. The waste of
the sandstone has undermined its edge, and a small portion of the
laccolite has fallen away. Near the opposite end a fragment of its cover
of arching sandstone survives—just enough to indicate that the
sedimentaries were once bent over it, and that the smooth low-arching
surface which now crowns it portrays the original form which the molten
lava assumed. The laccolite is about two and a half miles long and one
and one-half broad. The height of its eastward face, where it is sapped
by the erosion of the bad-land rock, is six hundred feet, and the
central depth must be more than eight hundred feet.
[Illustration:
FIG. 31.—Cross-section of the Pulpit arch, with ideal representation
of the Pulpit Laccolite. Scale, 1 inch = 3,500 feet. 1, Vermilion
Cliff Sandstone. 2, 2, Gray Cliff Sandstone. 3, 3, Flaming Gorge
Shale. 4, Henry’s Fork Conglomerate. 5, Tununk Shale.
]
_Pulpit arch_ is as high and as broad as the lesser arch of Mount
Holmes, but its place is not marked in the topography by an eminence for
the reason that the degradation of the land has not yet progressed so
far as to unearth its core of trachyte. The drainage from Mount Hillers
crosses it from west to east and has given it an oblique truncation, as
illustrated in the diagram. At the upper end of the slope the Henry’s
Fork conglomerate outcrops; at the lower end the base of the Flaming
Gorge series, and in the interval the Gray Cliff sandstone is lifted to
the surface. The same streams which planed away the crown of the arch
have now cut themselves deeper channels and divide the massive sandstone
by picturesque cañons, between which it is grotesquely carved into
pinnacles and ridges. A curious salient of the sandstone has given its
name to the arch. How deeply the Pulpit laccolite lies buried is not
known, no sheet nor dike of trachyte betraying its proximity. The valley
of the Colorado may have to be deepened thousands of feet before it will
be laid bare.
_The Jerry Butte_ is the most conspicuous adjunct to Mount Hillers and
topographically is more important and striking than the features which
have just been described, but its structure is less clear. Its crest is
formed by a great dike several hundred feet in width and two miles in
length, and with an even top like those observed on Mount Holmes. The
western end of the dike is the higher and forms the culminating point of
the butte, and from it there radiate three other dikes of notable size.
The inclosing strata, preserved from erosion only by the shelter of the
dikes, are the lower portion of the Cretaceous series, and they are so
little lifted above their normal level that there is room for no
considerable laccolite beneath them. The inclination of the beds is so
complicated by the dips of the Pulpit, Steward, and Hillers arches, all
of which are contiguous, that nothing can be made out of the form of the
laccolite, if it exists.
_The Howell laccolite_ lies apart from the cluster and is well exposed.
It differs from all that have been enumerated in its extreme thinness.
With a breadth of more than two thousand feet, it has a depth of only
fifty. Seen from the east, it might readily be mistaken for a _coulée_,
for on that side it is the thin, hard, black cap of a table carved out
of soft, sandy shale (Flaming Gorge Group) by circumdenudation. But
followed westward, the table is found gradually to lose its height by
the rising of the adjacent land, and at last the lava-bed runs into the
slope and disappears beneath the upper layers of the same sandy shale on
which it rests. How far it extends under ground can only be conjectured.
How far it originally stretched in the opposite direction cannot be
known because it is broken away. The original edge is concealed at one
end and has been undermined and destroyed at the other, so that the only
place where it can be seen is the point at which it emerges from the
shale. At this point, where erosion has bared but has not yet attacked
the lava, the form and character of the edge are exhibited. In place of
the tapering wedge which usually terminates intrusive sheets, there is a
blunt, rounded margin, and the lava scarcely diminishes in depth in
approaching it. The underlying strata, locally hardened to sandstones,
lie level; the overlying curve downward to join them, and between the
curved strata is interleaved a curved lava-sheet. In all these
characters the intrusive body is affiliated with the typical laccolites,
and distinguished from the typical sheets.
[Illustration:
FIG. 32.—The Howell Laccolite, as seen from the north.
]
[Illustration:
FIG. 33.—The Edge of the Howell Laccolite.
]
[Illustration:
FIG. 34.—Mount Pennell, from the west.
]
_The laccolite which is marked “D”_ on the diagrams of the Hillers
cluster is identical in nearly all its characters with the Howell; it is
thin and broad. One margin is wasting as its foundation is sapped; the
other is hidden from view. Its depth does not diminish toward the edge.
Moreover, a few dikes issue from its margin, or from beneath its margin.
It was intruded within the same formation as the Howell (the Flaming
Gorge shale), but at a horizon several hundred feet higher; and, what is
specially noteworthy, the rock of which it is composed is identical in
_facies_ with that of the Howell laccolite, and notably different from
all others which were observed in the Hillers cluster.
Thus there are grouped in this one cluster laccolites of the most varied
character, differing in form, in magnitude, in the stratigraphic depth
at which they were intruded, in the extent to which they have been
uncovered or demolished in the progress of erosion, and also, but very
slightly, in their lithologic characters. The greatest is one thousand
times as bulky as the least. The length of the most obese is three times
its depth; the length of the most attenuated is more than one hundred
times its depth. The one highest in the strata lies a thousand feet
above the base of the Cretaceous rock series; the lowest is not higher
than the summit of the Carboniferous. The latter has not yet been
touched by erosion, others have been completely denuded, and some have
been partially demolished and removed.
MOUNT PENNELL.
Mount Pennell and Mount Ellen are distinct mountain masses separated by
a low pass, but there is no interval between the clusters of laccolites
by which they are constituted.
Whether the site of a laccolite shall be marked by a mountain depends in
great measure on the relation of the laccolite to the progress of
erosion. In the Henry Mountains the laccolites which have not been
reached by the denudation scarcely affect the topography. The arched
sedimentaries above them are no harder than the same strata in the
surrounding plain, and they are brought substantially to the same level.
It is to those which the downward progress of erosion has reached and
passed that the mountains are due. In virtue of their hardness they
survive the general degradation, and conserve with them broad
foundations of more perishable material. Mounts Ellen and Pennell mark
the positions of the highest of a great cluster of laccolites, and the
pass between them marks a part of the cluster where all the laccolites
lie low in the strata.
[Illustration:
FIG. 35.—Cross-section of Mount Pennell. 1, Blue Gate Sandstone. 2, 2,
Blue Gate Shale. 3, 3, Tununk Sandstone. The full black lines
represent Trachyte.
]
Mount Pennell is not so easily studied as the lower mountains at the
south. Its summits are timbered and are carved into alpine forms which
do not portray the structure but rather mask it. Upon its flanks however
the topography and the structure are in so close sympathy that the
latter is easily read; and by their study some of the general features
of the mountain have been made out. From the east and south and west the
strata can be seen to rise toward it. The uprising strata on the west
are the Tununk sandstone and the shales above and below it. The
sandstone forms no revetments, but accords so closely in its dips with
the slopes of the mountain that it is the surface rock over a broad
area, outcropping wherever there is rock exposure. At the head of the
foot-slope trachyte sheets are associated with it, some overlying and
others underlying it; and at one point a gorge reveals a laccolite not
far below it—a laccolite that may or may not be the great nucleus of the
mountain. On the west and south flanks the uprising strata are the Blue
Gate and Tununk sandstones, with their shales. The Gate sandstone has
been worn away nearly to the foot of the slope, and forms a monoclinal
ridge circling about the base. The ridge is interrupted by a number of
waterways, and it sends salients well up upon the flank, but it is too
continuous to be regarded as a mere line of revetments. The Tununk
sandstone is not to be seen without search, being covered by heavy
trachyte sheets. Trachyte sheets also underlie it, and the whole are
carved into a conspicuous series of revet-crags.
[Illustration:
FIG. 36.—Mount Pennell, from the north.
]
The association of overarching strata with sheets of trachyte leaves no
doubt in my mind that the core of Mount Pennell is laccolitic, but
whether it is simple or compound is not so clear. The collation of all
the observed dips shows that if there be but one laccolite it has not
the simplicity of form which usually characterizes them.
There are low arches of the strata at the southern, northern, and
northeastern bases, which reveal no trachyte and give the impression
that there may be a foundation of low-lying laccolites upon which the
main trachyte mass or masses of the mountain are based. One of these low
arches, that at the northeast, is shown in the foreground of Figure 36.
The strata which portray it are of the Henry’s Fork conglomerate, and
the laccolite which they cover is of necessity distinct from that which
is revealed on the east flank of the mountain.
[Illustration:
FIG. 37.—Sentinel Butte.
]
In addition to the laccolites of the foundation and of the main body,
there is a series which jut forth from the northern flank like so many
dormer windows. They are comparatively small but are rendered
conspicuous by the removal of the soft rocks which originally inclosed
them. They are higher in the strata than any other observed laccolites,
their position being above the Tununk sandstone and in the Blue Gate
shale. The core of the main body of the mountain is probably inclosed in
the Tununk shale, and the laccolites under the low arches (at the north
at least) are entirely below the Cretaceous. The higher series stand on
top of the low arches, and are just outside of the sheets which inclose
the central body. The largest of them constitutes Sentinel Butte, and
stands guard over Penellen Pass. Sapped by the yielding of its soft
foundation it is rapidly wasting, and on three sides its faces are
precipitous. The huge blocks which cleave from it as they are undermined
strew the surrounding slopes for a great distance. Its depth of 400 feet
is made up of two layers, of which the lower is the deeper, and between
which there is a slight lithologic difference. The upper surface of the
butte is smooth and plane with an inclination to the south. It is
probably a portion of the original surface of the laccolite.
Thus we have in Mount Pennell a great central body consisting of a
single laccolite or of a number closely massed together; an inferior
group of three or more, evidenced by low broad arches; and a superior
group of not less than four, all of which are partially destroyed.
MOUNT ELLEN.
The crest of Mount Ellen is as lofty as that of Mount Pennell and it is
more extended. It stretches for two miles from north to south and is
buttressed by many spurs.
The sculpture of the crest is alpine, and the structure is consequently
obscured. There are dikes of trachyte and perhaps the remnants of
laccolites; there are Cretaceous sandstones greatly indurated; and there
are Cretaceous shales baked to clinking slate; but they are all carved
into smooth, pyramidal forms, and each is half hidden by the _débris_
from the rest so that their order and meaning cannot be seen.
The flanks however are full of interest to the geologist. The Ellen
cluster of laccolites is a broad one, and all but the central portion is
well exposed for study. In the spurs and foot-slopes and marginal buttes
no less than sixteen individual laccolites have been discriminated, a
number of them most beautifully displayed. They will be enumerated in
the order of their position, beginning at the west of Penellen Pass and
passing along the western, northern, and eastern flanks to the Scrope
Butte, east of the pass.
[Illustration:
FIG. 38.—Ground Plan of the Ellen Cluster of Laccolites.
]
In the chart of the Ellen cluster, Figure 38, an attempt is made to show
the horizontal grouping of the laccolites and the order of their
superposition wherever they overlap. It will be observed that where the
limits are imperfectly known the outlines are left incomplete, and that
the central area remains blank, not because it contains no trachyte
masses, but because its alpine sculpture has prevented their study.
Where it is not evident which of two encroaching laccolites is the
superior, they are separated by a straight line.
[Illustration:
FIG. 40.—Section of the Lewis Creek Cañon through the Newberry Arch.
1, Henry’s Fork Conglomerate. 2, Upper portion of Flaming Gorge
Shale. 3, Newberry Laccolite.
]
The line _a a_ in the chart is the springing line of a broad, flat arch
which underlies all the other arches of the western flank. If it covers
but one laccolite, that one is the rival in magnitude of the Hillers
nucleus, although widely different in proportions; but it is more
probable that it contains a greater number. The upper surface is rolling
and uneven, and has not the degree of symmetry which laccolites usually
display. At the points _b_, _c_, _d_, and _e_ the altitude of the arch
is 3,800, 3,500, 2,000 (estimated) and 2,500 feet. Nevertheless there is
nothing in its simple outline to indicate a compound structure; the
monoclinal ridges by which it is margined do not exhibit the flexuous
curves which are commonly seen about the bases of confluent arches.
Whether the nucleus is simple or compound it sends no branches to the
surface; the only outcrops of trachyte belong to the overlying arches.
The upper parts of the arch have been so carried away that the steepness
of the mountain flank is not increased by it, and inferior strata are
brought to the surface. At the north the crown of the arch bears the
Henry’s Fork conglomerate, while beyond its base the plateau is built of
Blue Gate sandstone. At the south the arch bears the Tununk sandstone,
and the Masuk lies outside.
[Illustration:
FIG. 39.—The Newberry Arch and Laccolite.
]
_The laccolite marked “E”_ on the chart rests upon the Tununk sandstone.
The Blue Gate shale which once buried it has been all washed away except
some metamorphosed remnants upon the top, and the trachyte itself has
wasted to such an extent that its original form cannot be traced. The
laccolite has no near neighbor, and the erosion has left it prominent
upon the mountain flank. A continuous and solitary spur joins it to the
central ridge.
_The Newberry laccolite_ makes a knob 1,700 feet high, and stands by
itself. Its cover of Henry’s Fork conglomerate is re-enforced by a
number of trachyte sheets, and is broken through at one point only. At
that point Lewis Creek cuts with a straight course across a flank of the
arch, and exposes a portion of the nucleus in section (Figures 39 and
40). The conglomerate does not rest directly upon the trachyte, but is
separated by one or two hundred feet of shale of the Flaming Gorge
Group.
[Illustration:
FIG. 41.—The Geikie Laccolite (_G_), overlapped by the Henry’s Fork
Conglomerate (_H H_), and the Shoulder Laccolite (_S_).
]
Two miles to the northward is the _Geikie laccolite_, smaller than the
rest but similar in character. It lies close to the top of the Flaming
Gorge shale, and is enwrapped by the Henry’s Fork conglomerate. Upon
three sides the conglomerate can be seen to curve over its margin from
top to bottom, and upon two of these sides the curves are so broken
through by erosion that the trachyte is visible within. On the north two
cañons cut down to the nucleus, and on the south there is a broad face
of trachyte framed all about by the cut edges of the conglomerate beds.
_The Shoulder laccolite_ overlaps the Geikie, and the two are exposed in
such manner as to show their relation in section. The conglomerate runs
under the one and over the other and separates them. The upper laccolite
is a broad and deep one, and takes its name from the fact that it makes
a great shoulder or terrace on the mountain side. Toward the mountain it
is buried; toward the valley it is uncovered, and in part bounded by a
cliff. It is deeply cleft by cañons. It has not been subjected to
measurement, but its depth is not overestimated at fifteen hundred feet
nor its area at five square miles.
In the sketch (Figure 42) many of these features can be traced—the
Geikie laccolite at the left and the Shoulder in the center, with an
outcrop of the conglomerate curving down from the roof of the one to the
floor of the other, and the Newberry arch in the foreground with its
cleft side. At the rear is the pyramidal Ellen Peak and the _F_
laccolite, overlooking the Shoulder. In the distance at the left is the
Marvine laccolite.
Little more can be said of the _F laccolite_ than that it exists and is
the nucleus of a lofty spur. Its summit rises too nearly to the crest of
the mountain to be well defined, and at its base the sedimentaries are
hidden by talus.
[Illustration:
FIG. 42.—The Western Flank of Mount Ellen. _N_, Newberry Arch; _S_,
Shoulder Laccolite; _G_, Geikie Laccolite.
]
[Illustration:
FIG. 43.—The Northern face of the Marvine Laccolite.
]
[Illustration:
FIG. 44.—The Western face of the Marvine Laccolite.
]
Not so the _Marvine laccolite_. Lying at the foot of the mountain where
erosion is so conditioned as to discriminate between hard and soft, and
surrounded by nothing firmer than the Tununk shale and Tununk sandstone,
it has suffered a rapid denudation, in which nearly the whole of its
cover has been carried away without seriously impairing its form. It
stands forth on a pedestal, devoid of talus, naked and alone. The upper
surface undulates in low waves preserving the original form as it was
impressed on the molten mass. Over a portion there is a thin coating of
sandstone, the layer next to the trachyte being saved from destruction
by the induration acquired during the hot contact. From the remainder
this also has disappeared, and the contact face of the trachyte is bare.
For some reason the exterior portion of the laccolite disintegrates more
slowly than the interior. It may be that there was some reaction from
the surrounding sedimentaries during the cooling, which modified the
crystallization. Or it may be that at a later epoch a reciprocal
metamorphism was induced along the contact of the diverse rocks. At all
events there is a crust a few feet in thickness which is specially
qualified to resist destructive agents, and which by this peculiarity
can be distinguished from the trachyte of the interior. All about the
northern and western faces, which are steep, this crust has been broken
through and the interior excavated; but it is only along the upper edge
of the face that the crust is completely destroyed. Along the lower edge
it is preserved in remnants sufficiently numerous to fully define the
outline of the base. Each remnant is a sort of revet-crag standing
nearly vertical and joined by a buttress to the cliff behind it. In the
accompanying sketches (Figures 43 and 44) the most of these features
find better expression than words can give them. The solemn order of the
tombstone-like revetments is not exaggerated, nor is the contrast
between the ruggedness of the cliff and the smoothness of the upper
crust. At the left in the upper view, and at the right in the lower,
there can be seen inclined strata, the remnants of the arch which once
covered the whole.
The extreme depth of the laccolite is 1,200 feet, and its diameters are
6,000 and 4,000 feet.
_The G arch_, the Dana, the Crescent, and the Maze agree in having no
visible laccolites. They are mere dome-like uplifts, by which inferior
beds are brought to light. In the middle of the Crescent arch, and there
only, is a small dike. They differ in their dimensions and in their
erosion and topography. The G arch is low and broad, and lifts the
Henry’s Fork conglomerate a few hundred feet only. It is covered by that
bed except where Bowl Creek and one or two others cross it in shallow
cañons.
_The Dana arch_ bears the same relation to Mount Ellen and Jukes Butte
that the Pulpit arch bears to Mount Hillers. The streams which flow down
have truncated it, and afterward carved a system of cañons below the
plane of truncation. The Henry’s Fork conglomerate overlooks it from the
base of the Jukes laccolite on one side, and on the other margins it
with a monoclinal ridge; and in the interval the Flaming Gorge and Gray
Cliff rocks come to the surface.
[Illustration:
FIG. 45.—The Jukes Butte, as seen from the southeast, showing the
Jukes Laccolite, resting upon the Dana Arch.
]
_The Crescent arch_ is so perfectly truncated by the existing mountain
streams that their flood-plains unite above it in a single broad slope.
On the side toward the mountain there are a few insular hills of the
conglomerate, and on the side toward the plateau the same rock lifts a
low monoclinal ridge which circles about the base of the arch in a
crescent. Within the crescent there are few outcrops, but it is probable
that the Gray Cliff Sandstone is brought to the surface. Near the center
a solitary thin dike juts forth, like a buoy set to mark the place where
a laccolite is sunk. Judging the magnitude of the laccolite by the
proportions of the uplift, it has a diameter of nearly four miles and a
depth of 2,500 feet; and these dimensions indicate a volume of three and
a half cubic miles.
_The Maze arch_ covers a smaller area than the Crescent, but its height
is greater. The drainage from the mountain crosses it on a number of
lines, but there is no indication that at any recent stage of the
degradation they have produced an even truncation. At present they
divide the Gray and Vermilion Sandstones by so intricate a labyrinth of
deep cañons that the whole area of the uplift is almost impassable. The
monotony of red sandstone, for here all members of the Jura-Trias are
brick-red, the variety of dip, complicated to the eye by oblique
lamination, the multiplicity of cañons and ridges, conspire to give an
impression of chaos from whatever point the tract is viewed; and even
from the most commanding stations I was unable to make out completely
the arrangement of the drainage lines. Still no faults were discerned,
and it is probable that the Maze arch, intricate as it seems, is a
simple circular dome. On the north it adjoins the Crescent arch, and the
monoclinal ridges of conglomerate which margin the two at the east are
confluent. On the south it adjoins rather more closely one of the low
arches which have been referred to the Pennell cluster. On the west, or
toward the mountain, it is probably met by other arches at its own
level; but these if they exist cannot be fully known until the
progressing degradation of the country shall have removed certain
laccolites which lie above them and slightly overlap the Maze arch.
The four arches just described are of the foundation series of the
eastern base, and cover laccolites of unknown depths. Higher in the
strata, and at the same time absolutely higher, is a second series,
which to a certain extent overlap them. The Bowl Creek encroaches on the
G laccolite; the Jukes, on the Dana and on the Bowl Creek; the H
laccolite, on the Crescent; the Peale and Scrope, on the Maze.
[Illustration:
FIG. 46.—Cross-section of the Bowl Creek Arch. _c_, _c_ marks the
water level of Bowl Creek; _l_, the Bowl Creek Laccolite; _a_, _a_,
Shales; and _s_, the Gryphea Sandstone. Scale, 1 inch = 1,000 feet.
]
_The Bowl Creek arch_ is so masked by the overlapping Jukes laccolite,
and by the encroachment of certain large dikes, that its general form
and proportions are not known; but it is laid bare at one point in a
fine natural section. Bowl Creek crosses it near the center, and in the
walls of the cañon are exhibited two hundred feet of the laccolite,
together with two hundred and fifty feet of superjacent beds. The curve
of the strata is unbroken by faults or dikes, and carries them below the
level of the creek at each end of the cañon. Next above the trachyte
lies a clay shale which has been baked to the hardness of limestone. It
is one hundred feet thick, and is more or less altered throughout, as is
also the sandstone which overlies it. The extent of the metamorphism
indicates that the trachyte mass by which it was produced is not a mere
sheet, but is the body of the laccolite itself.
_The Jukes laccolite_ encroaches upon the G, the Dana, and the Bowl
Creek arches, and is superior to them all; it may be said to stand upon
them. The trachyte has a depth of only one thousand feet, but it lies so
high with reference to the general degradation that it is a conspicuous
feature of the topography. The edges of the laccolite are all eaten
away, and only the central portion survives. All of its faces are
precipitous. The cover of shale or sandstone has completely disappeared,
and the upper surface seems uneven and worn; but a distant view (Figure
47) shows that its wasting has not progressed so far as to destroy all
trace of an original even surface. The eminences of the present surface
combine to give to the eye which is aligned with their plane the
impression of a straight line. The hill is loftier than the laccolite,
for under the one thousand feet of trachyte are five hundred feet of
softer rock which constitute its pedestal, and by their yielding
undermine the laccolite and perpetuate its cliffs.
[Illustration:
FIG. 47.—Profile of the Jukes Butte, as seen from the northwest.
]
_The H laccolite_ is not well exposed. It lies on one edge of the
Crescent arch and is covered by the Henry’s Fork conglomerate.
[Illustration:
FIG. 48.—The Peale Laccolite exposed in natural cross-section. _a_,
_b_, _c_, _d_, and _e_ are intrusive masses of trachyte. _H_ is the
Henry’s Fork Conglomerate.
]
[Illustration:
FIG. 49.—The East Flank of Mount Ellen, showing the Scrope, Peale, and
Jukes Laccolites, and the Maze and Crescent Arches.
]
Little is known of the form of the _Peale laccolite_. One edge is lost
in the obscurity of the alpine sculpture, and the other has been removed
along with the crest of the Maze arch, on which it rested. But by this
removal a section has been opened across the laccolite, revealing its
internal structure from top to bottom. It is shown to be composite,
attaining its height of 850 feet by the compilation of three distinct
beds of trachyte, separated by partings and wedges of shale. The lowest
bed is thin and of small extent. Next is the main bed, six hundred feet
thick and a mile or more broad; and on top is a bed two hundred feet
thick and proportionately narrow. Each of the beds is lenticular in
section, and the piling of the less upon the greater produces a
quasi-pyramidal form. The interleaved shale bands are metamorphosed to
the condition of slate. Under the laccolite are one or two hundred feet
of shale, apparently unaltered except at the contact. Then come the
Henry’s Fork conglomerate, 350 feet thick, and the Flaming Gorge shale,
several times thicker. Within the upper shale is a restricted trachyte
sheet and near the top of the lower shale is a broader one. The latter
is double throughout, its two layers having been intruded at different
times.
The top of the escarpment is at the top of the laccolite, and only a
short distance back from the brow are shale and sandstone resting upon
the trachyte and conforming to its uneven surface.
_The Scrope laccolite_ resembles the Jukes in everything except that its
erosion has progressed so far as to obliterate every trace of the
original upper surface. Its position in the strata is about the same,
and the erosion of its matrix has left it a conspicuous crag. It rests
on the flank of the Maze arch, just as the Jukes laccolite rests on the
Dana arch. The remnant of trachyte is less than one thousand feet high,
and has been carved into a subconical form in which no hint of the
original size and proportions of the trachyte body is conveyed.
Figure 49 is a view of the east flank of Mount Ellen, as seen from Mount
Hillers. It groups together many of the details that have been
enumerated. The conical hill at the left is the Scrope laccolite. The
spur from the mountain which ends just at the right of it is the Peale
laccolite. The bold butte which terminates the last spur of the mountain
in the distance is the Jukes laccolite. Across the base of the latter
one can trace the outcrop of a hard bed. This is the Henry’s Fork
conglomerate, and the upward curve which it shows belongs (probably) to
the Crescent arch. Nearer than the Jukes Butte and in the same direction
is a low hill, marking the dike within the Crescent. Just to the left of
it is an insular outcrop of the conglomerate, and nearer by is another
outcrop in the form of a cliff, which is continuous to the base of the
Peale laccolite. Between the Peale and Scrope laccolites the
conglomerate is hidden by an embayment of the cliff, and it reappears in
the base of the Scrope Butte. In all these outcrops the escarpments of
conglomerate face toward the east and at the same time toward the Maze
and Crescent arches. On the opposite side of the arches the same
conglomerate outcrops in a monoclinal ridge with its escarpment facing
to the west. The ridge can be traced in the sketch from a point in the
foreground under the Jukes Butte nearly to the eastern base of the
butte, the course at first being almost directly toward the butte and
then curving far to the right and forming the Crescent. Between the
Scrope and Peale laccolites on one side and the monoclinal ridge on the
other, lie the Maze and the Maze arch. Beyond the Maze arch and limited
at the right by the Crescent, is the Crescent arch. The Scrope, the
Peale, and the Jukes are visible laccolites; the Maze and Crescent
arches cover invisible laccolites.
The determinate laccolites and arches which compose the lower slopes of
Mount Ellen, while they are of prime importance for purposes of
investigation, must not be considered superior in number and magnitude
to those of the central region. That region is known to abound in
trachyte masses, and it far exceeds the marginal district in the extent
and degree of its metamorphism. There is every reason to believe that
the mountain crest marks the zone of greatest igneous activity, and that
the foot-hills are as truly subsidiary from a geological point of view
as they are from a topographic. There is no evidence of a great central
laccolite, such as the Hillers and Pennell clusters possess, and I am
disposed to regard the mountain as a great congeries of trachyte masses
of moderate size, separated and in the main covered by shales and
sandstones. The sedimentary strata of the summits are all of the
Cretaceous series, but they are too greatly altered to permit the
discrimination of the Masuk, Gate, and Tununk groups. The Henry’s Fork
conglomerate, which might have been recognized even in a metamorphic
condition, was not seen.
STEREOGRAM OF THE HENRY MOUNTAINS.
For the double purpose of mapping and studying the mountains a model in
relief was constructed, in which great pains was taken to give all the
principal features their proper altitudes and proportions. This model
was photographed, and has been reproduced by the heliotype process in
Plate III, at the end of the volume. After it had been completed, a
second model was constructed by adding to the surface of the first.
Wherever the Blue Gate sandstone appeared in the original model no
addition was made. Where the Tununk sandstone appeared there was added
an amount equivalent to the combined thickness of the Blue Gate
sandstone and the Blue Gate shale; and in general, enough was added in
every part to bring the surface up to the summit of the Blue Gate
sandstone. In this way a restoration was made of the form of that
sandstone previous to its erosion. It is not to be understood that the
mountains ever possessed this form; for when the surface of the Blue
Gate sandstone was unbroken by erosion it was unbroken only because it
was covered by other strata, which while they shielded it were
themselves eroded. If the sandstone had been indestructible, this is the
form which would have been developed by the washing away of all the
overlying beds; and in this form are embodied the arches and domes which
were impressed upon the sandstone by the upswelling of the laccolites.
The model became a stereogram of the displacements of the Henry
Mountains, and a photograph from the stereogram appears in Plate IV.
It will be observed that over the central district of Mount Ellen, where
by reason of the peculiar sculpture of the rock its structure was
concealed, the restoration of the sandstone was not carried; it seemed
better to represent our lack of knowledge by a blank than to bridge over
the interval by the aid of the imagination.
If the reader will study the plate, he will find that it expresses a
great body of the phenomena which have been described in this chapter.
The simplicity of the Ellsworth and Holmes arches is contrasted with the
complexity of the others; the greatness of the Hillers and Pennell domes
with the smallness of those which lie upon their flanks. One point that
is especially striking is the relation between the upper and lower domes
of the Ellen cluster. The upper, which give rise to all the conspicuous
features of the mountain flanks, are comparatively small, while the
lower, which might almost be overlooked in a rapid examination of the
mountain, are comparatively large and constitute the great mass of the
uplift. The smaller laccolites, because they are the upper, have been
denuded of their covers, and in virtue of their hardness stand forth
salient. The larger, because they are the lower, have not been laid
bare, and the comparatively feeble resistance which their covers have
opposed to erosion has impressed their forms but slightly on the
topography.
CHAPTER IV.
THE LACCOLITE.
The principal facts in regard to the laccolites of the Henry Mountains
having been set forth in the preceding chapter, an attempt will now be
made to deduce from them the natural history of the Laccolite.
There is a question which the critical geologist will be likely to
propound, and which should be answered at the outset. “What evidence”,
he may demand, “is there that the origin of the laccolite was subsequent
to the formation of the inclosing strata rather than contemporaneous
with it? May it not have been buried instead of intruded? May not the
successive sheets and masses of trachyte have been spread or heaped by
eruption upon the bottoms of Mesozoic seas, and successively covered by
the accumulating sediments?” The answer is not difficult.
1st. No fragment of the trachyte has been discovered in the associated
strata. The constitution of the several members of the Mesozoic system
in the Henry Mountain region does not differ from the general
constitution of the same members elsewhere. This evidence is of a
negative character, but if there were no other it would be sufficient.
For there is indubitable proof that at the end of the Shinarump period
the Henry Mountain region was lifted above the ocean, and if any of the
nuclei of the mountains had then existed they could not have escaped
erosion by shore waves, and must have modified the contemporaneous
deposits.
2d. The trachyte is in no case vesicular, and in no case fragmental. If
it had been extruded on dry land or in shallow water, where the pressure
upon it was not sufficient to prevent the dilatation of its gases, it
would have been more or less inflated, after the manner of recent lavas.
If it had issued at the bottom of an ocean, the rapidity of cooling
would have cracked the surface of the flow while the interior was yet
molten and in motion, and breccias of trachyte _débris_ would have
resulted. The absence of inflation and of brecciation are of course of
the nature of negative evidence, but they derive weight from the fact
that a great number of distinct bodies of trachyte have been examined in
the course of the investigation.
3d. The inclination of the arched strata proves that they have been
disturbed. If the laccolites were formed in each case before the
sediments which cover them, the strata must have been deposited with
substantially the dips which they now possess. This is incredible. The
steepest declivity of earth-slopes upon the land is 34° from the
horizontal, and they have not been found to equal this under water.
Prof. J. D. Whitney noted 23° as the original slope of a deposit on the
Pacific Coast, and regarded it as an extreme case. The writer has made
many measurements of the inclination of oblique lamination in massive
sandstones, and found the maximum to be 24°. But the strata which cover
the laccolites dip in many places 45° to 60°, and in the revetments of
the south base of Mount Hillers they attain 80°.
4th. It occasionally happens that a sheet, which for a certain distance
has continued between two strata, breaks through one of them and strikes
across the bedding to some new horizon, resuming its course between
other strata. Every such sheet is unquestionably _subsequent_ to the
bedding.
5th. The strata which overlie as well as those which underlie laccolites
and sheets, are metamorphosed in the vicinity of the trachyte, and the
greatest alteration is found in the strata which are in direct contact
with it. The alteration of superior strata has the same character as the
alteration of inferior. This could never be the case if the trap masses
were contemporaneous with the sediments; the strata on which they were
imposed would be subjected to the heat of the lava, but the superior
strata would accumulate after the heat had been dissipated. In the Henry
Mountains a large number of observations were made of the phenomena at
and near the contacts of sedimentary and igneous rocks, and in every
instance some alteration was found.
In fine, all the phenomena of the mountains are phenomena of intrusion.
There is no evidence whatever of extrusion. It is not indeed
inconceivable that during the period in which the subterranean chambers
were opened and filled, a portion of the lava found its way to the top
of the earth’s crust and there built mountains of eruption; but if such
ever existed, they have been obliterated.
The Henry Mountains are similar among themselves in constitution. They
all exhibit dome-like uplifts; they all contain intrusive rocks; and
their intrusive rocks are all of one lithologic type. They are moreover
quite by themselves; the surrounding country is dissimilar in structure,
and there is no gradation nor mingling of character. Thus similar and
thus isolated it is natural to regard the mountains as closely related
in origin, to refer their trachytes to a common source, and to look for
homology in all their parts. It was the search for such homology which
led to the hypothesis that the laccolite is the dominant element of
their structure. It is now time to examine this hypothesis, the truth of
which has been assumed in the preceding pages, and see how far it
accords with the facts of observation.
The facts to be correlated are the following:
1st. There are seven laccolites which lie so far above the local plane
of erosion that they are specially exposed to denuding agents. They have
no enveloping strata, and their only associated sheets lie in the strata
under them. Their original forms have been impaired or destroyed by
erosion. They are the Scrope, the Jukes, the Sentinel and its three
companions, and the A laccolites.
2d. There are two laccolites so nearly bared that their forms are
unmistakable, but which are still partially covered by arching strata,
and which have associated sheets and dikes. They are the Marvine and the
Steward laccolites.
3d. There are five supposed laccolites situated where the erosion planes
are inclined, which run under the slopes and are covered at one side or
end, and at the other project so far above them as to have lost
something by erosion. These are accompanied by overarching strata and by
sheets and dikes. They are the Peale, the Howell, the Shoulder, the D,
and the E.
4th. There are seven or eight supposed laccolites, of which only a small
part is in each case visible, but which are outlined in form by domes of
overarching strata. Their bases are not exposed. Associated with them
are dikes and sheets. They are the Hillers, the Pennell, the Geikie, the
Newberry, the Bowl Creek, the C, the H, and perhaps the F.
5th. There are five domes of strata accompanied by dikes and (with one
exception) by sheets, but showing no laccolite. They are the Ellsworth,
the Greater Holmes, the Crescent, the Jerry, and the B arches.
6th. There are nine or more domes of strata with no visible
accompaniment of trachyte. They are the Lesser Holmes, the Pulpit, the
G, the Dana, and the Maze arches, and those of the foundation of Mount
Pennell and of the west base of Mount Ellen.
Upon the hypothesis that all these phenomena are examples of the
laccolitic structure, they have the following explanation: Each
individual case comprised originally a laccolite, covered by a great
depth of uplifted and arching strata, and accompanied by dikes and
sheets which penetrated the strata to a limited distance. Lying at
different depths from the surface, they have borne and still bear
different relations to the progressive degradation of the country, and
have been developed by erosion in different degrees. In nine of the
instances cited the arch of strata has been truncated, but at so high a
level that no dikes nor sheets were unearthed. In five instances the
plane of truncation was so low that dikes and sheets were brought to
light, but not the main body of trachyte. The truncation was in most of
these cases less perfect because of the resistance to erosion by the
hard dikes and sheets and by the strata which their heat had hardened.
In eight other instances the erosion has left prominent the dome of
hardened strata with its sheets and dikes, but has somewhere broken
through it so as to reveal a massive core of trachyte. In five instances
one side of the dome of strata has been washed away, exposing the core
of trachyte to its base and showing undisturbed strata beneath it. In
two instances the soft matrix has been so far washed away from the
laccolite as to expose its form fully; and seven laccolites have not
only lost all cover but have themselves been partially demolished.
Certainly, the hypothesis accords with all the facts that have been
observed and unites them into a consistent whole. It explains fourteen
dome-like arches of sedimentary rock which are imperfectly exposed, by
classing them with seven other arches of the same region which have been
opened in section _to the base_ and found to contain laccolites; and it
strengthens the case by pointing to a connected series of intermediate
phenomena. Until some strata-dome of the Henry Mountains, or of a
closely allied mountain, shall be found to display some different
internal structure, it will be safe to regard the whole phenomena of the
group as laccolitic.
_Form of laccolites._—As a rule laccolites are compact in form. The
base, which in eleven localities was seen in section, was found flat,
except where it copied the curvature of some inferior arch. Wherever the
ground plan could be observed it was found to be a short oval, the ratio
of the two diameters not exceeding that of three to two. Where the
profile could be observed it was usually found to be a simple curve,
convex upward, but in a few cases and especially in that of the Marvine
laccolite the upper surface undulates. The height is never more than
one-third of the width, but is frequently much less, and the average
ratio of all the measurements I am able to combine is one to seven.
The ground plan approximates a circle, and the type form is probably a
solid of revolution—such as the half of an oblate spheroid.
_Internal Structure._—Of the laccolites which are best exhibited in
section, there are a number which appear to be built up of distinct
layers. The Peale exhibits three layers with uneven partings of shale.
The Sentinel shows two without visible interval. The Howell shows two.
The Pennell has a banded appearance but was not closely examined. The
Marvine shows at a distance a faint banding, which near by eludes the
eye. No division nor horizontal structure was seen in the Hillers,
Jukes, Scrope, or Steward laccolites, but observation was not
sufficiently thorough to satisfy me of its absence. It is probable that
all the larger laccolites are composite, having been built up by the
accession of a number of distinct intrusions.
There is little or no prismatic structure in the trachytes. It is
sometimes simulated by a vertical cleavage induced in sheets and
laccolites which are undergoing disintegration by sapping, but I did not
observe the peculiar prismatic cleavage which is produced by rapid
cooling in dikes, sheets, and _coulées_ of basalt. The two structures
are not often discriminated, but they are really quite distinct, and
their peculiar characters are easily recognized. The cleavage planes
produced by cooling are as a rule perpendicular to the cooling surface,
and the systems of prisms which are based on the opposite walls of a
dike or sheet, do not correspond with each other, and do not run across,
but meet midway in a confused manner. The cleavage planes which are
produced by the shearing force when a massive bed of trap or other rock
is undermined or sapped and yields under its own weight, extend from
base to top, and are perpendicular to the plane of the horizon instead
of the plane of the bed.[2]
Footnote 2:
See page 172 of the “Exploration of the Colorado River”, by J. W.
Powell.
_Vertical Distribution._—The range of altitudes at which laccolites have
been formed is not less than 4,500 feet, and neither the upper nor the
lower limit is known.
The highest that are known—those which were intruded at the highest
geological horizon—are near the base of the Blue Gate shale (Middle
Cretaceous), but it is quite possible that higher ones have been
obliterated by erosion. The lowest that is known was intruded in the
Shinarump shale, but it is known of the invisible Ellsworth laccolite
that upper Carboniferous strata lie above it, so that its horizon of
intrusion must be still lower. The plexus of dikes and sheets on the
Ellsworth arch indicates that the laccolite is not deeply buried; but in
the series of arches there are nine which show no trachyte, and we have
no data from which to infer their depth.
How the laccolites are distributed within these limits is more readily
comprehended by the aid of a diagram. In Figure 50 the triangles mark
the horizons of determined laccolites, and the crosses the horizons
which the invisible laccolites cannot exceed. For example, the base of
the Scrope laccolite is visible and is seen to lie on the Tununk shale
400 feet above the Henry’s Fork conglomerate; to represent it a triangle
is placed at about the middle of the space representing the Tununk
shale, and in the column devoted to the Ellen cluster. The upper surface
of the Geikie laccolite is visible and upon it rest, first a few feet of
Flaming Gorge shale, and then the Henry’s Fork conglomerate; to
represent it a triangle is placed near the top of the Flaming Gorge
space. The crown of the Pulpit arch has been so far eroded that half of
the Vermilion Cliff sandstone is shown in section, but no laccolite is
revealed. It is evident that the Pulpit laccolite is lower than the
middle of that sandstone; and to represent it a cross is placed below
the middle of the Vermilion Cliff space and in the column devoted to the
Hillers cluster.
[Illustration:
FIG. 50.—Diagram of the Vertical Distribution of the Laccolites of the
Henry Mountains.
]
The first feature which this graphic assemblage yields to the eye is
that there are at least two zones of laccolites. The upper ranges from
the lower part of the Blue Gate Group, through the Tununk and Henry’s
Fork, to the upper part of the Flaming Gorge Group. The lower has not
yet been fully developed by erosion, but its proximity is indicated. The
Hillers laccolite is uncovered at top, and the dikes of the Ellsworth,
the Greater Holmes, and the Crescent have been reached. All of the
invisible laccolites indicated in the Vermilion Cliff and Shinarump
spaces must be referred to this zone; and there is reason to suspect
that all but one of the invisible laccolites whose indication falls in
the Tununk and Flaming Gorge spaces belong also to the lower zone.
However this may be, it is not probable that the determination of the
depths of the invisible laccolites would vitiate the conclusion that
there is an upper zone of laccolitic frequency which is separated from a
lower zone by an interspace of laccolitic infrequency.
Another feature illustrated by the diagram is that all the determined
laccolites are inclosed by soft beds. They have been intruded into the
shales, but not the sandstones. They cluster about the Henry’s Fork
conglomerate, but none of them divide it. This selection of matrix is
confined however to the laccolites and is not exercised by sheets and
dikes. Trachyte sheets were seen within the Henry’s Fork, the Gray
Cliff, the Vermilion Cliff, and the Aubrey sandstones.
A third feature of the diagram is the restriction of the upper zone of
laccolites to the northward clusters; Mounts Ellsworth and Holmes
contain laccolites of the lower zone only. This fact of distribution is
correlated with a fact of denudation—namely, that in the general
degradation of the country, the region about the southern mountains has
lost two thousand feet more than the northern. One fact is probably the
cause of the other. The absence of laccolites of the upper zone at the
south may have permitted the greater degradation; or the greater
degradation may have caused the destruction of several of the upper
laccolites. The fact that the difference in degradation can be
independently accounted for is favorable to the latter supposition. The
Colorado River which is the main artery of drainage for the whole
region, flows close to the bases of the southern mountains, and the
rapid declivity from the mountain summits to the river has given and
still gives exceptionally great power to the agents of erosion. No other
cause is needed to explain the difference of degradation, and the
absence of laccolites of the upper zone is explicable without assuming
that they were never present.
The negative objection to the idea that the southern mountains
originally possessed laccolites of the upper zone being thus disposed
of, it is worth while to inquire whether there is any evidence in its
favor. If superior laccolites existed, they would be sure to leave
behind them a record of the conduits through which their lava was
injected. A dike or a chimney must always connect a laccolite with the
source of its material; and the removal of the laccolite necessarily
exposes a cross-section of its stem. The discovery of such a dike can be
regarded, not indeed as a proof of the former existence of a superior
laccolite, but as demonstrating its possibility. The summits and flanks
of Mounts Holmes and Ellsworth bear many dikes, which have been regarded
as subsidiary features of the laccolites beneath them, but it is quite
possible that any one of them formerly led to another laccolite above.
The upper laccolite may have been first formed and then have been
lifted, dike and all, when the lower was intruded; or it may have been
last formed, and been fed through a fissure which traversed the lower
after its congelation. The only dike which was discovered in the
vicinity of these mountains, without being upon them, stands midway
between them. It is the only observed dike of the Henry Mountains
(excepting always the alpine district of Mount Ellen), which is not so
closely associated with some laccolite as to seem an accessory feature,
and its exceptional position has led to the suspicion that it belonged
to an overlying laccolite.
Upon such uncertain evidence no positive conclusion can be based, and it
is vain to build laccolites in the air. The most that can be said is
that the southern mountains need not be distinguished from the northern,
because at the present stage of degradation they contain laccolites of
the lower zone only.
_The Material of the Laccolites._
The intrusive rocks of the Henry Mountains were sampled with care.
Specimens were selected which had undergone little decomposition and
which represented all the prominent lithologic varieties. They were
chosen from the trachytes of both zones and of each of the mountain
masses; and they represent dikes and sheets, as well as laccolites. From
about thirty specimens thin slices were cut for microscopic examination.
Captain C. E. Dutton, of Omaha, Nebraska, was so kind as to undertake
the study of the collection, and the letter which embodies his
conclusions is given below. In accordance with his diagnosis I shall
call the intrusive rocks _porphyritic trachyte_; and I am glad to have
the weight of his authority in support of my belief that all the rocks
of the series are of one type, their resemblances far outweighing their
differences.
NOTE.—Persons desiring to examine the Henry Mountain trachytes under
the microscope can obtain mounted thin sections from Mr. Alexis A.
Julien, School of Mines, Columbia College, New York.
REPORT ON THE LITHOLOGIC CHARACTERS OF THE HENRY MOUNTAIN INTRUSIVES.
BY CAPTAIN C. E. DUTTON.
“I have examined with great interest and attention the Henry Mountain
rocks you sent me, and proceed to acquaint you with such results as my
limited facilities have permitted me to derive from the examination. It
is a very well defined series, having some marked characters which
distinguish it from the nearest allied group with which I am acquainted.
This is all the more interesting, because I am inclined to think that
these peculiarities may have a definable association with or relation to
the manner in which the intrusive rocks occur in those “laccolites”, as
you term them.
“The hand specimens show in most cases large and unusually perfect
crystals of orthoclase imbedded in a very compact uniform paste through
which hornblende is also disseminated rather more abundantly than is
usually the case where the dominant felspar is monoclinic. Micaceous
crystals appear to be wholly wanting and this is a notable circumstance,
since the trachytes of the Plateau country, to which these rocks are
most nearly allied, are seldom without one or more of them. The only
other mineral which is of frequent occurrence in the specimens is
magnetite (or possibly titanic iron), which is diffused in the usual
form of minute granules in many of them, but is scarce in several of
them. In general there is a great scarcity of mineral species and any
others than those mentioned are of the greatest rarity.
“The dominant felspar is orthoclase, but a portion of it is triclinic,
and I presume this portion is albite, with an occasional occurrence of
oligoclase. The groundmass in which the crystals are included is in most
cases decidedly compact and without distinguishable crystals, but shows
between the crossed Nicols closely aggregated luminous points, which
with a ¼-inch objective are resolved indistinctly into felspar. In some
cases the crystals of the groundmass are quite apparent with an inch
objective, and their species determinable. But the greater portion of
the groundmass is quite amorphous, and does not polarize light at all.
The proportion of crystalline to amorphous matter in the paste is highly
variable—in some cases it is quite bright between crossed Nicols, in
others far less so, and in none is it entirely dark. Those specimens
which have the finer and more amorphous groundmass have the larger and
more perfect crystals of felspar—an association of properties which is
not wholly without qualification, but still sufficiently decided.
“Turning to the included felspars, their mode of occurrence is quite an
uncommon one, I believe, so far as the eruptive rocks of the Rocky
Mountain region are concerned, and give rise to some hesitation before
assigning them definitely to the trachytic group. In a great many cases
the felspathic crystals are well developed, and so large and so nearly
perfect that their aspect is decidedly porphyritic. This is especially
the case with the dikes of Mounts Ellsworth and Holmes (Nos. 56, 61, 68,
and 69). The orthoclase is invariably of the white “milky” variety, with
the exception of a single specimen from a Mount Ellsworth dike (No. 57),
where it is present as sanidin. (This is an exceptional rock in all
respects, and will be spoken of hereafter.) Nearly all of them appear to
have been subject to alteration by chemical action since their formation
as is indicated by their diminished power to polarize light. Whether
this is due to atmospheric weathering or to changes _en masse_ it is of
course impossible to distinguish with certainty, though I incline to the
latter view since it is manifested as decidedly in specimens which show
no external indications of weathering as in those which do show them. It
is not uncommon to find crystals which have almost entirely ceased to
polarize. The zonal arrangement is very common in the crystals, and some
of the zones contain numberless minute fluid cavities in the largest
crystals. The foreign substances included in the crystals present no
novelty, being the ordinary films of hornblende, minute needles of
felspar, granules of magnetite, and those dust-like points of brownish
yellow color which are the proper inclusives of the groundmass.
“The orthoclase occasionally presents the adular variety, but this never
becomes a marked feature. I have observed the same in many of the
trachytes of the High Plateaus and of the Great Basin. Another
phenomenon is the occurrence of crystals which are quite typically
monoclinic at one end (orthoclase), and at the other end have the
arrangement of plagioclase. This is well known elsewhere, and described
by Zirkel. (Mik. Beschaff der Mineralien u. Gesteine).
“In classifying these rocks therefore, we may observe that they present
a blending of the characteristics which are common to trachyte and
felsitic porphyry. Those who regard porphyry as a distinct class of
eruptive rocks would have no hesitation in calling Nos. 18, 56, 61, 68,
and 69 undebatable felsitic porphyries, and to the same series might
with propriety be added No. 33. With equal confidence Nos. 16, 20, 35,
and 43 may be called unqualified trachytes. The other rocks are
intermediate in character between these two extremes, and the whole may
be regarded as a series in which the individuals form a graduated scale.
“You will recall the fact that many lithologists object to the terms
porphyry or porphyritic being used to designate a distinct class or
group of rocks, holding that they merely characterize a single feature
which is more or less frequently presented by all igneous rocks and
having no necessary relation to any of them, and that this is no more
adequate to such an important distinction than the color or relative
degree of fineness or coarseness of texture. Although this latter view
seems to me to underrate the distinctive value of the porphyritic
character in general, I incline very decidedly to the belief that it is
true as applied to these Henry Mountain rocks. Here at least the
porphyritic character has but little significance. In some varieties the
crystals are larger and more perfect and the groundmass more
homogeneous; in others the crystals are smaller and imperfect, and the
groundmass more coarse and irregular; while still others are ‘betwixt
and between’. The most careful scrutiny fails to show any fundamental
differences in the groundmass or in the included minerals. Hence I think
these rocks would be accurately designated _as a group_ by calling them
porphyritic trachytes. Such varieties as Nos. 16, 20, and 35 may by
themselves be called simply trachyte, the porphyritic character being
insufficiently distinct in them to warrant any qualification of the
name.
“There is one specimen (No. 57, from dike in Shinarump shale, Mount
Ellsworth) which constitutes an exception to the foregoing. By
inspection of a hand specimen it might hastily pass for a very compact
andesite, but the observer will be instantly undeceived by applying the
microscope. It consists of imperfect crystals, which must be sanidin,
imbedded in a very close groundmass composed of a material which differs
from the foregoing trachytes in being wholly amorphous. Between crossed
Nicols the paste transmits no light whatever, though between the
parallel Nicols it closely resembles the others. The hand specimen is
very dark colored (gray), but the slide is sufficiently translucent. The
felspar crystals are either fragmental or very imperfectly developed as
to their edges and angles, but polarize very sharply. The amount of
twinning is very small, but it appears occasionally. The rock is
undoubtedly a trachyte, but an unusual one. Its dark color is not due to
hornblende nor to magnetite, both of which occur in it very sparingly,
especially the former.
“I have already remarked that the only frequent minerals besides felspar
are hornblende and magnetite. Apatite occurs and is tolerably plentiful
in a few of the specimens, but absent from most of them. The crystals
are all small, requiring a low power for the determination of the larger
and a high power for the smaller. They present no peculiarities. Of very
rare occurrence is nepheline. This mineral is usually associated with
the more basic volcanic rocks and seldom penetrates the trachytic group.
Quartz is almost equally rare. The absence of any great variety in the
mineral species is quite normal, except possibly the total absence of
mica, which is usually present and frequently the only associate of
felspar in the western trachytes. The Henry Mountain rocks do not so far
as I can discover contain a trace of it.
“In answer to your particular inquiries—
“1st. ‘Is the paste vesicular, or is there any evidence in the crystals
to indicate the pressure under which they were formed?’ I can only say
that the paste is extremely compact and contains no vesicles even in
those specimens of which the aspect is most decidedly trachytic. Some of
the larger crystals of felspar contain an abundance of pores or vesicles
which may have contained liquids, but with a ¼-inch objective they are
too small for treatment by the method you refer to. A few large
cavities, usually of irregular shape, occur, but I am quite unfamiliar
with the practical treatment of this subject and cannot advise you. I do
not find any cavities still containing fluids, and I presume that even
if they existed they would be difficult if not impossible to gauge on
account of the impellucidity of the felspar. I presume quartz is the
most favorable mineral for this investigation on account of its
transparency and the greater frequency of its large cavities. Quartz
however is almost the scarcest of the contents of these rocks.
“2d. ‘Do any mineralogic differences correlate with the superficial or
geographic distribution of the rocks?’ and
“3d. ‘Do any mineralogic differences correlate with the vertical
distribution of the trachytes?’ As it appears from your account of the
distribution of the masses that the upper zone belongs (with one
exception) to Mounts Ellen and Pennell and the lower to the other
mountains, the answer to one is the answer to both, and this is in the
negative. The mineralogical differences are exceedingly small,
considering the number of distinct masses, and this covers the inquiry
entirely. Regarding the texture or habitus, on the contrary, it appears
to me that the true trachytes predominate in the upper zone and the
porphyries in the lower, but not without exceptions.
“4th. ‘Do any mineralogic differences correlate with the size of the
intrusive masses?’ No mineralogical differences thus correlate, but I
find a preponderance of the porphyritic texture in the smaller masses
and of the trachytic texture in the larger. It is not without exception,
and the preponderance is small.”
_Metamorphism and Contact Phenomena._—Wherever the trachytes came in
contact with the sedimentaries the latter were more or less altered.
Large bodies of trachyte produced greater changes than small. The
laccolites both metamorphosed their walls more completely, and carried
their influence to a greater distance than the sheets and dikes. The
summits of the laccolites had a greater influence than the edges; a
phenomenon to which I shall have occasion to revert. The sandstones were
less affected than the shales, at least in such characters as readily
catch the eye. Clay shales were indurated so as to clink under the
hammer, and Captain Dutton discovered with the microscope that minute
crystals of felspar had been developed. Sandstones were usually modified
in color, and their iron was segregated so as to give a mottled or
speckled appearance to the fracture. They were indurated, but the
granular texture was always retained.
The trachyte carries numerous small fragments of sedimentary rock broken
apparently from its walls, and these are as thoroughly crystalline as
their matrix.
The altered rocks are usually jointed, but nothing approaching to slaty
cleavage was seen, nor has there been any crumpling.
The reciprocal influence of the sandstone and shale upon the trachyte
was small. Specimens broken from the contact surface of a laccolite and
from its interior cannot be distinguished. In the Marvine laccolite
however there is a difference between the exterior and interior portions
in their ability to withstand erosion.
_Historical._—Before leaving the subject of the structure of the
mountains it is proper to place on record certain observations by others
which antedated my own but have never been published.
While Professor Powell’s boat party was exploring the cañons of the
Colorado, Mr. John F. Steward a geologist and member of the party
climbed the cliff near the mouth of the Dirty Devil River and approached
the eastern base of the mountains. He reported that the strata had in
the mountains a quaquaversal dip, rising upon the flanks from all sides.
The following year Prof A. H. Thompson then as now in charge of the
geographic work of Professor Powell’s survey crossed the mountains by
the Penellen Pass and ascended some of the principal peaks. He noted the
uprising of the strata about the bases and the presence of igneous
rocks.
In 1873 Mr. E. E. Howell at that time the geologist of a division of the
Wheeler Survey traveled within twelve miles of the western base of the
mountains, and observed the uprising of the strata.
My own observations were begun in 1875, at which time a week was spent
among the mountains. They proved so attractive a field for investigation
that in the following year a period of nearly two mouths was devoted to
their study.
_Other Igneous Mountains._—The Henry Mountains are not the only igneous
group which the Plateau province comprises. They are scattered here and
there throughout its whole extent. From the summits of the Henry
Mountains one can see the Sierra La Sal ninety miles to the
northeastward, and the Sierra Abajo seventy miles to the eastward.
Beyond them and two hundred miles away are the Elk Mountains of
Colorado. Fifty miles to the southwestward stands the Navajo Mountain on
the brink of the Colorado; and one hundred and twenty miles to the
southeast the Sierra La Lata and the Sierra Carriso are outlined against
the horizon. Westward it is less than thirty miles to the Aquarius
Plateau, the nearest member of the great system of volcanic tables among
which the Sevier and Dirty Devil Rivers rise.
Beyond the horizon at the south and southwest and southeast are a series
of extinct volcanoes; Mount Taylor and the Marcou Buttes in New Mexico;
the Sierra Blanca, the Sierra Mogollon, the San Francisco Group and the
Uinkarets in Arizona; and the Panguitch Lake Buttes in Utah.
Of the groups which are visible, all but that of the Aquarius Plateau
are allied in character to the Henry Mountains.
The Sierra Abajo was studied in 1859 by Dr. J. S. Newberry, geologist of
the Macomb Expedition, who writes: “Within the last few weeks we have
been on three sides of this sierra, and have learned its structure quite
definitely. It is a mountain group of no great elevation, its highest
point rising some 2,000 feet above the Sage-plain, or perhaps 9,000 feet
above the sea. It is composed of several distinct ranges, of which the
most westerly one is quite detached from the others. All these ranges,
of which there are apparently four, have a trend of about 25° east of
north, but being arranged somewhat _en echelon_, the most westerly range
reaching farthest north, the principal axis of the group has a northwest
and southeast direction. The sierra is composed geologically of an
erupted nucleus, mainly a gray or bluish-white trachyte, sometimes
becoming a porphyry, surrounded by the upheaved, partially eroded,
sedimentary rocks. The Lower Cretaceous sandstones and Middle Cretaceous
shales are cut and exposed in all the ravines leading down from it,
while nearly the entire thickness of the Cretaceous series is shown in
spurs which, in some localities, project from its sides; apparently the
remnants of a plateau corresponding to, and once connected with, the
Mesa Verde. Whether the Paleozoic rocks are anywhere exposed upon the
flanks of the Sierra Abajo I cannot certainly say, though we discovered
no traces of them. It is, however, probable that they will be found in
some of the deeper ravines, where, as in most of these isolated
mountains composed mainly of erupted material, they are doubtless but
little disturbed, but are buried beneath the ejected matter which has
been thrown up through them.
“The relations of the Cretaceous rocks to the igneous nucleus of the
Sierra Abajo are very peculiar, for, although we did not make the entire
circuit of the mountain mass, and I can, therefore, not speak definitely
in regard to the western side, as far as our observations extended we
found the sedimentary strata rising on to the trachyte core, as though
it had been pushed up through them.” (Geology of the Macomb Expedition,
page 100.)
Of another group Dr. Newberry says in the same report (page 93): “Of the
composition of the Sierra La Sal we know nothing except what was taught
by the drifted materials brought down in the cañons through which the
drainage from it flows. Of this transported material we saw but little,
but that consisted mainly of trachytes and porphyry, indicating that it
is composed of erupted rocks similar to those which form the Sierra
Abajo, of which it is in fact almost an exact counterpart. From the
cliffs over Ojo Verde we could see the strata composing both the upper
and second plateaus, rising from the east, south, and southwest on to
the base of the Sierra La Sal, each conspicuous stratum being distinctly
traceable in the walls of the cañons and valleys which head in the
sierra. It is evident, therefore, that the rocks composing the Colorado
Plateau are there locally upheaved, precisely as around the Sierra
Abajo * * *.”
These mountain groups have been since visited by the geologists of Dr.
Hayden’s survey, Dr. A. C. Peale ascending the Sierra La Sal and Mr. W.
H. Holmes the Sierra Abajo. Mr. Holmes has also examined the La Lata and
Carriso Mountains and found in them the same upbending of Cretaceous
strata and the same association of igneous material.
The Navajo Mountain has been viewed by Mr. Howell and by the writer from
a commanding position on the opposite side of the Colorado River, and
fragments of its trachyte have been gathered on the river bank by
Professor Powell, but no geologist has yet climbed it. Still there can
be no question of its general structure. It is a simple dome of
Jura-Triassic sandstone, springing abruptly from a plateau of the same
material, and veined at the surface by sheets and dikes of trachyte—the
counterpart in fine of Mount Ellsworth, only of more imposing
proportions.
The La Sal, the Abajo, the La Lata, the Carriso, the Navajo, and the
Henry Mountains agree in their essential features. Structurally they
have no trends. Their phenomena are grouped about centers and not axes.
In all of them the strata are lifted into dome-like arches, and
associated with these arches are bodies of trachyte. The trachytes are
all of one lithologic type, and are so closely related that a collection
of rock specimens representing all the groups would show scarcely more
variety than a collection representing the Henry Mountain laccolites.
With so many characters in common they can hardly fail to agree in the
possession of laccolitic nuclei.[3]
Footnote 3:
While these pages are passing through the press a paper by Dr. A. C.
Peale “On a peculiar type of eruptive mountains in Colorado” (Bulletin
U. S. Geol. Sur., Vol. III, No. 3) comes to hand. He groups together
as of one type not only the Elk, La Sal, Abajo, La Lata, and Carriso
Mountains, but also the Spanish Peaks, Park View Mountain, Mount
Guyot, Silverheels Mountain, the San Miguel Mountains, the La Plata
Mountains, and certain smaller masses in Middle Park and near the
Huerfano River. He says, “Although modified in several instances, the
general plan appears to be the same. The igneous material came up
through fissures in the sedimentaries, sometimes tipping up their
ends, and sometimes passing through without disturbing them. On
reaching the Cretaceous shales, it generally spread out in them, and
pushed into and across them dikes and intrusive sheets of the same
igneous rock. The elevation in some cases appears to be due to actual
upheaval caused by the eruptive force. The mountains as they now exist
are doubtless largely the result of erosion, the hard igneous rock
opposing greater resistance to erosive influences than do the
surrounding soft sedimentary beds.”
The Elk Mountains are at the very margin of the plateau, and
geographically might be connected with the Sawatch Range which bounds
the plateau province on that side. But structurally they are a group
instead of a range, and affiliate with the groups which are insulated by
an environment of tables. Thanks to the labors of Mr. Holmes and Dr.
Peale their general structure is known. The Eastern Elk Mountains
consist of four great bodies of “eruptive granite”, over which are
arched not only Mesozoic but Paleozoic strata. Their foundation must be
a floor of Archæan metamorphics. Two of them, the Snow Mass and White
Rock laccolites, are joined by a continuous line of disturbance, in the
description of which by pen and pencil Mr. Holmes has made an important
contribution, not only to dynamical geology, but to the methods of
geological illustration. The others are more symmetric and are
complementary illustrations of the common structure. One, the Sopris, is
half truncated by erosion so that the core is exposed at top with an
encircling fringe of upturned sedimentaries; and the other, the
Treasury, retains a complete arch of Paleozoic strata. The Western Elk
Mountains are a cluster of smaller laccolites which are inserted between
strata of Cretaceous age. Their traps include porphyritic trachytes
undistinguishable from those of the Henry Mountains, and eruptive
granites identical with those of the Eastern Elk Mountains; and they
exhibit a gradation from one to the other. Indeed the two rocks are
nearly related, and their assignment to classes so diverse as trachyte
and granite is merely an illustration of the imperfection of our
classification of rocks. The description of the Elk Mountains will be
found on pages 61 to 71 and 163 to 168 of the Annual Report for 1874 of
the “Geological and Geographical Survey of the Territories.”
If we turn now to the distinctively volcanic mountains of the Plateau
province—to those which are built by eruption at the surface—we leave at
once the porphyritic trachytes. Mount San Francisco, Mount Bill
Williams, Mount Sitgreaves, Mount Kendrick, Mount Floyd and the Sierra
Blanca (of Arizona) are all composed of basic trachytes, and so are the
Aquarius Plateau and the many tables that lie beyond it.
The Mogollon group, the Marcou Buttes, the minor cones about Mount San
Francisco, the Uinkarets, and the Panguitch Lake group are basaltic. In
each of these instances the igneous rock issued above the surface and
there is no evidence by displacement that any portion of it was
deposited below.
Mount Taylor may be an exception. In the character of its lava and its
general features it resembles Mount San Francisco, but there are
disturbed strata on its southern flank, and it is possible the mountain
is both extrusive and intrusive. Extrusion and intrusion are probably
combined in some small tables lying fifty miles north of the Henry
Mountains. They are built of Flaming Gorge shale, preserved from erosion
by dikes, sheets, and (probably) outflows of basalt.
Combining all these facts we attain to a simple relation between two
types of igneous rock on the one hand, and two types of mountain
structure on the other. One type of rock is acidic, including
“porphyritic trachyte” and “eruptive granite”, and its occurrence is
without exception intrusive. The other type of rock is basic, including
basic trachyte and basalt, and its occurrence is almost uniformly
extrusive.
It is not possible to combine the two groups of phenomena by saying that
in one case the eruptive cones cover laccolites, and in the other the
laccolites have been covered by eruptive cones which have disappeared;
first, because many of the eruptive cones are too well exposed to admit
of the concealment of laccolitic arches beneath them; second, because
the two types of lava are essentially different. The acidic type if
extruded at the surface would be an ordinary trachyte; the basic type if
crystallized under pressure would be classed with the greenstones.
The basis for the generalization is exceedingly broad. I have enumerated
only seven groups of laccolitic mountains and ten groups of eruptive;
but with few exceptions each group is composed of many individuals, each
one of which is entitled to rank as a separate phenomenon. In the
Uinkaret Mountains Professor Powell has distinguished no less than one
hundred and eighteen eruptive cones, and in the Henry Mountains I have
enumerated thirty-six individual laccolites. In one locality basic lava
has one hundred and eighteen times risen to the surface by channels more
or less distinct, instead of opening chambers for itself below. In the
other locality porphyritic trachyte has thirty-six times built
laccolites instead of rising to the surface.
If our attention was restricted to these two localities we might as
naturally correlate the types of structure with some accidents of
locality as with types of lava; but when all the localities are taken
into account it is evident that there is no common mark by which either
the laccolitic or the volcanic are distinguished.
THE QUESTION OF CAUSE.
We are now ready to consider the question: Why is it that in some cases
igneous rocks form volcanoes and in other cases laccolites?
It is not necessary to broach the more difficult problem of the source
of volcanic energy. We may assume that molten rock is being forced
upward through the upper portion of the earth’s crust, and disregarding
its source and its propelling force may restrict our inquiry to the
circumstances which determine its stopping place.
Let us further assume, but for a moment only, that the cohesion of the
solid rocks of the crust does not impede the upward progress of the
fluid rock, nor prevent it from spreading laterally at any level. The
lava will then obey strictly the general law of hydrostatics, and assume
the station which will give the lowest possible position to the center
of gravity of the strata and lava combined.
(1) If the fluid rock is less dense than the solid, it will pass through
it to the surface and build a subaërial mountain.
(2) If the upper portion of the solid rock is less dense than the fluid,
while the lower portion is more dense, the fluid will not rise to the
surface but will pass between the heavy and light solids and lift or
float the latter.
(3) If the crust be composed of many horizontal beds of diverse and
alternating density, the fluid will select for its resting place a level
so conditioned that no superior group of successive beds, including the
bed immediately above it, shall have a greater mean specific gravity
than its (the fluid’s) own; and that no inferior group of successive
beds, including the bed immediately beneath, shall have a less mean
specific gravity than its own.
[Illustration:
FIG. 51.—Diagram to illustrate the application of the law of
Hydrostatic Equilibrium to the movements of lavas. The shaded bands
represent heavy strata; the open, light.
]
[In the diagram, a series of light and heavy beds are represented in
section by open and shaded spaces. A lava stream free to move upward or
laterally will intrude itself at some point (_c_) so placed that every
combination of superior beds (_a_), which includes the lowest, shall
have a less average density; and every combination of inferior strata
(_b_), which includes the highest, shall have a greater average density
than that of the lava.]
The first case is that of a volcano; the second is that of a laccolite;
and the third is the general case, including the others and applying to
all volcanoes and laccolites.
Conversely we may say that, given a series of strata of diverse and
alternating density, a very light lava will traverse it to the top and
be extruded; a heavier will intrude itself at some lower level; and a
series of dissimilar lavas may select an equal number of distinct
levels.
It is easy to imagine such a balancing of conditions that a slight
change in a lava will determine a great change in its level of
intrusion.
Having seen the general application of the hydrostatic law, it is time
to recall the condition which we laid aside at the start. Cohesion, or
rigidity, is never absent and must affect every phase of vulcanism. It
certainly opposes the free circulation of lavas, and it cannot but
modify their obedience to the hydrostatic law.
But granting this, and believing that a full comprehension of the
subject must include this condition, I am at a loss to tell in what way
it influences the selection by a lava flood of a subaërial or a
subterranean bourne. Whether it will on the whole oppose upward progress
more than lateral, or _vice versa_, is not clear. If it resists lateral
intrusion the more strongly, it favors the formation of volcanoes; if it
resists upward penetration the more strongly, it favors the formation of
laccolites; and in either case the working of the hydrostatic law is
modified.
But in neither case is the working of the law more than modified. The
law is not abrogated, and in obedience to it light lavas still _tend_ to
rise higher than heavy however much the rising of all lavas may be
hindered or favored.
In brief, since lavas are fluids they are subject to the law of fluid
equilibrium, and their behavior is conditioned by the relations of their
densities to the densities of the solids which they penetrate; and since
the latter solids are rigid and coherent, it is further conditioned by
the resistance which is opposed to their penetration. When the
resistance to penetration is the same in all directions, the relation of
densities determines the stopping place of the rising lava; but when the
vertical and lateral resistances are unequal, _their_ relation may be
the determining condition.
If we can decide whether the determinative condition in the Plateau
region was that of densities or that of penetrability, we shall have
solved our problem.
Assuming, first, that the essential condition is that of penetrability,
we should expect that some particular stratum or that a few particular
strata, being less penetrable than others, would check the rising lavas
and accumulate them in a system of laccolites, which would occupy one,
or a few definite horizons. Volcanoes would occur in districts from
which such impenetrable strata either were originally absent or had been
removed before the igneous epoch; and we should expect to find the same
variety of material in laccolites and in volcanoes.
Assuming, second, that the essential condition is that of densities, we
should expect as before to find certain stratigraphic horizons more
favorable than others to the accumulation of laccolites, and we should
also expect to find certain lavas usually volcanic and certain others
usually laccolitic.
That is to say—since the condition of impenetrability resides in the
solid rock only, and the condition of density pertains to both solid and
fluid, either condition might determine laccolites at certain
stratigraphic horizons, while the latter only could discriminate certain
lavas as intrusive and others as extrusive.
The vertical distribution of laccolites is not inconsistent with either
assumption. In the Henry Mountains there are two zones of occurrence; in
the Eastern Elk Mountains there is a third; and it is probable in the
present state of our knowledge that all other laccolites of the Plateaus
can be assigned to one or another of these. The fact that the laccolites
of the upper zone have a vertical range of two thousand feet is rather
favorable to the idea that their stations were determined by relations
of density, but is not decisive.
When however we turn to the relation between the constitutions and the
behaviors of lavas, we find the entire weight of the evidence in favor
of the assumption that conditions of density determine the structure.
The coincidence of the laccolitic structure with a certain type of
igneous rock is so persistent that we cannot doubt that the rock
contained in itself a condition which determined its behavior.
We are then led to conclude that the conditions which determined the
results of igneous activity were the relative densities of the intruding
lavas and of the invaded strata; and that the fulfillment of the general
law of hydrostatics was not materially modified by the rigidity and
cohesion of the strata.
Having reached this conclusion it is natural to seek for confirmation by
the investigation of the densities of the rocks concerned in the
phenomena. As will appear by a table given further on, the density of
the Henry Mountain trachyte has been determined to be 2.61.; but the
densities of the erupted lavas of the Plateaus are not yet known. There
can be no doubt however that the latter are heavier. Von Cotta in his
Lithology gives 2.9 to 3.1 as the density of basalt, and 2.6 to 2.9 as
the density of the more basic trachytes. And in general, it is well
established that where the state of aggregation is the same, basic
igneous rocks are always heavier than acidic. But in order that the
laccolitic structure should have been determined by density, the acidic
rock of the laccolites must have been heavier in its _molten_ condition
than the more basic rocks of the neighboring volcanoes; and since in the
_crystalline_ condition the acidic is the lighter, it follows that it
has gained less density in cooling than the basic.
If the amount of contraction of the several rocks in passing from their
natural molten condition to the crystalline condition could be
determined experimentally, a crucial test would be applied to our
conclusions as to the origin of laccolites. The matter is however beset
with difficulties. Bischof attempted by melting eruptive rocks in clay
crucibles to obtain their ratios of expansion and contraction, but his
method involved so many sources of error that his results have been
generally distrusted. He concluded that the contraction in passing from
the molten to the crystalline state is greater in acidic than in basic
rocks. Delesse by an extended series of experiments in which crystalline
rocks were melted and afterward cooled to glasses, showed that acidic
rocks increase in volume from 9 to 11 per cent. in passing from the
crystalline state to the vitreous, while basic increase only 6 to 9 per
cent. Mallet concluded from some experiments of his own that the
contraction of rocks in cooling from the molten condition is never more
than 6 per cent., and that it is greater with basic than with acidic
rocks; but considering that the substances which he treated were
artificial and not natural products, that his methods were not uniform,
and that he ignored the distinction between the vitreous and the
crystalline, of which Delesse had demonstrated the importance, no weight
can be given to his results.
If however all of these experiments were trustworthy and their results
were concordant, their bearing upon the problem of the laccolites would
still be slight. It is generally conceded that the fusion of lavas is
hydrothermal, while in all the experiments recourse was had to dry
fusion; and the densities attained in the two ways are necessarily
different. The practical difficulty in the way of restoring the natural
molten condition is great and may be insuperable, but unless it shall be
overcome we cannot learn experimentally the changes of density which
igneous rocks undergo in congelation.
There is a fact of observation which tends to sustain the view that the
laccolitic rocks contracted less in cooling than the volcanic. The
prismatic structure is produced by the contraction of cooling rocks
during and after solidification. That it does not occur in the Henry
Mountain trachytes indicates that their contraction was small. That it
does occur at numerous localities in Utah in basalts, indicates that
their contraction was relatively great. Mr. Jukes, in his Manual of
Geology, says that it is most frequently exhibited in “doleritic lavas
and traps, being especially characteristic of basalt, but occurs almost
as perfectly in some greenstones and felstones”; and in the range of my
own observation I can recall no instance of its occurrence in other than
basic rocks.
For the sake of comparing the densities of the intrusive rocks with
those of the strata which contain them, a number of determinations were
made of the specific gravities of specimens representative of the
trachytes and of the several sedimentary groups of the Henry Mountains.
Trachytes were selected to represent as great a variety of locality and
relation as possible, and at the same time exclude all specimens which
showed traces of decomposition. Hand specimens weighing from one hundred
to four hundred grains were used, and these were weighed first dry, and
then suspended in water. By using such large quantities averages were
obtained of a rock which, minutely considered, is heterogeneous; and by
using the blocks entire instead of pulverized or granulated, the state
of aggregation of its minerals was included as an element of the
specific gravity of the rock.
It will be observed that the range, 2.54 to 2.66, is very small.
_Table of Specific Gravities of Trachytes of the Henry Mountains._
───────────────────────────────────────────────────┬─────────────────
Locality. │Specific gravity.
───────────────────────────────────────────────────┼─────────────────
East flank of Mount Pennell; sheet │ 2.66
Marvine Laccolite; north base of Mount Ellen │ 2.65
Peale Laccolite; east flank of Mount Ellen │ 2.64
Dike on Mount Ellsworth │ 2.64
South base of Mount Hillers; sheet │ 2.63
Sheet under the Peale Laccolite │ 2.62
Scrope Laccolite; southeast base of Mount Ellen │ 2.60
Bowl Creek Laccolite; northeast base of Mount Ellen│ 2.58
North spur of Mount Pennell; dike │ 2.58
Sentinel Laccolite; north base of Mount Pennell │ 2.54
│ ————
Mean │ 2.61
───────────────────────────────────────────────────┴─────────────────
Specimens to represent the stratigraphic series were selected at the
_margins_ of the disturbed region so far as possible, to avoid the
effect of metamorphism. But as it was not practicable to eliminate this
source of error in every case, the densities of highly metamorphic
specimens were also measured for the purpose of indicating the effect of
the metamorphism. In order to restore so far as practicable the
condition of the rocks at the time of the lavic intrusion, the specimens
were saturated with water, and in this condition were weighed in air as
well as in water. The results for the porous sandstones are from
one-seventh to one-fourteenth lower than would have been obtained by the
usual method. Hand specimens were used as before.
_Table of Specific Gravities of Sedimentary Rocks of the Henry
Mountains._
──┬─────────────────────────────────┬────────────────┬─────────────────
│ Rock. │ Condition. │Specific gravity.
──┼─────────────────────────────────┼────────────────┼─────────────────
1│Masuk Sandstone │Unaltered │ 2.16
2│Blue Gate Sandstone │Unaltered │ 2.14
3│Blue Gate Shale │Unaltered │ 2.45
4│Flaming Gorge Shale │Unaltered │ 2.42
5│Gray Cliff Sandstone │Unaltered │ 2.13
6│Vermilion Cliff Sandstone, (top) │Unaltered (?) │ 2.21
7│Vermilion Cliff Sandstone, (base)│Unaltered (?) │ 2.28
8│Henry’s Fork Conglomerate │Slightly altered│ 2.25
9│Vermilion Cliff Sandstone, │Altered │ 2.48
10│Aubrey Sandstone │Altered │ 2.55
11│Tununk Shale │Altered │ 2.60
──┴─────────────────────────────────┴────────────────┴─────────────────
It is plain from this table that the effect of the metamorphism was to
increase the densities of the rocks affected. The Blue Gate shale which
_unaltered_ gave 2.45, is lithologically identical with the Tununk shale
which _altered_ gave 2.69. The Aubrey sandstone cannot be observed
unaltered in the vicinity of the mountains, but at a distance of forty
miles where it again comes to the surface it closely resembles the Gray
Cliff sandstone. If it has the same normal weight as the latter, then it
has increased from 2.13 to 2.55.
The specimens of the Vermilion Cliff sandstone numbered 6 and 7 were not
visibly changed, but as they were obtained from the flank of the Holmes
arch there was reason to suspect that their condition was not normal,
and the determined densities strengthen the suspicion. Judged by other
localities, the normal density of the Vermilion Cliff rock is not far
from that of the Gray Cliff rock, namely 2.13; and it is easy to believe
that the upper portion of the bed where it lay on the side of the Holmes
arch was changed in density to 2.21; while the lower portion lying
nearer the laccolite was changed to 2.28; and while the same bed among
the Ellsworth dikes acquired the density of 2.48.
Taking into account both these considerations and certain others which
need not be enumerated, I derive the following:
_Table of the Specific Gravities of the Henry
Mountain Sedimentary series in the Order of
Superposition._
───────────────────────────────────┬─────────────────
Bed. │Specific gravity.
───────────────────────────────────┼─────────────────
Masuk Sandstone │ 2.16
Masuk Shale estimated│ 2.40
Blue Gate Sandstone │ 2.14
Blue Gate Shale │ 2.45
Tununk Sandstone │ 2.15
Tununk Shale estimated│ 2.45
Henry’s Fork Conglomerate │ 2.25
Flaming Gorge Shale │ 2.42
Gray Cliff Sandstone │ 2.13
Vermilion Cliff Sandstone estimated│ 2.15
Shinarump Shale estimated│ 2.40
Aubrey Sandstone estimated│ 2.15
───────────────────────────────────┴─────────────────
Taking into account the thicknesses of the several beds enumerated in
the foregoing table, it is easy to obtain the mean specific gravity of
all which lie above a given horizon; and by making this determination
for the horizon of the base of each of the indicated beds, the following
table has been derived. The figures are based on the assumption that the
rock series included nothing above the Masuk sandstone. If (as is
probable) there were Tertiary beds also, the estimates are too low, for
the Tertiaries of the vicinity are calcareous and argillaceous and
consequently dense.
_Table showing the Mean Specific Gravities of the
Rock Series contained between certain horizons and
the summit of the Masuk Sandstone._
─────────────────────────────────┬─────────────────
Horizons. │ Specific
│ gravities.
─────────────────────────────────┼─────────────────
Base of Masuk Sandstone │ 2.16
Base of Masuk Shale │ 2.28
Base of Blue Gate Sandstone │ 2.23
Base of Blue Gate Shale │ 2.32
Base of Tununk Sandstone │ 2.31
Base of Tununk Shale │ 2.34
Base of Henry’s Fork Conglomerate│ 2.33
Base of Flaming Gorge Shale │ 2.36
Base of Gray Cliff Sandstone │ 2.33
Base of Vermilion Cliff Sandstone│ 2.32
Base of Shinarump Shale │ 2.33
─────────────────────────────────┴─────────────────
From this it appears that the laccolites of the upper zone, extending
from the lower part of the Blue Gate Shale to the upper part of the
Flaming Gorge Shale, bore loads of which the mean densities were from
2.31 to 2.34, and that laccolites of the lower zone, which has its upper
limit in the Shinarump Shale, bore loads of which the mean densities
were 2.32 and upward. If the positions of the laccolites were determined
purely by the law of hydrostatic equilibrium, then these figures define
the density of the molten trachyte, and show that its contraction in
cooling—from the density 2.34 to the density 2.61—was about one-tenth of
its volume.
THE STRETCHING OF STRATA.
It has been the opinion, not only of the writer but of other students of
the displacements of the West, that the ordinary sedimentary rocks,
sandstone, limestone, and shale, are frequently _elongated_ as well as
compressed by orographic movements, and that this takes place without
any appreciable metamorphism; but it is difficult to find opportunity
for the demonstration of the phenomenon by measurement. When a fold is
made in a level stratum, either of two things may take place; the
portions of the stratum which remain level at the sides may approach
each other; or the stratum may be stretched. But when a circular portion
of a continuous level stratum is lifted into a quaquaversal arch (as
illustrated in Figure 11), an approach of the level portions is out of
the question, and there must be a stretching or a fracture. Of the
unfractured quaquaversals of the Henry Mountains there is one which
combines all the essentials of a crucial case. The Lesser Holmes arch is
nearly isolated; on three sides it rises from the undisturbed plateau,
and on the fourth it joins a similar but fractured dome. The major part
of its surface is composed of one bed, the Vermilion Cliff sandstone,
broken only by erosion. Comparing the length of this bed in its present
curved form with the space it must have occupied before it was upbent, I
find that in a distance of three miles it has been elongated three
hundred feet. Moreover there is every reason to suppose that the
elongation was produced quickly, or at least by a succession of finite
rather than infinitesimal increments; for the lifting of the arch was
caused by the intrusion of a laccolite, and though the latter may have
been built by the addition of many separate lava flows, it could not
have risen with secular and continuous slowness. The molten trachyte,
rising through a passage and into a reservoir that were comparatively
cool, would have clogged itself by congelation had it not moved with a
certain degree of rapidity.
[Illustration:
FIG. 52.—Cross-section of an uplifted dome. The dotted lines show the
original position of a bed; the curved lines, the imposed.
]
The condition which rendered possible the elongation and the sudden
bending of so rigid and brittle a rock as a massive sandstone, was
pressure. At the time of the uplift the sandstone was buried by other
sediments to a depth of from five thousand to eight thousand feet, and
sustained a pressure of from five thousand to eight thousand pounds to
the square inch. Now the experiments which have been made upon building
stones show that the weight required to crush similar sandstones in a
dry condition, is three thousand to five thousand pounds to the inch;
and it is a fact familiar to quarrymen that sandstone and limestone
which are quarried below the water level are both softer and weaker
while they are still saturated than they are after drying. So we may
fairly assume that the Vermilion sandstone was loaded at the time of its
displacement with a crushing weight. No part could yield to the pressure
while it was sustained by the surrounding parts; but every part was
ready to yield whenever its support was withdrawn. It was in a
quasi-plastic state and abhorred a fissure as strongly as “nature abhors
a vacuum”, and for the same reason. A fissure could not be opened in it
unless it was coincidently filled by something—such as lava—which would
resist the tendency of its walls to flow together. The formation of a
gaping fissure being thus prevented, and the uplifting of the dome
requiring that the sandstone should cover a greater area, an extension
of the bed was the necessary result. It was not _stretched_ into the
dome form; it was _compressed_. The efficient force did not act in the
direction of the extension, but vertically. The sandstone was pushed,
not pulled.
If this explanation is the true one, then it is true in general that
just as for each rock there is a crushing weight, so there is for each
rock a certain depth at which it cannot be fissured and can be flexed.
The softer rocks are plastic at small depths. Fire-clays under coal
seams exude, or “creep”, even with the pressure of a few feet of
superincumbent strata. Springs of water rise at the outcroppings of soft
strata because the joints which intersect most rocks near the surface of
the ground cannot cross those which are soft enough to yield under the
pressure incident to them. If the soft beds were jointed they would not
intercept percolating water, and the distribution of springs would be
very different.
The phenomena of fissure veins are in point. When a fault takes place,
and one rock mass is slidden past another to which it had been joined it
is usually the case that the opposed surfaces no longer fit together as
they did before the movement, and interspaces are left. These become
filled, at first by water, and afterward by minerals deposited from the
water, and the mineral masses thus deposited are called fissure veins.
But the preservation of the interspaces depends upon the rigidity of the
rocks which inclose them; and it frequently happens that where a system
of rocks is traversed by a fault, the harder will keep somewhat apart
and maintain a fissure, while the softer will be crushed together
without an interspace. If the mineral vein which forms in such a fissure
is afterward explored in mining, it is found to be traceable and
continuous so far as it is walled by the hard rock upon both sides, but
when the hard is replaced by the soft in one or both walls, the vein is
either reduced to a mere fillet or disappears completely. If the fault
extends to a great depth, it will finally reach a region where the
hardest rocks which it separates are coerced by so great a pressure that
they cannot hold themselves asunder, but are forced together before the
fissure can be filled by mineral deposits. Thus there is a definable
inferior limit to the region of vein formation; and even while it is
impossible to assign a downward limit to the fault which made place for
a vein, it may be possible to assign a downward limit to the vein
itself.
Accordant with this view is the absence of fissure veins from the Henry
Mountains. Displacement and thermal disturbance are usually regarded as
the conditions of mineral concentration; and here were displacement and
lavic intrusion coincident in time and place. The heat which
metamorphosed great bodies of shale and sandstone was surely competent
to excite the currents and reactions which concentrate minerals in
veins; but the displacements did not open fissures, and the heated water
could circulate only through the pores of rocks. Fissure veins were
impossible, and the sluggish currents which were engendered in
continuous rock masses did not effect a great change in the distribution
of minerals.
THE CONDITIONS OF ROCK FLEXURE.
There are three known conditions under which strata of the most rigid
character may be bent without fracture; or in other words there are
three ways in which flexibility may be either induced or demonstrated.
At ordinary temperatures and at the surface of the earth a hard stratum
cannot be quickly flexed. But no rigidity is absolute, and a constant
strain, even though slight, will in the course of time produce
deformation. The same result may be accomplished quickly if the
temperature of the stratum is raised to near the point of fusion. Or it
may be accomplished with neither great heat nor great time if only the
stratum is so deeply buried that the weight of its cover keeps it from
opening fissures. The three conditions of flexure are time, heat, and
pressure; and whenever the circumstances of a displacement include none
of these, the rocks are broken. A fourth condition, moisture, is of
great importance as an accessory, but alone it is not sufficient to
prevent fracture. The whole body of strata of the earth’s crust is
saturated with water, except a very little at the surface, and all rock
movements are thereby facilitated. If the strata were dry, their flexure
would require much more time, or heat, or pressure, than is necessary in
their moist condition.
Often the three conditions complement each other; but not always.
We may say, the greater the load which strata bear the more rapidly they
can be flexed; and conversely, the more slowly strata are displaced the
less the pressure necessary to prevent fracture.
And we may say, the higher the temperature of strata the more rapidly
they can be flexed; and conversely, the more slowly strata are displaced
the lower the temperature necessary to prevent fracture.
For both these statements we find support in a great series of
homologies. But we cannot affirm that such a reciprocal relation exists
between the effects of heat and pressure. For all rocks are believed to
expand by heating, up to the point of fusion; and it is a recognized
physical law that in all bodies which heat expands, the effects of heat
are opposed by pressure. Hence we cannot say, “The heavier the load
which strata bear the lower the temperature necessary to prevent
fracture”, nor can we say, “The higher the temperature of strata the
less the load necessary to prevent fracture”.
THE QUESTION OF COVER AND THE QUESTION OF AGE.
It is evident that the laccolites of the Henry Mountains were formed
beneath the surface of the earth’s crust, but at what depth is not so
evident. The problem is involved with the problem of the age of the
laccolites, and the two are connected with the general history of the
Basin of the Colorado. Neither problem can be called, for the present at
least, determinate, but it is possible to narrow them down by the
indication of limits which their solutions will not exceed.
So much of the Colorado Plateau region as lies within Colorado and Utah
was covered during a geological age which it is convenient to call
Cretaceous, by a sea, the waters of which appear to have become fresh
toward the last. Then came elevation both general and differential. A
great part of the sea bed became dry land, and the accumulated sediments
together with many which underlay them were bent into great waves
thousands of feet in altitude. The crests of the waves were subjected to
erosion and truncated. Then came a second submergence which was purely
lacustrine. In some way that has not been ascertained a lake basin was
formed, and the region received a new system of sediments which it is
convenient to call Tertiary, and which not merely filled the troughs
between the great rock-waves but covered the truncated summits of the
waves themselves. Then followed the desiccation of the basin by the
cutting down of its rim where the water overflowed. The overflowing
river as it deepened its channel and gradually lowered the lake,
steadily extended its upper course to follow the receding shore; and
finally when the basin was completely drained the river remained, its
channel leading through what had been the deepest part of the Tertiary
sea. That river is the Colorado. As portions of the lake bottom were
successively drained they began at once to be eroded, and from that time
to this there has been progressive degradation. The regions nearest to
the central river were reduced most rapidly and have been completely
stripped of their Tertiary strata, but broad areas of the latter remain
at the west, and north, and east.
(The reader will understand that this succinct history is shorn for the
sake of clearness of all details and qualifications. There have been
complicating eruptions and displacements or oscillations at every stage,
and if the full story could be told, it would not be by a single
paragraph nor by a single chapter.)
When the Cretaceous strata were thrown into waves the site of the Henry
Mountains remained in a trough, and it probably was not dried, but
continued the scene of sedimentation while the crests of the surrounding
rock-waves were worn away. Certainly it was not greatly eroded at that
time; and when the Tertiary lake beds were thrown down it was favorably
disposed for a heavy deposit. It is not extravagant to assume that four
thousand feet of lake beds rested on the Masuk sandstone at the
beginning of the final desiccation.
In brief there may be distinguished—
1. The deposition of the Cretaceous.
2. The folding and erosion of the Cretaceous.
3. The deposition of the Tertiary.
4. The desiccation of the Tertiary lake basin.
5. The erosion which is still in progress.
It is evident that the laccolites were not formed until the Cretaceous
strata had been deposited; for their uplifts have bent and tilted all
Cretaceous rocks up to and including the Masuk sandstone.
They were not formed at any late stage of the final erosion, for they
conserve tables along their western base, which but for their shelter
would long since have disappeared. From the end of the Cretaceous period
to the end of the desiccation of the basin there is no event with which
the laccolites can be directly connected. There is however a
consideration which in an indirect way sanctions the opinion that the
epoch of igneous activity was after the deposition of the Tertiaries and
before their erosion.
The Masuk Sandstone is at once the summit of the Cretaceous and the
highest bed in the present Henry Mountain section. If it were restored
over the entire range, the laccolites of the upper zone would have on
the average thirty-five hundred feet of cover, and those of the lower
zone nearly seven thousand feet. This was the depth of their original
cover, if they were intruded at the close of the Cretaceous age. During
the epoch of Tertiary deposition and the subsequent epoch of erosion,
the cover first increased in depth and then diminished, having its
maximum at the end of the Tertiary deposition. If it can be shown that
the original cover of the upper laccolites exceeded thirty-five hundred
feet, the question of age will be reduced to comparatively narrow
limits. In order to discuss the problem of the original depth of cover
it will be necessary to consider another matter, of which the connection
will not at first be apparent.
_The size of laccolites._—It is a matter worthy of note that no
laccolite of inconsiderable extent is known in the Henry Mountains. The
smallest which has been measured is more than half a mile in diameter,
and the largest about four miles. The phenomenon does not occur upon a
small scale, but has a definite inferior limit to its magnitude. Let us
seek an explanation of this limit.
The dome of strata which covers a laccolite has for its profile on every
side a monoclinal curve. In Figure 52 the section of a dome exhibits a
monoclinal flexure in _s a_ and again in _s b_; and the dome being
approximately circular this flexure completely surrounds it. We may even
describe or define the dome as a monoclinal flexure encircling a point
or a space. Considering now that when the laccolite was injected the
overlying strata were lifted, and that this disturbance was communicated
upward to the then existing surface of the earth, we may properly speak
of the lifted body of rock as a cylinder bounded on every side by a
monoclinal flexure. Furthermore, since the monoclinal flexure is the
structural equivalent of the fault[4], we may render our conception
still simpler by replacing in imagination the encircling flexure by an
encircling fault, and picturing to ourselves the uplifted rock mass as a
simple cylinder, perfectly divided from the surrounding rock and slidden
upward so as to project above the surface an amount equal to the depth
of the laccolite.
Footnote 4:
Exploration of the Colorado, pp. 182–184. Explorations West of the
100th Meridian, Vol. III, p. 48. American Journal of Science, July,
1876, p. 21.
It is possible to give a mathematical expression to the force necessary
to produce such a circular fault. Disregarding lithologic differences,
the resistance to the rupture is measured by the area of the faulted
surface, or what is the same thing, the area of the convex surface of
the cylinder. Representing the resistance to be overcome by _r_, the
height of the cylinder (equal to the depth of the cover of the
laccolite) by _d_, and its circumference by _c_, we have
_r_ = _dcC_ (1),
in which _C_ is a function of the cohesion of the material and is
constant.
The force by which the cylinder is lifted and by which it is assumed
that the faulting is accomplished, is communicated through the molten
lava of the forming laccolite. Being thus communicated it is applied
equally to all parts of the base of the cylinder, and its efficient
total is measured by the area of that base. A part of it is devoted to
lifting the weight of the cylinder, and the remainder is devoted to the
making of the fault. Each of these parts is proportioned, like the
whole, to the area of the base of the cylinder, or to the area of the
laccolite. Representing the portion applied to the faulting by _f_, and
the area of the laccolite by _a_, we have
_f_ = _a C_{l}_
in which _C_{l}_, is a constant, and a function of the pressure under
which the lava is injected.
Substituting for _a_ its equivalent, _c_^²⁄₄π
_f_ = _c^2 C_{l}_/4π
and substituting _C_{ll}_ for the constant term _C_{l}_/4π
_f_ = _c^2 C_{ll}_ (2)
Equation 1 gives an expression for the resistance which cohesion can
oppose to the uplift of the cylinder. Equation 2 gives an expression for
the force exerted by the fluid laccolite toward overcoming the
resistance of cohesion. It is evident that for a given value of _d_ it
is possible to assign a value of _C_ so large that _f_ will be greater
than _r_, or so small that _f_ will be less than _r_. That is to say, at
a given depth beneath the surface a laccolite of a certain circumference
will be able to force upward the superjacent cylinder of rock, while a
laccolite of a certain smaller circumference will be unable to lift its
cover. Or in other words, there is a limit in size beneath which a
laccolite cannot be formed.
When a lava forced upward through the strata reaches the level at which
under the law of hydrostatic equilibrium it must stop, we may conceive
that it expands along some plane of bedding in a thin sheet, until its
horizontal extent becomes so great that it overcomes the resistance
offered by the rigidity of its cover, and it begins to uplift it. The
direction of least resistance is now upward, and the reservoir of lava
increases in depth instead of width. The area of a laccolite thus tends
to remain at its minimum limit, and may be regarded as more or less
perfectly an index of that limit.
In equations 1 and 2, if _f_ = _r_, then
_c_^2 _C_{ll}_ = _d c C_
or
_c_ = _d_(_C_/_C_{ll}_) (3)
That is to say, if the force exerted by the lava is barely sufficient to
overcome the resistance to uplift, then the circumference of the
laccolite is proportional to the depth of its cover. Or in other words,
the (linear) size of a laccolite is proportioned to its depth beneath
the surface.
If now we return from the faulted cylinder which for simplicity’s sake
has been hypothecated, to the actual cylinder which is surrounded by a
flexure instead of a fault, can we retain our conclusions? With certain
modifications I think we can. The strains developed in deformation by
flexure are less easy of analysis than those which arise in faulting,
but the two cases are in some degree analogous.
The expression (equation 2) for the force which the lava applies to
deformation is unaffected by the manner in which the strata yield.
The expression (equation 1) for the resistance to deformation by
faulting involves two terms, each in its simplest relations; the
resistance varies directly as the circumference of the laccolite, and it
varies directly as the depth of the cover. In order to pass to an
expression for the resistance to deformation by flexure, only one of
these terms need be changed. The resistance bears the same relation to
the circumference of the laccolite; but it is no longer simply
proportional to the depth of the cover. It varies more rapidly.
If the covering strata were all of a given thickness, were identical in
kind, and were free to slide upon each other without friction, their
total resistance to deformation would be equal to the resistance of a
single stratum multiplied by the number of strata. But since they are
not free to slide one upon another, they sustain each other, and the
resistance offered by the combination is greater than that product.
I am led by the analogy of allied problems in mechanics to assume that
the resistance of the body of strata varies with some power of its
depth, but I am unable to say _what_ power. So far as I am aware,
neither mathematical analysis nor experimentation has been directed to
the problem in question. According to Rankine “the resistances of
flexure of similar cross-sections [of elastic beams] are as their
breadths and as the _squares_ of their depths” (“Applied Mechanics”,
page 316), and it is possible that the same law applies to the
resistances which continuous strata oppose to the uplifts of domes. But
it appears more probable that the greater complexity of the strains
developed in the formation of domes causes the depth to enter into the
formula with a higher power than the second.
On the other hand, some allowance should be made for the fact that the
elasticity of the resisting strata is imperfect.
If we call the power with which the depth enters the formula a, equation
1 becomes
_r_ = _d^a_ _c_ _C_{lll}_ (4).
and equation 3 becomes
_c_ = _d^a_ (_C_{lll}_/_C_{ll}_) (5).
It is probable that the true value of _a_ is not less than 2, nor more
than 3.
Interpreting these equations in the same manner as those applying to
deformation by faulting, we reach the following conclusions:
1st. At a given depth beneath the surface, lava injected under a given
pressure cannot form a laccolite of less than a certain area. This may
be called its _limital area_.
2d. The pressure of injection remaining constant, the limital area of a
laccolite is a direct function of its depth beneath the surface. The
limital area is greater when the depth is greater, and less when the
depth is less.
3d. A laccolite of small volume will not exceed the limital area, but
will grow by lifting its cover. If however the volume of intruded lava
be great, its own weight becomes a factor in the equilibrium of forces
and modifies the distribution of the pressures. As the rock bubble
rises, the weight of the contained fluid is progressively subtracted
from the pressure against its top, and this proceeds until the upward
and lateral pressures become proportional to the resistances which
severally oppose them. Further expansion is then both upward and
outward.
4th. There is a limit to upward expansion, dependent on the fact that
the pressure due to the combined weight of the laccolite and cover
cannot exceed the pressure of the intrusive lava. Regarding the
intrusive pressure as constant, it is divisible into three parts, of
which one sustains the weight of the cover, also constant; another
sustains the weight of the fluid laccolite, and is measured by its
thickness or depth; and the third produces deformation. When the sum of
the weights of the cover and laccolite equals the total pressure of the
intrusive lava, uplift ceases, and the maximum depth or thickness is
attained. We may call this the _limital thickness_. With regard to
simple laccolites the limit is absolute, but it applies only to the
distinct layers of those which are composite; for a composite laccolite,
built by successive intrusions at wide intervals of time, may be
relieved of part of its load by the erosion of the mound which its
expansion causes at the surface of the land.
A laccolite formed beneath the bottom of a sea has a greater limital
thickness than one formed beneath a land surface; for the superjacent
water being displaced and thrust aside, is to that extent subtracted
from the load to be lifted.
5th. The laccolite in its formation is constantly solving a problem of
“least force”, and its form is the result. Below, above, and on all
sides its expansion is resisted, and where the resistance is greatest
its contour is least convex. The floor of its chamber is unyielding, and
the bottom of the laccolite is flat. The roof and walls alike yield
reluctantly to the pressure, but the weight of the lava diminishes its
pressure on the roof. Hence the top of the laccolite becomes broadly
convex, and its edges acutely. Local accidents excepted, the walls
oppose an equal resistance on every side; and the base of the laccolite
is rendered circular.
The second of the conclusions enunciated above is susceptible of test by
observation. By selecting those laccolites of which the dimensions are
known with the best degree of approximation, the following table has
been formed:
│ Formations. │Titles of Laccolites.│Diameters│Means.
│ │ │in miles.│
───────────┬───────────────────┬─────────────────────┬─────────┬───┬───
Upper Zone │Blue Gate Shale │Sentinel │ .7│ .7│1.2
───────────┼───────────────────┼─────────────────────┼─────────┼───┼───
„ │Tununk Shale │Geikie │ .8│1.2│ „
„ │ „ │A │ .9│ „ │ „
„ │ „ │Marvine │ 1.0│ „ │ „
„ │ „ │Jukes │ 1.4│ „ │ „
„ │ „ │Peale │ 1.8│ „ │ „
───────────┼───────────────────┼─────────────────────┼─────────┼───┼───
„ │Flaming Gorge Shale│Steward │ 1.0│1.4│ „
„ │ „ │B │ 1.1│ „ │ „
„ │ „ │Newberry │ 1.8│ „ │ „
„ │ „ │C │ 1.9│ „ │ „
───────────┴───────────────────┼─────────────────────┼─────────┼───┼───
Lower Zone │Dana │ 2.0│ │2.6
„ │Greater Holmes │ 2.1│ │ „
„ │Lesser Holmes │ 2.1│ │ „
„ │Ellsworth │ 2.3│ │ „
„ │Pulpit │ 2.3│ │ „
„ │Maze │ 2.8│ │ „
„ │Crescent │ 3.6│ │ „
„ │Hillers │ 3.9│ │ „
There is no laccolite of the upper zone so large as the smallest in the
lower zone; and the mean diameter of those in the lower zone is double
the mean of those in the upper. The measurements do not give the
diameters of limital areas, but it is presumable that the actual areas
bear substantially the same relation to the limital in the two zones. If
we select the smallest laccolites in each group as those most likely to
express the limital areas, the result is practically the same.
│ Formations. │Diameters.│ Means.
───────────┼───────────────────┼──────────┼────┬────
Upper Zone │Tununk Shale │ .8│ .9 │1.0
„ │ „ │ .9│ „ │ „
„ │ „ │ 1.0│ „ │ „
───────────┼───────────────────┼──────────┼────┼────
„ │Flaming Gorge Shale│ 1.0│1.05│ „
„ │ „ │ 1.1│ „ │ „
───────────┴───────────────────┼──────────┼────┼────
Lower Zone │ 2.0│ │2.1
„ │ 2.1│ │ „
„ │ 2.1│ │ „
The mean for the lower zone is still double the mean for the upper.
The confirmation of the conclusion is as nearly perfect as could have
been anticipated. There is no room to doubt that a relation exists
between the diameters of laccolites and the depths of their intrusion.
Having determined by observation the mean size of the laccolites in the
upper and lower zones, as well as the interval which separates the two
zones, and knowing approximately the law which binds the size of the
laccolite to its depth of intrusion, we can compute the depth of
intrusion of each zone. Our result will doubtless have a large probable
error, but it will not be entirely without value.
Let _x_ represent the thickness in feet of the original cover of the
laccolites of the upper zone; and _x_ + 3300 the thickness of the cover
of the laccolites of the lower zone. The mean circumference in feet of
the upper laccolites is 1.2 π × 5280 = 6336 π. The mean circumference of
the lower laccolites is 2.6 π × 5280 = 13728 π. Substituting these
values in equation 5, we obtain
6336 π = _x^a_(_C_{lll}_/_C_{ll}_)
and
13728 π = (_x_ + 3300)_^a_(_C_{lll}_/_C_{ll}_)
Dividing the second equation by the first and reducing, 3300
_x_ = 3300/ª√(¹³⁷²⁸⁄₆₃₃₆ − 1) (6).
To obtain a minimum result, assume a = 2; then
_x_ = 3300/√(¹³⁷²⁸⁄₆₃₃₆ − 1) = 7000 feet,
and
_x_ + 3300 = 10300 feet.
The summit of the Masuk sandstone is 3,500 feet above the mean level of
the upper laccolites; subtracting this from the value of _x_ gives 3,500
feet as the depth of Tertiary strata which overlay the Masuk beds during
the epoch of laccolitic intrusion.
To obtain a maximum result, assume _a_ = 3; then
_x_ = 3300/∛(¹³⁷²⁸⁄₆₃₃₆ − 1) = 11200 feet,
_x_ + 3300 = 14500 feet,
and the result for the depth of the Tertiary strata is 7,700 feet.
I am far from attaching great weight to this speculation in regard to
the original depths of the laccolite covers. It is always hazardous to
attempt the quantitative discussion of geological problems, for the
reason that the conditions are apt to be both complex and imperfectly
known; and in this case an uncertainty attaches to the law of relation,
as well as to the quantities to which it is applied. Nevertheless after
making every allowance there remains a presumption that the cover of the
laccolites included some thousands of feet of Tertiary sediments.
What evidence we have then, indicates that the epoch of laccolitic
intrusion was after the accumulation of deep Tertiary deposits and
before the subsequent degradation had made great progress—that it was at
or near the close of the epoch of local Tertiary sedimentation.
If the reader would realize the relation between the eroded material and
the surviving mountains, let him turn to the Frontispiece. A perspective
view is there given of a tract ten miles square, with Mount Ellsworth in
the center. It is represented as cut out from all surroundings by
vertical planes which descend to the level of the ocean. The southern or
nearer half of the block shows the present aspect of the country; the
remote half shows the form it is supposed to have had if the uplift was
completed before the erosion began, or what is the same thing, the form
it would have, had there been no erosion. The difference between the two
represents the total amount of the material that has been washed away
since the completion of the Tertiary sediments.
Partly in review, let us now sketch the
HISTORY OF THE LACCOLITE.
When lavas forced upward from lower-lying reservoirs reach the zone in
which there is the least hydrostatic resistance to their accumulation,
they cease to rise. If this zone is at the top of the earth’s crust they
build volcanoes; if it is beneath, they build laccolites. Light lavas
are more apt to produce volcanoes; heavy, laccolites. The porphyritic
trachytes of the Plateau Province produced laccolites.
The station of the laccolite being decided, the first step in its
formation is the intrusion along a parting of strata, of a thin sheet of
lava, which spreads until it has an area adequate, on the principle of
the hydrostatic press, to the deformation of the covering strata. The
spreading sheet always extends itself in the direction of least
resistance, and if the resistances are equal on all sides, takes a
circular form. So soon as the lava can uparch the strata it does so, and
the sheet becomes a laccolite. With the continued addition of lava the
laccolite grows in height and width, until finally the supply of
material or the propelling force so far diminishes that the lava clogs
by congelation in its conduit and the inflow stops. An irruption is then
complete, and the progress of the laccolite is comparable with that of a
volcano at the end of its first eruption. During the irruption and after
its completion, there is an interchange of temperatures. The laccolite
cools and solidifies; its walls are heated and metamorphosed. At the
edges, where the surface of the laccolite is most convex, the heat is
most rapidly dissipated, and its effect in metamorphism is least. A
second irruption may take place either before or after the first is
solidified. It may intrude above or it may intrude beneath it; and
observation has not yet distinguished the one case from the other. In
any case it carries forward the deformation of cover that was begun by
the first, and combines with it in such way that the compound form is
symmetric, and is substantially the same that would have been produced
if the two irruptions were combined in one. Thus the laccolite grows by
successive accretions until at length its cooled mass, heavier and
stronger than the surrounding rocks, proves a sufficient obstacle to
intrusion. The next irruption then avoids it, opens a new conduit, and
builds a new laccolite at its side. By successive shiftings of the
conduit a group of laccolites is formed, just as by the shifting of
vents eruptive cones are grouped. Each laccolite is a subterranean
volcano.
[Illustration:
FIG. 53.—Diagram to illustrate the relation of Dikes and Sheets to the
Strains which are developed in the uplifting of laccolitic arches.
]
The strata above the laccolite are bent instead of broken, because their
material is subjected to so great a pressure by superincumbent strata
that it cannot hold an open fissure and is _quasi-plastic_. But although
quasi-plastic it is none the less solid, and can be cracked open if the
gap is instantaneously filled, the cracking and the filling being one
event. This happens in the immediate walls of the laccolite, and they
are injected by dikes and sheets of the lava. The directions of the
cracks are normal to the directions of the extensive strains (strains
tending to extend) where they occur. From the top of the laccolite dikes
run upward into the roof, marking horizontal strains (_a a_). From the
sides smaller vertical dikes run outward, marking horizontal, tangential
strains. And parallel to the sides near the base of the laccolite, are
numerous sheets, marking strains directed outward and upward (_c c_).
These last especially serve to show that the rigidity of the strata is
not abolished, although it is overpowered, by the pressure which warps
them.
Here we are brought face to face with a great fact of dynamic geology
which though well known is too often ignored. The solid crust of the
earth, and the solid earth if it be solid, are as plastic _in great
masses_ as wax is in small. Solidity is not absolute but relative. It is
only a low grade of plasticity. The rigidity or strength of a body is
measured by the square of its linear dimensions, while its weight is
measured by the cube. Hence with increase in magnitude, the weight
increases more rapidly than the strength; and no very large body is
strong enough to withstand the pressure of its own weight. However solid
it may be, it must succumb and be flattened. When we speak of rock
masses which are measured by feet, we may regard them as solid; but when
we consider masses which are measured by miles, we should regard them as
plastic.
The same principle is illustrated by the limital area of laccolites. A
small laccolite cannot lift its small cover, but a large laccolite can
lift its correspondingly large cover. The strength or rigidity which
resists deformation is overcome by magnitude.
_Laccolites of Other Regions._—In many lands geologists have observed
intrusive rocks occurring in great bodies, but I am not aware that such
a system as that of the Henry Mountains has ever been described.
Doubtless all such bodies are laccolitic, but the combination of
conditions which this field presents can rarely be repeated. In the
first place the strata which here contain the laccolites lay level. They
had suffered no displacement before the epoch of irruption, and they
have suffered none since. The laccolitic phenomena stand by themselves,
with nothing to mar their symmetry or complicate their study. In the
next place the laccolites are here assembled in such number and with
such variety of size, form, and horizon that there is little danger of
mistaking accidental features for essential. Again, the region having
been recently elevated is the scene of rapid degradation. Waterways are
deeply corraded, slopes are steep, and escarpments abound. And finally
the climate is so arid that vegetation is exceedingly scant. The rocks
are for the most part bare and their examination is unobstructed.
If the conditions of erosion and climate had been unfavorable in the
Henry Mountains, they could not have yielded the key to the laccolitic
structure; but the key once found, it is to be anticipated that the
structure will be recognized in other laccolites of which the exposures
are less perfect.
If the strata had experienced anterior displacements so as to be
inclined, folded, and faulted, a symmetrical growth of laccolites would
have been impossible, and the mountains would not have yielded a
knowledge of the type form. But the type form being known, it is to be
anticipated that in disturbed regions aberrant forms will be recognized
and referred to the type.
_Possible Analogues of the Laccolite._—All the arches of the Henry
Mountains have been ascribed to laccolites, whether their nuclei were
visible or concealed, and the evidence upon which the latter were
included appears to admit of no controversy. The question arises whether
the great flexures of the Plateau region may not be allied in structure.
The volcano having its homologue in the laccolite, may not broad lava
fields have their homologues beneath displacements of the Kaibab type?
The idea is naturally attractive to one who has made a special study of
laccolites, but it is hardly tenable. There are indeed many points of
resemblance between such flexures as the Waterpocket, and the uplifts of
the Henry Mountains; but the points of contrast are equally conspicuous,
and seem to mark a radical difference.
There is a certain symmetry of form which is characteristic of the
laccolitic arches, but which is rarely seen in the great flexures. And
there is a linear element which is characteristic of the latter, but not
of the former. The great flexures always have direction or trend, and
often exhibit parallelism; the laccolitic arches betray no trend either
individually or collectively.
These features are well shown in Plate II, where the Waterpocket flexure
is contrasted with the Henry Mountain arches.
CHAPTER V.
LAND SCULPTURE.
The Basin of the Colorado offers peculiar facilities for the study of
the origin of topographic forms, and its marvelous sculpture has excited
the interest of every observer. It has already made notable
contributions to the principles of earth sculpture[5], and its resources
are far from exhausted. The study of the Henry Mountains has not proved
entirely unfruitful, and for the sake of showing the bearing of its
peculiar features upon the general subject, I shall take the liberty to
restate certain principles of erosion which have been derived or
enforced by the study of the Colorado Plateaus.
Footnote 5:
Geology of the “Colorado Exploring Expedition”, by J. S. Newberry, p.
45.
“Exploration of the Colorado River of the West”, by J. W. Powell, p.
152.
“Geology of the Uinta Mountains”, by J. W. Powell, p. 181.
“Explorations West of the 100th Meridian”, Vol. III, Part I, by G. K.
Gilbert, pp. 67 and 554.
“The Colorado Plateau Region” in American Journal of Science for
August, 1876, by G. K. Gilbert.
A portion of the last paper is repeated, after modification, in the
first section of this chapter.
I.—EROSION.
The sculpture and degradation of the land are performed partly by shore
waves, partly by glaciers, partly by wind; but chiefly by rain and
running water. The last mentioned agencies only will be here discussed.
The erosion which they accomplish will be considered (A) as consisting
of parts, and (B) as modified by conditions.
A. PROCESSES OF EROSION.
All indurated rocks and most earths are bound together by a force of
cohesion which must be overcome before they can be divided and removed.
The natural processes by which the division and removal are accomplished
make up erosion. They are called disintegration and transportation.
Transportation is chiefly performed by running water.
Disintegration is naturally divided into two parts. So much of it as is
accomplished by running water is called corrasion, and that which is
not, is called weathering.
Stated in their natural order, the three general divisions of the
process of erosion are (1) _weathering_, (2) _transportation_, and (3)
_corrasion_. The rocks of the general surface of the land are
disintegrated by _weathering_. The material thus loosened is
_transported_ by streams to the ocean or other receptacle. In transit it
helps to _corrade_ from the channels of the streams other material,
which joins with it to be transported to the same goal.
_Weathering._
In weathering the chief agents of disintegration are solution, change of
temperature, the beating of rain, gravity, and vegetation.
The great solvent of rocks is water, but it receives aid from some other
substances of which it becomes the vehicle. These substances are chiefly
products of the formation and decomposition of vegetable tissues. Some
rocks are disintegrated by their complete solution, but the great
majority are divided into grains by the solution of a portion; and
fragmental rocks usually lose by solution the cement merely, and are
thus reduced to their original incoherent condition.
The most rigid rocks are cracked by sudden changes of temperature; and
the crevices thus begun are opened by the freezing of the water within
them. The coherence of the more porous rocks is impaired and often
destroyed by the same expansive force of freezing water.
The beating of the rain overcomes the feeble coherence of earths, and
assists solution and frost by detaching the particles which they have
partially loosened.
When the base of a cliff is eroded so as to remove or diminish the
support of the upper part, the rock thus deprived of support is broken
off in blocks by gravity. The process of which this is a part is called
cliff erosion or _sapping_.
Plants often pry apart rocks by the growth of their roots, but their
chief aid to erosion is by increasing the solvent power of percolating
water.
In general soft rocks weather more rapidly than hard.
_Transportation._
A portion of the water of rains flows over the surface and is quickly
gathered into streams. A second portion is absorbed by the earth or rock
on which it falls, and after a slow underground circulation reissues in
springs. Both transport the products of weathering, the latter carrying
dissolved minerals and the former chiefly undissolved.
Transportation is also performed by the direct action of gravity. In
sapping, the blocks which are detached by gravity are by the same agency
carried to the base of the cliff.
_Corrasion._
In corrasion the agents of disintegration are solution and mechanical
wear. Wherever the two are combined, the superior efficiency of the
latter is evident; and in all fields of rapid corrasion the part played
by solution is so small that it may be disregarded.
The mechanical wear of streams is performed by the aid of hard mineral
fragments which are carried along by the current. The effective force is
that of the current; the tools are mud, sand, and bowlders. The most
important of them is sand; it is chiefly by the impact and friction of
grains of sand that the rocky beds of streams are disintegrated.
Streams of clear water corrade their beds by solution. Muddy streams act
partly by solution, but chiefly by attrition.
Streams transport the combined products of corrasion and weathering. A
part of the _débris_ is carried in solution, and a part mechanically.
The finest of the undissolved detritus is held in suspension; the
coarsest is rolled along the bottom; and there is a gradation between
the two modes. There is a constant comminution of all the material as it
moves, and the work of transportation is thereby accelerated. Bowlders
and pebbles, while they wear the stream bed by pounding and rubbing, are
worn still more rapidly themselves. Sand grains are worn and broken by
the continued jostling, and their fragments join the suspended mud.
Finally the detritus is all more or less dissolved by the water, the
finest the most rapidly.
In brief, weathering is performed by solution; by change of temperature,
including frost; by rain beating; by gravity; and by vegetation.
Transportation is performed chiefly by running water. Corrasion is
performed by solution, and by mechanical wear.
Corrasion is distinguished from weathering chiefly by including
mechanical wear among its agencies, and the importance of the
distinction will be apparent when we come to consider how greatly and
peculiarly this process is affected by modifying conditions.
B. CONDITIONS CONTROLLING EROSION.
The chief conditions which affect the rapidity of erosion are (1)
declivity, (2) character of rock, and (3) climate.
_Rate of Erosion and Declivity._
In general _erosion is most rapid where the slope is steepest_; but
weathering, transportation, and corrasion are affected in different ways
and in different degrees.
With increase of slope goes increase in the velocity of running water,
and with that goes increase in its power to transport undissolved
detritus.
The ability of a stream to corrade by solution is not notably enhanced
by great velocity; but its ability to corrade by mechanical wear keeps
pace with its ability to transport, or may even increase more rapidly.
For not only does the bottom receive more blows in proportion as the
quantity of transient detritus increases, but the blows acquire greater
force from the accelerated current, and from the greater size of the
moving fragments. It is necessary however to distinguish the ability to
corrade from the rate of corrasion, which will be seen further on to
depend largely on other conditions.
Weathering is not directly influenced by slope, but it is reached
indirectly through transportation. Solution and frost, the chief agents
of rock decay, are both retarded by the excessive accumulation of
disintegrated rock. Frost action ceases altogether at a few feet below
the surface, and solution gradually decreases as the zone of its
activity descends and the circulation on which it depends becomes more
sluggish. Hence the rapid removal of the products of weathering
stimulates its action, and especially that portion of its action which
depends upon frost. If however the power of transportation is so great
as to remove completely the products of weathering, the work of
disintegration is thereby checked; for the soil which weathering tends
to accumulate is a reservoir to catch rain as it reaches the earth and
store it up for the work of solution and frost, instead of letting it
run off at once unused.
Sapping is directly favored by great declivity.
In brief, a steep declivity favors transportation and thereby favors
corrasion. The rapid, but partial, transportation of weathered rock
accelerates weathering; but the complete removal of its products retards
weathering.
_Rate of Erosion and Rock Texture._
Other things being equal, _erosion is most rapid when the eroded rock
offers least resistance_; but the rocks which are most favorable to one
portion of the process of erosion do not necessarily stand in the same
relation to the others. Disintegration by solution depends in large part
on the solubility of the rocks, but it proceeds most rapidly with those
fragmental rocks of which the cement is soluble, and of which the
texture is open. Disintegration by frost is most rapid in rocks which
absorb a large percentage of water and are feebly coherent.
Disintegration by mechanical wear is most rapid in soft rocks.
Transportation is most favored by those rocks which yield by
disintegration the most finely comminuted _débris_.
_Rate of Erosion and Climate._
The influence of climate upon erosion is less easy to formulate. The
direct influences of temperature and rainfall are comparatively simple,
but their indirect influence through vegetation is complex, and is in
part opposed to the direct.
Temperature affects erosion chiefly by its changes. Where the range of
temperature includes the freezing point of water, frost contributes its
powerful aid to weathering; and it is only where changes are great and
sudden that rocks are cracked by their unequal expansion or contraction.
All the processes of erosion are affected directly by the amount of
rainfall, and by its distribution through the year. All are accelerated
by its increase and retarded by its diminution. When it is concentrated
in one part of the year at the expense of the remainder, transportation
and corrasion are accelerated, and weathering is retarded.
Weathering is favored by abundance of moisture. Frost accomplishes most
when the rocks are saturated; and solution when there is the freest
subterranean circulation. But when the annual rainfall is concentrated
into a limited season, a larger share of the water fails to penetrate,
and the gain from temporary flooding does not compensate for the
checking of all solution by a long dry season.
Transportation is favored by increasing water supply as greatly as by
increasing declivity. When the volume of a stream increases, it becomes
at the same time more rapid, and its transporting capacity gains by the
increment to velocity as well as by the increment to volume. Hence the
increase in power of transportation is more than proportional to the
increase of volume.
It is due to this fact chiefly that the transportation of a stream which
is subject to floods is greater than it would be if its total water
supply were evenly distributed in time.
The indirect influence of rainfall and temperature, by means of
vegetation, has different laws. Vegetation is intimately related to
water supply. There is little or none where the annual precipitation is
small, and it is profuse where the latter is great—especially where the
temperature is at the same time high. In proportion as vegetation is
profuse the solvent power of percolating water is increased, and on the
other hand the ground is sheltered from the mechanical action of rains
and rills. The removal of disintegrated rock is greatly impeded by the
conservative power of roots and fallen leaves, and a soil is thus
preserved. Transportation is retarded. Weathering by solution is
accelerated up to a certain point, but in the end it suffers by the
clogging of transportation. The work of frost is nearly stopped as soon
as the depth of soil exceeds the limit of frost action. The force of
rain drops is expended on foliage. Moreover a deep soil acts as a
distributing reservoir for the water of rains, and tends to equalize the
flow of streams.
Hence the general effect of vegetation is to retard erosion; and since
the direct effect of great rainfall is the acceleration of erosion, it
results that its direct and indirect tendencies are in opposite
directions.
In arid regions of which the declivities are sufficient to give thorough
drainage, the absence of vegetation is accompanied by absence of soil.
When a shower falls, nearly all the water runs off from the bare rock,
and the little that is absorbed is rapidly reduced by evaporation.
Solution becomes a slow process for lack of a continuous supply of
water, and frost accomplishes its work only when it closely follows the
infrequent rain. Thus weathering is retarded. Transportation has its
work so concentrated by the quick gathering of showers into floods, as
to compensate, in part at least, for the smallness of the total rainfall
from which they derive their power.
Hence in regions of small rainfall, surface degradation is usually
limited by the slow rate of disintegration; while in regions of great
rainfall it is limited by the rate of transportation. There is probably
an intermediate condition with moderate rainfall, in which a rate of
disintegration greater than that of an arid climate is balanced by a
more rapid transportation than consists with a very moist climate, and
in which the rate of degradation attains its maximum.
Over nearly the whole of the earth’s surface there is a soil, and
wherever this exists we know that the conditions are more favorable to
weathering than to transportation. Hence it is true in general that the
conditions which limit transportation are those which limit the general
degradation of the surface.
To understand the manner in which this limit is reached it is necessary
to look at the process by which the work is accomplished.
_Transportation and Comminution._
A stream of water flowing down its bed expends an amount of energy that
is measured by the quantity of water and the vertical distance through
which it descends. If there were no friction of the water upon its
channel the velocity of the current would continually increase; but if,
as is the usual case, there is no increase of velocity, then the whole
of the energy is consumed in friction. The friction produces
inequalities in the motion of the water, and especially induces
subsidiary currents more or less oblique to the general onward movement.
Some of these subsidiary currents have an upward tendency, and by them
is performed the chief work of transportation. They lift small particles
from the bottom and hold them in suspension while they move forward with
the general current. The finest particles sink most slowly and are
carried farthest before they fall. Larger ones are barely lifted, and
are dropped at once. Still larger are only half lifted; that is, they
are lifted on the side of the current and rolled over without quitting
the bottom. And finally there is a limit to the power of every current,
and the largest fragments of its bed are not moved at all.
There is a definite relation between the velocity of a current and the
size of the largest bowlder it will roll. It has been shown by Hopkins
that the weight of the bowlder is proportioned to the sixth power of the
velocity. It is easily shown also that the weight of a suspended
particle is proportioned to the sixth power of the velocity of the
upward current that will prevent its sinking. But it must not be
inferred that the total load of detritus that a stream will transport
bears any such relation to the rapidity of its current. The true
inference is, that the velocity determines the size-limit of the
detritus that a stream can move by rolling, or can hold in suspension.
Every particle which a stream lifts and sustains is a draft upon its
energy, and the measure of the draft is the weight (weighed in water) of
the particle, multiplied by the distance it would sink in still water in
the time during which it is suspended. If for the sake of simplicity we
suppose the whole load of a stream to be of uniform particles, then the
measure of the energy consumed in their transportation is their total
weight multiplied by the distance one of them would sink in the time
occupied in their transportation. Since fine particles sink more slowly
than coarse, the same consumption of energy will convey a greater load
of fine than of coarse.
Again, the energy of a clear stream is entirely consumed in the friction
of flow; and the friction bears a direct relation to its velocity. But
if detritus be added to the water, then a portion of its energy is
diverted to the transportation of the load; and this is done at the
expense of the friction of flow, and hence at the expense of velocity.
As the energy expended in transportation increases, the velocity
diminishes. If the detritus be composed of uniform particles, then we
may also say that as the load increases the velocity diminishes. But the
diminishing velocity will finally reach a point at which it can barely
transport particles of the given size, and when this point is attained,
the stream has its maximum load of detritus of the given size. But fine
detritus requires less velocity for its transportation than coarse, and
will not so soon reduce the current to the limit of its efficiency. A
greater percentage of the total energy of the stream can hence be
employed by fine detritus than by coarse.
(It should be explained that the friction of flow is in itself a complex
affair. The water in contact with the bottom and walls of the channel
develops friction by flowing past them, and that which is farther away
by flowing past that which is near. The inequality of motion gives rise
to cross currents and there is a friction of these upon each other. The
ratio or coefficient of friction of water against the substance of the
bed, the coefficient of friction of water against water, or the
viscosity of water, and the form of the bed, all conspire to determine
the resistance of flow and together make up what may be called the
coefficient of the friction of flow. The friction depends on its
coefficient and on the velocity.)
Thus the capacity of a stream for transportation is enhanced by
comminution in two ways. Fine detritus, on the one hand, consumes less
energy for the transportation of the same weight, and on the other, it
can utilize a greater portion of the stream’s energy.
It follows, as a corollary, that the velocity of a fully loaded stream
depends (_ceteris paribus_) on the comminution of the material of the
load. When a stream has its maximum load of fine detritus, its velocity
will be less than when carrying its maximum load of coarse detritus; and
the greater load corresponds to the less velocity.
It follows also that a stream which is supplied with heterogeneous
_débris_ will select the finest. If the finest is sufficient in quantity
the current will be so checked by it that the coarser cannot be moved.
If the finest is not sufficient the next grade will be taken, and so on.
_Transportation and Declivity._
To consider now the relation of declivity to transportation we will
assume all other conditions to be constant. Let us suppose that two
streams have the same length, the same quantity of water, flow over beds
of the same character, and are supplied to their full capacities with
detritus of the same kind; but differ in the total amount of fall. Their
declivities or rates of fall are proportional to their falls. Since the
energy of a stream is measured by the product of its volume and its
fall, the relative energies of the two streams are proportional to their
falls, and hence proportional to their declivities. The velocities of
the two streams, depending, as we have seen above, on the character of
the detritus which loads them, are the same; and hence the same amount
of energy is consumed by each in the friction of flow. And since the
energy which each stream expends in transportation is the residual after
deducting what it spends in friction from its total energy, it is
evident that the stream with the greater declivity will not merely have
the greater energy, but will expend a less percentage of it in friction
and a greater percentage in transportation.
Hence declivity favors transportation in a degree that is greater than
its simple ratio.
There are two elements of which no account is taken in the preceding
discussion, but which need to be mentioned to prevent misapprehension,
although they detract in no way from the conclusions.
The first is the addition which the transported detritus makes to the
energy of the stream. A stream of water charged with detritus is at once
a compound and an unstable fluid. It has been treated merely as an
unstable fluid requiring a constant expenditure of energy to maintain
its constitution; but looking at it as a compound fluid, it is plain
that the energy it develops by its descent is greater than the energy
pertaining to the water alone, in the precise ratio of the mass of the
mixture to the mass of the simple water.
The second element is the addition which the detritus makes to the
friction of flow. The coefficient of friction of the compound stream
upon its bottom will always be greater than that of the simple stream of
water, and the coefficient of internal friction or the viscosity will be
greater than that of pure water, and hence for the same velocity a
greater amount of energy will be consumed.
It may be noted in passing, that the energy which is consumed in the
friction of the detritus on the stream bed, accomplishes as part of its
work the mechanical corrasion of the bed.
_Transportation and Quantity of Water._
A stream’s friction of flow depends mainly on the character of the bed,
on the area of the surface of contact, and on the velocity of the
current. When the other elements are constant, the friction varies
approximately with the area of contact. The area of contact depends on
the length and form of the channel, and on the quantity of water. For
streams of the same length and same form of cross-section, but differing
in size of cross-section, the area of contact varies directly as the
square root of the quantity of water. Hence, _ceteris paribus_, the
friction of a stream on its bed is proportioned to the square root of
the quantity of water. But as stated above, the total energy of a stream
is proportioned directly to the quantity of water; and the total energy
is equal to the energy spent in friction, plus the energy spent in
transportation. Whence it follows that if a stream change its quantity
of water without changing its velocity or other accidents, the total
energy will change at the same rate as the quantity of water; the energy
spent in friction will change at a less rate, and the energy remaining
for transportation will change at a greater rate.
Hence increase in quantity of water favors transportation in a degree
that is greater than its simple ratio.
It follows as a corollary that the running water which carries the
_débris_ of a district loses power by subdivision toward its sources;
and that, unless there is a compensating increment of declivity, the
tributaries of a river will fail to supply it with the full load it is
able to carry.
It is noteworthy also that the obstruction which vegetation opposes to
transportation is especially effective in that it is applied at the
infinitesimal sources of streams, where the force of the running water
is least.
A stream which can transport _débris_ of a given size, may be said to be
_competent_ to such _débris_. Since the maximum particles which streams
are able to move are proportioned to the sixth powers of their
velocities, competence depends on velocity. Velocity, in turn, depends
on declivity and volume, and (inversely) on load.
In brief, the capacity of a stream for transportation is greater for
fine _débris_ than for coarse.
Its capacity for the transportation of a given kind of _débris_ is
enlarged in more than simple ratio by increase of declivity; and it is
enlarged in more than simple ratio by increase of volume.
The competence of a stream for the transport of _débris_ of a given
fineness, is limited by a corresponding velocity.
The _rate_ of transportation of _débris_ of a given fineness may equal
the capacity of the transporting stream, or it may be less. When it is
less, it is always from the insufficiency of supply. The supply
furnished by weathering is never available unless the degree of fineness
of the _débris_ brings it within the competence of the stream at the
point of supply.
The chief point of supply is at the very head of the flowing water. The
rain which falls on material that has been disintegrated by weathering,
begins after it has saturated the immediate surface to flow off. But it
forms a very thin sheet; its friction is great; its velocity is small;
and it is competent to pick up only particles of exceeding fineness. If
the material is heterogeneous, it discriminates and leaves the coarser
particles. As the sheet moves on it becomes deeper and soon begins to
gather itself into rills. As the deepening and concentration of water
progresses, either its _capacity_ increases and the load of fine
particles is augmented, or, if fine particles are not in sufficient
force, its _competence_ increases, and larger ones are lifted. In either
case the load is augmented, and as rill joins rill it steadily grows,
until the accumulated water finally passes beyond the zone of
disintegrated material.
The particles which the feeble initial currents are not competent to
move, have to wait either until they are subdivided by the agencies of
weathering, or until the deepening of the channels of the rills so far
increases the declivities that the currents acquire the requisite
velocity, or until some fiercer storm floods the ground with a deeper
sheet of water.
Thus rate of transportation, as well as capacity for transportation, is
favored by fineness of _débris_, by declivity, and by quantity of water.
It is opposed chiefly by vegetation, which holds together that which is
loosened by weathering, and shields it from the agent of transportation
in the very place where that agent is weakest.
When the current of a stream gradually diminishes in its course—as for
example in approaching the ocean—the capacity for transportation also
diminishes; and so soon as the capacity becomes less than the load,
precipitation begins—the coarser particles being deposited first.
_Corrasion and Transportation._
Where a stream has all the load of a given degree of comminution which
it is capable of carrying, the entire energy of the descending water and
load is consumed in the translation of the water and load and there is
none applied to corrasion. If it has an excess of load its velocity is
thereby diminished so as to lessen its competence and a portion is
dropped. If it has less than a full load it is in condition to receive
more and it corrades its bottom.
A fully loaded stream is on the verge between corrasion and deposition.
As will be explained in another place, it may wear the walls of its
channel, but its wear of one wall will be accompanied by an addition to
the opposite wall.
The work of transportation may thus monopolize a stream to the exclusion
of corrasion, or the two works may be carried forward at the same time.
_Corrasion and Declivity._
The rapidity of mechanical corrasion depends on the hardness, size, and
number of the transient fragments, on the hardness of the rock-bed, and
on the velocity of the stream. The blows which the moving fragments deal
upon the stream bed are hard in proportion as the fragments are large
and the current is swift. They are most effective when the fragments are
hard and the bed-rock is soft. They are more numerous and harder upon
the bottom of the channel than upon the sides because of the constant
tendency of the particles to sink in water. Their number is increased up
to a certain limit by the increase of the load of the stream; but when
the fragments become greatly crowded at the bottom of a stream their
force is partially spent among themselves, and the bed-rock is in the
same degree protected. For this reason, and because increase of load
causes retardation of current, it is probable that the maximum work of
corrasion is performed when the load is far within the transporting
capacity.
The element of velocity is of double importance since it determines not
only the speed, but to a great extent the size of the pestles which
grind the rocks. The coefficients upon which it in turn depends, namely,
declivity and quantity of water, have the same importance in corrasion
that they have in transportation.
Let us suppose that a stream endowed with a constant volume of water, is
at some point continuously supplied with as great a load as it is
capable of carrying. For so great a distance as its velocity remains the
same, it will neither corrade (downward) nor deposit, but will leave the
grade of its bed unchanged. But if in its progress it reaches a place
where a less declivity of bed gives a diminished velocity, its capacity
for transportation will become less than the load and part of the load
will be deposited. Or if in its progress it reaches a place where a
greater declivity of bed gives an increased velocity, the capacity for
transportation will become greater than the load and there will be
corrasion of the bed. In this way a stream which has a supply of
_débris_ equal to its capacity, tends to build up the gentler slopes of
its bed and cut away the steeper. It tends to establish a single,
uniform grade.
Let us now suppose that the stream after having obliterated all the
inequalities of the grade of its bed loses nearly the whole of its load.
Its velocity is at once accelerated and vertical corrasion begins
through its whole length. Since the stream has the same declivity and
consequently the same velocity at all points, its capacity for corrasion
is everywhere the same. Its rate of corrasion however will depend on the
character of its bed. Where the rock is hard corrasion will be less
rapid than where it is soft, and there will result inequalities of
grade. But so soon as there is inequality of grade there is inequality
of velocity, and inequality of capacity for corrasion; and where hard
rocks have produced declivities, there the capacity for corrasion will
be increased. The differentiation will proceed until the capacity for
corrasion is everywhere proportioned to the resistance, and no
further,—that is, until there is an equilibrium of action.
In general, we may say that a stream tends to equalize its work in all
parts of its course. Its power inheres in its fall, and each foot of
fall has the same power. When its work is to corrade and the resistance
is unequal, it concentrates its energy where the resistance is great by
crowding many feet of descent into a small space, and diffuses it where
the resistance is small by using but a small fall in a long distance.
When its work is to transport, the resistance is constant and the fall
is evenly distributed by a uniform grade. When its work includes both
transportation and corrasion, as in the usual case, its grades are
somewhat unequal; and the inequality is greatest when the load is least.
It is to be remarked that in the case of most streams it is the flood
stage which determines the grades of the channel. The load of detritus
is usually greatest during the highest floods, and power is conferred so
rapidly with increase of quantity of water, that in any event the
influence of the stream during its high stage will overpower any
influence which may have been exerted at a low stage. That relation of
transportation to corrasion which subsists when the water is high will
determine the grades of the waterway.
_Declivity and Quantity of Water._
The conclusions reached in regard to the relations of corrasion and
declivity depend on the assumption that the volume of the stream is the
same throughout its whole course, and they consequently apply directly
to such portions only of streams as are not increased by tributaries. A
simple modification will include the more general case of branching
streams.
Let us suppose that two equal streams which join, have the same
declivity, and are both fully loaded with detritus of the same kind. If
the channel down which they flow after union has also the same
declivity, then the joint stream will have a greater velocity than its
branches, its capacity for transportation will be more than adequate for
the joint load, and it will corrade its bottom. By its corrasion it will
diminish the declivity of its bed, and consequently its velocity and
capacity for transportation, until its capacity is equal to the total
capacity of its tributaries. When an equilibrium of action is reached,
the declivity of the main stream will be less than the declivities of
its branches. This result does not depend on the assumed equality of the
branches, nor upon their number. It is equally true that in any river
system which is fully supplied with material for transportation and
which has attained a condition of equal action, the declivity of the
smaller streams is greater than that of the larger.
Let us further suppose that two equal streams which join, are only
partially loaded, and are corrading at a common rate a common rock. If
the channel down which they flow after union is in the same rock and has
the same declivity, then the joint river will have a greater velocity,
and will corrade more rapidly than its branches. By its more rapid
corrasion it will diminish the declivity of its bed, until as before
there is an equilibrium of action,—the branch having a greater declivity
than the main. This result also is independent of the number and
equality of the branches; and it is equally true that in any river
system which traverses and corrades rock of equal resistance throughout,
and which has reached a condition of equal action, the declivity of the
smaller streams is greater than that of the larger.
In general we may say that, _ceteris paribus, declivity hears an inverse
relation to quantity of water_.
(There is an apparent exception to this law, which is specially
noteworthy in the sculpture of bad-lands, and will be described in
another place).
II. SCULPTURE.
Erosion may be regarded from several points of view. It lays bare rocks
which were before covered and concealed, and is thence called
_denudation_. It reduces the surfaces of mountains, plateaus, and
continents, and is thence called _degradation_. It carves new forms of
land from those which before existed, and is thence called _land
sculpture_. In the following pages it will be considered as land
sculpture, and attention will be called to certain principles of erosion
which are concerned in the production of topographic forms.
_Sculpture and Declivity._
We have already seen that erosion is favored by declivity. Where the
declivity is great the agents of erosion are powerful; where it is small
they are weak; where there is no declivity they are powerless. Moreover
it has been shown that their power increases with the declivity in more
than simple ratio.
It is evident that if steep slopes are worn more rapidly than gentle,
the tendency is to abolish all differences of slope and produce
uniformity. The law of uniform slope thus opposes diversity of
topography, and if not complemented by other laws, would reduce all
drainage basins to plains. But in reality it is never free to work out
its full results; for it demands a uniformity of conditions which
nowhere exists. Only a water sheet of uniform depth, flowing over a
surface of homogeneous material, would suffice; and every inequality of
water depth or of rock texture produces a corresponding inequality of
slope and diversity of form. The reliefs of the landscape exemplify
other laws, and the law of uniform slopes is merely the conservative
element which limits their results.
_Sculpture and Structure; the Law of Structure._
We have already seen that erosion is influenced by rock character.
Certain rocks, of which the hard are most conspicuous, oppose a stubborn
resistance to erosive agencies; certain others, of which the soft are
most conspicuous, oppose a feeble resistance. Erosion is most rapid
where the resistance is least, and hence as the soft rocks are worn away
the hard are left prominent. The differentiation continues until an
equilibrium is reached through the law of declivities. When the ratio of
erosive action as dependent on declivities becomes equal to the ratio of
resistances as dependent on rock character, there is equality of action.
In the structure of the earth’s crust hard and soft rocks are grouped
with infinite diversity of arrangement. They are in masses of all forms,
and dimensions, and positions; and from these forms are carved an
infinite variety of topographic reliefs.
In so far as the law of structure controls sculpture, hard masses stand
as eminences and soft are carved in valleys.
_The Law of Divides._
We have seen that the declivity over which water flows bears an inverse
relation to the quantity of water. If we follow a stream from its mouth
upward and pass successively the mouths of its tributaries, we find its
volume gradually less and less and its grade steeper and steeper, until
finally at its head we reach the steepest grade of all. If we draw the
profile of the river on paper, we produce a curve concave upward and
with the greatest curvature at the upper end. The same law applies to
every tributary and even to the slopes over which the freshly fallen
rain flows in a sheet before it is gathered into rills. The nearer the
watershed or divide the steeper the slope; the farther away the less the
slope.
It is in accordance with this law that mountains are steepest at their
crests. The profile of a mountain if taken along drainage lines is
concave outward as represented in the diagram; and this is purely a
matter of sculpture, the uplifts from which mountains are carved rarely
if ever assuming this form.
[Illustration:
FIG. 54.—Typical profile of the Drainage Slopes of Mountains.
]
Under the _law of Structure_ and the _law of Divides_ combined, the
features of the earth are carved. Declivities are steep in proportion as
their material is hard; and they are steep in proportion as they are
near divides. The distribution of hard and soft rocks, or the geological
structure, and the distribution of drainage lines and watersheds, are
coefficient conditions on which depends the sculpture of the land. In
the sequel it will be shown that the distribution of drainage lines and
watersheds depends in part on that of hard and soft rocks.
In some places the first of the two conditions is the more important, in
others the second. In the bed of a stream without tributaries the grade
depends on the structure of the underlying rocks. In rock which is
homogeneous and structureless all slopes depend on the distribution of
divides and drainage lines.
The relative importance of the two conditions is especially affected by
climate, and the influence of this factor is so great that it may claim
rank as a third condition of sculpture.
_Sculpture and Climate._
The Henry Mountains consist topographically of five individuals,
separated by low passes, and practically independent in climate. At the
same time they are all of one type of structure, being constituted by
similar aggregation of hard and soft rocks. Their altitudes appear in
the following table.
Altitude above the sea.
Mount Ellen 11,250 feet.
Mount Pennell 11,150 feet.
Mount Hillers 10,500 feet.
Mount Ellsworth 8,000 feet.
Mount Holmes 7,775 feet.
The plain on which they stand has a mean altitude of 5,500 feet, and is
a desert. A large proportion of the rain which falls in the region is
caught by the mountains, and especially by the higher mountains. Of this
there is abundant proof in the distribution of vegetation and of
springs.
The vegetation of the plain is exceedingly meager, comprising only
sparsely set grasses and shrubs, and in favored spots the dwarf cedar of
the West (_Juniperus occidentalis_).
Mount Ellen, which has a continuous ridge two miles long and more than
11,000 feet high, bears cedar about its base, mingled higher up with
piñon (_Pinus edulis_), and succeeded above by the yellow pine (_P.
ponderosa_), spruce (_Abies Douglasii_), fir (_A. Engelmanni_), and
aspen (_Populus tremuloides_). The pines are scattering, but the cedars
are close set, and the firs are in dense groves. The upper slopes where
not timbered are matted with luxuriant grasses and herbs. The summits
are naked.
Mount Pennell sends a single peak only to the height of the Ellen ridge.
Its vegetation is nearly the same, but the timber extends almost to the
summit.
Mount Hillers is 650 feet lower. Its timber reaches to the principal
summit, but is less dense than on the higher mountains. The range of
trees is the same.
Mount Ellsworth, 2,500 feet lower than Mount Hillers, bears neither fir,
spruce, pine nor aspen. Cedar and piñon climb to the summit, but are not
so thickly set as on the lower slopes of the larger mountains. The
grasses are less rank and grow in scattered bunches.
Mount Holmes, a few feet lower, has the same flora, with the addition of
a score of spruce trees, high up on the northern flank. Its summits are
bare.
In a word, the luxuriance of vegetation, and the annual rainfall, of
which it is the index, are proportioned to the altitude.
Consider now the forms of the mountain tops.
In Figure 55 are pictured the summit forms of Mount Ellen. The crests
are rounded; the slopes are uniform and smooth. Examination has shown
that the constituent rocks are of varying degrees of hardness, trachyte
dikes alternating with sandstones and shales; but these variations
rarely find expression in the sculptured forms.
In Figure 56 are the summit crags of Mount Holmes. They are dikes of
trachyte denuded by a discriminating erosion of their encasements of
sandstone, and carved in bold relief. In virtue of their superior
hardness they survive the general degradation.
[Illustration:
FIG. 55.—The Crest of Mount Ellen, as seen from Ellen Peak.
]
[Illustration:
FIG. 56.—The Crest of Mount Holmes.
]
The other mountains are intermediate in the character of their
sculpture. Mount Pennell is nearly as smooth as Mount Ellen. Mount
Ellsworth is nearly as rugged as Mount Holmes. One may ride to the crest
of Mount Ellen and to the summit of Mount Pennell; he may lead his
sure-footed cayuse to the top of Mount Hillers; but Mounts Ellsworth and
Holmes are not to be scaled by horses. The mountaineer must climb to
reach their summits, and for part of the way use hands as well as feet.
In a word, the ruggedness of the summits or the differentiation of hard
and soft by sculpture, is proportioned inversely to the altitude. And
rainfall, which in these mountains depends directly on altitude, is
proportioned inversely to ruggedness.
The explanation of this coincidence depends on the general relations of
vegetation to erosion.
We have seen that vegetation favors the disintegration of rocks and
retards the transportation of the disintegrated material. Where
vegetation is profuse there is always an excess of material awaiting
transportation, and the limit to the rate of erosion comes to be merely
the limit to the rate of transportation. And since the diversities of
rock texture, such as hardness and softness, affect only the rate of
disintegration (weathering and corrasion) and not the rate of
transportation, these diversities do not affect the rate of erosion in
regions of profuse vegetation, and do not produce corresponding
diversities of form.
On the other hand, where vegetation is scant or absent, transportation
and corrasion are favored, while weathering is retarded. There is no
accumulation of disintegrated material. The rate of erosion is limited
by the rate of weathering, and that varies with the diversity of rock
texture. The soft are eaten away faster than the hard; and the structure
is embodied in the topographic forms.
Thus a moist climate by stimulating vegetation produces a sculpture
independent of diversities of rock texture, and a dry climate by
repressing vegetation produces a sculpture dependent on those
diversities. With great moisture the law of divides is supreme; with
aridity, the law of structure.
Hence it is that the upper slopes of the loftier of the Henry Mountains
are so carved as to conceal the structure, while the lower slopes of the
same mountains and the entire forms of the less lofty mountains are so
carved as to reveal the structure; and hence too it is that the arid
plateaus of the Colorado Basin abound in cliffs and cañons, and offer
facilities to the student of geological structure which no humid region
can afford.
Here too is the answer to the question so often asked, “whether the
rains and rivers which excavated the cañons and carved the cliffs were
not mightier than the rains and rivers of to-day.” Aridity being an
essential condition of this peculiar type of sculpture, we may be sure
that through long ages it has characterized the climate of the Colorado
Basin. A climate of great rainfall, as Professor Powell has already
pointed out in his “Exploration of the Colorado,” would have produced
curves and gentle slopes in place of the actual angles and cliffs.
_Bad-lands._
Mountain forms in general depend more on the law of divides than on the
law of structure, but their independence of structure is rarely perfect,
and it is difficult to discriminate the results of the two principles.
For the investigation of the workings of the law of divides it is better
to select examples from regions which afford no variety of rock texture
and are hence unaffected in their erosion by the law of structure. Such
examples are found in _bad-lands_.
Where a homogeneous, soft rock is subjected to rapid degradation in an
arid climate, its surface becomes absolutely bare of vegetation and is
carved into forms of great regularity and beauty. In the neighborhood of
the Henry Mountains, the Blue Gate and Tununk shales are of this
character, and their exposures afford many opportunities for the study
of the principles of sculpture. I was able to devote no time to them,
but in riding across them my attention was attracted by some of the more
striking features, and these I will venture to present, although I am
conscious that they form but a small part of the whole material which
the bad-lands may be made to yield.
If we examine a bad-land ridge, separating two drainage lines and
forming a divide between them, we find an arrangement of secondary
ridges and secondary drainage lines, similar to that represented in the
diagram, (Figure 58.)
[Illustration:
FIG. 57.—General view of the Plateaus lying East of the Henry
Mountains.
]
The general course of the main ridge being straight, its course in
detail is found to bear a simple relation to the secondary ridges.
Wherever a secondary joins, the main ridge turns, its angle being
directly toward the secondary. The divide thus follows a zigzag course,
being deflected to the right or left by each lateral spur.
The altitude of the main ridge is correspondingly related to the
secondary ridges. At every point of union there is a maximum, and in the
intervals are saddles. The maxima are not all equal, but bear some
relation to the magnitudes of the corresponding secondary ridges, and
are especially accented where two or more secondaries join at the same
point. (See profile in Figure 59.)
I conceive that the explanation of these phenomena is as follows: The
heads of the secondary drainage lines laid down in the diagram are in
nature tolerably definite points. The water which during rain converges
at one of these points is there abruptly concentrated in volume. Above
the point it is a sheet, or at least is divided into many rills. Below
it, it is a single stream with greatly increased power of transportation
and corrasion. The principle of equal action gives to the concentrated
stream a less declivity than to the diffused sheet, and—what is
especially important—it tends to produce an equal grade in all
directions upward from the point of convergence. The converging surface
becomes hopper-shaped or funnel-shaped; and as the point of convergence
is lowered by corrasion, the walls of the funnel are eaten back equally
in all directions—except of course the direction of the stream. The
influence of the stream in stimulating erosion above its head is thus
extended radially and equally through an arc of 180°, of which the
center is at the point of convergence.
Where two streams head near each other, the influence of each tends to
pare away the divide between them, and by paring to carry it farther
back. The position of the divide is determined by the two influences
combined and represents the line of equilibrium between them. The
influences being radial from the points of convergence, the line of
equilibrium is tangential, and is consequently at right angles to a line
connecting the two points. Thus, for example, if _a_, _b_, and _c_
(Figure 58) are the points of convergence at the heads of three drainage
lines, the divide line _ed_ is at right angles to a line connecting _a_
and _b_, and the divides _fd_ and _gd_ are similarly determined. The
point _d_ is simultaneously determined by the intersection of the three
divide lines.
[Illustration:
FIG. 58.—Ground plan of a Bad-land Ridge, showing its relation to
Waterways. The smooth lines represent Divides.
FIG. 59.—Profile of the same ridge.
]
Furthermore, since that point of the line _ed_ which lies directly
between _a_ and _b_ is nearest to those points, it is the point of the
divide most subject to the erosive influences which radiate from _a_ and
_b_, and it is consequently degraded lower than the contiguous portions
of the divide. The points _d_ and _e_ are less reduced; and _d_, which
can be shown by similar reasoning to stand higher than the adjacent
portion of either of the three ridges which there unite, is a local
maximum.
There is one other peculiarity of bad-land forms which is of great
significance, but which I shall nevertheless not undertake to explain.
According to the law of divides, as stated in a previous paragraph, the
profile of any slope in bad-lands should be concave upward, and the
slope should be steepest at the divide. The union or intersection of two
slopes on a divide should produce an angle. But in point of fact the
slopes do not unite in an angle. They unite in a curve, and the profile
of a drainage slope instead of being concave all the way to its summit,
changes its curvature and becomes convex. Figure 60 represents a profile
from _a_ to _b_ of Figure 58. From _a_ to _m_ and from _b_ to _n_ the
slopes are concave, but from _m_ to _n_ there is a convex curvature.
Where the flanking slopes are as steep as represented in the diagram,
the convexity on the crest of a ridge has a breadth of only two or three
yards, but where the flanking slopes are gentle, its breadth is several
times as great. It is never absent.
Thus in the sculpture of the bad-lands there is revealed an exception to
the law of divides,—an exception which cannot be referred to accidents
of structure, and which is as persistent in its recurrence as are the
features which conform to the law,—an exception which in some
unexplained way is part of the law. Our analysis of the agencies and
conditions of erosion, on the one hand, has led to the conclusion that
(where structure does not prevent) the declivities of a continuous
drainage slope increase as the quantities of water flowing over them
decrease; and that they are great in proportion as they are near
divides. Our observation, on the other hand, shows that the declivities
increase as the quantities of water diminish, up to a certain point
where the quantity is very small, and then decrease; and that
declivities are great in proportion as they are near divides, unless
they are _very_ near divides. Evidently some factor has been overlooked
in the analysis,—a factor which in the main is less important than the
flow of water, but which asserts its existence at those points where the
flow of water is exceedingly small, and is there supreme.
[Illustration:
FIG. 60.—Cross-profile of a Bad-land Divide.
]
_Equal Action and Interdependence._
The tendency to equality of action, or to the establishment of a dynamic
equilibrium, has already been pointed out in the discussion of the
principles of erosion and of sculpture, but one of its most important
results has not been noticed.
Of the main conditions which determine the rate of erosion, namely,
quantity of running water, vegetation, texture of rock, and declivity,
only the last is reciprocally determined by rate of erosion. Declivity
originates in upheaval, or in the displacements of the earth’s crust by
which mountains and continents are formed; but it receives its
distribution in detail in accordance with the laws of erosion. Wherever
by reason of change in any of the conditions the erosive agents come to
have locally exceptional power, that power is steadily diminished by the
reaction of rate of erosion upon declivity. Every slope is a member of a
series, receiving the water and the waste of the slope above it, and
discharging its own water and waste upon the slope below. If one member
of the series is eroded with exceptional rapidity, two things
immediately result: first, the member above has its level of discharge
lowered, and its rate of erosion is thereby increased; and second, the
member below, being clogged by an exceptional load of detritus, has its
rate of erosion diminished. The acceleration above and the retardation
below, diminish the declivity of the member in which the disturbance
originated; and as the declivity is reduced the rate of erosion is
likewise reduced.
But the effect does not stop here. The disturbance which has been
transferred from one member of the series to the two which adjoin it, is
by them transmitted to others, and does not cease until it has reached
the confines of the drainage basin. For in each basin all lines of
drainage unite in a main line, and a disturbance upon any line is
communicated through it to the main line and thence to every tributary.
And as any member of the system may influence all the others, so each
member is influenced by every other. There is an interdependence
throughout the system.
III.—SYSTEMS OF DRAINAGE.
To know well the drainage of a region two systems of lines must be
ascertained—the drainage lines and the divides. The maxima of surface on
which waters part, and the minima of surface in which waters join, are
alike intimately associated with the sculpture of the earth and with the
history of the earth’s structure; and the student of either sculpture or
history can well afford to study them. In the following pages certain
conditions which affect their permanence and transformations are
discussed.
THE STABILITY OF DRAINAGE LINES.
In corrasion the chief work is performed by the impact and friction of
hard and heavy particles moved forward by running water. They are driven
against all sides of the channel, but their tendency to sink in water
brings them against the bottom with greater frequency and force than
against the walls. If the rate of wear be rapid, by far the greater part
of it is applied to the bottom, and the downward corrasion is so much
more powerful than the lateral that the effect of the latter is
practically lost, and the channel of the stream, without varying the
position of its banks, carves its way vertically into the rock beneath.
It is only when corrasion is exceedingly slow that the lateral wear
becomes of importance; and hence as a rule the position of a stream bed
is permanent.
The stability of drainage lines is especially illustrated in regions of
displacement. If a mountain is slowly lifted athwart the course of a
stream, the corrasion of the latter is accelerated by the increase of
declivity, and instead of being turned aside by the uplift, it
persistently holds its place and carves a channel into the mountain as
the mountain rises. For example the deep clefts which intersect the
Wasatch range owe their existence to the fact that at the time of the
beginning of the uplift which has made the range, there were streams
flowing across the line of its trend which were too powerful to be
turned back by the growing ridge. The same relation has been shown by
Professor Powell where the Green River crosses the uplift of the Uinta
Mountains, and in many instances throughout the Rocky Mountain region it
may be said that rivers have cut their way through mountains merely
because they had established their courses before the inception of the
displacement, and could not be diverted by an obstruction which was
thrown up with the slowness of mountain uplift.
THE INSTABILITY OF DRAINAGE LINES.
The stability of waterways being the rule, every case of instability
requires an explanation; and in the study of such exceptional cases
there have been found a number of different methods by which the courses
of streams are shifted. The more important will be noted.
_Ponding._
When a mountain uplift crosses the course of a stream, it often happens
that the rate of uplift is too rapid to be equaled by the corrasion of
the stream, and the uprising rock becomes a dam over which the water
still runs, but above which there is accumulated a pond or lake.
Whenever this takes place, the pond catches all the _débris_ of the
upper course of the stream, and the water which overflows at the outlet
having been relieved of its load is almost powerless for corrasion, and
cannot continue its contest with the uplift unless the pond is silted up
with detritus. As the uplift progresses the level of the pond is raised
higher and higher, until finally it finds a new outlet at some other
point. The original outlet is at once abandoned, and the new one becomes
a permanent part of the course of the stream. As a rule it is only large
streams which hold their courses while mountains rise; the smaller are
turned back by ponding, and are usually diverted so as to join the
larger.
The disturbances which divert drainage lines are not always of the sort
which produce mountains. The same results may follow the most gentle
undulations of plains. It required a movement of a few feet only to
change the outlet of Lakes Michigan, Huron, and Superior from the
Illinois River to the St. Clair; and in the tilting which turned Lake
Winipeg from the Mississippi to the Nelson no abrupt slopes were
produced. If the entire history of the latter case were worked out, it
would probably appear that the Saskatchewan River which rises in the
Rocky Mountains beyond our northern boundary, was formerly the upper
course of the Mississippi, and that when, by the rising of land in
Minnesota or its sinking at the north, a barrier was formed, the water
was ponded and Lake Winipeg came into existence. By the continuance of
the movement of the land the lake was increased until it overflowed into
Hudson’s Bay; and by its further continuance, combined with the
corrasion of the outlet, the lake has been again diminished. When
eventually the lake disappears the revolution will be complete, and the
Saskatchewan will flow directly to Hudson’s Bay, as it once flowed
directly to the Gulf of Mexico. (See the “Physical Features of the
Valley of the Minnesota River,” by General G. K. Warren.)
_Planation._
It has been shown in the discussion of the relations of transportation
and corrasion that downward wear ceases when the load equals the
capacity for transportation. Whenever the load reduces the downward
corrasion to little or nothing, lateral corrasion becomes relatively and
actually of importance. The first result of the wearing of the walls of
a stream’s channel is the formation of a flood-plain. As an effect of
momentum the current is always swiftest along the outside of a curve of
the channel, and it is there that the wearing is performed; while at the
inner side of the curve the current is so slow that part of the load is
deposited. In this way the width of the channel remains the same while
its position is shifted, and every part of the valley which it has
crossed in its shiftings comes to be covered by a deposit which does not
rise above the highest level of the water. The surface of this deposit
is hence appropriately called the _flood-plain_ of the stream. The
deposit is of nearly uniform depth, descending no lower than the bottom
of the water-channel, and it rests upon a tolerably even surface of the
rock or other material which is corraded by the stream. The process of
carving away the rock so as to produce an even surface, and at the same
time covering it with an alluvial deposit, is the process of
_planation_.
It sometimes happens that two adjacent streams by extending their areas
of planation eat through the dividing ridge and join their channels. The
stream which has the higher surface at the point of contact, quickly
abandons the lower part of its channel and becomes a branch of the
other, having shifted its course by planation.
The slopes of the Henry Mountains illustrate the process in a peculiarly
striking manner. The streams which flow down them are limited in their
rate of degradation at both ends. At their sources, erosion is opposed
by the hardness of the rocks; the trachytes and metamorphics of the
mountain tops are carved very slowly. At their mouths, they discharge
into the Colorado and the Dirty Devil, and cannot sink their channels
more rapidly than do those rivers. Between the mountains and the rivers,
they cross rocks which are soft in comparison with the trachyte, but
they can deepen their channels with no greater rapidity than at their
ends. The grades have adjusted themselves accordingly. Among the hard
rocks of the mountains the declivities are great, so as to give
efficiency to the eroding water. Among the sedimentary rocks of the base
they are small in comparison, the chief work of the streams being the
transportation of the trachyte _débris_. So greatly are the streams
concerned in transportation, and so little in downward corrasion
(outside the trachyte region), that their grades are almost unaffected
by the differences of rock texture, and they pass through sandstone and
shale with nearly the same declivity.
The rate of downward corrasion being thus limited by extraneous
conditions, and the instrument of corrasion—the _débris_ of the hard
trachyte—being efficient, lateral corrasion is limited only by the
resistance which the banks of the streams oppose. Where the material of
the banks is a firm sandstone, narrow flood-plains are formed; and where
it is a shale, broad ones. In the Gray Cliff and Vermilion Cliff
sandstones flat-bottomed cañons are excavated; but in the great shale
beds broad valleys are opened, and the flood-plains of adjacent streams
coalesce to form continuous plains. The broadest plains are as a rule
carved from the thickest beds of shale, and these are found at the top
of the Jura-Trias and near the base of the Cretaceous. Where the streams
from the mountains cross the Blue Gate, the Tununk, or the Flaming Gorge
shale at a favorable angle, a plain is the result.
The plain which lies at the southern and western bases of Mount Hillers
is carved chiefly from the Tununk shale (see Figure 27). The plain
sloping eastward from Mount Pennell (Figure 36) is carved from the Blue
Gate and Tununk shales. The Lewis Creek plain, which lies at the western
base of Mount Ellen, is formed from the Blue Gate, Tununk, and Masuk
shales, and the planation which produced it has so perfectly truncated
the Tununk and Blue Gate sandstones that their outcrops cannot be traced
(Figures 61, 39, and 42). The plain which truncates the Crescent arch
(Figure 49) is carved in chief part from the Flaming Gorge shale. Toward
the east it is limited by the outcrops of the Henry’s Fork conglomerate,
but toward the mountain it cuts across the edge of the same conglomerate
and extends over Tununk shale to the margin of the trachyte.
[Illustration:
FIG. 61.—Cross-section of the Lewis Creek Plain. M, Masuk Shale. BG,
Blue Gate Group. T, Tununk Group. HF, Henry’s Fork conglomerate.
Scale, 1 inch = 4,000 feet.
]
The streams which made these plains and which maintain them, accomplish
their work by a continual shifting of their channels; and where the
plains are best developed they employ another method of shifting—a
method which in its proper logical order must be treated in the
discussion of alluvial cones, but which is practically combined in the
Henry Mountains with the method of planation. The supply of detritus
derived from the erosion of the trachyte is not entirely constant. Not
only is more carried out in one season than another and in one year than
another, but the work is accomplished in part by sudden storms which
create great floods and as suddenly cease. It results from this
irregularity that the channels are sometimes choked by _débris_, and
that by the choking of the channels the streams are turned aside to seek
new courses upon the general plain. The abandoned courses remain plainly
marked, and one who looks down on them from some commanding eminence can
often trace out many stages in the history of the drainage. Where a
series of streams emerge from adjacent mountain gorges upon a common
plain, their shiftings bring about frequent unions and separations, and
produce a variety of combinations.
[Illustration:
FIG. 62.—Ideal sketch to illustrate the Shifting of waterways on a
slope of Planation.
]
The accompanying sketch, Figure 62, is not from nature, but it serves to
illustrate the character of the changes. The streams which issue from
the mountain gorges _a_ and _b_ join and flow to _z_; while that which
issues at _c_ flows alone to _x_. An abandoned channel, _n_, shows that
the stream from _b_ was formerly united with that from _c_, and flowed
to _x_; and another channel, _m_, shows that it has at some time
maintained an independent course to _y_. By such shiftings streams are
sometimes changed from one drainage system to another; the hypothetical
courses, _x_, _y_, and _z_, may lead to different rivers, and to
different oceans.
An instance occurs on the western flank of the mountains. One of the
principal heads of Pine Alcove Creek rises on the south slope of Mount
Ellen and another on the northwest slope of Mount Pennell. The two unite
and flow southward to the Colorado River. They do not now cross an area
of planation, but at an earlier stage of the degradation they did; and
the portions of that plain which survive, indicate by the direction of
their slopes that one or both of the streams may have then discharged
its water into Lewis Creek, which runs northward to the Dirty Devil
River.
As the general degradation of the region progresses the streams and
their plains sink lower, and eventually each plain is sunk completely
through the shale whose softness made it possible. So soon as the
streams reach harder rock their lateral corrasion is checked, and they
are no longer free to change their ways. Wherever they chance to run at
that time, there they stay and carve for themselves cañons. Portions of
the deserted plains remain between the cañons, and having a durable
capping of trachyte gravel are long preserved. Such stranded fragments
abound on the slopes of the mountains, and in them one may read many
pages of the history of the degradation. They form tabular hills with
sloping tops and even profiles. The top of each hill is covered with a
uniform layer of gravel, beneath which the solid rock is smoothly
truncated. The slope of the hill depends on the grade of the ancient
stream, and is independent of the hardness and dip of the strata.
The illustration represents a _hill of planation_ on the north slope of
Mount Ellsworth. It is built of the Gray Cliff sandstone and Flaming
Gorge shale, inclined at angles varying from 25° to 45°; but
notwithstanding their variety of texture and dip the edges of the strata
are evenly cut away, so that their upper surface constitutes a plane.
The stream which performed this truncation afterward cut deeper into the
strata and carved the lower table which forms the foreground of the
sketch. It has now abandoned this plain also and flows through a still
deeper channel on the opposite side of the hill.
[Illustration:
FIG. 63.—A Hill of Planation.
]
The phenomena of planation are further illustrated in the region which
lies to the northwest of the Henry Mountains. Tantalus and Temple
Creeks, rising under the edge of the Aquarius Plateau, transport the
trachyte of the plateau across the region of the Waterpocket flexure to
the Dirty Devil River. Their flood-plains are not now of great extent,
but when their drainage lines ran a few hundred feet higher they appear
to have carved into a single plain a broad exposure of the Flaming Gorge
shale, which then lay between the Waterpocket and Blue Gate flexures.
At the Red Gate where the Dirty Devil River passes from a district of
trachyte plateaus to the district of the Great Flexures, it follows for
a few miles the outcrop of the Shinarump shale, and the remnants of its
abandoned flood-plains form a series of terraces upon each bank. Small
streams from the sides have cut across the benches and displayed their
structure. Each one is carved from the rock _in situ_, but each is
covered by a layer of the rounded river gravel. The whole are results of
planation; and they serve to connect the somewhat peculiar features of
the mountain slopes with the ordinary terraces of rivers.
[Illustration:
FIG. 64.—Ideal cross-section of a Terraced River Valley, after
Hitchcock. A, B, C, D, E, and F, Alluvial deposits. G, Indurated
rock, _in situ_.
]
River terraces as a rule are carved out, and not built up. They are
always the vestiges of flood-plains, and flood-plains are usually
produced by lateral corrasion. There are instances, especially near the
sea-coast, of river-plains which have originated by the silting up of
valleys, and have been afterward partially destroyed by the same rivers
when some change of level permitted them to cut their channels deeper;
and these instances, conspiring with the fact that the surfaces of
flood-plains are alluvial, and with the fact that many terraces in
glacial regions are carved from unconsolidated drift, have led some
American geologists into the error of supposing that river terraces in
general are the records of sedimentation, when in fact they record the
stages of a progressive corrasion. The ideal section of a terraced river
valley which I reproduce from Hitchcock (Surface Geology, Plate XII,
figure 1) regards each terrace as the remnant of a separate deposit,
built up from the bottom of the valley. To illustrate my own idea I have
copied his profile (Figure 65) and interpreted its features as the
results of lateral corrasion or planation, giving each bench a capping
of alluvium, but constituting it otherwise of the preëxistent material
of the valley. The preëxistent material in the region of the Henry
Mountains is always rock _in situ_, but in the Northern States it often
includes glacial drift, modified or unmodified.
There is a kindred error, as I conceive, involved in the assumption that
the streams which occupied the upper and broader flood-plains of a
valley were greater than those which have succeeded them. They may have
been, or they may not. In the process of lateral corrasion all the
material that is worn from the bank has to be transported by the water,
and where the bank is high the work proceeds less rapidly than where it
is low. A stream which degrades its immediate valley more rapidly than
the surrounding country is degraded (and the streams which abound in
terraces are of this character) steadily increases the height of the
banks which must be excavated in planation and diminishes the extent of
its flood-plain; and this might occur even if the volume of the stream
was progressively increasing instead of diminishing.
[Illustration:
FIG. 65.—Ideal cross-section of a Terraced River Valley, regarded as a
result of Planation. A, B, C, &c., Alluvial deposits. G, Preëxistent
material from which the valley was excavated.
]
Of the same order also is the mistake, occasionally made of ignoring the
excavation which a stream has performed, and assuming that when the
upper terraces were made the valley was as open as at present, and the
volume of flowing water was great enough to fill it.
_Alluvial Cones._
Wherever a stream is engaged in deposition instead of corrasion—wherever
it deposits its load—there is a shifting of channel by a third process.
The deposition of sediment takes place upon the bottom of the channel
and upon its immediate banks, and this continues until the channel
bottom is higher than the adjacent country. The wall of the channel is
then broken through at some point, and the water abandons its old bed
for one which is lower. Such occurrences belong to the histories of all
river deltas, and the devastation they have wrought at the mouths of
large rivers has enforced attention to their phenomena and stimulated a
study of their causes.
The same thing happens among the mountains. Wherever, as in Nevada and
Western Utah, the valleys are the receptacles of the detritus washed out
from the mountains, the foot-slopes of the mountains consist of a series
of alluvial cones. From each mountain gorge the products of its erosion
are discharged into the valley. The stream which bears the _débris_
builds up the bed of its channel until it is higher than the adjacent
land and then abandons it, and by the repetition of this process
accumulates a conical hill of detritus which slopes equally in all
directions from the mouth of the mountain gorge. At one time or another
the water runs over every part of the cone and leaves it by every part
of its base; and it sometimes happens that the opposite slopes of the
cone lead to different drainage systems.
An illustration may be seen in Red Rock Pass at the north end of Cache
Valley, Idaho. Lake Bonneville, the ancient expansion of Great Salt
Lake,[6] here found outlet to the basin of the Columbia, and the channel
carved by its water is plainly marked. For a distance of twelve miles
the bed of the channel is nearly level, with a width of a thousand feet.
Midway, Marsh Creek enters it from the east, and has built an alluvial
cone which extends to the opposite bank and divides it into two parts.
In the construction of the cone Marsh Creek has flowed alternately to
the north and to the south, being in one case a tributary to the Snake
and Columbia Rivers and to the Pacific Ocean, and in the other to the
Bear River and Great Salt Lake. So far as the creek is known to white
men it is a tributary of the Snake, but an irrigating ditch that has
been dug upon its cone carries part of its water to the Bear.
Footnote 6:
Lake Bonneville is described in volume III (Geology) of the “U. S.
Geog. Surveys West of the 100th Meridian,” pp. 88–104; and less fully
in the American Naturalist for November, 1876, and the American
Journal of Science for March, 1876, p. 228. See also Johnson’s
Cyclopedia, article “Sevier Lake.”
Another illustration exists at the mouth of the Colorado River. As has
been shown by Blake in the fifth volume of the Pacific Railroad Reports
(p. 236), the delta of the Colorado—or in other words the alluvial cone
which is built at its mouth—has extended itself completely across the
Gulf of California, severing the upper end from the lower and from the
ocean, and converting it into a lake. In continuing the upbuilding of
the delta the river has flowed alternately into the lower gulf and into
its severed segment. At the present day its mouth opens to the lower
gulf; but at rare intervals a portion of its water runs by the channel
known as “New River” to the opposite side of the delta. While it is
abandoned by the river the lake basin is dry, and it is known to human
history only as the Colorado Desert. Its bottom, which is lower than the
surface of the ocean, is strewn with the remains of the life its waters
sustained, and its beaches are patiently awaiting the cycle of change
which is slowly but surely preparing to restore to them their parent
waves.
[Illustration:
FIG. 66.—Cross-section of inclined strata, to illustrate Monoclinal
Shifting of waterways.
]
_Monoclinal Shifting._
In a fourth manner drainage lines are unstable.
In a region of inclined strata there is a tendency on the part of
streams which traverse soft beds to continue therein, and there is a
tendency to eliminate drainage lines from hard beds. In Figure 66, _S_
represents a homogeneous soft bed, and _H_ and _K_, homogeneous hard
beds. _A_ and _B_ are streams flowing through channels opened in the
soft rock, and in the hard. As the general degradation progresses the
stream at _a_ abrades both sides of its channel with equal force; but it
fails to corrade them at equal rates because of the inequality of the
resistance. It results that the channel does not cut its way vertically
into the hard rock, but works obliquely downward without changing its
relation to the two beds; so that when the degradation has reached the
stage indicated by the dotted line, the stream flows at _a_, having been
shifted horizontally by circumstances dependent on the dip and order of
the strata.
At the same time the stream at _B_, encountering homogeneous material,
cuts its way vertically downward to _b_; and a continuance of the
process carries it completely through the hard rock and into the soft.
Once in the soft it tends like the other streams to remain there; and in
the course of time it finds its way to the lower edge and establishes a
channel like that at _A_.
[Illustration:
FIG. 67.—Ground plan of outcrops of inclined strata, to illustrate the
results of Monoclinal Shifting.
]
The effect of this process on the course of a stream which runs
obliquely across inclined beds is shown in Figure 67. The outcrops of a
series of hard and soft strata, _H_, _H_, _H_ and _S_, _S_, are
represented in ground plan, and the direction of their dip is indicated
by the arrow. Supposing that a stream is thrown across them in the
direction of the dotted lines and that the land is then degraded, the
following changes will take place. The portion of the stream from _c_ to
_d_ will sink through the soft rock down to the surface of the hard, and
then follow down the slope of the hard, until at last its whole course
will be transferred to the line of separation between the two, and its
position (with reference to the outcrops which will then have succeeded
the original) will be represented by the line _g c_. The portion from
_e_ to _d_ sinking first through the hard bed and then through the soft,
will be deflected in the same manner to the position _e h g_. The points
_e_ and _c_ will retain their original relations to the strata. The same
changes will affect the portion from _e_ to _f_; and the original
oblique course will be converted into two sets of courses, of which one
will follow the strike of the strata and the other will cross the strike
at right angles.
The character of these changes is independent of the direction of the
current. They are not individually of great amount, and they do not
often divert streams from one drainage system to another nor change
their general directions. Their chief effects are seen in the details of
drainage systems and in the production of topographic forms. The
tendency of hard strata to rid themselves of waterways and of soft
strata to accumulate them, is a prime element of the process which
carves hills from the hard and valleys from the soft. Where hard rocks
are crossed by waterways they cannot stand higher than the adjacent
parts of the waterways; but where they are not so crossed they become
divides, and the “law of divides” conspires with the “law of structure”
to carve eminences from them.
The tendency of waterways to escape from hard strata and to abide in
soft, and their tendency to follow the strike of soft strata and to
cross hard at right angles, are tendencies only and do not always
prevail. They are opposed by the tendency of drainage lines to
stability. If the dip of the strata is small, or if the differences of
hardness are slight, or if the changes of texture are gradual instead of
abrupt, monoclinal shifting is greatly reduced.
Waterpocket Cañon is one of the most remarkable of monoclinal valleys;
and it serves to illustrate both the rule of monoclinal shifting and its
exception. The principal bed of soft rock which outcrops along the line
of the Waterpocket flexure is the Flaming Gorge shale, having a
thickness of more than one thousand feet. Through nearly the whole
extent of the outcrop a valley is carved from it, but the valley is not
a unit in drainage. At the north it is crossed by the Dirty Devil River
and by Temple and Tantalus Creeks, and the adjacent portions slope
toward those streams. At the south it is occupied for thirty miles by a
single waterway—the longest monoclinal drainage line with which I am
acquainted. The valley here bears the name of Waterpocket Cañon, and
descends all the way from the Masuk Plateau to the Colorado River. The
upper part of the cañon is dry except in time of rain, but the lower
carries a perpetual stream known as Hoxie Creek. Whatever may have been
the original meanderings of the latter they are now restrained, and it
is limited to the narrow belt in which the shale outcrops. As the cañon
is worn deeper the channel steadily shifts its position down the slope
of the underlying Gray Cliff sandstone, and carves away the shale. But
there is one exceptional point where it has not done this. When the
bottom of the cañon was a thousand feet higher the creek failed, at a
place where the dip of the strata was comparatively small, to shift its
channel as it deepened it, and began to cut its way into the massive
sandstone. Having once entered the hard rock it could not retreat but
sank deeper and deeper, carving a narrow gorge through which it still
runs making a detour from the main valley. The traveler who follows down
Waterpocket Cañon now comes to a place where the creek turns from the
open cañon of the shale and enters a dark cleft in the sandstone. He can
follow the course of the water (on foot), and will be repaid for the
wetting of his feet by the strange beauty of the defile. For nearly
three miles he will thread his way through a gorge walled in by the
smooth, curved faces of the massive sandstone, and so narrow and devious
that it is gloomy for lack of sunlight; and then he will emerge once
more into the open cañon. Or if he prefer he can keep to his saddle, to
the open daylight, and to the outcrop of the shale, and riding over a
low divide can reach the mouth of the gorge in half the distance.
[Illustration:
FIG. 68.—Waterpocket Cañon and the Horseshoe Bend of Hoxie Creek.
]
THE STABILITY OF DIVIDES.
The rain drops which fall upon the two sides of a divide flow in
opposite directions. However near to the dividing line they reach the
earth the work of each is apportioned to its own slope. It disintegrates
and transports the material of its own drainage slope only. The divide
is the line across which no water flows—across which there is no
transportation. It receives the minimum of water, for it has only that
which falls directly upon it, and every other point receives in addition
that which flows from higher points. It is higher than the surfaces
which adjoin it, and since less water is applied to its degradation it
tends to remain higher. It tends to maintain its position.
Opposed to this tendency there are others which lead to
THE INSTABILITY OF DIVIDES,
and which will now be considered.
_Ponding, Planation, and Alluviation._
Whenever by ponding, a stream or a system of streams which have belonged
to one drainage system are diverted so as to join another there is
coincidently a change of divides. The general divide between the two
systems is shifted from one side to the other of the area which changes
its allegiance. The line which was formerly the main divide becomes
instead a subordinate divide separating portions of the drainage system
which has increased its area; and on the other hand a line which had
been a subordinate divide is promoted to the rank of a main divide. In
like manner the shifting of streams from one system of drainage to
another by the extension of flood-plains, or by the building of alluvial
cones or deltas, involves a simultaneous shifting of the divides which
bound the drainage systems.
The changes which are produced by these methods are per saltum. When a
pond or lake opens a new outlet and abandons its old one there is a
short interregnum during which the drainage is divided between the two
outlets, and the watershed separating the drainage systems is double.
But in no other sense is the change gradual. The divide occupies no
intermediate positions between its original and its final. And the same
may be said of the changes by planation and alluviation. In each case a
tract of country is transferred bodily from one river system to another,
and in each case the watershed makes a leap.
But there are other methods of change, by which dividing lines move
_slowly_ across the land; and to these we will proceed.
_Monoclinal Shifting._
In regions of inclined strata, the same process which gathers the
waterways into the outcrops of the softer beds converts the outcrops of
the harder into divides. As the degradation progresses the waterways and
divides descend obliquely and retain the same relations to the beds. The
waterways continuously select the soft because they resist erosion
feebly, and the watersheds as continuously select the hard because they
resist erosion strongly. If the inclination of the strata is gentle,
each hard bed becomes the cap of a sloping table bounded by a cliff, and
the erosion of the cliff is by sapping. The divide is at the brow of the
cliff, and as successive fragments of the hard rock break away and roll
down the slope the divide is shifted. The process is illustrated in the
Pink Cliffs of Southern Utah. They face to the south, and their
escarpment is drained by streams flowing to the Colorado. The table
which they limit inclines to the north and bears the headwaters of the
Sevier. As the erosion of the cliffs steadily carries them back and
restricts the table, the drainage area of the Colorado is increased and
that of the “Great Basin”, to which the Sevier River is tributary, is
diminished.
[Illustration:
FIG. 69.—Ideal cross-section of inclined strata, to show the Shifting
of Divides in Cliff Erosion. Successive positions of a divide are
indicated at _a_, _b_, and _c_.
]
_Unequal and Equal Declivities._
[Illustration:
FIG. 70.—Cross-profile of a bad-land divide separating slopes of
Unequal Declivity. Two stages of erosion are indicated, to
illustrate the horizontal shifting of the divide.
]
In homogeneous material, and with equal quantities of water, the rate of
erosion of two slopes depends upon their declivities. The steeper is
degraded the faster. It is evident that when the two slopes are upon
opposite sides of a divide the more rapid wearing of the steeper carries
the divide toward the side of the gentler. The action ceases and the
divide becomes stationary only when the profile of the divide has been
rendered symmetric.
It is to this law that bad-lands owe much of their beauty. They acquire
their smooth curves under what I have called the “law of divides”, but
the symmetry of each ridge and each spur is due to the law of equal
declivities. By the law of divides all the slopes upon one side of a
ridge are made interdependent. By the law of equal declivities a
relation is established between the slopes which adjoin the crest on
opposite sides, and by this means the slopes of the whole ridge, from
base to base, are rendered interdependent.
One result of the interdependence of slopes is that a bad-land ridge
separating two waterways which have the same level, stands midway
between them; while a ridge separating two waterways which have
different levels, stands nearer to the one which is higher.
It results also that if one of the waterways is corraded more rapidly
than the other the divide moves steadily toward the latter, and
eventually, if the process continues, reaches it. When this occurs, the
stream with the higher valley abandons the lower part of its course and
joins its water to that of the lower stream. Thus from the shifting of
divides there arises yet another method of the shifting of waterways, a
method which it will be convenient to characterize as that of
_abstraction_. A stream which for any reason is able to corrade its
bottom more rapidly than do its neighbors, expands its valley at their
expense, and eventually “abstracts” them. And conversely, a stream which
for any reason is able to corrade its bottom less rapidly than its
neighbors, has its valley contracted by their encroachments and is
eventually “abstracted” by one or the other.
The diverse circumstances which may lead to these results need not be
enumerated, but there is one case which is specially noteworthy on
account of its relation to the principles of sculpture. Suppose that two
streams which run parallel and near to each other corrade the same
material and degrade their channels at the same rate. Their divide will
run midway. But if in the course of time one of the streams encounters a
peculiarly hard mass of rock while the other does not, its rate of
corrasion above the obstruction will be checked. The unobstructed stream
will outstrip it, will encroach upon its valley, and will at last
abstract it; and the incipient corrasion of the hard mass will be
stopped. Thus by abstraction as well as by monoclinal shifting, streams
are eliminated from hard rocks.
_Résumé._—There is a tendency to permanence on the part of drainage
lines and divides, and they are not displaced without adequate cause.
Hence every change which is known to occur demands and admits of an
explanation.
(_a_) There are four ways in which abrupt changes are made. Streams are
diverted from one drainage system to another, and the watersheds which
separate the systems are rearranged,
(1) by _ponding_, due to the elevation or depression of portions of
the land;
(2) by _planation_, or the extension of flood-plains by lateral
corrasion;
(3) by _alluviation_, or in the process of building alluvial cones and
deltas; and
(4) by _abstraction_.
(_b_) There are two ways in which gradual changes are effected:
(1) When the rock texture is variable, it modifies and controls by
_monoclinal shifting_ the distribution in detail of divides and
waterways.
(2) When the rock texture is uniform, the positions of divides
are adjusted in accordance with the principle of _equal
declivities_.
The abrupt changes are of geographic import; the gradual, of
topographic.
The methods which have been enumerated are not the only ones by which
drainage systems are modified, but they are the chief. Very rarely
streams are “ponded” and diverted to new courses through the damming of
their valleys by glaciers or by volcanic _ejecta_ or by land-slips. More
frequently they are obstructed by the growing alluvial cones of stronger
streams, but only the smallest streams will yield their “right of way”
for such cause, and the results are insignificant.
The rotation of the earth, just as it gives direction to the trade-winds
and to ocean currents, tends to deflect rivers. In the southern
hemisphere streams are crowded against their left banks and in northern
against the right. But this influence is exceedingly small. Mr. Ferrel’s
investigations show that in latitude 45° and for a current velocity of
ten miles an hour, it is measured by less than one twenty-thousandth
part of the weight of the water (American Journal of Science, January,
1861). If its effects are ever appreciable it must be where lateral
corrasion is rapid; and even there it is probable that the chief result
is an inclination of the flood-plain toward one bank or the other,
amounting at most to two or three minutes.
CONSEQUENT AND INCONSEQUENT DRAINAGE.
If a series of sediments accumulated in an ocean or lake be subjected to
a system of displacements while still under water, and then be converted
to dry land by elevation _en masse_ or by the retirement of the water,
the rains which fall on them will inaugurate a drainage system perfectly
conformable with the system of displacements. Streams will rise along
the crest of each anticlinal, will flow from it in the direction of the
steepest dip, will unite in the synclinals, and will follow them
lengthwise. The axis of each synclinal will be marked by a watercourse;
the axis of each anticlinal by a watershed. Such a system is said to be
_consequent_ on the structure.
If however a rock series is affected by a system of displacements after
the series has become continental, it will have already acquired a
system of waterways, and _provided the displacements are produced
slowly_ the waters will not be diverted from their accustomed ways. The
effect of local elevation will be to stimulate local corrasion, and each
river that crosses a line of uplift will inch by inch as the land rises
deepen its channel and valorously maintain its original course. It will
result that the directions of the drainage lines will be independent of
the displacements. Such a drainage system is said to be _antecedent_ to
the structure.
But if in the latter case the displacements are produced rapidly the
drainage system will be rearranged and will become consequent to the
structure. It has frequently happened that displacements formed with
moderate rapidity have given rise to a drainage system of mixed
character in which the courses of the larger streams are antecedent and
those of the smaller are consequent.
There is a fourth case. Suppose a rock series that has been folded and
eroded to be again submerged, and to receive a new accumulation of
unconforming sediments. Suppose further that it once more emerges and
that the new sediments are eroded from its surface. Then the drainage
system will have been given by the form of the upper surface of the
superior strata, but will be independent of the structure of the
inferior series, into which it will descend vertically as the
degradation progresses. Such a drainage system is said to be
_superimposed by sedimentation_ upon the structure of the older series
of strata.
Fifth. The drainage of an alluvial cone or of a delta is independent of
the structure of the bed-rock beneath; and if in the course of time
erosion takes the place of deposition and the alluvial formation is cut
through, the drainage system which is acquired by the rocks beneath is
not consequent upon their structure but is _superimposed by
alluviation_.
Sixth. The drainage of a district of planation is independent of the
structure of the rock from which it is carved; and when in the progress
of degradation the beds favorable to lateral corrasion are destroyed and
the waterways become permanent, their system may be said to be
_superimposed by planation_.
In brief, systems of drainage, in their relation to structure, are
(A) _consequent_
(a) by emergence, when the displacements are subaqueous, and
(b) by sudden displacement;
(B) _antecedent_; and
(C) _superimposed_
(a) by sedimentation, or subaqueous deposition,
(b) by alluviation, or subaërial deposition, and
(c) by planation.
THE DRAINAGE OF THE HENRY MOUNTAINS
is consequent on the laccolitic displacements. The uplifting of a
laccolite, like the upbuilding of a volcanic cone, is an event of so
rapid progress that the corrasion of a stream bed cannot keep pace with
it. We do not know that the site of the mountains was dry land at the
time of their elevation; but if it was, then whatever streams crossed it
were obstructed and turned from their courses. If it was not, there were
no preëxistent waterways, and the new ones, formed by the first rain
which fell upon the domes of strata, radiated from the crests in all
directions. The result in either case would be the same, and we cannot
determine from the present drainage system whether the domes were lifted
from the bed of the Tertiary lake or arose after its subsidence.
But while the drainage of the Henry Mountains is consequent as a whole,
it is not consequent in all its details, and the character of its
partial inconsequence is worthy of examination.
Let us begin with the simplest case. The drainage system of Mount
Ellsworth is more purely consequent than any other with which I am
acquainted. In the accompanying chart the point _c_ marks the crest of
the Ellsworth dome; the inner circle represents the line of maximum dip
of the arching strata and the outer circle the limit of the disturbance.
It will be seen that all the waterways radiate from the crest and follow
closely the directions in which the strata incline. At _a_ the Ellsworth
arch touches that of Mount Holmes and at _b_ that of Mount Hillers; and
the effect of the compound inclination is to modify the directions of a
few of the waterways.
[Illustration:
FIG. 71.—Drainage system of the Ellsworth Arch.
]
Turning now to Mount Holmes, we find that its two domes are not equally
respected by the drainage lines. The crest of the Greater arch (see
Figure 72) is the center of a radiating system, but the crest of the
Lesser arch is not; and waterways arising on the Greater traverse the
Lesser from side to side. More than this, a waterway after following the
margin of the Lesser arch turns toward it and penetrates the flank of
the arch for some distance. In a word, the drainage of the Greater arch
is consequent on the structure, while the drainage of the Lesser arch is
inconsequent.
[Illustration:
FIG. 72.—Drainage system of the Holmes Arches.
]
There are at least two ways in which this state of affairs may have
arisen.
First, the Greater arch may have been lifted so long before the Lesser
that its waterways were carved too deeply to be diverted by the gentle
flexure of the latter. The drainage of the Lesser would in that case be
classed as antecedent. If the Lesser arch were first formed and carved,
the lifting of the Greater might throw a stream across its summit; but
it could not initiate the waterways which skirt the slopes of the
Lesser, especially if those slopes were already furrowed by streams
which descended them. If the establishment of the drainage system
depended on the order of uplift, the Greater arch is surely the older.
Second, the drainage of the Lesser arch may have been imposed upon it by
planation at a very late stage of the degradation. Whatever was the
origin of the arches, and whatever was the depth of cover which they
sustained, the Greater is certain to have been a center of drainage from
the time of its formation. When it was first lifted it became a drainage
center because it was an eminence; and afterward it remained an eminence
because it was a drainage center. When in the progress of the denudation
its dikes were exposed, their hardness checked the wear of the summit
and its eminence became more pronounced. It was perhaps at about this
time that the last of the Cretaceous rocks were removed from the summits
and slopes of the two arches and the Flaming Gorge shale was laid bare,
and so soon as this occurred the conditions for lateral corrasion were
complete. With trachyte in the peaks and shale upon the slopes planation
would naturally result, and a drainage system would be arranged about
the dikes as a center without regard to the curves of the strata. The
subsequent removal of the shale would impart its drainage to the
underlying sandstones.
Either hypothesis is competent to explain the facts, but the data do not
warrant the adoption of one to the exclusion of the other. The waterways
of the Lesser arch may be either antecedent, or superimposed by
planation. The Greater arch may have been the first to rise or the last.
The drainage of Mount Hillers is consequent to the main uplift and to
the majority of the minor, but to the Pulpit arch it is inconsequent. In
this case there is no question that the arch has been truncated by
planation. (Figure 73.) The Hillers dome, rising five times as high as
the Pulpit, became the center of drainage for the cluster, and the
trachyte-laden streams which it sent forth were able to pare away
completely the lower arch while it was still unprotected by the hardness
of its nucleus. The foot-plain of Mount Hillers, which extends unbroken
to the outcrop of the Henry’s Fork conglomerate, is continued on several
lines across the Pulpit arch, although in the intervals the central area
is deeply excavated. The planation stage is just completed, and an epoch
of fixed waterways is inaugurated.
[Illustration:
FIG. 73.—Cross-section of the Pulpit Arch, showing its truncation.
]
The drainage of Mount Pennell is consequent in regard to the main
uplift, but inconsequent to some of the minor. A stream which rises on
the north flank not merely runs across one of the upper series of
laccolites,—a companion to the Sentinel,—but has cut into it and divided
it nearly to the base. It is probable that the position of the waterway
was fixed by planation, but no remnant of the plain was seen.
Too little is known of the structure of the central area of Mount Ellen
to assert its relation to the drainage. About its base there are five
laccolites which have lost all or nearly all their cover, and each of
these is a local center of drainage, avoided by the streams which head
in the mountain crest. Four others have been laid bare at a few points
only, and these are each crossed by one or two streams from higher
levels. The remainder are not exposed at all, and their arches are
crossed by numerous parallel streams. The Crescent arch is freshly
truncated by planation, and the Dana and Maze bear proof that they have
at some time been truncated. The laccolites which stand highest with
reference to the general surface are exempt from cross-drainage, and the
arches which lie low are completely overrun.
If we go back in imagination to a time when the erosion of the mountain
was so little advanced that the stream beds were three thousand feet
higher than they now are, we may suppose that very little trachyte was
laid bare. As the surface was degraded and a few laccolites were
exposed, it would probably happen that some of the then existing streams
would be so placed as to run across the trachyte. But being unarmed as
yet by the _débris_ of similar material they would corrade it very
slowly; and the adjoining streams having only shale to encounter, would
so far outstrip them as eventually to divert them by the process of
“abstraction”. In this way the first-bared laccolites might be freed
from cross-drainage and permitted to acquire such radiating systems of
waterways as we find them to possess. At a later stage when trachyte was
exposed at many points and all streams were loaded with its waste, the
power to corrade was increased, and the lower-lying laccolites could not
turn aside the streams which overran them.
The work of planation is so frequently seen about the flanks of the
Henry Mountains that there seems no violence in referring all the
cross-drainage of lateral arches to its action; and if that is done the
history of the erosion of the mountains takes the following form:
When the laccolites were intruded, the mounds which they uplifted either
rose from the bed of a lake or else turned back all streams which
crossed their sites; and in either case they established upon their
flanks a new and “consequent” set of waterways. The highest mounds
became centers of drainage, and sent their streams either across or
between the lower. All the streams of the disturbed region rose within
it and flowed outward. The degradation of the mounds probably began
before the uplift was complete, but of this there is no evidence. As it
proceeded the convex forms of the mounds were quickly obliterated and
concave profiles were substituted. The rocks which were first excavated
were not uniform in texture, but they were all sedimentary and were soft
as compared to the trachyte. The Tertiary and probably the Upper
Cretaceous were removed from the summits before any of the igneous rocks
were brought to light, and during their removal the tendency of divides
to permanence kept the drainage centers or maxima of surface at
substantially the same points. When at length the trachyte was reached
its hardness introduced a new factor. The eminences which contained it
were established more firmly as maxima, and their rate of degradation
was checked. With the checking of summit degradation and the addition of
trachyte to the transported material, planation began upon the flanks,
and by its action the whole drainage has been reformed. One by one the
lower laccolites are unearthed, and each one adds to the complexity and
to the permanence of the drainage.
If the displacements were completed before the erosion began, the
mountains were then of greater magnitude than at any later date. Before
the igneous nuclei were laid bare and while sedimentary rocks only were
subject to erosion, the rate of degradation was more rapid than it has
been since the hardness and toughness of the trachyte have opposed it.
If the surrounding plain has been worn away at a uniform rate, the
height of the mountains (above the plain) must have first diminished to
a minimum and afterward increased. The minimum occurred at the beginning
of the erosion of the trachyte, and at that time the mountains may even
have been reduced to the rank of hills. They owe their present
magnitude, not to the uplifting of the land in Middle Tertiary time, but
to the contrast between the incoherence of the sandstones and shales of
the Mesozoic series and the extreme durability of the laccolites which
their destruction has laid bare. And if the waste of the plain shall
continue at a like uniform rate in the future, it is safe to prophesy
that the mountains will for a while continue to increase in relative
altitude. The phase which will give the maximum resistance to
degradation has been reached in none of the mountains, except perhaps
Mount Hillers. In Mount Ellen the laccolites of the upper zone only have
been denuded; the greater masses which underlie them will hold their
place more stubbornly. The main bodies of Mounts Ellsworth, Holmes, and
Pennell are unassailed, and the present prominence of their forms has
been accomplished simply by the valor of their skirmish lines of dikes
and spurs. In attaching to the least of the peaks the name of my friend
Mr. Holmes, I am confident that I commemorate his attainments by a
monument which will be more conspicuous to future generations and races
than it is to the present.
CHAPTER VI.
ECONOMIC.
There is little to add to what has already been said of the economic
value of the mountains, and this chapter is hardly more than a
regrouping of facts scattered through those that precede.
_Coal._—Possibly some valuable though restricted deposit was overlooked;
but it is safe to say that no thick and continuous bed will be found.
The Cretaceous sandstones all contain thin and local beds—enough to mark
them as coal-bearing rocks—but there are no seams of value. The best
outcrop was seen in the bank of the south branch of Lewis Creek where it
crosses the upturned edge of the Blue Gate sandstone. The seam has a
thickness of four feet only, and is not well disposed for mining.
But if the Cretaceous coals were well developed it is to be doubted if
they would ever be used. They could have no local market. They could not
be carried to the east or south on account of the cañons. If taken
northward they would have to compete with the coal of Castle Valley,
which is more convenient and very abundant. If taken westward to the
metal mines of Nevada and Western Utah they would be undersold by the
more accessible coals found on the headwaters of the Virgin River and
Kanab Creek, and even by those of the Kaiparowits Plateau.
The _Gypsum_ and _Building Stones_ of the region need not be described.
They are plentiful in many parts of Utah, and however abundant in this
remote place can never be in demand.
_Gold, Silver, etc._—Three parties of “prospectors” have at different
times made unsuccessful search for metalliferous veins. In the course of
my survey I spent more than a month’s time among the crystalline and
metamorphic formations of the mountain tops, and although directing my
attention constantly to the rocks, did not discover a fissure vein.
Combining these negative data with certain theoretic considerations
which are set forth in the fourth chapter, I am led to the very
confident opinion that the essential conditions for the production of
fissure veins have not existed in the Henry Mountains, and hence that
there are no valuable deposits of the precious metals. The same
theoretic considerations apply to other mountains of the same character,
and I venture to predict that gold and silver will not be found in
paying quantity in Navajo Mountain, the Sierra la Sal, the Sierra Abajo,
the Sierra Carisso, or the Sierra La Lata.
_Agricultural Land._—Bowl Creek, both branches of Lewis Creek, and the
south branch of Trachyte Creek can readily be led to tracts of land
sufficiently level for farming, and each furnishes enough water to
irrigate several hundred acres. It is possible that these tracts will
prove useful for farming, but they lie a little too high to be assured
of a favorable climate. The lowest has an altitude of 6,000, and the
highest of 6,800 feet.
_Grazing Land._—Above the altitude of 7,500 feet there are many tracts
of good grass, available for grazing through the greater part of the
year but covered by snow in the winter. Below that level there is a
greater area of inferior grass, available through the whole year. By
using one portion in summer and the other in winter the mountains could
be made to give permanent support to a herd of 3,000 or 4,000 cattle.
With such overstocking as is often practiced in Utah they may subsist
10,000 animals for one or two years.
_Timber._—The trees worthy to be classed as timber are of three
species—fir (_Abies Engelmanni_), Douglass Spruce (_A. Douglasii_), and
yellow pine (_Pinus ponderosa_). The pine is the most valuable and the
fir the most abundant. The fir grows upon the mountain slopes, above the
level of 7,500 feet and forms thick-set forests. The total area which it
covers is not far from twenty-five square miles. The spruce mingles with
the fir at the lower edges of the forests; and the pine forms a few open
groves a little lower down the slopes.
It is to be doubted if the trees will ever be cut. Other timber of the
same quality and superior in quantity lies between it and the
settlements, and neither railroad nor mine nor town is likely to create
a local demand.
Coal, building stone, gypsum, and timber have no value for lack of a
market, either present or prospective; gold and silver are not found;
and there is little or no land that can be successfully farmed. Only for
grazing have the mountains a money value.
INDEX.
Abajo Mountains, 67, 69, 152
Agricultural lands, 152
Alluvial cones and alluviation, 133, 139
Alpine sculpture, 36, 38
Altitudes of peaks and passes, 3, 117
Analogues of the laccolite, 98
Ancient rivers no larger than modern, 133
Antecedent drainage, 143
Arch, Bowl Creek, 45
, Crescent, 44
, Dana, 43
, G, 43
, Maze, 44
, Pulpit, 33, 147
Arches, Forces which produced laccolitic, 87
Aridity favorable to geological examination, 2, 98
of the Colorado Basin in former times, 120
Aubrey Sandstone, 8
Bad-lands, Sculpture of, 120, 140
Bischof, on contraction of igneous rocks in cooling, 76
Blue Gate flexure, 13
Sandstone, 4
Shale, 4
Bonneville, Outlet of Lake, 134
Bowl Creek arch, 45
Cañons of the Colorado as obstructions to travel, 1, 151
as evidence of arid climate, 120
Carboniferous strata, 8
Carriso Mountains, 69, 152
Circle Cliffs, 14
Cleavage, Slaty, not found, 66
, Vertical, produced by sapping, 55
Cliffs, Circle, 14
Climate, Arid, favorable to geological examination, 2, 98
, General influence of, on erosion, 103
, Influence of, on sculpture, 117
Coal, 5, 151
Colorado Basin, History of the, 84
Desert, 134
River, Origin of the, 85
Colors of sandstones, Local variation of, 7
Comminution of detritus aids transportation, 106
Competence, 110
Conditions of rock flexure, 83
Cones, Alluvial, 133
Consequent drainage, 143
Contact phenomena, 65
Contraction of igneous rocks by cooling, 75, 80
Corrasion, 100, 101
, Relation of, to declivity, 112
, to friction of flow, 109
, to transportation, 111
Cover of the laccolites, Depth of the, 86, 94
Crescent arch, 44
Cotta, Prof. Bernhard von, cited, 75
Cretaceous period, Disturbance at the close of the, 10, 85
rock series, 4
Cross-lamination, Inclination of, 7
Dana arch, 43
Declivity, General influence of, upon erosion, 102
of stream beds, relation to corrasion, 112
, relation to transportation, 108
, Relation of, to quantity of water, 113
Declivities, Influence of unequal, on divides, 140
Degradation (see Erosion), 99, 115
Delesse on contraction of igneous rocks in cooling, 76
Densities of igneous rocks, 75
of sedimentary rocks, 77
Density a condition of laccolitic intrusion, 73, 74
of porphyritic trachyte, 77
Denudation (see Erosion), 99, 115
of laccolites, 21
Deposition by running water, 111
Diameters of laccolites, 92
Dike near Mount Ellsworth, 59
Dikes and sheets defined, 20
, Flat-topped, 28
of Mount Ellsworth, 23
of Mount Holmes, 28
, Relation of, to strains, 96
Dinah Creek Pass, 3
Dirty Devil River, Flood-plains of, 131
Distances, Table of, 17
Distribution of laccolites, Horizontal, 20, 30
, Vertical, 21, 56
Divides in bad-lands, Rounding of, 122
, Instability of, 139
, Law of, 116
, Stability of, 138
Drainage lines, Instability of, 125
, Stability of, 124
, Systems of, 124, 143
Dutton, Capt. C. E., on the intrusive rocks, 61
Economic geology, 151
Elk Mountains, 69
Ellen Mount (see Mount Ellen).
Ellsworth Mount (see Mount Ellsworth).
Energy of muddy streams, 108
streams, how measured, 106
Equal action, Principle of, 123
Erosion, Conditions which control, 102
, Influence of climate upon, 103
, Influence of declivity upon, 102
Erosion, Influence of rock texture upon, 103
, Influence of vegetation upon, 104
of Mount Ellsworth, 25
, Principles of, 99
, Processes of, 99
Farming lands, 152
Faults on Mount Ellsworth, 23
Mount Holmes, 28
, Region of, 12, 15
Fish Lake Valley, Structure of, 14
Fissure veins limited in depth, 82
not found in the Henry Mountains, 83, 151
Flaming Gorge Group, 6
Flat-topped dikes, 28
Flexure of rocks, Conditions of, 83
Flood-plain defined, 127
Flow of streams, Friction of, 107, 109
Folds, The Great, 10, 11, 85
Forces which produce laccolitic arches, 87
Form of laccolites, 55
Frost, 104
Friction of flow of streams, 107, 109
Geikie laccolite, 41
Gold and silver, 151
Granite, Eruptive, 70
Graves, Mr. Walter H., 1
Gray Cliff Group, 6
Grazing lands, 152
Heights, 3, 117
Henry, Prof. Joseph, 1
Henry Mountains, Altitudes of the, 2, 117
caused by resistance of trachyte to erosion, 25, 35
, Contributions of the, to the principles of erosion, 99
, Detailed description of the, 22
, Drainage of the, 144
, Economic value of the, 152
, Planation in the, 127, 149
, Relation of vegetation to type of sculpture in the, 118
, Route of travel to the, 14;
their structure laccolitic, 19, 53
Henry’s Fork Group, 4
Hillers, Mount (see Mount Hillers).
Hills of planation, 130
Historical note, 66
History of the Colorado Basin, 84
Holmes, Mount (see Mount Holmes).
Holmes, Mr. W. H., on the Elk Mountains, 70
La Lata and Carriso Mountains, 69
Hopkins on the power of currents, 106
Howell laccolite, 34
, Mr. Edwin E., Fossils discovered by, 5;
observation of the Henry Mountains, 66;
observation of the Navajo Mountain, 69
Hydrostatic equilibrium, Law of, 72
Igneous mountains of the Plateau Province, 67
Igneous rocks, Contraction of, by cooling, 75, 80
Inconsequent drainage, 143
Instability of divides, 139
drainage lines, 125
Interdependence of drainage slopes, 123, 141
Internal structure of laccolites, 55
Intrusive rocks of the Henry Mountains, 59, 61
Isolation of the Henry Mountains, 2, 18
Jerry Butte, 34
Jukes Butte, 46
Jukes, Prof. J. Beete, on prismatic structure, 76
Jura-Trias rock series, 5
Kaibab structure, 11
Laccolite A, 32
B, 32
C, 32
D, 35
E, 41
F, 42
, Geikie, 41
H, 46
, Hillers, 30
, History of the, 95
, Howell, 34
, Jukes, 46
, Marvine, 42
, Newberry, 41
of Mount Ellsworth, 27
of Mount Pennell, 36
of the western base of Mount Ellen, 40
, Peale, 47
, Possible analogues of the, 98
, Scrope, 47
, Sentinel, 38
, Shoulder, 42
, Steward, 32
, The, defined, 19
Laccolites, Age of the, discussed, 84
, Composite, 55
, Denudation of, 21
, Depth of cover of, 86, 94
, Detailed description of, 22
, Diameters of, 92
, Form of, 55, 91
, Holmes, 27
, Horizontal, distribution of, 20
intruded in soft beds, 58
, Limital area of, 90, 97
thickness of, 91
, Material of, 59
not prismatic, 55
of the Elk Mountains, 70
of other regions, 97;
size limited, 86
subsequent to the strata which inclose them, 51
, Vertical distribution of, 21, 56, 65
Laccolitic mountains of the Plateau Province, 67, 69
structure characteristic of the Henry Mountains, 19, 53
Sierra La Sal, etc., 69
conditioned by densities, 74
correlated with acidic lavas, 71
, Discussion of the origin of, 72
Lake Bonneville, Outlet of, 134
, Great Salt, Outlet of, 134
Winipeg, Origin of, 126
Land, Agricultural, 152
, Grazing, 152
sculpture, Principles of, 99, 115
, Timber, 152
La Lata Mountains, 68, 69, 152
La Sal Mountains, 68, 69, 152
Law of divides, 116
, Exception to the, 122
Lewis Creek plain, 128
Limital area of laccolites, 90, 97
thickness of laccolites, 91
Load, Relation of, to comminution, 107
Mallet, Robert, C. E., on contraction of igneous rocks in cooling, 76
Marvine laccolite, 42
Ma-suk’ Plateau, 13
Sandstone, 4
Shale, 4
Maze arch, 44
Metamorphism in the Henry Mountains, 65
on Mount Ellen, 38
Ellsworth, 24
Hillers, 31
Holmes, 28
Minerals of the Henry Mountains, 61, 64
Monoclinal shifting, 135, 140
Mount Ellen, Character of sculpture of, 118
, Drainage of, 148
, Structure of, 38
, Topography of, 3
, Vegetation of, 118
Ellsworth, Character of sculpture of, 25, 118
, Drainage of, 145
, Stereogram of, 23, 95
, Structure of, 22
, Topography of, 3
, Vegetation of, 118
Hillers, Character of sculpture of, 119
, Drainage of, 147
, Structure of, 30
, Topography of, 3
, Vegetation of, 118
Holmes, Character of sculpture of, 118
, Drainage of, 146
, Structure of, 27
, Topography of, 3
Mount Holmes, Vegetation of, 118
Pennell, Character of sculpture of, 119
, Drainage of, 148
, Structure of, 35
, Topography of, 3
, Vegetation of, 118
Mount Taylor, 71
Mud-cracks, Fossil, 9
Navajo Mountain, 69, 152
Newberry, Dr. J. S., on the Sierra Abajo, 67
La Sal, 68
laccolite, 41
Oblique lamination, Inclination of, 7
Peale, Dr. A. C., on igneous mountains of Colorado, 69
on the Elk Mountains, 70
Peale laccolite, 47
Pennell, Mount (see Mount Pennell).
Pennellen Pass, 3
Pine Alcove Creek, Former course of, 130
Planation, 126, 139
, Hills of, 130
Plasticity of solids, 82, 83, 97
Plateaus enumerated, 13
Precipitation from running water, 111
Pressure a condition for rock flexure, 81, 83, 96
Prismatic structure, Absence of, 55, 76
defined, 55
Ponding, 125, 139
Porphyritic trachyte, 60, 64
Porphyry; use of term discussed, 63
Powell, Prof. J. W., on the climate of the Colorado Basin, 120
Pulpit arch, 33, 147
Rankine on flexure of beams, 90
Red Gate flexure, 13
, Flood-plains of, 131
, The, 16
Revet-crags defined, 25
of Mount Hillers, 31
of Mount Holmes, 29
of Mount Pennell, 37
Ripple-marks, Fossil, 16
River terraces produced by planation, 132
Rivers, Ancient, no greater than modern, 133
Rock texture, Influence of, on sculpture, 115, 135, 140
, General influence of, on erosion, 103
Rocks of the Henry Mountains, Intrusive, 59, 61
, Sedimentary, 3
Rotation of the earth, 142
Route to the Henry Mountains, 14
San Rafael fold, 16, 18
Scrope laccolite, 47
Sculpture, Alpine, 36, 38
influenced by climate, 117
distribution of divides, 116
rock structure, 115, 135, 140
Sculpture of bad-lands, 120
of Mount Ellsworth, 25
, Principles of, 99, 115
Sentinel Butte, 38
Sheets and dikes defined, 20
of Mount Ellsworth, 24
of Mount Holmes, 28
, Relation of, to strains, 96
Shin-ar’-ump Group, 6
Shoulder laccolite, 42
Sierra Abajo, 67, 69, 152
Carriso, 79, 152
La Lata, 68, 69, 152
La Sal, 68, 69, 152
Solidity of rocks not absolute, 82, 83, 97
Specific gravities (see Densities).
Specimens of Henry Mountain trachyte, 60
Springs; relation to soft strata explained, 82
Stability of divides, 138
of drainage lines, 124
Stereogram, how made, 11, 49
of Mount Ellsworth, 23, 95
of the Henry Mountains, 49
and Waterpocket flexure, 11
Steward laccolite, 32
, Mr. John F., cited, 66
Strains shown by dikes and sheets, 96
Stretching of strata, 80
Structure, Law of, 115
Suncracks, Fossil, 9
Superimposed drainage, 144
Systems of drainage, 124, 143
Table of altitudes, 117
densities of sedimentary rocks, 78
in order of superposition, 79
trachytes, 77
diameters of laccolites, 92
distances, 17
mean densities of certain rock series, 80
Tantalus Creek, 17
, Flood-plains of, 131
Taylor, Mount, 71
Temple Creek, 16
, Flood-plains of, 131
Terraces, River, produced by planation, 132
Tertiary strata protected by lava, 12
Thicknesses of individual strata, 7
Thompson, Prof. A. H., 1, 66
Thousand Lake Mountain, View from, 15
Timber land, 152
Trachyte, Porphyritic, 60, 64
Trachytes of the Henry Mountains not vesicular, 51, 64
Transportation, 100, 101
by streams, analyzed, 106
Conditions which determine rate of, 110
favored by comminution of material, 106
declivity, 108
quantity of water, 109
, Relation of, to corrasion, 111
Trend, Absence of, 2, 30, 98
Tu-nunk’ Sandstone, 4
Shale, 4
Unconformities, 8, 85
Unconformity at Salina, 14
Vegetation, General influence of, on erosion, 104
Veins, Absence of fissure, 83, 151
, Fissure, limited in depth, 82
Vermilion Cliff Group, 6
Vertical cleavage by sapping, 55
Volcanoes of the Plateau Province, Extinct, 67, 70
Volume of streams, Influence of, on erosion, 104
transportation, 109
, Relation of, to declivity, 113
Warren, General G. K., on the valley of the Minnesota River, 126
Waterpocket Cañon, 137
flexure, described, 12
, Passes across the, 16
Watersheds (see Divides).
Weathering, 100
White, Dr. C. A.; identification of Henry’s Fork Group, 4
Whitney, Prof J. D., on inclination of bedding, 52
Winipeg, Origin of Lake, 126
Zones of laccolites, 57, 58, 74
[Illustration: Plate I. MAP of the HENRY MOUNTAINS AND VICINITY.
Photographed from a model in relief. Triangulation by A. H. Thompson.
Topography by W. H. Graves.]
[Illustration: Plate II. STEREOGRAM of the HENRY MOUNTAINS AND
WATERPOCKET FOLD. Including the same area as Plate I.]
[Illustration: Plate III. MAP of the HENRY MOUNTAINS. by G. K. Gilbert.
Photographed from a model in relief.]
[Illustration: Plate IV. STEREOGRAM of the DISPLACEMENTS of the HENRY
MOUNTAINS including the same area as Plate III. The Blue Gate Sandstone
is restored so as to exhibit the form it received by the uplift of the
laccolitic domes.]
[Illustration: Plate V. MAP of the HENRY MOUNTAINS by G.K. Gilbert. From
a model in relief.]
------------------------------------------------------------------------
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