Report on the geology of the high plateaus of Utah, with atlas

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220 Devonshire St.

DEPARTMENT OF THE INTERIOR

U. 8. GEOGRAPHICAL AND GEOLOGICAL SURVEY OF THE ROCKY MOUNTAIN REGION
J. W. POWELL, IN CHARGE

REPORT

Vertebrate Paleontology
U, S. National Museum

ON THE

GEOLOGY OF THE HIGH PLATEAUS OF UTAT

WITH ATLAS

13357 Oe 1}, 1D) OIG

CAPTAIN OF ORDNANCE, U. S. A.

WASHINGTON
GOVERNMENT PRINTING OFFICE
1880

Wasuineron, D. C., April 19, 1880.

Sir: Herewith I have the honor to transmit a report of explorations
and studies in Utah Territory prosecuted during the years 1875, 1876, and
1877, in connection with the survey of Maj. J. W. Powell, under the In-
terior Department. This report is made in conformity with Special Orders
of the War Department No. 90, May 13, 1875; No. 134, July 3, 1876; No.
89, April 26, 1877, which require that the report be made to the Secretary
of War.

I respectfully request that the report may be forwarded to the hon-
orable the Secretary of the Interior, with a view to its publication in con-
nection with the survey work of Major Powell.

Very respectfully, sir, your obedient servant,
C. E. DUTTON,
Captain of Ordnance.
The Hon. Secretary oF War,
(Through the Chief of Ordnance, U. 8. A.)

[Indorsement. ]

OrpDNANCE Orrick, War DEPARTMENT,

Washington, April 20, 1880.
Respectfully submitted to the Secretary of War. Approved.
S. V. BENET,

Brigadier-General, Chief of Ordnance.

ll

War DeEpPAaRTMENT,
Washington City, April 22, 1880.
Sir: I have the honor to transmit herewith a report of Capt. C. E.
Dutton, of the Ordnance Department, of explorations and studies in Utah,
prosecuted during the years 1875, 1876, and 1877, in connection with the
survey of J. W. Powell, under the Interior Department.

In accordance with the wishes of Captain Dutton I respectfully
request that the report referred to may be published in connection with the
survey work of Major Powell.

Very respectfully, your obedient servant,
ALEXANDER RAMSEY,
Secretary of War.

The Hon. SECRETARY OF THE INTERIOR.
[Indorsement. ]
DEPARTMENT OF THE INTERIOR,
April 23, 1880.

Respectfully referred to Maj. J. W. Powell.
GEO. M. LOCKWOOD,
Chief Clerk.

Vv

PREFATORY NOTE.

BY THE DIRECTOR OF THE SURVEY.

The Colorado Plateaus extend from southern Wyoming through
western Colorado and eastern Utah far into New Mexico and Arizona.
They are bounded on the north by the Wind River and Sweetwater
Mountains, on the east by the Park Mountains, on the south by the Desert
Range Region, and on the west by the Basin Range Region.

The Plateaus are chiefly drained by the Colorado River, but a small
area on the northwest is drained into Shoshone River, another on the north-
east into the Platte River, still another on the southeast into the Rio Grande
del Norte, and finally the western margin is drained by the upper portions
of the Sevier, Provo, Ogden, Weber, and Bear Rivers. The general eleva-
tion is about 7,000 feet above the level of the sea—varying from 5,000 to
12,000 feet. The ascent from the low, desert plains on the south is very
abrupt—in many places by a steep and almost impassable escarpment In
the Plateau Province an extensive series of sedimentary formations appear,
embracing Paleozoic, Mesozoic, and Tertiary strata, but crystalline schists
and granites are found in some of the deep cations.

A marked unconformity exists between the Silurian and Devonian
rocks; another between the Devonian and Carboniferous; another, but
not so well marked, between the Carboniferous and Mesozoic, and lastly
an unconformity between Cretaceous and_ Tertiary is usually well defined.
The Plateaus have been above the sea since the close of the Cretaceous
period but during early Tertiary times extensive lakes existed through-
out the Province. In Mesozoic and Tertiary times the Basin Province to
the west was the principal source of the materials deposited in the Pla-

Vii

Vili GEOLOGY OF THE HIGH PLATEAUS.

teau Province. In general, each formation is exceedingly persistent and
homogeneous in its characteristics, but in passing from one formation to
another in the vertical-scale great heterogeneity is observed. Toa very
large extent the formations still lie in a horizontal or nearly horizontal
position. The entire surface is traversed by faults or their homologues,
monoclinal flexures, having in general a north and south direction. Fol-
lowing any given line of displacement frequent transitions from faulting to
flexure are observed. The method of transition is variable; sometimes the
flexed beds are found to be partially faulted so that the throw is part by
faulting and part by flexure; sometimes a great fault divides into two or
more minor ones in such amanner that the entire throw is accomplished”
by a series of steps. Still other important phenomena are observed in
these faults; to explain them, the terms throw and upheaval are used as
relative to each other. In the cases to be described the upheaved beds
have their edges flexed upwards. This is explained in the following man-
ner: First, a displacement occurred by flexure; second, another displace-
ment, reversing the first, occurred by faulting, so that the thrown beds of
the first displacement were the upheaved beds of the second. The evi-
dence of this reversed action is sometimes exhibited in beds deposited at a
time intervening between the two movements; in this manner the beds
last deposited are displaced only by the last movement. This reversal of
displacement along the same plain or zone is frequently seen. It is some-
times by faulting and sometimes by flexure, thus giving rise to many com-
plications in the positions of strata. The great displacements began in
early Tertiary time, and are probably yet in progress. The evidences of
the recency of some of these movements appear in the escarpments fre-
quently seen along the line of faults where Quaternary beds have been
broken at a time so recent that the escarpments have not been destroyed
by atmospheric agencies, and further evidence is exhibited in the small
amount of talus frequently found at the foot of a recently formed fault-
scarp. By these displacements the region is divided into blocks with a
north and south trend; but this geologic characteristic serves only in part
to divide the region into plateaus.

The streams which traverse the region have their sources in the Wind

PREFATORY NOTE. ix

River Mountains on the north; in the Park Mountains on the east, and a
number of tributaries come from the west. In their courses through the
plateaus they run in canons. These canons are profound gorges corraded
by the streams themselves. The ‘‘country rock” of the region is composed
of sedimentary beds, nearly horizontal, as already stated. The region is
also excessively arid, but the mountains that stand on the rim of the basin
precipitate a large proportion of moisture, and in this manner streams
of comparatively large volume head in the mountains, run through the
plateaus and descend rapidly to the level of the sea, while the country
through which they pass is very meagerly supplied with moisture. Under
these conditions the profound gorges have been cut, as the process of cation
cutting is more rapid than the lateral degradation of the country. In this
manner every river runs in a deep gorge, and these canons further serve to
divide the region into plateaus.

The division is completed by lines of cliffs. These cliffs are bold escarp-
ments hundreds and thousands of feet in altitude—grand steps by which
the region is terraced. As the rivers corrade their channels more rapidly
than general degradation is carried on, the stratigraphic conditions of the
horizontal beds play a very important part in the method of degradation.
Here degradation by surface erosion is less and degradation by sapping
greater, and thus the walls of the canons retreat slowly in a series of steps
by this sapping process. Softer beds easily yield to atmospheric agencies,
while harder beds resist and stand in bold escarpments.

Thus by faults and monoclinal flexures, by deep cations, and by lines
of cliffs the surface is cut into a great number of plateaus.

In addition to the Plateaus proper, there are mountains due to upheaval
and degradation. The more important of these are the Zuni Range, to the
south, and the Uinta Range, far to the north. The Uinta Range is carved
from a broad upheaval having an east and west axis. On either flank of the
upheaval there is a line or zone of maximum displacement where the
upheaval is by flexure or by faulting. Between these zones there is a gentle
flexure either way to the axis. Thus the upheaval is in part by general
flexure from the axis as an anticlinal, and in part by faulting and monoclinal
flexure, as in the Kaibab structure. Again there are small areas which are

xg GEOLOGY OF THE HIGH PLATEAUS.

zones of diverse displacement: these districts are broken into smaller
blocks by faults and flexures, and often the blocks have been excessively
tilted and warped in diverse directions. On the flanks of plateaus and
mountain systems of the Uinta type where monoclinal flexures occur mono-
clinal ridges are frequentiy seen The position of these monoclinal ridges
is frequently varied by the occurrence of transverse faults. Where a great
Kaibab, Uinta, or anticlinal upheaval is found broken by a transverse fault,
that portion of the grand upheaval which has the greater amplitude will
have its monoclinal ridges placed more distant from the axis of upheaval and
that portion which has the less amplitude will have its monoclinal ridges
nearer the axis. In this manner, by vertical movements in transverse
faulting, the monoclinal ridges may be placed back and forth from the axis
of grand upheaval in such a manner as to give the appearance of lateral
faulting, 7. e., faulting in a horizontal direction.

On the plateaus stand buttes, lone mountains, and groups of mountains.

The buttes are mountain cameos, composed of horizontal strata with
escarped sides—they are mountains of circumdenudation.

The mountains are composed in whole or in part of extravasated matter
and may be classed structurally under three types.
I. Those having the Henry Mounrain Sraucture—where the locus of vol-

canic deposition is below the base level of degradation.
II. Those having the Tusnar Srrucrure—where the locus of volcanic

deposition is at the base level of degradation.

III. Those having the Urnkarer Srrucrurre—where the locus of extrava-
sation is above the base level of degradation.

In the first, the mountains are composed in part of voleanic and in
part of sedimentary materials. The volcanic matter exists as laccolites, over
which sedimentary strata have extended in great mountain domes, but such
strata may have been carried away, more or less, by atmospheric degra-
dation. In this class each mountain is a mass of volcanic material, with
sedimentary beds upon its flanks, and often these sedimentary beds extend
high up or even quite over the volcanic materials.

In the second, the mountains are composed wholly of volcanie mate-

rials erected upon a base of sedimentary strata. The mass is composed of

PREFATORY NOTE. xi

many outflows, which are often separated by unconformities due to inter-
vening atmospheric degradation.

In the third, the mountains are composed in part of sedimentary and
in part of extravasated materials. The sedimentary beds constitute the
central masses, over which extravasated rocks are spread. The locus of
extravasation being above the general base level of degradation, as the
adjacent country was carried away by atmospheric agencies the underlying
sedimentaries were protected and left as mountain masses. Usually the
extravasation has been continued from time to time through a series of
vents marked by cinder cones, and in a general way the earlier ones appear
nearer the summit of the mountain masses, the later ones nearer the base.
In this manner the several sheets are inversely imbricated; that is, the
upper edge of the lower sheet is placed on the lower edge of the upper
sheet. ‘Table Mountains,” with caps of lava, are the simplest forms of
this structure.

There are many varieties of each of these grand classes, and through
them the systems of structure coalesce in such a manner that the charac-
teristics of demarkation are not absolute. :

The Colorado Plateaus may be divided into a number of groups, based
on topographic and geologic characteristics, of which the High Plateaus
constitute one of the most important. The great tabular masses are com-
posed of sedimentary formations of early Tertiary and late Cretaceous age,
nearly or quite horizontal and usually capped with formations of extrava-
sated matter. These lavas are of exceedingly complex arrangement. The
period of voleanic activity was long, and between the outbreaks atmos-
pheric degradation, local transportation, and deposition intervened. ‘To
unravel these complexities and discover the line of sequence has been a
task of great magnitude. In the earlier explorations of this country under
the direction of the writer, the general sequence of sedimentary formations
was discovered, as well as the general characteristics of displacement,
many of its principal faults had been traced, and the origin of the cliffs and
canons was known. All this was the result of a series of reconnaissance
surveys. But the principal work of the geological survey of the region still
awaited accomplishment. It was necessary that the sedimentary formations

>

Xll GEOLOGY OF THE HIGH PLATEAUS.

should be studied in detail, that the great structure lines, the faults and
flexures, should be carefully traced, and the displacements determined quan-
titatively; but the most important part of the investigation to be made was
presented in the study of the volcanic formations, which are the chief char-
acteristics of the group of High Plateaus. No systematic work had been
done in this field. Our knowledge of it was chiefly confined to its geo-
graphic extent and to a general belief that an extensive series of volcanic
rocks would be found, and that the subject was of great complexity. At
this stage Capt. C. E. Dutton, of the Ordnance Corps, was induced to under-
take the investigation. Three seasons were devoted by him to field labor,
und the intervening months were chiefly given to laboratory study of the
materials collected in the field. With ereat labor and skill the work has
been accomplished, and its results are presented in this volume, which will
be found to extend our knowledge of the geology of the United States and
to be an important contribution to geologic philosophy.

To a large extent the sedimentary region embraced in the survey of
which this volume treats is destitute of vegetation and soil and its rocks
are so naked that good sections are obtainable on every hand. Again, the
region is dissected by deep cations. From both of these reasons the geology
is plainly revealed. Every fault, every flexure, the relations of successive
strata, unconformities, and all facts of structure are seen at once. But
there are two sources of obscurity. First, some of the highest plateaus are
covered with forests and vegetation. Second, the extravasated rocks are
ageregated in a much more confused manner than the sedimentary beds,
and greater labor and care is required in tracing them, and after the utmost
care uncertainties and doubts remain. Thus it is that in describing the
structural geology of the region the details of examination do not appear as
in reports on regions of country less favorable to geologic examination.
To a large extent, also, the details of structure are omitted from the text
and appear in the graphic illustrations which accompany the report. It
has been the policy of the survey to relieve its reports to the utmost extent
of burdensome details of verbiage, by presenting them, as far as possible,
through graphic methods to the eye.

The early reconnaissance of the country was in part made by Mr.

PREFATORY NOTE. xii

I. E. Howell, whose elaborate notes were placed in the hands of Captain
Dutton, and from time to time he has in his volume given Mr. Howell
eredit for the material which he has used. It was unfortunate for Mr.
Howell that his labor was suspended prematurely, and that he was not
able to elaborate a report upon the country studied by him.

The geography of the district, as exhibited in the atlas accompanying
this volume, was the study of Prof. A. H. Thompson, who was my assistant
in charge of that branch of the work during the earlier years of explora-
tion and survey. Through his skill and industry the geography has been
represented with all the accuracy and detail that the adopted scale will
permit.

I am especially indebted to Brig. Gen. S. V. Benét, chief of the Ord-
nance Bureau, for the interest he has taken in the geologic and geographic
researches prosecuted by the survey under my direction. Through the
wise policy of administration adopted by him, Captain Dutton has been
enabled to carry on his labors as «a geologist outside of the general oper-
ations of the Ordnance Bureau. The contribution to science which he here
presents will abundantly justify the course pursued by his distinguished
chief. :

To the Secretary of War and the General of the Army, the survey is
indebted for assistance rendered in various ways—especially in furnishing
subsistance to field parties from the commissariat of the Army, but chiefly
in the opportunity given Captain Dutton to prosecute his researches.

J. W. POWELL.
Apri. 1880.

PREFACE.

In the year 1874 my kind friend Prof. J. W. Powell proposed to me
that I should undertake, under his direction, the study of a large volcanic
“tract in the Territory of Utah, provided the consent of proper authority
could be entertained. Distrusting my own fitness for the work, I felt that
it would be better for him if his proposals were thankfully declined. In
1875, however, he renewed the proposition in such a friendly and compli-
mentary manner that a refusal seemed ungracious. Ile therefore laid the
matter before the Secretary of War, the General of the Army, and the
Chief of Ordnance, all of whom gave their cordial approbation; and by
order of the War Department I was detailed for duty in connection with
the survey of the Rocky Mountain Region in charge of Professor Powell.
The field which he assigned me to study was the District of the High
Plateaus, and the investigations were made during the summers of 1875,
1876, and 1877. The preparation of a report or monograph upon the dis-
trict has several times been interrupted by the pressure of other official
duties to which the writer has been assigned during the last three years.

In submitting this work, the dominant feeling in my own mind isa
keen sense of its many imperfections and a consciousness that it falls far
short of my hopes and expectations. The defects have arisen in a great
measure from want of experience in western geological field work prior to
the inception of this undertaking, and especially from want of observation
in the class of phenomena of which the work principally treats. Probably,
also, the magnitude of the task proposed was too great even for much more
experienced observers to accomplish within the time allotted to it. It
involved not only a study of the immediate district under discussion, but the
investigation of large areas surrounding it to which the district stands in

xV

Xvi GEOLOGY OT THE HIGH PLATEAUS.

intimate relations. Inthe brief season during which work in such a region
is practicable the investigation must be pushed with the utmost vigor and
rapidity, and the greatest portion of the time must be devoted to acquiring
a general and connected view of the broader features, while details cannot
often receive the attention which their importance really demands. From
the nature of the case, therefore, the work must be somewhat superficial in
many respects.

In preparing a monograph upon this district, it has been necessary to
lay the greatest stress upon a few subjects of inquiry, and these would natu-
rally be those which the facts most fully exemplify. It was important, —
however, at the beginning to discuss it as a part of a great geological prov-
ince, in which are found certain categories of facts possessing a peculiar
interest, displayed in a remarkable manner, and of the highest importance
to physical geology. The “Plateau Country” of the west is, I firmly
believe, destined to become one of the most instructive fields of research
which geologists in the future will have occasion to investigate. Of its sub-
divisions the District of the High Plateaus is one of the most important,
and the relations of the district to the province were studied with great care.
The results of those studies are set forth in general terms in the first two
chapters.

In the treatment of geological phenomena occurring within the district
the investigation has been devoted chiefly to three lines of inquiry. The
first is geological structure—those attitudes of the strata and the topo-
graphical forms which have been caused by the vertical movements of the
rocks. The displacements which have occurred there are very striking
both in respect to their magnitude and to their systematic arrangement. In
their forms and modes of occurrence they are also somewhat peculiar,
especially when brought into comparison with displacements found in other
regions. Ultimately such facts must take their place in that branch of
geological philosophy which treats of the evolution of the earth’s physical
features, the building of mountains, and the elevation of continents and
plateaus; but at present the observed facts do not appear to group them-
selyes into the relation of effects to causes. The broader facts relating to
structure are discussed in the second chapter.

AUTHORS PREFACE. XVil

The second and principal subject of investigation comprises volcanic
phenomena. The High Plateaus are in chief part a great volcanic area, in
which eruptions have occurred upon a grand scale. The period of activity
has been a very long one, its initial epoch having been not far from the
Middle Eocene; and the eruptions have oceurred with probably long inter-
vals of repose throughout the remainder of Tertiary and Quaternary time,
the most recent ones having to all appearances taken place only a few cen-
turies ago. The variety of eruptive products is exceedingly great, all of
the commoner kinds from the very acid to the very basic groups being well
represented. The preponderating masses are trachytic, but rhyolites, ande-
sites (including propylites), and basalts are found in great abundance.
Perhaps the most striking masses were the accumulations of fragmental
volcanic products—the beds of conglomerate and tufa, which occur in pro-
digious volume, especially in the central and southern portions of the
district. These proved to be extremely interesting, yielding many themes
of inquiry and speculation.

It would have been impossible, under the circumstances, to apply to a
region so extensive, so varied, and so ancient, the exhaustive analysis which
Serope has given to the volcanoes of the Auvergne or Geikie to the volcanic
rocks of the Basin of the Forth. Of all geological investigations the most
difficult are those relating to voleanology. Where the accumulations are
of great extent the student for a long time recognizes nothing but confusion,
and the difficulty of evoking anything like order and a succession of events
is about proportional to the amount of extravasation. And where the
atmospheric forces have through long periods been at work destroying the
piles which have been built up by eruption, the difficulty is still further
augmented. Individual facts, indeed, are numerous and even bewildering
by their number and variety. But we want something more than facts;
we want their order, their relations, and their meaning; and it is rare to
find the facts and relations so displayed that they are readily discerned and
comprehended. It seemed best, therefore, to limit the inquiry to a very
few questions. The one which was regarded with the most interest had
reference to the Order of Succession of Volcanic Eruptions. Since the

publication of Richthofen’s “Memoir on a Natural System of Volcanic
H P—ii

XViil GEOLOGY OF THE HIGH PLATEAUS.

Rocks,” this subject has been of peculiar interest to American students of
western geology. ‘The discussion of it as applied to the District of the
High Plateaus will be found in the third chapter.

The great conglomerates composed of fragmental volcanic materials
also furnished an interesting subject of inquiry. There are many other dis-
tricts in the West where similar masses are found sometimes in even greater
quantity, and their origin and mode of accumulation became an attractive
problem. That these formations are accumulations of ejected fragments
seemed inadmissible, and the turther the investigation proceeded the more
untenable did this view appear to be. While great bodies of tufaceous
matter are usually found surrounding volcanic orifices, the conglomerates
in question do not conform either in the structure of the beds or in the dis
tribution of their masses to those of ordinary tufa cones. At the present
time there are now accumulating in the valleys between the great tables
extensive alluvial formations, which upon careful examination seem to cor-
respond closely to the older conglomerates now exposed in the palisades of
the plateaus, and the conclusion was reached that the ancient conglomerates
and modern alluvia were produced by the same process. The discussion
of these formations is contained in the tenth chapter, and the conclusions
are embodied in the latter part of the third chapter.

Another interesting subject was the metamorphism of clastic beds
derived from the detritus of volcanic rocks, and it is treated in the latter
part of the eleventh chapter relating to the East Fork Canon in the Sevier
Plateau.

Very naturally one of the most prominent objects of investigation was
to find the localities in which were situated the vents or orifices from which
the great eruptive masses were outpoured. In the case of the basalts,
which are comparatively recent in their dates of eruption, there was in most
cases no difficulty. But with the older rocks, the rhyolites, trachytes, and
andesites, it is quite different. Some of the rhyolites show very plainly
even to the most superficial investigation whence they came. Others do
not. So powerfully have the destroying agents wrought upon the old vol-
canic piles, and so vast is the mass which has been torn down and scattered,
that the work of restoration is exceedingly difficult. The task of finding

AUTHOR’S PREFACE. x1X

the old centers, however, is by no means impossible. In a considerable
number of cases the larger and more important centers are still discernible,
though some are doubtful and exceedingly indistinct. The obscurity prob-
ably arises in many cases from the fact that while the greater accumula-
tions of lavas outflowed from great central vents or from /oct within which
numerous vents were thickly clustered in close proximity, there were
numberless scattered orifices from which a few eruptions or even a single
eruption took place. And these dispersed vents were probably scattered
about in the intervals between the central localities of eruption. Such
craters would in the lapse of ages be wholly obliterated, and their out-
poured masses reduced to mere remnants The general effect of secular
decay has been to level the volcanic piles and build up the lowlands with
the debris. On the other hand, the great faults have brought up to daylight
masses of bedded lavas which otherwise would have been concealed, and
erosion has in many places attacked the faulted edges of the upraised
blocks and sawed deep ravines and chasms in which the igneous masses are
tolerably well displayed. Thus we are enabled to gain information con-
cerning the location of the centers of eruption which would otherwise have
been unattainable. But the knowledge so gained is far less perfect than is
desirable.

Although it may seem that an investigation of such importance ought
to be easy, it is by no means so. The vastness of the masses displayed at
any center of eruption is such that no conception of their totality or of their
general arrangement can be gained without a somewhat protracted investi-
gation of a large area. But so rugged and formidable are the physical
features that such an investigation is about as difficult an undertaking as ever
falls to the lot of a geologist.

The petrographic work has not been embodied in this volume. It has
not yet been completed, though considerable progress has been made.
Yet if it had been practicable to obtain the means to prosecute this branch
of research to the end, and to publish the results in such form and with
such illustration as the scientific student of the present day demands, it
would have been done. It was originally intended to make a thorough
series of chemical analyses of the volcanic rocks of this district. Many

xx GEOLOGY OI THE HIGH PLATEAUS.

hundreds of thin sections for microscopic investigation have long since
been made. It was intended, also, to describe these rocks thoroughly and
illustrate the microscopic characters with a large collection of colored plates.
But the contemplated work was too costly for the very limited appropriation
at the disposal of Professor Powell. A considerable number of chemical
analyses have been made by myself, but petrographers have very properly
adopted the habit of relying upon other parties to furnish their chemical
analyses, and I have therefore omitted to publish them. My conviction is
that the chemical analysis of voleanic rocks: should, whenever practicable,
accompany the description of microscopic characters, for it seems to me that
the two lines of investigation are mutually dependent. It is hoped that at
no distant day the contemplated work may be brought to completion in a
supplementary volume, for the want of it is most deeply felt in presenting -

the present one.
THE ATLAS.

The atlas which accompanies this work has been prepared with great
care. The first double sheet represents by contours the topography of the
country. The primary triangulation is by Prof A. H. Thompson, and the
topographical work by Messrs. J. H. Renshawe and Walter H. Graves, under
Professor Thompson’s supervision. Having been in immediate contact with
these gentlemen during much of the time occupied by their field work, and
having familiarized myself with their methods, I can testify to the great
care and accuracy with which that work has been performed. The detail
work has been done with plane-tables upon sheets on which the primary
and secondary triangulations had been accurately plotted. These sheets
were carefully filled up with details in the field, and when they were
brought back to Washington contained the material which was used in the
preparation of the final map. Whatever could be sighted from the stations
occupied has been located by triangulation and plane-table sights and not
by sketching. Messrs. Renshawe and Graves acquired great skill in the
use of the plane-table, and worked with surprising accuracy and rapidity.
Each of them covered more than 2,000 square miles in a season.

The geological map has been colored by myself. The northern half
of the sheet is for the most part held to be accurate in details. In the

AUTHOR'S PREFACE. Xxi

Pavant the Carboniferous is represented as occupying exclusively the west-
ern side of the range. It is believed, however, that a few remnants of
Triassic beds are to be found in that locality, but Iam not able to desig-
nate accurately their positions. On the northwestern side of the Tushar
also I am informed that there are some Archzean rocks, of which the exact
location cannot be specified. A portion of the northwestern flank of the
Tushar and the western side of the Pavant I have not visited, and the geo-
logical coloring is adopted in those portions as representing merely the
dominant rocks. A considerable portion of the country lying south of the
Wasatch Plateau is colored from data derived in part from my own observa-
tions and in part from those of Mr. Edwin E. Howell. There was some
difficulty here in fixing in the field the demarkation between the Tertiary
and Cretaceous, since the two series are not always well distinguished either
by lithological characters or by fossils. But if the horizon chosen was
properly selected the delineation is believed to be accurate. South and
southwest of the Markigunt Plateau a similar difficulty occurred in sepa-
rating the Jura from the Trias, and the uncertainty here is somewhat
greater. The boundary between those two formations, as delineated upon
the map, may, upon more thorough investigation, receive some notable
modifications, though I believe it represents very approximately the truth.
In the valley of the Paria some slight modifications also may be necessary in
locating with precision the same boundary line; and again upon the south-
eastern slopes of the Aquarius Plateau, around the net-work of canons
tributary to the Escalante, the Trias and the Jura were utterly inaccessible,
and the location of the separating horizon was inferred from the colors of
the beds and the arrangement of the rocky ledges viewed from a distance.
The colors and sculptural forms are most exceptionally characteristic in
these two formations, and in this locality there is no possibility of mistak-
ing them whenever they can be distinctly seen, whether from great or small
distances

The large area of the map devoted to the trachytes should be under-
stood as meaning that in that area the trachytes are the dominant rocks.
Commingled with them are the principal bodies of conglomerate and very
extensive masses of andesite and dolerite. To define these intercalary

¥Xil GEOLOGY OF THE HIGH PLATEAUS.

lavas and the conglomerates would obviously be impossible. With the
foregoing exceptions the distribution of the strata is given with great con-
fidence In the exceptional cases the errors are believed to be so small as
not to sensibly impair the accuracy of the map.

The relief map was prepared in the following manner: A plaster cast
about five feet square was made, the horizontal and vertical scale being the
same. ‘The data for the cast were obtained from the contour map. The
cast was then photographed, and a copy of the photograph was drawn upon
stone.

The map (Sheet No. 4), showing the arrangement of the faults and
flexures, was designed to show at a glance the connection, relations, and in
some cases the continuity of the greater structure lines of the High Plateaus
with those of the Kaibab district around the Grand Canon of the Colorado.
The Kaibab or Grand Canon faults have been already worked out in an
admirable manner by Powell. The importance of connecting the two dis-
tricts by these common features is very great, and is not only essential to
the present work, but will have, if possible, still greater importance when
the geology of the southwestern part of the Plateau Province is discussed.
Only the greater displacements are here given. There are very many
smaller ones which are not so well known nor so well identified. Those
which are given have been traced rigorously mile by mile so far as they
are represented, excepting, however, the portions which extend south of the
Colorado. The course of these faults south of the Grand Cation has been
given to me by Mr. G. K. Gilbert, who has in part identified their existence
in that region, though I presume that he would not wish to be understood
as attaching a very high degree of accuracy to his designations, having
made merely a preliminary reconnaissance in that region.

The stereogram has been worked out with great care. It is the con-
solidated expression of a very large number of sections made in the field,
together with the results obtained by tracing continuously each fault along
its course This mode of illustrating displacements is by no means all that
could be desired and has some serions defects But it seems to be a great
improvement in the means of illustrating structure, since it groups the

dominant features together in their proper relations. Probably the greatest

AUTHOR'S PREFACE. XXlil

value of it is the facility it affords the student of testing the accuracy of
his work. He cannot commit a serious error in making his stereogram
without knowing it. He cannot proceed far in his work without becoming
conscious of the defects and gaps in his knowledge, and, best of all, he
obtains an index pointing to the very localities which he must revisit in
order to supplement the deficiencies. A stereogram is a laborious work, but
it abundantly repays the labor expended upon it. The writer who achieves
one will know the structure of the objects he is describing in a way and with
a thoroughness he could never hope for from any other means. Unfor-
tunately this method of systematizing observation is of very limited appli-
cability. Much disturbed regions and countries which have preserved very
obscurelythe records of their displacements are hardly capable of such a
discussion. The stereogram cannot take the place of the ordinary geologi-
cal sections, though it can embody in one illustration some of the most
important features of a hundred or more.

It is my pleasant duty to acknowledge the obligations which I owe to
Professor Powell for the earnest support he has given me during the work
of exploration and while the report has been in process of preparation.
Every facility which he could supply has been placed at my disposal,
whether in the field or in the office. But the greatest debt which I owe
him is for the scientific advice and assistance he has given me. He has
been not merely the director and administrator of his survey, but in the
most literal sense its chief geologist. During the period of his field work
in the Plateau Country (from 1869 to 1874) he had mastered with great
rapidity and acumen the broader facts and had co-ordinated them into a
system which was novel in many respects and which further research has
proved to be perfectly sound. The geological phenomena encountered in
that region are indeed governed by the same fundamental laws which prevail
elsewhere, but the conditions under which those laws operate are altogether
novel and peculiar, and the results which they produce are so singular that
they seem at first anomalous and then mysterious. The geologist who is
skilled in the conventional methods of investigation, the older applications
of principles, and the routine logic which have long been in vogue, might
well have been excused if he had found in this strange land little else than

4

XXIV GEOLOGY OF THE HIGH PLATEAUS.

paradoxes. But with Powell it was not so. His industry and energy in
the collection of facts, his stubborn resolution and dauntless courage in over-
coming the physical obstacles which nature has there placed in the way of
investigation, would alone have secured his fame; but even these are less
admirable than the analytic power with which he traced the facts back to
their causes, and the synthetic skill with which he grouped them together.
He has made the Plateau Country a most alluring field of geological study,
and evolved from it a new range of geological thought and philosophy.
The principles and fundamental generalizations with which he wrought are
indeed old and long established, but the facts being new and strange, it
required in order to comprehend them, a sagacity and penetration analogous
to that which is necessary for the citizen of one civilization to understand
the ethics of another. Not only has he grasped the details of his subject—
the salient features of the geological history, the stratigraphy, the erosion,
the displacements, the sculpture, the structure, the drainage, the origin of
the cliffs and cations of the Plateau Country—but he has woven all these
details and many others into a compact and consistent whole, in which each
part of the scheme gives support and bond to all the others. The pressure
of administrative duties and the prosecution of other work which he could
not avoid, chiefly ethnographic, have retarded the appearance of the great
work he has contemplated upon the Plateau Country; but those whose
privilege it has been to continue the study of that region under his direc-
tion, to consult with him daily, to benefit by his advice and thorough knowl-
edge of the field are deeply sensible of the fact that their own work has
been merely tributary to the broader scheme which originated with him and
of which he is unquestionably the founder and master.

I must also acknowledge my indebtedness to Mr. Edwin E. Howell
for some very important material which has been embodied in this work.
In the year 1873 Mr. Howell was attached to the survey of Lieut. (now
Capt.) George M. Wheeler, of the Corps of Engineers, and under the able
and energetic direction of that officer he rapidly traversed a large portion
of the Plateau Country. His brief but very instructive report is con-
tained in Vol. III, Geology, Surveys West of the One Hundredth Merid-
ian, Lieut. George M. Wheeler in charge. In the year 1874 Mr. Howell

AUTHOR’S PREFACE. XXV

joined Professor Powell’s survey and rapidly traversed the District of the
High Plateaus and portions of the region southwest of that district. Dur-
ing that year he succeeded in fixing the geological horizons of the chief
sedimentary beds there occurring, and also began the study of the struc-
tural features of the northern part of the district in the Wasatch Plateau
and in the Pavant. In the following winter he withdrew from the survey
in order to engage in business, and left copious notes of his observations
and drawings of geological sections which I have had the privilege of con-
sulting. His drawings of the sections made by him in the northern part
of the district are embodied in this volume. He is entitled to high praise
for the ability and accuracy of his work, and it is much to be regretted
that he was induced to abandon geological fieldwork.

I am also indebted to Mr. G. K. Gilbert for many valuable suggestions.
He has traversed this district several times on his way to and from his
own field of research and has given me information which has often proved
of great utility.

The atlas has been lithographed by Mr. Julius Bien, of New York, and
bears abundant evidence of his great skill and intelligence in that kind of
work.

C. E. DUTTON,
Captain of Ordnance.

CONTENTS.

CHAPTER I.

Page.
GENERAL CONSIDERATIONS RELATING TO THE TOPOGRAPHY AND GEOLOGICAL HISTORY OF THE
HiGH PLATEAUS AND THEIR RELATIONS TO THE PLATEAU PROVINCE OF WHICH THEY FORM
OTA nn einle ns mnica espe Sa Roraceh Denote a GmcG,. Saco SlconSopmpoon Ge anu Boceie po uetorceesobe 1
Situation of the High Plateaus.—The several ranges and intervening valleys.—Relations of High
Plateaus to the Plateau Province at large.—Geological history of the province.—Its lacustrine
strata.—Its emergence and desiccation.—Its erosion.—Its drainage system.—Origin of its
peculiar features. 1-24.
CHAPTER II.
STRUCLURATNGHOLOGYAORALHM PHI GHePTARWAUSM as =ea see ciaseniceeicereitcieemeericniccccenicesciense 25

Faults and monoclinal flexures.—The principal faults described.—A discussion of their age.—
Ancient displacements.—Parallelism of faults to the ancient shore line.—A comparison of
the structural forms prevailing in the Park, Plateau, and Basin Provinces. 25-54.

CHAPTER III.

VOLCANIC PHENOMENA PRESENTED IN THE DISTRICT AND A GENERAL DISCUSSION OF THEM..-- 55

Initial epochs of eruption.—Order of succession of eruptions.—Richthofen’s law of succession.—
Fragmental volcanic rocks.—Tufas.—Voleanic conglomerates.—Origin of the clastic beds.—
Metamorphism of tufas. 55-81.

CHAPTER IV.

CEASSIBICATION) OFTHE VOLCANIC) ROCKS: scien -tsisercleet easels sieele eels ielsieiesielasiaeieleiao= eo -e--eeeeee 82

A discussion of principles of classification and the objects to be gained.—Classification primarily
in accordance with chemical constitution.—Correlations between chemical constitution on
the one hand and mineral and physical constitution on the other.—Lithological texture.—
Correlation between texture and geological age.—Von Cotta’s view adopted.—The porphy-
ritic texture.—Acid and basic rocks.—Subdivisions—rhyolite, trachyte, andesite, basalt.

82-112.
XXVii

XXVIil GEOLOGY OF THE HIGH PLATEAUS.

CHAPTER V.

SPECULATIONS CONCERNING THE CAUSES OF VOLCANIC ACTION .-.. .--.---- e200 -- cece cece ee cone

The probable locus of voleanic activity.—Volcanism inconsistent with the notion of an all-liquid
interior.—Localization of the phenomena.—Independence of vents. —Comparison of lavas with
metamorphic rocks.—Synthetic character of basalt.—Dynamical causes of eruptions.—Local
increments of subterranean temperature.—Mechanies of eruption.—Application of hydrostatic
principles.—Explanation of the sequence of eruptions.—A compound function of tempera-
ture, density, and fusibility.—Discussion of the hypothesis.—Exceptions and anomalies.—The
ultimate cause unknown. 113-142.

(Ost AIO UID Ia “\F IEG

SEDIMENTARY FORMATIONS OF THE DISTRICT OF THE HIGH PLATEAUS .----.--...---------eeee

The Paleozoic.—The Shinérump or Lower Trias.—The Vermilion Cliffs or Upper Trias.—The Ju-
rassic.—The Cretaceous.—The Eocene. 143-159.

CHAPTER VII.
Abr MW AERGI Lele AG RDY NO oScmco oScmos cdoc5o.coCacS OND SeSnibS cae soanoS Gnad SoUdaS EESocOEROSSECaGO
Its structure.—Strata composing its mass.—The great monoclinal.—Gunnison Valley.—Salina

Cafion.—The Jurassic Wedge.—San Pete Platean.—Sedimentary beds of the Wasatch Mono-
cline.—Bitter Creek, Lower and Upper Green River beds. 160-168.

CHHEAGE BER Velen:

Sevier Valley from Gunnison southward.—General structure of the northern part of the range.—
Its intermediate character between the basin and plateau types.—Rugged character of the
northern portion.—Bullion Canon.—Rhyolitic eruptions.—Southern portion of the Tushar.—
The great conglomerates.—History of the range.—Alternations of volcanic activity and re-
pose.—The Tushar fault.—Succession of eruptions. 169-187.

CHAPTER Ix.
ene MARKAGUNT Wei AUEAU ans toes sae ee els eia aetna ee ohne nape ee ele = melee eae
General description.—Dog Valley and its eruptive masses.—Bear Valley.—Little Creek Peak.—
Tufas and conglomerates.—General surface of the Markégunt.—Succession of eruptions.—

Basalt fields.—Panquitch Lake and recent basaltic outpours.—Sedimentary formations.—Out-
look from the southern verge of the plateau. 188-210.

CHAPTER X.
SEVIER VALLEY AND ITS ALLUVIAL CONGLOMERATES ....--. -- 200-2220 cone -one cone oon ne oon
Upper Sevier or Panquitch Valley.—Panquitch Cation.—Circle Valley.—Origin of Circle Val-

ley.—Modes of accumulation of conglomerates.—Alluvial cones.—Identity of origin of the old
conglomerates and the alluvia now accumulating in the valleys. 211-224.

143

160

169

188

211

CONTENTS. XX1X

CHAPTER XI.
Page.
SEVIERPANDEPAUNSAGUNIW PEATIOAU Shee ese neee ene cee ee ne eee ect oe eee alee eee eee

General structure and form of the Sevier Plateau.—Monroe Amphitheater.—Eastern side of the
Plateau and Blue Mountain.—Northern lava floods.—The central portions of the plateau and
their eruptive masses.—Volcanie conglomerates.—Southern eruptive center of the plateau.—
East Fork Cafion.—Its tufas.—Their metamorphism.—Grass Valley, its structure and ori-
gin.—Alluyial cones and tufas of Grass Valley.—The Paunségunt.—Lower Eocene beds.—
The southern terraces.—Scenery of Pdria Amphitheater and Pink Cliffs.—Basaltic cones.
225-255.

CHAPTER XII.

THe Fisa Lake PLATEAU.—THE AWAPA.—THOUSAND LAKE MOUNTAIN ..-.------------------ 257

Southern extension of-the Wasatch Monocline.—Grass Valley faults.—Summit Valley.—Fish Lake
Plateau and the grand gorge.—Fish Lake.—Terminal moraines.—Succession of volcanic
beds.—Mount Terrill and Mount Marvine.—Tertiary formations.—Origin of Summit Valley.—
Moraine Valley.—Mount Hilgard and its rocks.—The Awapa Plateau.—Trachytes and con-
glomerates.—Ancient basalt fields.—Rabbit Valley and its alluvial beds.—Tertiary strata.—
Thousand Lake Mountain.—Jura and Trias.—The Red Gate. 256-283.

CHAPTER XIII.
THEVA QUARTUS: PLATEAU) st oo scais os!cicisiecsta cteis ciatecaloelsia ce ceie els) stctein want cicle wialeicjomte cieepwe esc seinen Od

Distant views and the approach to the Aquarius.—Its grandeur.—Panorama from its south-
eastern salient.—The Water Pocket Fold.—Inconsequent drainage.—The cations of the Esca-
lante.—The great Kaiparowits Cliff.—Circle Cliffs.—Navajo Mountain.—Potato Valley.—Pre-
Tertiary flexures and erosion.—Central faults of the Aquarius.—Its lava cap.—Western wall
of the Plateau.—Table Cliff—Kaiparowits Peak. 284-298,

LIST OF HELIOTY PES.

HELIOTYPE I.—THE GATE OF MONROE.

This picture represents the narrow gorge through which the drainage of the Monroe Amphitheater
passes to join the Sevier River. It is situated in the western wall of the Sevier Plateau, near its loftiest
part. The gorge is cut in a large mass of hornblendic propylite, and forms a cleft about 20 feet wide
and nearly 400 feet deep. In the background is seen one of the large hills within the amphitheater,
composed of trachyte and augitic andesite.

HeELioTtyPe Il.—CONGLOMERATE IN THE TUSHAR.

The cliff here exhibited is upon the eastern flank of the Tushar facing Circle Valley. In the face
of the cliff are seen about 1,300 feet of conglomerate surmounted by 400 feet of lava. The bedding here
is much less conspicuous than is usually the case in such formations.

HeELIoTyPE III.—Tura.—MARrKAGUNT PLATEAU.

This material has been derived from the complete decay of lavas, and consists of aluminous
silicate, accumulated as a deposit in the bed of a small lake, where it was consolidated and subse-
quently eroded. Such formations are not very uncommon on the Markagunt and elsewhere.

Heriotyes [V.—VOLCANIC ALLUVIAL CONGLOMERATE ON TRACHYTE.—PANQUITCH CANON.

The beds here exhibited were derived from the break-up of older volcanic masses situated in the
vicinity. At a fermer epoch the river flowed at a level as high as the summit of the cation wall, and the
upper portion of the conglomerate was eroded. An uplifting of the locality subsequently took place,
and the river cut its caiion, exposing the structure of the beds. It will be noted that the layers pre-
sent an arrangement suggestive of {alse stratification or cross-bedding, since their planes of stratifica-
tion do not conform to the surface of the trachyte below. This is the normal structure of all alluvial
cones.

HELiotyre V.—METAMORPHOSED TUFAS.—EAsT Fork Canon.
The beds here seen are all water-laid and occur within the inner gorge of the cation. The upper
member exhibited is a massive rock, with all the lithologic characters of an intrusive igneous rock.
Some of the thin layers below have the same character. (See Chap. XI.)

HeLi1otyrr VI.—Tura AND CONGLOMERATE.—EAsT ForK CANON.

On the right are seen the continuations of the same beds as in the preceding illustration. The
hill in the distance is com))osed of the same rocks below with coarse volcanic conglomerate above.

XXxi

XXX GEOLOGY OF THE HIGH PLATEAUS.

HELIOTYPE VII.—PINK CLirrs.—LOWER EOCENE.—PAUNSAGUNT PLATEAU.
The picture represents the southern termination of the Paunsdégunt, and is a good example of the
sculpture which is seen in this formation around the rim of the Paria Amphitheater for a distance of
40 miles. The rocks are exquisitely colored.

HELIOTYPE VIII.—CROSS-BEDDED JURASSIC SANDSTONE.

Taken in Johnson’s Cafion, on the road from Sevier Valley to Lower Kanab. Much finer instances
may be seen in any of the deep cafions cut in this formation.

HELIOTYPE IX.—CROSS-BEDDED JURASSIC SANDSTONE.
The same as the preceding.
HELIOTYPE X.—THE RED GaTE.—LOWER TRIAS.—SHINARUMP.
Taken atthe southeast flank of Thousand Lake Mountain. The beds in the cliff are variegated
in color, being banded horizontally, and the colors are very deep and rich. The sculpture is very charac-

teristic of the formation.

HELIOTYPE XI.—PHONOLITE.—EAsST FORK CANON.

GEOLOGY OF THE HIGH PLATEAUS.

BY CAPT. C. E. DUTTON.

CHAPTER I.

GENERAL CONSIDERATIONS RELATING TO THE TOPOGRAPHY AND GEOLOGICAL HIS-
TORY OF THE HIGH PLATEAUS AND THEIR RELATIONS TO THE PLATEAU PROVINCE
OF WHICH THEY FORM A PART.

Situation of the High Plateaus.—The westernmost range comprising the Pdvant, Tushar, and Mark4-
guut.—Sevier Valley.—The second or middle range comprising the Sevier and Paunségunt Pla-
teaus.—Grass Valley.—The third range comprising the Wasatch, Fish Lake, Awapa, and Aquarius
Plateans.—Structural features of the Park, Plateau and Basin Provinces.—The High Plateaus form
the western district of the Plateau Province.—Relations of the High Plateaus to the Plateau Province
at large.—Geological history in outline during Cretaceous time.—Interruption of continuity be-
tween the Upper Cretaceous and Tertiary.—Unconformity between Cretaceous and Tertiary.— Early
Tertiary history.—The lacustrine condition of the entire Plateau Province during early Eocene
time.—Gradual desiccation of this Eocene lake.—Cretaceous-Eocene strata occupying its locus
at the close of the Eocene.—Their vast bulk and gradual subsidence pari passu with deposition.—
The counterpart of this subsidence, viz, the elevation of the surrounding mountain chains.—
Post-Eocene history.—Erosion.—Its conspicuous display and the certainty of its evidence.—The
drainage system of the Colorado River.—Its origin.—Its stability of loeation.—Priority of drainage
channels to structural features.—Their persistence.—The methods of erosion.—Centers of erosion
and the recession of cliffs.—The San Rafael Swell.—Vastness of the results accomplished by
erosion.—Iffect of the removal of great bodies of strata from large areas.—The erosion chiefly
accomplished in the Miocene.—Summary of the relations of the High Plateaus to the Plateau
country at large and to the Basin Province adjoining them on the west.

The region to be discussed in this work is centrally situated in the
Territory of Utah, occupying a belt of country extending from a point
about 15 miles east of Mount Nebo in the Wasatch, south-southwest, a
distance of about 175 miles, and having a breadth varying from 25 to 80

miles. The total area of this field of study may approach 9,000 square
iL 936 1p

Y INTRODUCTORY.

miles If we examine the old War Department maps of the western half
of the United States and those maps which have been derived from them,
we shall find the Wasatch Mountains laid down as extending southward
with an increasing westerly trend until the range reaches a point near the
southwestern corner of Utah. This delineation conveys to the eye the
general truth that along this belt of country there is a lofty and, in a
qualified sense, a mountainous barrier separating the drainage system of
the Colorado River from that of the Great Basin of the West. Jt would
be impracticable upon a map of small scale to designate clearly the fact
that the Wasatch as a distinct mountain range ends at Mount Nebo, 75
miles south of Great Salt Lake, and that it is here overlapped en échelon
by a chain of plateau uplifts which extend southward, gradually swinging
around the southeastern rim of the Great Basin.

These plateaus are not a part, either structurally or topographically,
of the Wasatch, but belong to another age, and are totally different
in their forms and geological relations. The extension of the name
“Wasatch Mountains” south of Nebo is a misnomer. The region south of
that mountain has nothing in common with the belt to the north of it, except
the mere fact that it carries the boundary line between the two drainage
systems; otherwise the two belts constitute one of the most decided of those
strong contrasts of topography and geological relations which are some-
times presented in adjacent portions of the Rocky Mountain Region. Those
who have studied these plateaus have recognized their distinct character,
and it seems necessary to give effect to this recognition to the extent of
employing for purposes of geological discussion a distinguishing name. It
has seemed to me that for these purposes the belt of country which they
occupy would be sufficiently characterized by giving to it the name of the
District oF THE Hicu Puateaus of Utah

These uplifts have certain analogies to mountain ranges, but in most

cases are distinguished by their well-marked tabular character.

COMPONENT MEMBERS OF THE GROUPS OF HIGH PLATEAUS.

There are three ranges of plateaus within the district, and each range
: ] ’ gs
can be subdivided into individual tables. The westernmost range is made

~~

INDIVIDUAL PLATEAUS. 3

up of three component masses, or members—the Pavant at the north end,
the Tushar in the middle, and the Markégunt at the south. The Pavant
is a curious admixture of plateau and sierra, the eastern side being tabular
in form and detail, while the western side is a common mountain front, like
many others found in the Great Basin. The Tushar is also a composite
structure, its northern half being a wild bristling cordillera of grand dimen-
sions and altitudes, crowned with snowy peaks, while the southern half is
conspicuously tabular. The Markdgunt isa true plateau, of the normal type
and of great expanse, and though very lofty (about 11,000 feet), is in utter
contrast to a mountain uplift. A narrow, and in some portions profound,
valley separates the western from the middle range of plateaus. This is
the Sevier Valley, bearing a small river of the same name, which collects
the drainage of the greater part of the district and pours it into a wretched
salina of the Great Basin, where it is evaporated. But the valley is an
important one, because it is one of the principal highways of travel, and,
still more, because it has already become the granary of Utah, and prom-
ises to increase in importance as an agricultural district.

The second range of plateaus consists of the Sevier Plateau on the
north and the Paunsdgunt Plateau on the south. The Sevier Plateau is
80 miles in length and only 12 to 20 in width. Its great elongation and
the bold sculpture of its fronts would assimilate it to a mountain range,
and such it seems to be in some portions of its extent as we look up to its
grand pediments from the valley below. but its structure and topography
are seen to be conspicuously tabular when viewed from lofty standpoints.
It is cut in twain near the middle by a tremendous gorge, which carries the
East Fork of the Sevier River, which drains the plateaus to the eastward
and southward.

The Paunsdgunt Plateau is a flat-topped mass, projecting southward
in the continuation of the long axis of the Sevier Plateau, bounded on
three sides by lofty battlements of marvelous sculpture and glowing color.
Its terminus looks over line after line of cliffs to the southward and down
to the forlorn wastes of that strange desert which constitutes the district of
the Kaibabs and the drainage system of the Grand Cation of the Colorado
River.

Aye: INTRODUCTORY.

Between the second and third range of plateaus is a second valley
parallel to that of the Sevier. This is called Grass Valley. It is long
and rather narrow, walled upon the west by the long barrier of the Sevier
Plateau and upon the east by the battlements of the third chain. It is
treeless yet not wholly barren, for it is situated at that altitude where the
possibility of agriculture is extremely doubtful, and where the grasses are
rich enough for profitable pasturage. It carries the drainage of portions of
both the second and third chains of plateaus, and the streams uniting from
north and south near the southern end of the valley burst through the
profound gorge of East Fork Canon in the Sevier Plateau and join the
Sevier River. |

The third range of plateaus begins much farther north than the others.
The northernmost member of it is the Wasatch Plateau, which overlaps the
southern end of the Wasatch Mountain Range en échelon to the eastward.
It is a noble structure, nearly as lofty as the summits of the Wasatch Mount-
ains, but is a true plateau, or rather the remnant of one left by the erosion
of the country tothe east of it. It has not been studied as yet with the care
and thoroughness it deserves, because it lies too far from the more compact
district to the southward; is, in a certain sense, an outlier of the main
group. Its southern terminus is walled by great cliffs, which look down
upon a broad depression separating it from the next member of the range.

This next member to the south is the Fish Lake Plateau. It is small
in area, but one of the loftiest (11,400 feet), and is a true table Its length
does not exceed 15 miles, while its breadth is about 4 or 5. Its southeast-
ern escarpment looks down into a profound depression nearly filled by a
beautiful lake about 6 miles long and rarely picturesque. This plateau is
difficult to separate from the next member, the Awapa. Indeed, it is nearly
confluent with it. The Awapa is of less altitude, and this constitutes the
principal reason for separating it. This plateau feebly slopes to the east-
ward, somewhat after the manner of the half of a watch-glass. Its extent
is very great, being 30 miles in length and nearly 20 in breadth. It is
quite treeless, though it stands at an altitude where timber usually flour-
ishes luxuriantly; and the scarcity of water combines with the monotonous

THE THREE GEOLOGICAL PROVINCES. 5

rolling prairie of its broad expanse to make it as cheerless and repulsive a
locality as can well be conceived.

But south of the Awapa stands the grandest of all the High Plateaus,
the Aquarius. It is about 35 miles in length, with a very variable width.
and its altitude is about 11,600 feet. Its broad summit is clad with dense
forests of spruces, opening in grassy parks, and sprinkled with scores of
lakes filled by the melting snows. On three sides

south, west, and east—it
is walled by dark battlements of volcanic rock, and its long slopes beneath
descend into the dismal desert in the heart of the ‘‘PLarrau Country.”

THE THREE GEOLOGICAL PROVINCES.

For convenience of geological discussion, Professor Powell has divided
that belt of country which lies between Denver City and the Pacific and
between the 34th and the 43d parallels into provinces, each of which, so far
as known, possesses structural and topographical features which distinguish
it from the others.* The easternmost division he has named the Park Prov-
ince. It ts characterized by lofty mountain ranges, consisting of granitoid
and metamorphic rocks, pushed upward and protruded through sedimentary
strata, the latter being turned upwards upon the flanks of the ranges and
their edges truncated by erosion. ‘The general transverse section presented
by these ranges, on the assumption that the sedimentaries prior to uplifting
extended over their present loci,t is that of a broad and extensive anticlinal
sometimes profoundly faulted parallel to the trend, the sedimentary strata
which may once have existed being removed by erosion. The intervening
valleys still retain the sedimentary series, including the Tertiary beds.
This form of mountain structure, with its resulting topographical features,
gradually passes as we proceed westward into another type, arising from the
decreasing frequency of the greater displacements or differential vertical
movements of the earth’s surface; but such movements as have occurred
have been vast in extent and involve greater masses, though the displace-
ments have been fewer in number. Great blocks of country have been

lifted with a singular uniformity with comparatively little flexing and with

* Geology of the Uinta Mountains. J. W. Powell.
t This assumption may be regarded as generally true for Palaozoric and Mesozoic beds, but not for
Cenozoic.

6 INTRODUCTORY.

little disarrangement, except at the fault planes which bound the several
blocks. These divisional lines are sometimes sharp, trenchant faults, some-
times that peculiar form of displacement to which Messrs. Powell and Gil-
bert have given the name of monoclinal flexures,* but most frequently the
dislocation is a combined monoclinal flexure and a fault or series of faults
with all shades of relative emphasis. If we look solely at the amount of
energy displayed in the vertical differential movements, we shall probably
reach the conviction that it does not fall much, if any, below that required
to build the most imposing mountain ranges; yet within the limits of any
one of the great blocks into which this country has been divided the strata
have preserved their original attitudes with a singularly small amount of
warping, flexing, and comminution. Sometimes the blocks are slightly
tilted, causing a slight dip, and in the immediate neighborhood of a great
dislocation a single flexure of the beds is usually seen; but, on the whole,
the amount of bending and undulation is very small. This small amount
of departure from horizontality of the beds as they now lie has played its
part in the determination of the topographical features as they appear in
the landscape, and justifies the name which has been applied to it with one
accord by all observers—Tae Puiareau Country.

the Great Basin. Its topog-

West of this province lies a third one
raphy and structure are characterized by jagged ranges of mountains,
ordinarily of very moderate length, and separated by wide intervals of
barren plains. These ranges are usually monoclinal ridges produced by
the uptilting of the strata along one side of a fault. Sometimes the faults
are multiple; that is, consist of a series of parallel faults, the intervening
blocks being careened in the same manner and direction. This repetitive
faulting is of frequent occurrence. Other modifications, and even different
types of structure, are presented; but there is throughout the Great Basin a
striking predominance of monoclinal ridges, in which one side of a range
slopes with the dip of the strata, while the other slopes lie across the
upturned edges. The forms impressed upon these masses by erosion are
rugged, bristling, and sierra-like, and their peculiarities are aggravated by

*Mr. Jukes describes a great flexure of similar nature in Ireland under the name uniclinal flexure,
which name is evidently defective in etymology. The nature of monoclinal flexures is most ably dis-
cussed by Professor Powell in Expl. of Colorado River, 1569-1572.

BORDER LINES OF PROVINCES. of

the fact that before these ‘‘ mountains were brought forth” the platform
of the country from which they arose had been plicated, and the plications
planed down again by erosion. The Basin area is the oldest of the West,*
its final emergence being of older date than the Jurassic, and most probably
as ancient as the close of the Carboniferous.

Between the Plateau and Park Provinces there is no definite boundary.
Gradually as we proceed westward from the easternmost ranges of the Rocky
system the valleys widen out, and the country gradually expands into a
medley of terraces bounded by lofty cliffs, which stretch their tortuous
courses across the land in every direction, yet not without system.. The
boundary separating the Plateau Province from the Basin is, on the contrary,
tolerably definite, and in some portions of its extent remarkably so. It
lies along the eastern flank of the Wasatch, south of the Uintas, as far as
Nebo; thence along the Juab Valley, in the Pavant Range, as far as the
Tushar Mountains. Here for a time it is concealed by immense floods of
old lavas, and is not seen for a distance of 50 miles. It reappears near the
southern end of that range, continuing south-southwest along the western
base of the Markégunt Plateau, near a string of Mormon settlements scat-
tered along the route from Beaver to Saint George, and follows the great
fault which makes the Hurricane Ledge to the Arizona boundary. Here
an offset carries it to the westward to another fault which walls the Grand
Wash, and it then extends southward to the mouth of the Grand Canon
of the Colorado and crosses the river. Here is the maximum westing of
the Plateau Province. <A few miles south of the crossing it swings back
to the southeastward, and continues beyond the explorations of this sur-
vey. This boundary is frequently very sharp and distinct, and throughout
the greater portion of its extent the breadth of the doubtful or transitional
zone lies wholly within the limits of a narrow valley or a narrow moun-
tain range. The Pavant is a range of which the eastern side presents
conspicuously the features of the Plateau type, while the western side pre-
sents those of the Basin type The Tushar Range shows a distinct plateau
form in its southern half, while the northern half is masked by floods of vol-
canic rock. From Toquerville to Parowan the Markdigunt Plateau faces

*I refer only to large areas. There may be, and probably are, small areas of equal or greater
antiquity.

8 INTRODUCTORY.

the westward, looking across a valley floored with recent alluvium to typi-
cal Basin Ranges lying to the westward. The district of the High Plateaus
is therefore a portion of the western belt of the Plateau Province, and its
western boundary is the trenchant one just described.

THE PLATEAU PROVINCE AT LARGE.

‘To the eastward of the High Plateaus is spread out a wonderful region.
Standing upon the eastern verge of any one of these lofty tables where
the altitudes usually exceed 11,000 feet, the eye ranges over a vast expanse
of nearly level terraces, bounded by cliffs of strange aspect, which are truly
marvelous, whether we consider their magnitude, their seemingly intermi-
nable length, their great number, or their singular sculpture. They wind
about in all directions, here throwing out a great promontory, there reced-
ing in a deep bay, but continuing on and on until they sink below the
horizon, or swing behind some loftier mass, or fade out in the distant haze.
Each cliff marks the boundary of a geographical terrace sloping gently
backward from its crestline to the foot of the next terrace behind it, and
each marks a higher and higher horizon in the geological scale as we
approach its face. Very wonderful at times is the sculpture of these
majestic walls. Panels, pilasters, niches, alcoves, and buttresses, needing
not the slightest assistance from the imagination to point the resemblance ;
grotesque forms, neatly carved out of solid rock, which pique the imagina-
tion to find analogies; endless repetitions of meaningless shapes fretting the
entablatures are presented to us on every side, and fill us with wonder as
we pass. But of all the characters of this unparalleled scenery, that which
appeals most strongly to the eye is the color. The gentle tints of an east-
ern landscape, the rich blue of distant mountains, the green of vernal and
summer vegetation, the subdued colors of hillside and meadow, all are
wanting here, and in their place we behold belts of fierce staring red,
yellow, and toned white, which are intensified rather than alleviated by
alternating belts of dark iron gray. The Plateau country is also the land
of canons. Gorges, ravines, canadas are found in every high country, but
canons belong to the region of the Plateaus. Like every other river, the

Colorado has many tributaries, and in former times had many more than

RELATIONS OF HIGH PLATEAUS TO PLATEAU PROVINCE. 9

now, and every branch and every twig of a stream runs in canons. The
land is thoroughly dissected by them, and in many large tracts so intricate
is the labyrinth and so inaccessible are their walls, that to cross such regions
except in specified ways is a feat reserved exclusively to creatures endowed
with wings. The region at levels below 7,000 feet is a desert. A few
miserable streams meander through it in profound abysses. The surface
springs will not average one in a thousand square miles, for the canons in
their lowest depths absorb the subterranean water-courses. But in the
High Plateaus above we find a moist climate with an exuberant vegetation

and many sparkling streams.
RELATIONS OF THE HIGH PLATEAUS TO THE PLATEAU PROVINCE AT LARGE.

It is impossible to gain any adequate conception of the broader and
more general features of the High Plateaus apart from their relations to the
. Plateau Province at large. The geological history of the district is insepara-
ble from that of the province of which it is a part, and that history is full of
interest and instruction. Beyond Cretaceous time it is unfortunately vague
and uncertain at present; and even during the Cretaceous our knowledge is
limited as yet to a few salient facts too conspicuous to be overlooked, but
of very great geological importance. We now know that during Cretaceous
time the ocean stretched from the Wasatch to Eastern Kansas, Nebraska,
and Dakota, and from the Gulf of Mexico far northwards toward the Arctic
Circle. The area now occupied by the Great Basin was then a large island,
or possibly a portion of some unknown continental mass. East of it proba-
bly lay numerous islands. -Around the southern border of this area the
Cretaceous ocean joined the Pacific, covering the entire extent of the Plateau
Province and more to the southwestward. We find throughout the plateaus
vast bodies of Cretaceous stata which seem in a geueral way or collectively
to correspond with those which have been studied and described by Meek
and Hayden in the Great Plains of Nebraska, Dakota, Montana, Wyoming,
and Colorado, and by Newberry in New Mexico and Arizona. Although
the subdivisions of the Plateau Province have not been wholly correlated
with the marine Cretaceous of the other territories north and east, there
can be little doubt that the series as a whole agrees in general. The lower

10 INTRODUCTORY.

member (Dakota group) can probably be correlated very approximately,
although presenting a somewhat different fauna; but the upper members
(2, 3, 4, and 5 of Meek and Hayden) cannot be so satisfactorily distin-
guished nor subdivided in the same way as elsewhere, though it seems
probable, in a high degree, that all these members are represented. The
lithological characters show the same agreement, though not an observed
correspondence of details. In one respect, however, there is a notable
distinction. The entire Cretaceous series of the Plateau Province abounds
in coal and carbonaceous shales, while in the more eastern exposures coal
appears to be confined to the higher members.

CLOSE OF THE CRETACEOUS—UNCONFORMITIES.

The closing period of the Cretaceous marks a change in the physical
condition of the region. The ocean gave place to brackish waters. What
orographic movements or what uplifts of broad areas may have accom-
plished this change we do not know in detail, and it is at present impossible
to form any very definite idea of the geography of the region during that
period. We only know that the uppermost Cretaceous strata have hitherto
furnished only brackish-water fossils, and we naturally infer from them that
the Cretaceous ocean was subdived into a number of Baltics or Euxines by
the rearing of mountain chains and broad land areas around their borders,
but leaving narrow straits communicating with the sea. The brackish-
water fossils either mean that or they are at present inexplicable. These
movements, however, involved no other changes in the physical condition
of the country, for the deposit of shaly, marly, and arenaceous strata with
seams of lignite went on as before, and continued through a long period
until the accumulations reached in many places a thickness of nearly 2,000
feet without any interruption which can be specified. These Upper Creta-
ceous beds are without much doubt the equivalents of the Judith River
beds of Meek and Hayden and the Laramie beds of King.

The continuity of deposition was at last broken. Resting upon these
Laramie beds is a series of calcareous shales alternating with sandstones,
which, through a thickness of 100 to 250 feet from the base, contain also a

brackish-water fauna, but which as we ascend gives places to molluscan

UNCONFORMITY OF CRETACEOUS AND TERTIARY. 11

fossils of purely fresh-water types. The junction of the two series is uncon-
formable, and is often highly so. This unconformity is seen in many
localities on both sides of the Uintas, along the eastern slopes of the
Wasatch, and becomes even more strongly pronounced to the southwest-
ward. During the course of this work, localities will be mentioned where
it is conspicuously displayed, the Upper Cretaceous (Laramie) beds being
flexed at a high angle, the flexures planed off by erosion, and the overlying
series resting across the beveled edges, or even upon the Jurassic beds
below. It was at this unconformity that Professor Powell drew the divid-
ing horizon between the Tertiary and Cretaceous. Quite independently of
any physical break, Professor Meek had chosen the division at the same
horizon upon the evidence of invertebrate fossils, though that evidence was
regarded by him as being too meager and the species too few and indecisive
to justify an unqualified opinion.* Professor Marsh also reached a similar
conclusion much more decisively from mammalian fossils from beds just
above the unconformity which he referred approximately to the horizon of
the London clay or the base of the Kocene.t The physical break which
separates these divisions of time is of wider distribution and more emphatic
than was supposed when first detected, for the Upper Cretaceous (=Lara-
mie) beds are often greatly flexed and eroded beneath the Tertiary, and
these occurrences are frequent throughout the province. Very often, and
probably in most of the exposures distant from the mountains, the contact is
apparently conformable, for the obvious reason that neither series has been
sensibly disturbed from original horizontality, or the disturbances have
been of late occurrence, involving both series alike. The separation in
such cases then becomes a purely lithological one, or sometimes none can
be detected. The fossils do not indicate any break, since the base of the
Tertiary and the summit of the Cretaceous are lignitic, and furnish only
brackish-water mollusca, which are indecisive and have a very great vert-
ical range in nearly all the species.

* Invertebrate Paleontology (1876), Dr. F. V. Haydey’s Survey, pp. xlvii et seq.
tExpl. 40th Parallel, C. King, vol. ii, p. 329.

12 INTRODUCTORY.

THE EOCENE OR LACUSTRINE AGE.

The early Tertiary history of the Plateau Province is much clearer
than its history during prior epochs. The shore of the great Eocene lake
which covered its expanse and received its sediments can be defined with
tolerable accuracy throughout those portions of it which lay within the
area constituting the field of this survey. Its northern and the greater part
of its western shore line has been traced from the Uintas to the Colorado,
and most of the way coincides with the boundary already described as
separating the Plateau Province from the Great Basin. South of the High
Plateaus, however, the Eocene lacustrine beds stretch westward beyond this
boundary, and are found among the southern Basin Ranges. We know,
too, the origin of a large portion of its sediment. Much of it came from the
Great Basin, and probably still more from the degradation of the Wasatch,
the Uintas, and the mountains of Western Colorado, which girt about its
northern half. The southern shore line is not at present known, and there
is much uncertainty at present as to the exact course of its southeastern
coast. From what is known, however, we may wonder at the vast dimen-
sions of such a lake, which must have had an area more than twice that of
Lake Superior, and may even have exceeded that of the five great Canadian
lakes combined. Still more astonishing is the vastness of the mass of strata
thrown down upon its bottom. Around the flanks of the Uintas and South-
ern Wasatch the thickness of the Eocene beds exceeds 5,000 feet, though
they attenuate as we recede from the mountains, but never fall below 2,000
feet so far as yet observed. And where this minimum is observed there is
good evidence that the deposition had terminated long before it ceased
elsewhere, and that the series was never completed.

The deposition ended in the southern and southwestern part of the
lake area much earlier than in the northern part. Around the southern
portions of the High Plateaus no later beds than the Bitter Creek (which
constitute the lower one-third of the local Eocene) were deposited so far
as known at present. ‘The inference is that about that time the southern
and southwestern portions of the lake began to dry up, while to the north-

ward around the Uintas the lacustrine condition persisted for a much longer

°

SUBSIDENCES. iB

period. In other words, the lake contracted its area from south to north
during at least the latter half of the Eocene, and at the close of that age
finally disappeared.

SUBSIDENCE OF CRETACEOUS-EOCENE SEDIMENTS.

A most interesting but perplexing problem is suggested when we con-
sider the enormous bulk of the Cretaceous-Kocene strata of the Plateau
Province and the peculiar circumstances under which they were deposited.
The whole series abounds in coal and carbonaceous shales, and remains of
land plants are abundant, even where carbonaceous matter is absent. If
current theories of the formation of coal are not radically wrong, we seem
compelled to believe that throughout that vast stretch of time which
extended from the base of the Cretaceous to the summit of the Eocene the
whole province, with the exception of afew possible but unknown land areas,
maintained its level almost even with that of the ocean. ‘The Dakota sand-
stone could not have been deposited here much if any below that level,
nor the Wasatch beds much if any above it. And yet we have the paradox
that 6,000 to 15,000 feet of strata were deposited over an area of more than
100,000 square miles with comparatively few unconformities and contem-
porary disturbances, while the level of the uppermost stratum always
remained at sensibly the same geographical horizon!

It is incredible that the Cretaceous ocean at the commencement of
that age could have had a depth equal to the thickness of the strata and
that the sediments filled it up. The facts are wholly against such a sup-
position, and_ point clearly to shallow waters. The only conclusion which
appears tenable is that the strata sank as rapidly as they were deposited.
The case is analogous to that-of the Appalachians during Paleozoic time,
and especially during the Carboniferous; and the more we reflect upon the
similarity the stronger does it become. It fails, however, when we come
to consider the phenomena presented in the two regions in the period sub-
sequent to the deposition; the Appalachian strata were flexed and plicated
to an extreme degree, while those of the west are for the most part calm
and even. Only in the vicinity of the mountains and shore lines do we find

them much disturbed.

14 INTRODUCTORY.

But if we are to admit that the strata sank as rapidly as they accu-
mulated we cannot shake off some ulterior questions. By virtue of what
condition of the underlying magmas was such a subsidence possible? If
they sank, they must have displaced matter beneath them, and what became
of the displaced matter? If we look around the borders of the area and
partially within it, we shall find a problem of an inverse order. The Uintas,
the Wasatch, the Great Basin have suffered an amount of degradation by
erosion, which is perhaps one of the most impressive facts which the physi-
cal geologist has yet been brought to contemplate. From the Uintas more
than 30,000 feet of strata have been removed since their emergence. From
the Wasatch the removal has been much more; from the Great Basin the
degradation has been many, we know not how many, thousand feet. We
are not prepared to believe that the Uintas ever stood 8 miles high, nor
the Wasatch 12 miles high, but we know that their altitudes are merely
the difference between elevation and erosion. It was from these ranges
that the heaviest masses of the Cretaceous-Hocene sediments were derived.
As fast as, or even faster than, the mountains were devastated to supply
mass for the new strata, they continued to rise. But if they rose, fresh matter
must have been thrust under their foundations, replacing the rising strata.
Whence came the replacing matter? It may be premature as yet to say
that the elevation of the mountains and subsidence of the strata are cor-
related in the way which these inquiries suggest, but the juxtaposition of

the facts must be regarded as significant.
POST-EOCENE HISTORY—EROSION.

With the desiccation of the Eocene lake began a new order of. events
in the history of the Plateau Country; in truth, its most instructive and
impressive chapter. The lessons which may be learned from this region
are many, but the grandest lesson which it teaches is Erosroy. It is one
which is taught, indeed, by every land on earth, but nowhere so clearly as
here. If we could but find the evidence, we might be able in other regions
to point to erosions of much greater amount. We may suspect that in the
Appalachians a denudation has occurred compared with which the denuda-
tion of the Plateaus is small; and such an inference has no intrinsic

EROSION. 115)

improbability, though the proofs are difficult beyond a certain amount.
The great value of the Plateau Country is the certainty and fullness of the
evidence. Nature here is more easily read than elsewhere. She seems at
times amid those solitudes to have lifted from her countenance the veil of
mystery which she habitually wears among the haunts of men. Elsewhere
an enormous complexity renders the process difficult to study; here it is
analyzed for us. The different factors are presented to us in such a way
that we may pick out one in one place, another in another place, and study
the effect of a single variable, while the other factors remain constant. The
land is stripped of its normal clothing; its cliffs and canons have dissected
it and laid open its tissues and framework, and ‘he who runs may read” if
his eyes have been duly opened. As Dr. Newberry most forcibly remarks:
“Though valueless to the agriculturist, dreaded and shunned by the emi-
grant, the miner, and even the adventurous trapper, the Colorado Plateau
is to the geologist a paradise. Nowhere on the earth’s surface, so far as we
know, are the secrets of its structure so fully revealed as here.”

In the new era, beginning with the desiccation of the lake, we have
the history of a process which resulted in the destruction and dissipation
of those great bodies of sediment which had been gathered and stratified
during Mesozoic and Eocene time. ‘Then, too, appears to have begun in
earnest the gradual elevation of the entire region which has proceeded from
that epoch until the present time, and which even yet may not have cul-
minated. The two processes of uplifting and erosion are here inseparably
connected, so much so, that we cannot comprehend the one without keeping
constantly in view the other.

From the very inception of the process the drainage system of the
Plateau Province has been that plexus of streams which unite in the Colo-
rado River. This is the trough through which the waste of the land has
been carried to the Pacific. Its origin goes back to the emergence of the
land now drained by it from its lacustrine condition. Even prior to that
we may conjecture the existence of a Cretaceous-Eocene strait connecting
with the ocean that area which was covered by the Laramie beds and the
brackish water deposits at the base of the local Eocene ; and many consid-
erations lead to the inference that this Hellespont occupied the same position

16 INTRODUCTORY.

as the lower course of the Colorado from the mouth of the Virgin to the
Pacific. Whether the connection was at first elsewhere and at an early
epoch in Tertiary time shifted to this place may be doubtful, but the prob-
abilities at present are. that the connection was southwestward along the
lower course of the present river. But after the desiccation of the lake
began in the latter part of the Eocene, the course of the Colorado was fixed
for the remainder of Tertiary time. In order to conceive the growth and
evolution of this river, let us endeavor to imagine what might happen if the
whole region of the Canadian lakes were to be progressively uplifted sev-
eral thousand feet. In due time the St. Lawrence would sink its channel
by the increasing corrasive power of its waters, and would drain in succes-
sion Ontario, Erie, Huron, and Superior, becoming a great river with many
branches, while the lakes would be emptied. Such was the early history
of the Colorado ; first a Hellespont, then a St. Lawrence, then a large river
heading in the interior of a continent.

The relations of the Colorado to the strata through which it runs present
certain phenomena which, when rightly understood, become a master-key
in the solution of a whole category of problems of a most interesting and
instructive character. It would be difficult to point out an instance of a
river under conditions more favorable to stability in respect to the location
of its course than the Colorado and its principal tributaries. Since the
epoch when it commenced to flow it has been situated in a rising area. Its
springs and rills have been among the mountains, and throughout its history
its slope has been increasing. The relations of its tributaries in this respect
have been the same, and indeed the river and its tributaries constitute a
system and not merely an aggregate, the latter dependent upon and thor-
oughly responsive to the former. Now, the grand truth which meets us
everywhere in the Plateau Country, which stands out conspicuous and self-
evident, which is so utterly unmistakable, even by the merest tyro in geol-
ogy, is this: The river is older than the structural features of the country.
Since it began to run, mountains and plateaus have risen across its track
and those of its tributaries, and the present summits mark less than half the

total uplifts. The streams have cleft them to their foundations. Nothing

DRAINAGE SYSTEM—MIOCENE EROSION. 7

can be clearer than the fact that the structural deformations (unless older
than Tertiary time) never determined the present courses of the drainage.
The rivers are where they are in spite of faults, flexures, and swells, in
spite of mountains and plateaus. As these irregularities rose up the streams
turned neither to the right nor to the left, but cut their way through in the
same old places. It is needless to multiply instances. The whole province
is a vast category of instances of river channels running where they never
could have run if the structural features had in any manner influenced them.
What, then, determined the present distribution of the drainage? The
answer is that they were determined by the configuration of the old Eocene
lake bottom at the time it was drained. Then, surely, the water-courses ran
in conformity with the surface of the uppermost Tertiary stratum. Soon
afterward that surface began to be deformed by unequal displacement, but
the rivers had fastened themselves to their places and refused to be diverted.
Many of the smaller streams have dried up and perished through the fail-
ure of their springs and the advent of an arid climate. These have left
traces here and there in the shape of dry canons and gulches. Many more
are still perishing. But the larger streams heading far up in moist Alpine
highlands still meander through the desert, and have never ceased to flow
from the beginning.

In order to comprehend the relations of the High Plateaus to the
province at large, it is necessary to advert to some of the salient features
of the general erosion of the Plateau Country which followed the desicca-
tion of the great lake, and which continued without interruption during
Miocene time and down to the present day. Its history during Miocene time
must be spoken of only in general terms. In truth, during that great age
there is no evidence of the occurrence of any critical event aside from the
general processes of uplifting and erosion which affected the province as a
whole. What forms and what topography were sculptured we know not:
Of its climatal condition we can only suppose that it was similar to that of
neighboring regions similarly situated—moist and subtropical. The vast
erosion of the region has swept away so much of its mass, that most of the
evidence as to details has vanished with its rocks. But the more important

features of the work, its general plan in outline, have left well-marked
2) 3st 1D

18 INTRODUCTORY.

traces, and these can be unraveled. It was a period of slow uplifting, reach-
ing a great amount in the aggregate; and it was also a period of stupendous
erosion. The uplifting was, however, unequal. The comparatively even
floor of the old lake was deformed by broad gentle swells rising a little
higher than the general platform. In consequence of their greater altitudes,
these upswellings at once became objects of special attack by the denuding
agents, and were wasted more rapidly than the lower regions around them.
Here were formed centres, or short axes, from which erosion proceeded
radially outward, and the strata rising very gently toward these centres, or
axes, from all directions, were bevelled off. As erosion progressed, so also
did the local upliftings, thus maintaining the maximum erosion at the same
localities.

It is a most significant fact that the brunt of erosion throughout
the Plateau Country is directed against the edges of the strata and not
against the surfaces. This is directly traceable to the fact that the strata
are nearly horizontal, the dips rarely exceeding four or five degrees, and
even then only where a great monoclinal flexure occurs. The rains wash
and disintegrate most rapidly where the slopes are steepest, and where the
strata are flat the steepest slopes are the valley sides and chasm walls.
Thus the battering of time is here directed against the scarps and falls but
lightly on the terrepleins.

Ordinarily, the local uplifts have one diameter longer than the others,
and we may call the greatest the major axis. The strata dissolved away
in all directions from this axis, and after the lapse of long periods the
newest or uppermost stratum encircled the centre of erosion at a great dis-
tance from it, the next group below encircled it a little nearer, and so on.
This has been the history of each of the subdivisions of the central part of
the Plateau Country. Upon the western and northern sides of the Colorado
five of these centres are now easily discerned. By far the largest and
probably the oldest is around the Grand Canon; a second lies east of the
Kaiparowits Plateau; a third is found about 50 miles south-southwest of
the junction of the Grand and Green; the fourth is the Henry Mountains,
and the fifth is what is known as the San Rafael Swell, lying between the

SAN RAFAEL SWELL 19

Green River and the Wasatch Plateau. All these had their inception in
Miocene time except the one around the Grand Canon, which goes back
into the latter part of the Eocene. This gradual dissolution of the strata
by the waste of their edges constitutes what Powell has called the Recession
of Cliffs.

Of these five centres of maximum erosion, the San Rafael Swell is by
far the best suited for study, and may be regarded as the type of them all.
If we stand upon the eastern verge of the Wasatch Plateau and look east-
ward, we shall behold one of those strange spectacles which are seen only
in the Plateau Province, and which have a peculiar kind of impressiveness,
and even of sublimity. From an altitude of more than 11,000 feet the eye
can sweep a semicircle with a radius of more than 70 miles. It is not the
wonder inspired by great mountains, for only two or three peaks of the
Henry Mountains are well in view; and these, with their noble Alpine
forms, seem as strangely out of place as Westminster Abbey would be
among the ruins of Thebes. Nor is it the broad expanse of cheerful plains
stretching their mottled surfaces beyond the visible horizon. It is a pic-
ture of desolation and decay; of a land dead and rotten, with dissolution
apparent all over its face. It consists of a series of terraces, all inclining
upwards to the east, cut by a labyrinth of deep narrow gorges, and
sprinkled with numberless buttes of strange form and sculpture. We stand
upon the Lower Tertiary, and right beneath our feet is a precipice leaping
down across the edges of the level strata upon a terrace 1,200 feet below.
The cliff on which we stand stretches far northward into the hazy distance,
gradually swinging eastward, and then southward far beyond the reach of
vision and below the horizon. It describes, as we well know, a rude semi-
circle around a centre more than 40 miles to the eastward. At the foot of
this cliff is a terrace about 6 miles wide of Upper Cretaceous beds, inclining
upwards towards the east very slightly, and at that distance it is cut off by
a second cliff, plunging down 1,800 feet upon Middle Cretaceous beds.
This second cliff describes a smaller semicircle like the first and concentric
with it. From its foot the strata again rise gently towards the east through
a distance of 10 miles, and are cut off by a third series of cliffs as before.

20 INTRODUCTORY.

There are five of these concentric lines of cliffs. In the centre there is an
elliptical area about £0 miles long and 12 to 20 broad, its major axis lying
north and south, and as completely girt about by rocky walls as the valley
of Rasselas. It has received the name of the San Rafael Swell. Its floor
is covered with the lowest Triassic strata, and probably in some portions
of it the Carboniferous is laid bare, though it has not yet been seem. But,
at all events, we know that the Carboniferous is very thinly covered, even
if it be not exposed.

Thus, as we pass from the summit of the Wasatch Plateau to the floor
of the Red Amphitheatre, we cross the outcrops of nearly 10,000 feet of
strata. The Tertiary is found only at a distance of 40 miles from it. Yet
if we look back to Eocene time we shall find that the whole stratigraphic
series from the base of the Mesozoic to the summit of the Eocene covered
this amphitheatre. One after another, in orderly succession, the vast
stratigraphic members have been stripped off, and the edges of the remain-
ing portions are seen in the successive cliffs which bound the encircling
terraces.

Still more vast has been the erosion which took place in the vicinity
of the Grand Canon of the Colorado. Here the Carboniferous now forms
the floor of the country, though a few patches of Trias still remain in the
vicinity of the river. But the main body of Triassic rocks now stands 50
miles north of the river, and beyond them, in a series of terraces, rise the
Jura, the Cretaceous, and the Tertiary, the latter usually capped by great
masses of volcanic rock.

We may note here another question which presents itself in connection
with the differential movements among the various parts of the province.
Those areas which have been uplifted most have suffered the greatest
amount of denudation. Is it not possible in some cases and under certain
restrictions to invert this statement and say that those regions which have
been most denuded have been most uplifted, thereby assuming the removal
of the strata as a cause and the uplifting as the effect? May not the
removal of such a mighty load as 6,000 to 10,000 feet of strata from an area

of 10,000 square miles have disturbed the earth’s equilibrium of figure, and

EFFECTS OF GREAT DENUDATIONS. 21

the earth, behaving as a quasi-plastic body, have reasserted its equilibrium
of figure by making good a great part of the loss by drawing upon its
whole mass beneath? Few geologists question that great masses of sedi-
mentary deposits displace the earth beneath them and subside. Surely
the inverse aspect of the problem is a priori equally palpable. That some
such process as this has operated in the Plateau Country looks at least
plausible, and if there could be found independent reasons for believing in
its adequacy the facts certainly bear it out. Yet its application is not
without some difficulties, and the explanation is not quite complete. Grant-
ing the principle, it will still be difficult to explain how these local uplifts
were inaugurated, and we can only refer them to the operations of that
mysterious plutonic force which seems to have been always at work, and
the operations of which constitute the darkest and most momentous problem
of dynamical geology. On the whole, it seems to me that we are almost
driven to appeal to this mysterious agency to at least inaugurate and in part
to perpetuate the upward movement, but that we must also recognize the
co-operation of that tendency which indubitably exists within the earth to
maintain the statical equilibrium of its levels The only question is whether
that tendency is merely potential or becomes in part kinetic, and this again
turns upon the rigidity of the earth. But it is easy to believe that where
the masses involved are so vast as those which have been stripped from the
Kaibabs and from the San Rafael Swell, the rigidity of the earth may be-
come a vanishing quantity.

The great erosion of the Plateau Province was most probably accom-
plished mainly in Miocene time, but continued with diminishing rapidity
throughout the Pliocene. But it is necessary to say that the terms Mio-
cene and Pliocene have here no definition. They cannot be correlated
except in a very general manner with events occurring outside the province.
We have only a vast stretch of time, with an initial epoch near the close of
the local Eocene. The greater part of the denudation is assigned to the Mio-
cene, because the conditions appear to have been more favorable to a rapid
rate of destruction in that age than subsequently. The climate appears to
have been humid, while the elevation was at the same time gradually increas-

ing, both conditions being favorable to a rapid disintegration and removal

2D, INTRODUCTORY.

of the rocks. The Pliocene witnessed the gradual development of an arid
climate similar to that now prevailing there. To this age belong the canons
and the great cliffs, which could not have been produced in an ordinary or
humid climate, nor at low altitudes. That this aridity is by no means a
condition of recent establishment is indicated by many evidences. They
consist of remnants of a former topography, preserved in a few localities
from the general wreck of the land, and which show the same general facies
of cliffs and canons as those of more recent formation. And as the more
recent sculpture owes its peculiarities in great part to the aridity, so, we
conclude, must these more ancient remnants The Kaiparowits Plateau
presents an excellent example. Its surface is in many places rendered
utterly impassable by a plexus of sharp narrow canons, of which the heads
have been cut off by the recession of the gigantic cliff which forms the
eastern wall of the plateau. They have long been dug, and have remained
with but little change for an immense period of time.

And now the relation of the High Plateaus to the Plateau Province at
large becomes evident They are the remnants of great masses of Tertiary
and Cretaceous strata left by the immense denudation of the Plateau Prov-
ince to the south and east. From the central part of the province the
Tertiary beds have been wholly removed and nearly all of the Upper
Cretaceous. A few remnants of the Lower Cretaceous stretch far out into
the desert, and one long narrow causeway, the Kaiparowits Plateau, extends
from the southeastern angle of the district of the High Plateaus far into
the Central Province and almost joins the great Cretaceous mesas of North-
eastern Arizona, being severed from them only by the Glen Canon of the
Colorado. The Jurassic has also been enormously eroded. This forma-
tion, which iv »f great importance and bulk in the northern and north-
western portion of the province, and especially around the High Plateaus,
appears to have thinned out towards the south and southeast. In large
portions of New Mexico it is wholly wanting and was probably never
deposited there. In the northwestern portion of that Territory only a few
thin beds of that age are found. But in the northern part of the province
a conspicuous and wonderful sandstone formation of most persistent char-

acter is found, overlaid and underlaid by shales holding a distinctly Jurassie¢

FINAL RESULTS OF EROSION AND UPLIFTING. 23

fauna. This formation once extended over the Grand Cation area prob-
ably as far south as the river itself, and possibly farther, but has all been
swept away as far north as the southern end of the district of High
Plateaus. From the region east of the High Plateaus also very large areas
of it have been removed. The Upper Trias has also been greatly denuded,
and the Lower Trias nearly as much so. The erosion of the Carboniferous
has been small, being confined chiefly to the cutting of canons—most
notably the Grand and Marble Canons, which are sunk wholly in that
series, and in several places have been cut through the entire Paleozoic
series system.

The average denudation of the Plateau Province since the closing
periods of the local Eocene can be approximately estimated, and cannot fall
much below 6,000 feet,* and may, nay, probably does, slightly exceed that
amount. Of course this amount varies enormously, being in some locali-
ties practically nothing and in others nearly or quite 12,(00 feet. It is a
minimum in the High Plateaus Within that district the average denuda-
tion will fall much below 1,000 feet in the sedimentary beds. The enor-
mous floods of volcanic emanations have protected them, and these have
borne the brunt of erosion, and their degradation has given rise to local
accumulations of sub-aerial conglomerates in all the valleys and plains sur-
rounding the volcanic areas, thus increasing the protection.

The general cause which has enabled these strata to survive within the
limits of the High Plateaus while they have been so terribly wasted else-
where may be stated briefly. Until near the close of the Pliocene the High
Plateaus were not only the theatre of an extended vulcanism, but those
portions which never were sheeted over by lavas were low-lying areas,
where alluvial strata tended to accumulate. They remained, in fact, base
levels of erosion during the greater part of Tertiary time.

Turning now to the Great Basin, which lies even lower than the mean
level of the Plateau Country, we find that the pre-eminence of the High
Plateaus is due to a totally different cause. Here the difference of altitude
is due altogether to differences in the amounts of uplifting. Since the

*My own estimate exceeds by a few hundred feet that of Professor Powell and also that of Mr.
Gilbert. The latter places it at about 5,500 feet.

Q4 GEOLOGY OF THE HIGH PLATEAUS.

Eocene, the High Plateaus have risen from 10,000 to 12,000 feet, while the
adjoining Basin areas have risen from 5,000 to 6,000. As we pass from
the Basin eastward and ascend the High Plateaus we mount the long slopes
of great monoclinal flexures, or scale the giant cliffs which had their origin
in the long major faults which traverse the district from south to north.
As we pass westward from the heart of the Plateau Province and ascend
the High Plateaus, we ascend cliffs of erosion. The fact that those cliffs
which had their origin in displacement, with very rare exceptions, face west-
ward, has attracted much attention and has received various interpretations.
It seems to me that the explanation is exceedingly, almost amusingly,
simple. The country to the east of them, and also the belt of country
- which they occupy, has been elevated from 5,000 to 6,500 feet above the
country to the west of them. These figures express, of course, relative
vertical displacements. The passage from west to east across the belt of
country, which may be called the border-land between the two provinces,
discloses a succession of faults and monoclinal flexures which are the
obvious results of such a displacement.

CHAPTER II.

STRUCTURAL GEOLOGY.

Homology of faults and monoclinal flexures.—Their systematic arrangement.—Those of the High
Plateaus belong to the same system as those of the Kaibabs.—The Grand Wash fault.—Hurri-
ricane fault.—Tushar fault.—Toroweap fault.—Sevier fault.—Western and LEastern Kaibab
faults.—Thousand Lake fault.—Musinia faults.—Age of these displacements.—Their relative
recency.—Difficulty of assigning their periods in definite terms.—Argument of recency from
amounts of erosion.—Argument from the amounts of accumulation of valley deposits.—Age of the
faults with reference to evidences of glaciation.—Importance of knowing the ages of these faults.—
Some are more recent than others.—An older system of faults of Cretaceous-Eocene age.—Water-
Pocket flexure.—San Rafael flexure.—Parallelism of recent major faults to the old Cretaceous-
Eocene shore-line.—Evidences of recent uplifting in the cations.—Comparison of structural forms
in the three provinces, the Basin, the Plateaus, and the Parks.—Types of the Parks.—Effects of
erosion upon structure.—Absence of horizontal forces in the elevation of the Plateaus.

The great structural features of the High Plateaus are the faults and
monoclinal flexures. Faulting is an almost universal concomitant of great
disturbances of the strata and of the uplifting of mountains and plateaus.
Of their causes geology has taught us but little beyond the bare fact that
they are produced in the great majority of cases by differential uplifting
by vertical forces, which is hardly more than an identical proposition. The
nature of the forces we know not, and can only speculate vaguely about
them. We do not always know even whether a fault is produced by uplitt-
ing upon one side of a given vertical plane or by sinkage on the other, and
there must always be an implicit reservation when we speak of them as
produced by upliting, so that nothing more is meant than that the strata
have been sheared vertically, and that one portion is left on a higher plane
than the other. Why the vertical forces should undergo an abrupt change
or even total extinction in passing from one side of a given line to the other
is a mystery which we cannot hope to solve until we know the origin of
the force itself. Allthat is left us at present is to study the faults them-
selves carefully, ascertaining, as far as practicable, what movements have

25

26 GEOLOGY OF THE HIGH PLATEAUS.

really taken place, how they are related to each other, what dislocations
have been produced by them, and what are the present and what were
probably the former attitudes of the disturbed masses; and yet there are
very few subjects in the range of geology so difficult to study. It seems as
if Nature were ashamed of her scars, and resorted to numberless tricks and
devices to hide them from sight ; here smoothing over the break and deftly
hiding it with a mantle of soil; there confusing the inquisitive student by a
multiplicity. of perplexing forms, which are sure to worry if not to mislead
him; ana always shy of the truth. Throughout the greater part of the
Plateau Province, Nature is so poorly clad in the raiment of soil and vege-
tation and the earth is so well dissected by erosion that these features do
not easily escape the scrutiny of the determined and experienced investi-
gator. In the High Plateaus, however, the faults are less readily scruti-
nized than in some other parts of the province, though much more conspic-
uously displayed than in smoother and moister countries or than in countries
of more complicated structure. While I suspect that many minor faults
have escaped detection, I am confident that all of the grander ones have been
discovered and their principal features and relations unraveled.

All of the greater displacements of the district present certain well-
marked habitudes. Most important among them is the strict homology of
the faults with monoclinal flexures. In truth, so close is the homology, that
we are justified in calling a monoclinal in some of its aspects a modified
fault. The only difference for structural purposes is that in the case of a
typical fault of the simplest form the shearing is along one plane, while in
the monoclinal the shearing lies between two planes. We have also cumu-
lative or repetitive or “‘step-faults,” where the shearing is subdivided among
several planes. All have this in common, that the passage from the uplifted
to the lowest thrown side is through a very narrow zone, which has its width
reduced to zero in the case of the single or simple fault. All of the great
lines of displacement assume all of these modifications in different parts of
their extent. In one place the fault is simple. A few miles farther along
its course it may become subdivided into aseries of ‘“step-faults ;” still far-
ther on, into a perfect unbroken monoclinal; it may be at another locality

a faulted monoclinal—a part of the displacement being through flexing and

SYSTEM OF PLATEAU FAULTS. Part

apart through shearing. In any case the effect is in its broader aspects
the same. One side has been uplifted, the other side ‘ thrown.”

The true monoclinal in its perfect form is much more common in the
sedimentary than in the voleanic beds ‘The latter seem to lack that flexi-
bility or rather adaptability which enables strata to undergo differential
distortion without fracture. In the sedimentaries, on the other hand, the
monoclinal seems to be the favored form of displacement, though trenchant
faults are common enough. In the volcanics there is a tendency to the
monoclinal form, but the unyielding nature of the rocks has produced com-
minuted fracture in places where a monoclinal would doubtless have been
produced had the strata been more compliant. Hence the volcanics seldom
preserve the unbroken monoclinal, though there is one good example of this
preservation. This comminution is a source of perplexity in resolving the
displacement into its constituents, and frequently renders it necessary to
stay long and scrutinize abundantly before the extent of it and its true
method can be properly ascertained.

Another striking characteristic of these displacements is their sys-
tematic arrangement. Viewed in one way they approach parallelism, but
there is a noticeable convergence of the lines as we trace them from south
to north. In disturbed regions the faults and flexures usually tend to paral-
lelism, and while the tendency is as decided here as it is elsewhere, yet the con-
verging tendency is a noticeable characteristic. These great displacements
of the High Plateaus are the northward continuations of those which have
been described by Powell and Gilbert in the vicinity of, and crossing, the Col-
rado River at.the Grand Canon. But in the Grand Canon district (where
they gave origin to the Kaibabs) the belt of faulted country is wider and
the intervals between the faults and flexures are greater than in the High
Plateaus. This width diminishes northward, and several of the grander
faults at length become merged into one vast monoclinal flexure, forming
the western flank of the Wasatch Plateau. South of the Colorado these
faults have not been studied, but the indications now are that they also
converge in that direction, giving the greatest expansion to the system just
where the Colorado cuts across it. It is impossible to separate the faults of
the High Plateaus from their systematic association with those of the Kai-

28 GEOLOGY OF THE HIGH PLATEAUS.

babs, for the two districts have a common history, so far as relates to their
more recent structure. The individual faults overlap, and both districts
sympathized in the vertical movements. Indeed, the Hurricane and Eastern
Kaibab faults form structure lines of the first magnitude in both districts,
with no break in the continuity. The indications are unmistakable that the
upliftings of the Kaibabs and High Plateaus were sensibly synchronous
and formed one movement, and that any attempt to separate them would
be to ignore their proper relations.

The westernmost of the series is the Grand Wash fault. It crosses. the
Colorado at the lower end of the Grand Canon. Southward it curves
gradually in its trend, and at the farthest point to which it has been traced
its course is to the southeast. Northward from the river the curvature of
the trend is still preserved though much less distinct, and its course is
nearly due north. It runs out apparently about 35 miles from the river.
Its maximum displacement is about 5,500 feet, and the lifted side forms the
Sheavwits Plateau.

Next in order comes the Hurricane fault. Its southern terminus south
of the Colorado is unknown. It crosses the river just west of Mounts
Trumbull and Logan, forming the Hurricane Ledge, and its course is nearly
north, with a very slight swerving to the eastward. At the Grand Canon
its displacement is about 1,800 feet, and this amount is maintained with
little variation for about 40 miles north of the canon. Here its throw (to
the west) rapidly increases. It becomes the western boundary of the great
Markdgunt uplift—the southwesternmost of the High Plateaus, and is at
the same time the boundary which sharply separates the Plateau Province
from the Great Basin. Continuing on past the Mormon town, Cedar, and just
before reaching Parowan, it suddenly swings eastnortheast, making almost
a sharp angle. Thereafter it swings slowly back towards the north until it
reaches the western flank of the Tushar, where its throw has much dimin-
ished. The precise point where it runs out is not known, since it is covered
by basaltic eruptions, but it is not seen beyond the middle of the western
flank of the Tushar. Its maximum throw is near Cedar, on the western
flank of the Markigunt, where it reaches on an average, along 20 miles of

its course, a displacement of about 5,000 feet.

HURRICANE AND TUSHAR FAULTS. 29

From the Grand Canon northward for 40 miles it is a nearly simple
fault, though in some places it shows comminution of the rocks in the
vicinity of the fault plane, and in a few places the beds on the thrown side
are turned up. Along the southwestern base of the Markagunt the fracture
becomes very complicated. The upper beds have been eroded backward
from the fault plane on the lifted side of the fault, and the lower beds on that
side have in several places been turned up with a sharp flexure and stand
nearly vertical—in one instance have been turned past the vertical. This
movement seems to be exceptional, no other instance of the same kind
having been seen anywhere. It is difficult to understand by what applica-
tion of forces such a contortion could have been effected. The Carbonif-
erous has been brought up by it so as to abut against the Tertiary on the
thrown side of the fault, and right at the plane of shearing the displace-
ment of the lower beds seems to bé about 12,000 or 13,000 feet. But
away from the fault plane the beds quickly come back to their normal
position, with an uplift of about 4,000 feet. A few miles south of this point
another equally abnormal displacement occurs. A small branch of the
fault runs into the uplift and a huge block seems to have cracked off and
rolled over, the beds opening with a V, and forming a valley of grand
dimensions. About six miles north of the great upturn all trace of that
peculiar flexure has vanished and the beds are neatly sheared. The Hur-
ricane fault nowhere appears to take on the true monoclinal form. The
length of this great displacement is probably more than 200 miles.

The third great fault is that which lies at the eastern base of the Tu-
shar. Most of the faults have their throws to the west, but the throw of
the Tushar is to the east. It commences with two branches at the south-
eastern base of the range and the branches converge near the middle of its
eastern flank They are obscure and difficult to locate exactly on account
of their concealment by the alluvial débris, resulting from the waste of the
ancient lava beds and the somewhat chaotic nature of the tract through
which they run; for this tract is one of the old centers of eruption. But
some well preserved beds of conglomerate turned up on the thrown side
and matched with beds appearing above at last revealed them, and the
discovery of a series of peculiar trachytic beds on both sides of the fault

30 GEOLOGY OF THE HIGH PLATEAUS.

planes confirmed the belief that the faults really existed. In the middle of
the range the obscurity is still greater. Volcanic activity, producing great
distortion and destruction of the stratification, has made it impossible to
unravel the complications of the displacement. I only know that the upper
Jurassic beds appear at the base and again high up in the heart of the
range and in a very distorted and more or less metamorphic condition at
intermediate places. I have cut the knot, and represented the movement
in the stereogram as a simple fault. Near the northern end of the Tushar
the fault is shown more clearly, and is there relatively simple, though not
without some slight complexities arising from undulation of the strata. The
same line of displacements extends beyond the Tushar along the eastern
flank of the Pdvant, which is the northern continuation of that range.
Here it is at first a simple fault, but gradually becomes a monoclinal beyond
the town of Richfield by the thrown strata flexing gradually upward until
they meet the ends of the beds on the lifted side.

Opposite Salina it suddenly changes its trend to the northwest and
forms the western wall of Round Valley—a depression cutting through the
Pavant obliquely. The length of this displacement is about 80 miles.

The Toroweap* fault cannot be reckoned among the greater faults,
though it is so noticeable and conspicuously exhibited that it deserves men-
tion. It crosses the Grand Canon near Mount Trumbull, about 11 miles east
of the Hurricane fault, with a throw to the west of about 700-800 feet, but
in the course of about 20 miles to the northward it probably runs out.
Very little is known concerning it south of the river. It is a fault of the
simplest order.

The fourth great displacement is the Sevier fault. It commences about
35 miles north of the Grand Canon. It makes its first appearance at ‘Pipe
Spring,” at the base of the Vermilion Cliffs, and presents a remarkable atti-

tude.t Approaching it from the west, the beds are turned down on the

“The Toroweap is a valley opening upon the middle terrace of the Grand Cation from the north
side, It was excavated and its stream dried up before the commencement of the cutting of the inner
chasm, and its floor, therefore, remains about on a level with the middle terrace. It is a magnificent
avenue of approach to a sublime spectacle of the Grand Cation, bringing the observer to the brink of the
inner abyss, where he may look vertically downwards more than 3,000 feet and with more than 2,000
feet of wall above him. The name Toroweap signifies ‘‘a clayey locality.”

+There are some indications that it extends a few miles south of Pipe Spring, but it is covered
with soil and sand.

SEVIER FAULT. dl

thrown side and remain horizontal on the other. The beds, five miles from
the fault on the thrown side, come back to horizontality at about the same
levels which they occupy on the other side of the fault, Fig. 3. The trend
of the fault at first is northeast. Ten miles from Pipe Spring it is a simple
fault. Farther on, in Long Valley, it is “stepped” with two branches.
Passing on to the base of the Paunsdgunt at Upper Kanab the beds on the
thrown side are flexed upward, while on the lifted side (east) they are hori-
zontal. This form continues northward from Upper Kanab for about 13
miles, when branch faults appear on the thrown side and the fault is
stepped and here and there somewhat comminuted, but with one predomi-
nant shear, forming the western wall of the Paunsdgunt Plateau. These
modifications disappear about 6 miles farther on, and the fault becomes
simple with a diminished throw; the displacement opposite the village
of Hillsdale not exceeding 800 feet. Beyond Hillsdale the throw is nearly
uniform for about 10 miles and then increases again. ‘The increase is
slow but steady for the next 60 miles. Along the east side of Panquitch
Valley it is very difficult to study, because it cuts the volcanic rocks,
which are much confused, and here is one of the great eruptive cen-
ters. It is probably somewhat complicated, though the principal dis-
placement is distinctly revealed in the great plateau wall on the east, and
inthe great ravines and chasms which cut across it and open into the valley
below. Opposite Circle Valley the fault splits off a large piece from the
Sevier Plateau by means of a branch which leaves the main displacement
and then reunites with it. At East Fork Canon the thrown beds, consisting
of volcanic conglomerate, are turned up monoclinally, but are sundered by
the fault at the summit, with a shear of 3,000 feet. A little north of this
cation a branch diverges from the main displacement, running off into the
Sevier Valley, where it rapidly dies out. The maximum displacement is
apparently attained a few miles south of the Mormon village Monroe, and
from that point northward it rather rapidly diminishes. Between Glenwood
and Salina the apparent shear has become zero. But the circumstances are
remarkable. The fault from Monroe northward is asecondary displacement
superposed upon an older one. The zero point of the fault is quickly suc-
ceeded in the same line by a resumption of the shear, but in the opposite

32 GEOLOGY OF THE HIGH PLATEAUS.

direction; 7. e., the throw north of the zero point is to the east while south
of this point it is to the west. The fault with its throw reversed now con-
tinues northward, crossing the lower end of San Pete Valley, and becomes
‘the eastern wall of the San Pete Plateau, its shear increasing until it reaches
nearly to Mount Nebo. It has not been traced farther, but where it has
last been verified it is still in considerable force. The length of this dis-
placement, so far as now known, is nearly 220 miles. It forms the western
fronts of the Paunsdgunt and Sevier Plateaus and the eastern front of the
San Pete Plateau.

The Western Kaibab fault is the fifth great displacement. It is supposed
at its southern extension across the Grand Canon to unite with the Eastern
Kaibab fault, as it is known to do at its northern end at Paria, about 40
miles north of the head of Marble Cation. Its trend describes a large bow,
of which the Eastern Kaibab fault is the chord. Between them the Kaibab
Plateau has been uplifted. Through the portions immediately north of the
Grand Canon it is stepped, but the steps unite into a true monoclinal flexure
opposite the middle of the Plateau. Towards the north it gradually dies
out, and near the junction with the Eastern Kaibab displacement it is but
a gentle monoclinal swell and hardly perceptible.

The Eastern Kaibab fault is the longest line of displacement of which
I have ever heard. It comes up out of unknown regions in Arizona from
the vicinity of the San Francisco Mountain, and appears near the mouth of
the Little Colorado River as a double displacement, but probably consider-
ably complicated.* The displacement has two parallel branches, which
appear to be faults where they cross the Colorado, but about 10 miles
northward they gradually pass into two beautiful monoclinal flexures, the
strata being unbroken, except by erosion at the surface. At House Rock Val-
ley the two flexures merge into one, which continues northward past Paria,
trending fixst northnortheast, but gradually swinging in a curve around to
the northwest, always preserving its true monoclinal form. As it approaches
Table Cliff, it dwindles as if about to die out; but opposite the southwest angle

* Professor Powell is probably the only geologist who has seen these faults in this locality. The
place is a terrible one to reach unless by boats through the entire length of the Marble Cafion, and even
then the approach is formidable. He would be a bold men who should endeavor to reach the locality
from above.

KAIBAB AND THOUSAND LAKE FAULTS. 33

of the Aquarius Plateau it is joined by an important fault coming from the
southsouthwest. This is the Paunsdgunt fault, which lies near the eastern
base of that plateau. As its throw is in the opposite direction to that of the
Kaibab fault, the two are apparently distinct, though they really are
branches of one displacement. The displacement now continues north
along the western front of the Aquarius Plateau, and presents complication
with subordinate faults. Still northward it has the Awapa Plateau for its
uplifted and Grass Valley for its thrown side, the minor faults gradually
merging with the principal one.

Near the north end of Grass Valley it rapidly passes into a sharply-
flexed monoclinal, forming the northwest shoulder of Fish Lake Plateau, and
the monoclinal so formed gradually expands into a broader flexure, with an
increasing displacement, and becomes the great monoclinal of the Wasatch
Plateau, one of the grandest flexures of the Plateau Country. This flexure
forms the southeast side of San Pete Valley for about 50 miles. It has not yet
been traced beyond the northern end of this valley, but from the topography
it is supposed to extend far beyond it, being in full force where it has been
last observed. Its total length, reckoning as one displacement the Wasatch,
Grass Valley, Table Cliff, and Eastern Kaibab portions, cannot fall much
short of 300 miles, and may considerably exceed that after the termini have
been discovered. It presents many phases or modifications, but the domi-
nant feature is the monoclinal form. The maximum displacement is at the
Wasatch Plateau, and reaches nearly 7,000 feet.

The easternmost fault (Thousand Lake fault) of this system begins upon
the southern slopes of the Aquarius Plateau, trending due north. It crosses
that plateau with a dislocation of 500-600 feet, and splits into two faults,
which reunite upon the northern base. Crossing the lower end of Rabbit
Valley, it passes along the western base of Thousand Lake Mountain, and
then swings to the northeast The throw is to the west, and in passing
from the foot of the Aquarius to the base of Thousand Lake Mountain the
displacement rapidly increases to about 3,500 feet, and then as rapidly
diminishes, again becoming zero about 20 miles northnortheast of the mount-
ain. But it immediately recommences with a ‘throw in the opposite diree-

tion (east), repeating the phenomenon presented by the Sevier fault a little
@) isl IP

34 GEOLOGY OF THE HIGH PLATEAUS.

south of Salina. Resuming its northerly trend, the fault with a reversed
throw passes along the west side of Gunnison Valley with a shear of at
least 3,000 feet, and runs obliquely up on the great Wasatch Monoclinal,
forming a superimposed displacement, and then cuts obliquely down into
San Pete Valley, where it disappears. It may continue farther northward,
but it has not been traced in that direction beyond San Pete Valley. Its
total observed length is very nearly 100 miles. It is everywhere a true
fault, though at several places it is complicated by minor fractures and some
flexing of the thrown beds.

I have not included the East Musinia fault among the greater displace-
ments, though it has considerable length—perhaps 45 miles—and at one
place in Gunnison Valley the shear reaches more than 2,000 feet, and pos-
sibly near to 3,000 feet. It is, however, an important feature, and almost
entitled to rank with the greater faults of the system. It is parallel to the
northern portion of the Thousand Lake fault last described, and might be
called a mate to it, since the two hold between them the sunken block of
Gunnison Valley and the continuation of that block obliquely across the
ereat Wasatch Monoclinal.

This sunken block is an interesting occurrence, and belongs to that
kind of complicated fracture which Powell has named “Zone of Diverse
Displacement.” he part of it which lies in the lowest portion of Gunnison
Valley has been analyzed and described by Mr. Gilbert. It extends both
north and south from this locality, and in the former direction continues to
display the same comminuted fracture in great variety for a distance of
more than 20 miles, while the width of the zone does not exceed 3 miles.
It appears to be a very clear case of a block dropping through the drawing
apart of the strata and sinking to fill the gap thus produced. Another in-
stance occurs along the western base of the Aquarius Plateau in the south-
ernmost portion of Grass Valley. Here the block between the faults,
instead of shearing sharply on both sides, has partly careened and settled
down synclinally.

These displacements do not belong wholly to any one period. There
is evidence that different faults belong to different ages—not widely separ-

ated probably, but recoguizably distinct. There is evidence that different

COMPARATIVE RECENCY OF FAULTS. By

portions of some of the faults did not occur simultaneously, or, perhaps
more properly, at the same rate of progress. There is evidence that some
portions of a fault progressed through intervals of alternate repose and
activity. But while the entire Tertiary history of this district, or at least
that portion of its history since the Eocene, was marked by the recurrence
of disturbing forces here and there, there is one period which appears
to have been pre-eminently a period of faulting and uplifting, standing out
conspicuously as a culminating period in the movements. It was this period
which more than any other gave, not indeed birth, but certainly the maxi-
mum growth and expansion to the structural features of the district. This
period was a comparatively recent one. To name it in terms of the ordi-
nary geological calendar would probably convey the impression that the
means of determining and correlating the ages of events occurring within
the district with reference to those occurring outside of it are greater than
they really are. Since the middle Eocene all direct connection of the Ter-
tiary -history of the Plateau Province with external regions ceases. Since
then everything is relative. The order of sequence is plain, but so far as
time is concerned we are out of sight of stars and landmarks, and run through
the succeeding periods only by dead reckoning. The next age which
we can fix after the Eocene is the Glacial period. We recognize high up
in the plateaus and mountains the traces of local glacial action, and it has
the same general traces of geological recency and historic or prehistoric
antiquity as elsewhere. But between these two ages we are conscious only
of a vast stretch of time, in which great results were accomplished in a
certain definite order. Each individual feature in that progressive evolution
was one which by its very nature required long periods to accomplish, and
the last of them all was the great uplifting and fracturing of the rocks which
had previously accumulated,

I place the age of the principal displacement in a period which had its
commencement in the latter part of Pliocene time, and extended down to an
epoch which, even in a historical sense, may not be extremely ancient, and
which certainly falls on this side of the Glacial period. Perhaps it is still
in progress. Perhaps the plateaus are to-day growing higher and the faults
increasing their shear. But the beginning of this last period of faulting,

36 GEOLOGY OF THE HIGH PLATEAUS.

whether the period is closed or not, goes, I believe, only back into the late
Pliocene. These faults are so important not only to the history of the High
Plateaus, but also to the general history of the Plateau Province at large,
that it seems proper to enter at some length upon the considerations which
have led to this opinion concerning their age.

Recognizing the great magnitude of the results accomplished in this
region by erosion since the Eocene, we are naturally led to inquire whether
we may not here and there gain some conception of the relative ages of cer-
tain events by ascertaining the amount. of erosion which has been effected
since their occurrence. The laws of erosion, both generally and in their
somewhat abnormal application to this strange region, are sufficiently un-
derstood to enable us to decide where erosion ought to be most rapid and
where most sluggish. Of all portions of the Plateau Province the best
watered is the District of the High Plateaus. It is also the loftiest, and gives,
therefore, to its water-courses the swiftest descents and the greatest trans-
porting power. On the other hand, its rocks are the hardest and most dura-
ble. Thus the altitude and copious rainfall favor a rapid rate of erosion,
while the greater durability of the rocks retards it. Not all of the rocks,
however, are of this adamantine character. Indeed, some of the most
voluminous formations are conglomerates, some well consolidated, but most
of them only moderately so. Around the borders of the district are the
sedimentaries, differing lithologically in no material respect from those of
the province at large. By comparing the effects of erosion in rocks of dif-
ferent classes similarly situated we find great irregularities, but so far as can
be seen these irregularities are due chiefly to the relative durability of the
rocks. The sedimentaries are most powerfully eroded, and clearly disin-
tegrate far more rapidly than the volcanics, and considerably more so than
the conglomerates. There is seldom difficulty in distinguishing the erosion
which has occurred during or since the faulting from that which may have
occurred before it; and when we first separate this erosion from the earlier
we find that in the sedimentaries it is very considerable. Vast ravines have
been scored and deep canons cut into the risen blocks. The fronts have been
battered and scoured by the storms of unknown millenniums and pared off
until they stand back of the fault-planes which mark the rifts where they

RECENCY OF FAULTS. 37

were severed from the platforms below. Realizing how slowly to human
senses these processes operate, the thought of the long ages through which
they have been at work at first oppresses us, and we are conscious only of
a duration which we can no more comprehend than we can comprehend
eternity. Yet, when we come to compare the work which has been done
upon the flanks of the plateaus with what we are sure has been done upon
the regions they overlook, the former sinks into insignificance.

Since the commencement of the faulting ravines have been exca-
vated 2,000 or 3,000 feet in depth; some of the living streams have sunk
their cations from a few hundred to a thousand feet; here and there a patch
of exposed country has lost some hundreds of feet of strata; old volcanic
vents on which possibly stood cones have moldered away and left barely
a heap of unintelligible ruins. More than this: we know that since the
same epoch the inner gorge of the Grand Canon has sunk under the inces-
sant grinding of its turbid waters 3,000 feet into the earth, and its side gorges
near the river have deepened an equal amount. Doubtless many other
changes have occurred, the precise nature and extent of which we can only
conjecture. Such as we recognize seem stupendous to us and even stagger
us when we look at the instrumentality to which we must attribute them.
But these are only the last touches of the work which has denuded an
empire, sweeping from its surface 6,000 feet of strata.

When we study more closely the later erosion, we find that by far the
greater part of its results are of that class which is effected with the greatest
ease and rapidity. Slow as the process seems to our senses which has cut”
gorges and cafions, it is swift and trenchant when compared with the
moldering of cliffs and the decay of buttes and mesas; and this slow decay
is far less slow than the decay of platforms and terrace summits. It is in
ravines and canons that the denuding forces work to the utmost advantage.
Let a plateau or mountain range arise, and the streams will dissect it to its
core before it will have materially suffered otherwise. Such uplifts as we
find in the Plateau Province have given to the streams which flow from
them the most favorable opportunity to corrade, and they have cut profound
gorges; but the amount of waste upon the summits and even upon the
great palisades which bound them has been insufficient to sensibly modify

38 GEOLOGY OF THE HIGH PLATEAUS.

their general outlines or even their larger details along the structure lines
The same is true of the heart of the province. The evidence is clear and
irrefragable that at a comparatively recent epoch there has been a wide-
spread uplifting coming upon the country suddenly as it were after an im-
mense period of repose. Before its advent the streams had long remained
at the limiting levels where they could sink no more, and the slower pro-
cesses of decay, the recession of cliffs, the widening of valleys, the shrink-
age of mesas, the lateral expansion of canons, had been in progress long
enough to have produced very extensive results. As this uplifting came
upon the land the rivers were at once disturbed and resumed their occupa-
tion of deepening their channels, and sank them almost as fast as the coun-
try rose. But they remain to-day with walls but little affected by lateral
waste. Every indication points to the conclusion that they are freshly cut
and are still cutting.

Thus the study of the effect of erosion upon the uplifted sides of the
ereat displacements of the High Plateaus everywhere indicates relative re-
cency. The time during which these displaced edges have been subject to
the action of the elements is trifling when compared with the interval which
separates us from the Eocene. It is represented only by a work which is
relatively small and easy of accomplishment and performed under circum-
stances most favorable to rapidity and efficiency. But the general denuda-
tion which dates back to the Eocene is incomparably greater in amount,
considering only equal areas; and represents in chief part the kind of
degradation which is relatively slow, performed under circumstances not
always favorable to rapidity.

There is another point of view from which we arrive at the same con-
clusion, that the great displacements are very young. The volcanism of
the country has a history which we are able to unravel as to its broader
features. It began after the disappearance of the Eocene lake which cov-
ered the Plateau Province. How long after the desiccation we cannot
say even relatively. The lake had withdrawn apparently from the High
Plateau District soon after the close of the Upper Green River epoch, which
represents a period in the latter part (but before the close) of the local

Eocene. Resting unconformably upon the Upper Green River beds is a

RECENCY OF FAULTS. 39

series of beds, displayed in all parts of the district, composed of the waste
of volcanic rocks. The rocks which furnished these sands and marls are
nowhere discernible. Either they have been buried beneath the later lava-
floods or have been wholly removed by erosion. Deep in the recesses of
some of the plateaus, at a very few places where the grander gorges have
eaten their way into them, the oldest observed Tertiary eruptives, the pro-
pylites, are revealed. Of these earliest propylitic eruptions we know ex-
ceedingly little historically. They are covered with great floods of andesite
and trachyte. There is evidence that these eruptions had their periods of
activity alternating with long periods of repose. These periods represent
an immense amount of devastation wrought upon the older voleanic mount-
ains by the elements, for their débris is found in the form of huge beds of
conglomerate stratified in a manner which leaves no doubt in my mind that
the process of accumulation was the exact counterpart of that which is now
building similar beds in the valleys—a purely alluvial process. The earlier
andesitic mountains were almost utterly destroyed by this process. Then
came another period of activity, followed by another period of denudation.
We have older and younger conglomerates. The older contain the andesitic
and some trachytic fragments; the younger contain trachytic, doleritic, and
even basaltic fragments. But both conglomerates represent an enormous
period of denudation, for the aggregate thickness of the beds will frequently
exceed 2,000 feet, covering very large areas. At length a period of fault-
ing set in. These conglomerate beds were sheared or flexed, and now form
the walls and summits of the great plateaus for many scores of miles in
alternation with the remnants of the old volcanic sheets. Again the process
of degradation set to work tearing down these tables, the streams rolling
the fragments down into the valleys and building up along the foot of each
wall a row of very low alluvial slopes, often beautifully stratified, and the
exact counterparts of the conglomeritic strata which are now seen edgewise
in the plateau-walls. Since the uplifting began the amount of accumula-
tion in this way will probably reach three or four hundred feet in some
places, though it is not probable that the average will exceed 200 feet. But
this modern accumulation has been made under peculiarly advantageous
circumstances. The process will become slower and more difficult as the

40 GEOLOGY OF THE HIGH PLATEAUS.

streams sink their channels and every additional yard of deposit will be
accumulated at a slower rate.

It was the uplifting along great lines of dislocation which set this cone-
building process going. The abrupt descents gave the creeks and brooks
their power to transport this coarse débris, and those slopes are now long
and steep. But as the work proceeds the mountains and tables are gradu-
ally rounded and smoothed down and the valley plains built up. As yet
comparatively little has been accomplished in this direction, but the work
is under full headway. In comparing what has been effected since the
beginning of the displacements with work of the same character which
has been accomplished in ages prior to the displacements, we shall be most
forcibly impressed with the littleness of the one and the greatness of the
other. It is a comparison of hundreds with thousands. More than that:
the hundreds of feet of modern valley cones represent the utmost activity
of a process which has worked without interruption and under conditions
the most favorable, while the thousands of feet of ancient accumulations
represent the same process in all degrees of activity, now intense, now fad-
ing and dying out, and then probably long intervals of cessation.

Thus, whether we view the denudation of the High Plateaus or the
accumulations in the valleys at their bases, we reach the same conclusions.
The faults are very late occurrences in the history of the district. But when
we come to ask what is the age, in terms of the geological chronology, to
which they must be referred, we can give no further answer than this: they
belong to a very late one. There is no record of Miocene or Pliocene in
this disturbed region, and we have nothing to mark the lapse of time, except
relatively, since the close of the Eocene. But in other parts of the world,
where we have some knowledge of the strata, we infer that the Miocene
was a longer age than the Pliocene and the Pliocene longer than the Qua-
ternary, though these are impressions rather than conclusions, and to be
held lightly. Judging, however, by the magnitude of results accomplished
by erosion in the High Plateaus since the faults were started, and compar-
ing these results with similar work accomplished in other localities, and
taking into the account the conditions under which they were accomplished,
it seems perfectly safe to say that if we carry back the faulting to the mid-

RECENCY OF FAULTS. 4]

dle of the Pliocene we shall have dealt generously with any one who may
be disposed to push them back to the remotest possible epoch.

But it may be asked if erosion may not after all have proceeded slowly
in this region on account of the arid climate, and whether there may not
have been long intervals when its rate was insignificant. I think the answer
must be decidedly in the negative so far as the time is concerned which
lies on this side of the epoch of displacement. The High Plateaus are not
arid, but are watered copiously

less, indeed, than the regions east of the
Mississippi, but far more abundantly than the deserts which lie to the east
and to the west of them. It must be remembered that their altitude is
great, and that their length and breadth is far greater than most of the
Rocky Ranges. They are the most prominent topographical barrier which
the westerly winds strike after leaving the Sierra Nevada, and though the
plains and even the ragged ridges of the Great Basin are parched and dry,
yet the High Plateaus wring from the air notable quantities of moisture.
The rainfall is not known, but 30 inches per annum is a small estimate of
the probable precipitation on the Plateau summits. In the valley plains of
the Great Basin the rainfall seldom exceeds 8 inches, and in the painted
desert to the east of the High Plateaus it could not reasonably be expected
to amount to so much as 4 inches. But there is evidence that in the past—
in Glacial and Post-glacial time—the rainfall was far more abundant than
now. ‘The drainage of three-fourths of the district was gathered in those
periods into the grand expanse of Lake Bonneville, of which Great Salt
Lake and Sevier Lake are the remnants. At present this drainage is ab-
sorbed and finally evaporated in Sevier Lake alone. Very abundant must
have been the rainfall and moist the atmosphere which, with such a relatively
moderate water-shed, could have kept such a lake as Bonneville brimming.

Nor is there at present any evidence that the erosion was materially
affected either in degree or kind by the presence of ice during the Glacial
epoch. On the contrary, the evidence is strongly in favor of the conclu-
sion that in that period the climate was not glacial in this district. The
ravines and valleys are conspicuously water-carved and conspicuously
not ice-carved. As if to furnish proof that the absence of all indications
of ice action in the valleys and plateau flanks should be construed as

42 GEOLOGY OF THE HIGH PLATEAUS.

meaning that none existed, we do find at the very summits unmistak-
able indications of the action of local and very small glaciers, with beauti-
fully preserved terminal morains. But I have never seen a morain in the
High Plateaus at a lower level than 8,500 feet, and 9,000 feet may be con-
sidered as the mean level at which they are first encountered. We find
even these only on portions of flanks which bound the loftiest parts of the
tabular summits, showing that the loftiest parts alone accumulated ice and
generated small glaciers. This will not seem surprising even to those who
hold strongly pronounced views on the subject of the Glacial period if we
assume that during that period the plateaus stood considerably lower than
at present. That they did stand lower then is not improbable. We cannot
look to the Glacial period, therefore, for the discovery of any cause which
would retard the process of erosion; but, on the contrary, we find in its
moister climate reasons for thinking that it may have been notably more
rapid than now.*

I have discussed this subject at some length, because the age od these
faults is very important in the geology of the region, and is even more im-
portant to the southern and southwestern portions of the Plateau Province,
if possible, than to the High Plateaus. They are associated with the later
history of the canons and cliffs and with the climatal changes of the prov-
ince in the most intimate manner. The evolution of that region has long
since showa a tendency to cluster; it has even taken form; around certain
marked events of which one of the most prominent was the faulting, and
the consequences of these faults reach out in a manner which cannot be
appreciated until the whole region is described and the history of its con-
stituent parts delineated ; a work which I trust will be accomplished in the
near future. They everywhere betray in numberless ways their recency,
and I have presented only that evidence which strikes the eye at once where
we first encounter them.

But while they are all comparatively recent some are older than others.
The two Kaibab faults in particular are apparently older than the rest, at
least in part ‘Those greater faults which cut through the heart of the

“Whether erosion would proceed faster under the action of ice than of running water is a ques-
tion which I do not raise. I+ has no present bearing.

DIFFERENT EPOCHS OF DISPLACEMENT. 43

eruptive district seem to have had portions of their shearing before the
beginning of the principal epoch of displacement. But these earlier symp-
toms are usually like old wounds which had once healed and afterwards
broke out again with increased disorder. The Sevier fault, in particular,
shows signs of two epochs of activity in some portions of its extent. Be-
tween Monroe and Gunnison it appears as a fault cutting along the axis of
a small but sharp monoclinal flexure. The flexure is clearly older than
the fault. The Musinia faults cut obliquely across the great monoclinal of
the Wasatch Plateau, and show little sympathy with it. The Paunsagunt
fault, uniting with the northern extension of the Hast Kaibab flexure, is
plainly independent of it, and is decidedly younger. It is a most curious
circumstance that where we find this two-period displacement the motion
of the fault is often reversed—the lift of the first period is the throw of
the second. It is not always so, but I believe it to be true in a majority
of cases where the double movement has been detected. On the other
hand, where the shearing of both periods has been in the same direction,
the movements would be much more difficult to separate, and many such
double movements doubtless have escaped observation.

All of the displacements thus far discussed belong to the same system.
Whether older or younger, they lie along the same lines and very seldom
show any interferences. None of them will go back of the Pliocene in age,
and I think it probable that none of them will go behind the middle Plio-
cene. Older displacements along these lines, if they exist, are wholly coy-
ered up and obliterated, and cannot be separated at present from the later
ones of this system.

There is, however, a totally distinct system of displacements, belong-
ing to a much earlier age, which the grander and more general erosion of
the country has brought to light, but which can never be confounded with
the Pliocene-Quaternary system. They make a wide angle with the lat-
ter series and have a history wholly independent of them. They are only
occasionally revealed in a fragmentary manner in places where deep gorges
have cut through thousands of feet of Tertiary formations and volcanic
emanations, or where erosion has swept off corresponding amounts of strata

from broad districts. Only in two or three places in the heart of the High

44 GEOLOGY OF THE HIGH PLATEAUS.

Plateaus are they brought to light; but around the southeastern borders of
the district they are displayed conspicuously. The age of these flexures is
apparently Post-Cretaceous and Pre-Tertiary; that is, they occupy, in respect
to time, an interval which separates the Mesozoic from the Tertiary.* They
consist of a series of monoclinal flexures, quite perfect in form, which trend
from northwest to north-northwest. They involve the Mesozoic beds, but not
the Tertiary. They come up from the southeast, and disappear under the
Aquarius Plateau, and on the southern and southeastern flanks are laid bare
by a vast erosion. Just before they reach this plateau they are seen to
be eroded, and near the summit the Eocene beds are seen to lie unconform-
ably across the beveled edges, and still farther on near the lava cap they
rest upon the Jurassic. All around the southern and eastern flanks of the
Aquarius and along a part of the northern flank, also entirely around the ~
circumference of Thousand Lake Mountain (with the possible exception of
its northern end), the contact of the Tertiary with the Jurassic is obvious.
Farther eastward in the heart of the Plateau Province, outside of the
district of the High Plateaus, are three more displacements of grand pro-
portions, of which I can make but a passing mention. The southernmost
is the Echo Cliff flexure, a great monoclinal seen south of the Colorado near
the Moquis towns. Trending a little west of north, it crosses the river at
the head of Marble Canon, and continuing along the Paria River dies out
near Paria settlement at the base of the Vermilion Cliffs. Farther east is
the Water-Pocket flexure, one of the grandest monoclinals of the West. It
crosses the Colorado in the heart of Glen Canon, and running north-north-
west between the Henry Mountains and Aquarius for nearly 60 miles, swings
around to the west in a great curve and disappears under Thousand Lake
Mountain. The third is the San Rafael flexure, beginning as a branch of
the Water-Pocket flexure, where the latter changes its trend, and running
north-northeast along the eastern side of the San Rafael swell, passes off into
the northeast and dies out again. These are all monoclinal flexures of impos-
ing dimensions and of perfect form. Their age I cannot speak of at present
in any detail, though it is hardly doubtful that they go far back in Tertiary

‘Here, as elsewhere in this work, the Laramie beds are reckoned with the Cretaceous, of which
they form the upper group of beds.

RELATION OF FAULTS TO ANCIENT SHORE LINES. 45

time and possibly are Pre-Tertiary. Mr. Gilbert has studied the Water-
Pocket flexure, and believes that its epoch belongs to the interval which
separates Tertiary from Cretaceous time. The Echo Cliff flexure is proba-
bly much younger. The San Rafael flexure remains to be studied. None
of them appear as yet to have any sympathy with the Pliocene-Quaternary
faults of the High Plateaus.

It yet remains to speak of another interesting relation of the later
system of faults. They have throughout preserved a remarkable and _ per-
sistent parallelism to the old shore line of the Eocene lake, following the ~
broader features of its trend in a striking manner. The cause of this rela-
tion is to me quite inexplicable, so much so, that I am utterly at a loss to
think of any subsidiary facts which may be mentioned in connection with
it and which can throw light upon it. It seems best, therefore, to allow
the main fact to stand by itself, and not to confuse it with any others with
which it has no certain relation.

The faulting and flexing has been associated with a general increase in
the altitude not only of the district of the High Plateaus, but of the country
south and east of them. The uplifting has by no means been confined to
the few tabular masses. Wherever we look in the western part of the Pla-
teau Province the signs of this elevation are unmistakable. In some local-
ities it was much greater than in others, but the signs of it are common to
all. Itis betrayed in the drainage channels. At a comparatively recent
epoch there has been a sudden renewal of activity on the part of the
streams, by which they have taken to canon-cutting with renewed energy
as if their slopes had been increased, and this is especially observable in
the Colorado itself, where the effect has been a maximum. The tribu-
taries have responded and have acted in like manner. Just prior to the
advent of this regional uplifting, the aspect of the region appears to
have been that which would naturally have resulted from a long period
of stability at the same altitude. The canons and intervales were wide,
and long stretches of the rivers were at or near their base-levels, having
eroded as deeply as possible, then slowly widened their valleys and made
flood-plains. All at once a new era of canon-cutting set in, and profound

narrow chasms were sawed in the strata and are to-day sinking deeper.

46 GEOLOGY OF THE HIGH PLATEAUS.

These traces are less conspicuous on the eastern terraces than upon the
southern, but are seldom absent. In the Great Basin west of the plateaus
there is no evidence of any such general uplifting in the later pericds, at
least within many leagues of the High Plateaus, although local disturb-
ances of no small magnitude have occurred, and doubtless the southwestern
ranges have gained notably in altitude.

It is interesting to compare the structural forms produced by the
displacements of the High Plateaus and Kaibabs with those observed in
other countries and in other parts of the Rocky Mountain Region. The
earliest ideas acquired by geologists concerning mountain structure were
derived from the study of the Alps and Jura’ The conspicuous fact
there presented is plication—waves of strata like the billows of the ocean
rolling into shallow waters, and often a more extreme flexing until the folds
become closely appressed. With the extension of observation among the
other mountain belts of Europe, and wherever the traces of great disturb-
ance among the strata were found, the same phenomenon of repetitive flex-
ing was discerned, seldom amounting to “close plication,” but undulating
in greater or less degree. Ata later period, when geology was colonize
in America, its systematic researches were first prosecuted in the Apala-
chians, where the same order of facts was presented in a degree of perfec-
tion and upon a scale of magnitude far surpassing the original types of
Switzerland. At a still later period the geologists who inaugurated in the
Sierra Nevada and Coast Ranges the study of the Rocky system disclosed
another grand example of the same relations. Thus the increase of obser-
vation has been for many years strengthening the original induction that
plication and mountain-building are correlative terms.

But the rapid and energetic surveys of the remaining portions of the
Rocky Mountain Region have within a few years brought to light facts of a
different order. From the eastern base of the Sierra Nevada to the Great
Plains are very many mountain ranges, a large proportion of which have come
under the scrutiny of geologists; and of those which have been hitherto
studied sufficiently to justify any conclusions concerning their structure
not one has been found to be plicated. Not one of them presents any
recognizable analogy to the structure which is so remarkably typified in

ABSENCE OF PLICATION IN THE ROCKY SYSTEM. 47

the Apalachians. It is certainly true that the study of these mountains
has not been so minutely detailed nor so long continued as that of mount-
ains situated in populous countries; that a considerable portion of them
have not been examined geologically at all. But, on the one hand, the
number of which we already possess a preliminary knowledge is considera-
ble, and on the other hand the remarkable distinctness with which structural
facts are there displayed, and the comparative ease with which they may
be read, justify more confidence in our conclusions than might otherwise
have been admissible. No one familiar with the progress of knowledge in
this special direction can fail to recognize the conspicuous absence of plica-
tion in the mountain structures which are found east of the Sierra Nevada.

Yet in some portions of this great expanse of territory there are im-
portant flexings and warpings of the strata. This is particularly true ot
the Basin Ranges. But a very significant distinction is necessary here.
These flexures are not, so far as can be discerned, associated with the build-
ing of the existing mountains in such a manner as to justify the inference
that the flexing and the rearing of the ranges are correlatively associated.
On the contrary, the flexures are in the main older than the mountains, and
the mountains were blocked out by faults from a platform which had been
plicated long before, and after the inequalities due to such pre-existing flex-
ures had been nearly obliterated by erosion. It may well be that this ante-
rior curvation of the strata has been augmented and complicated by the
later orographic movements. But it is not impossible to disentangle the
distortions which ante-date the uplifting from the bending and warping of
the strata which accompanied it, and it is only the latter that we can prop-
erly associate and correlate with the structures of the present ranges. These
present no analogy to what is usually understood by plication. The amount
of bending caused by the uplifting of the ranges is just enough to give the
range its general profile, and seldom anything more. The same fact is pre-
sented in the noble ranges of Colorado. Along their flanks the sedimentary
strata roll up usually with a single sweep, and high on the slopes are cut off
by erosion. The typical anticlinal axis is not a characteristic feature of
the Rocky Mountain system |

The type-section of the Park Mountains of Colorado, as given by the

48 GEOLOGY OF THE HIGH PLATEAUS.

late A. R. Marvine, shows a series of broad platforms, uplifted with a single
monoclinal flexure or a fault on either side. The width of these platforms
varies from 20 to 45 miles, and from these masses the individual mountain-
piles have been carved by erosion. The restored profiles obtained by re-
placing the material removed by erosion are not indeed horizontal nor
straight lines, but ordinarily convex upwards, with slight curvature, becom-
ing abrupt or even passing into a great fault at the margin of the uplift.
Inasmuch as almost any configuration of the strata which is convex upwards,
be it never so little, is called an anticlinal, these platforms would probably
be so characterized by most geologists. But what a contrast to the short,
sharp waves of the Apalachians! If we analyze the form carefully, it will
become apparent that we have’ to do with a structure which has nothing
in common with a true anticlinal except this slight convexity, and which
possesses characters which the true anticlinal does not.

It has already been indicated that faults and monoclinal flexures are
homologous terms. They represent varying degrees of abruptness in the
passage from the thrown to the lifted side of a displacement. In the case
of the fault the shearing is confined to a single plane; in the case of a mo-
noclinal flexure the shearing is distributed through a narrow zone between
two planes. Both mean essentially the same thing. In the Park Mount-
ains we have uplifts with a fault or equivalent monoclinal on one side or on
both. Most frequently it is on both sides, but the shearing is almost inva-
riably more strongly emphasized on one side than on the other. It rarely
happens that the fault is clean and trenchant, but is accompanied with much
fracturing and shattering of the thrown edges of the strata, and there are
cases when the dragging of the fault has been accompanied by the over-
turning of a great slice of strata torn from the thrown edges. Instances are
abundant where the rocks in the flanks of these ranges in the vicinity of
the faults have been subjected to the most “heroic” treatment; but at short
distances from the faults in both directions the disorganization quickly
diminishes. Upon the summits of the platforms the traces of violence and
distortion attending the upward movement are much less. Where erosion

‘thas laid bare the most ancient rocks they are ordinarily found to be more

COMPARISON OF OROGRAPHIC FORMS. 49

or less flexed, but the flexing, according to Mr. Marvine, is chiefly of very

ancient date—certainly Pre-Tertiary.

Thus the lifting of these platforms has no significance corresponding
to an anticlinal fold. It is expressed by the conception of a block of strata
having a fault or equivalent monoclinal flexure upon both sides. But while
these characteristics predominate strongly throughout the more easterly
ranges of the Rocky system numberless changes are rung upon them. One
dislocation is usually greater than the other. One fades out to a mere in-
clined plane, while the other becomes a gigantic fault; all shades of differ-
ence are found from the evanishment of one to the sensible equality of
both The relative courses of the two displacements constantly vary; here
parallel, there converging, and again diverging. But throughout this diver-
sity the dominant type-form is still persistent. These broad platforms have
upon their surfaces in most cases a certain amount of minor flexing and un-
dulation. Occasionally a sharp turn of the strata upwards or downwards
produces a minor or superimposed wave with a well marked anticlinal and
synclinal profile. Minor faults and local shattering are also seen here and
there. But those systematic repetitive parallel waves of strata which are
conveyed to the mind when we speak of plication are not found in any
known region east of the Sierra Nevada and west of the Apalachians.

In the Uintas we find a repetition of the Park Mountain type upon a
grand scale. This has been illustrated admirably by Professor Powell in
his work on the geology of the Uinta Mountains. It consists of a block
somewhat broader than those of Colorado, but otherwise the type presents
no essential modification. It has a great monoclinal upon the southern
flank and a colossal fault upon the northern. Between the dislocations
there is a notable amount of superimposed undulation and subordinate
fracturing and flexing; but the greater part of it antedates the Tertiary
history of the range, and very much of it is at least as old as the Carbon-
iferous.

In the Plateau Province there are very few mountains, and such as
occur are of volcanic origin. Some of them are constructed in a most
singular manner, presenting in their genesis and structure an utter contrast
to the Alpine and most of the Colorado forms. Lenticular masses of igneous

4 HP

50 GEOLOGY OF THE HIGH PLATEAUS.

rock have been intruded between the Carboniferous and Mesozoic strata,
hoisting the upper beds into great domes. Mr. G. K. Gilbert has studied in
great detail the Henry Mountains of southeastern Utah, which present this
singular phenomenon in perfection. This group of mountains consists of
five individual masses, two of which are of great magnitude, and all of
them have been domed up by lava rising from the depths and accumulating
in reservoirs several thousand feet below the surface. Each of the mount-
ains has a considerable number of these reservoirs and the two larger masses
have many of them. The lava intruded itself at various horizons and con-
gealed, leaving lenticular masses, which are now laid bare and admirably
dissected by erosion. There are no indications that any notable quantity
of the lava ever outflowed. To these intrusive masses Mr. Gilbert has given
the name of “laccolites.” These are by no means isolated instances of
this extraordinary origin of mountains. The Sierra Abajo on the east wall
of the Colorado and a small neighboring range called El] Late present the
same phenomenon. The Navajo Mountain at the mouth of the San Juan
River is similarly constructed.* Several of the Colorado ranges, according
to Dr. Peale, owe their structure in part to “laccolitic” intrusion. But
mountains on the whole are rare occurrences in the Plateau Province. The
uplifts there are almost wholly of the tabular form. Yet, when we come to
examine their structure, we find that those plateaus which are due to dis-
placement have a construction strikingly similar to the broad platform-ranges
of Colorado and to the Uintas. They are found along the western belt of
the Plateau Province in the Kaibabs and in still more perfect development in
the High Plateaus. Here the uplifts have been blocked out by the usual
faults and monoclinal flexures. Most of them have a single fault upon the
western side, inclining at a very small angle towards the east. The western
limit is the lifted side of the fault; the eastern limit is the thrown side of
the next fault. All traces of the anticlinal have vanished and the structure
is of the simplest possible order. In a few of these uplifts we have a block
between two faults or monoclinals of opposite throws. Such is the Kaibab

Plateau itself. But the great predominance of the faults which face the west

“The Navajo Mountain is a solitary dome-like mass of grand dimensions upon the very brink of
the Glen Cation. The cain slices off a segment of its base, and the spectacle of rock-work, looking at
it from the end of the Kaiparowits Plateau across the gulf, is overpoweringly grand.

COMPARISON OF OROGRAPHIC FORMS. il

is very striking. If we compare these uplifts with the Park Ranges and
with the Uintas, the similarity of the structural profiles is very conspic-
uous. But in the plateaus there is greater simplicity, less subordinate
flexing (indeed almost none at all), and an absence of convexity in the
section lines.

Crossing the abrupt boundary which separates the plateaus from the
Great Basin, we are at once among mountains of a very different order.
The Basin Ranges are many in number and inferior in magnitude to those
of Colorado, though of no mean dimensions. They are strongly individ-
ualized, each being separated from its neighbors by broad expanses of plains
as lifeless and expressionless as Sahara. It is as difficult to find a type-form
representing the construction of these ranges as for those of Colorado. Yet
there are common features of almost universal prevalence among them and
at the same time thoroughly distinctive of the group. There is on one side
of the range, sometimes a single great fault, or more frequently a repetition
of faults throwing in the same direction, while upon the other side the
strata slope down to the neighboring plains and there smooth out again.
There is much variety in the details of the dislocations, and so complicated
do they become in certain localities, that they sometimes mask the general
plan until we carefully unravel it. The strata also are almost invariably
tilted to high degrees of inclination, thus contrasting strongly with the low
and almost insensible slopes of the plateaus. Hence on one side of the
range the slope of the profile is along the dip of the strata, on the other
side it is across their upturned edges.

We may now compare the orographic forms prevailing in the three
great provinces—the Park system, the Plateau system, and the Basin sys-
tem. The uplits of the plateaus approach in the forms of their displace-
ments more nearly to those of the Park Ranges than to those of the Basin,
but are much simpler, much less complicated by subordinate fracture and
flexing, and have undergone a much smaller amount of vertical movement.
There is, however, one very striking contrast between the Plateaus and
the Park Ranges. In the latter, erosion has played a most important part in
their history and development. The mountain platforms have undergone
an amount of degradation which never fails to revive astonishment when-

52 GEOLOGY OF THE HIGH PLATEAUS.

ever the mind recurs to it. Many thousands—nay, even tens of thou-
sarids—of feet of strata have been stripped off from their summits and
scattered far and wide. As fast as they were denuded they arose, maintain-
ing, and probably even increasing, their altitudes in spite of the waste.
Much of the denuded material has been redistributed in strata around their
flanks upon the old lake-bottoms of Tertiary time, where there has been,
relatively at least, a gradual subsidence as sedimentation progressed. The
great faults and monoclinal flexures where the strata are now hog-backed
against the flanks of the ranges are the apparent results of the shearing
motion set up by the rise of the mountain platforms on one side and the
sinking of the newer deposits on the other. In the plateaus the action of
erosion has been strikingly different. The tables have been affected only
in comparatively slight degree more than the adjoining lowlands. Indeed,
erosion has wrought almost equally upon high and upon low levels. In
some portions the denudation has been stupendous, but the denuded
material has not been carried down and redistributed in the plains below,
but has found its way into the deep cafons which cut below its lowest plat-
forms and has been swept through the Colorado to the ocean. Now, it is
unquestionably a true law of nature that the denuding agencies operate
more vigorously against highlands than against lowlands, and it is quite as
true in the Plateau Country as elsewhere. But the recency of the differen-
tial elevations of the Plateau Province has not permitted any very great
difference to show itself as yet, though it is easy to see that a difference
really exists, and is even conspicuous. Furthermore, the peculiar fact that
the deeply sunken drainage channels of the province do not allow of great
accumulation and restratification at the bases of the loftier masses is a suffi-
cient reason why lower levels should be eroded as well as higher ones,
though to a less extent.

We cannot, therefore, attribute the faulting and monoclinal flexing of
the plateaus to erosion of the uplifts and the deposition of the débris at their
flanks, for no such (relatively greater) amount of erosion is found upon the
uplifts, and no such depositions take place upon their flanks. The Kaibabs
have been enormously denuded, but not much more upon the highest than
upon the lowest portions. The High Plateaus have, compared with the

COMPARISON OF OROGRAPHIC FORMS. 53

Kaibabs, suffered but little from erosion. In neither district can we look
for the same causation of faults and flextures as we might at first feel in-
clined to employ to explain those of Colorado and the Uintas. In the first
chapter I have alluded to the possible effects attending the removal of great
loads of strata from one locality of considerable area and the deposition of
the same materials in adjoining areas; and while we may rationally sup-
pose this transfer of loads to have important consequences in respect to ver-
tical movements, we seem compelled to postulate additional forces, which
for want of any definite conception as to their real nature we call Plutonic
forces. The necessity for such a postulate seems perfectly obvious in the
plateaus, and a little consideration will, I think, make its necessity apparent
in the mountains of Colorado and the Uintas. It is not impossible that the
differences existing between the structural profiles of the Plateaus on the
one hand and those of the Parks and Basin Ranges on the other may be
largely, or even wholly, due to the fact that in the latter regions the débris
has been deposited at the bases of the mountains, while in the Plateau
country it is carried away through the canons to another part of the world.
Hence in the Plateaus we have the result of the uplifting forces, almost
pure and simple, while elsewhere it is complicated, and generally reinforced,
by the effects of the transfer of great loads from the mountain platforms to
the plains and valleys around their bases, followed by a readjustment of
the plastic earth to a statical equilibrium of its profiles.

In comparing the plateaus with the Basin Ranges we have to deal with
the fact that the displacements of the latter are in the main older than those
of the former, though younger than those of the Eastern Rocky Ranges.
Erosion has operated powerfully upon all of the Basin Ranges, and the.
ageregate displacements are greater than in the plateaus. The strata ordi-
narily incline at larger angles and exhibit a greater amount of subordinate
fracturing and dislocation. There is, however, some similarity between the
plateau and basin uplifts. Both present a succession of inclined platforms,
sloping in the same direction, with greater dislocations upon the uplifted
sides. In the Basin Ranges, the uplifting being greater, the inclination is
correspondingly greater, so much so, that we pass from the notion of a
plateau or platform to that of a mountain slope. The inclination of the

54 GEOLOGY OF THE HIGH PLATEAUS.

plateau summits is rarely so great as 3°; the inclination of the structure-
slopes of the Basin Ranges is rarely so little as 8° or 10°.

As bearing upon the general hypothesis that the great structural feat-
ures are produced by the action of tangential forces generated by the secu-
lar contraction of the earth’s interior, it may be remarked that the displace-
ments of the Plateau Province do not furnish any evidence of the operation
of such forces. A careful study of the system of the Kaibabs and High
Plateaus has established the conviction that in those districts no such force
has operated. Evidence, however, is often discerned that the strata, while
undergoing displacement, have been subject to tension arising from the
increased length of profile caused by the undulations so produced. This
lengthening of profiles in the vicinity of the monoclinals is indicated by
the repetitive faults with an oblique hade or underlie; and sometimes also
by the dropping of a long wedge of strata between two faults with con-
verging hades: Complications of this character often appear as super-
imposed features upon the great monoclinal flexures.

CHAPTER III.

VOLCANIC GHOLOGY.

A region of extinct voleanism.—Initial epochs.—Tufas.—The most ancient eruptive rocks.—Propy-
lites.—Hornblendic andesites.—Trachytes.—Rhy olites.—Basalts.—The order of succession of the
eruptions.—Richthofen’s generalization sustained by the succession presented by the High Pla-
teaus.—Certain modifications of the order given by Richthofen.—Resolution of the order into two
semi-series.—F'ragmental volcanic rocks.—Their great extent and mass.—Two classes of frag-
mental deposits.—Tufas.—Considerations as to their origin and mode of accumulation.—They
are the detritus of more ancient lavas.—Their age.—Volcanic conglomerates.—Their texture and
petrographic characters.—Modes of stratification.—They originate from the break up of massive

lavas, and are chiefly alluvial accumulations.—Metamorphism of the clastic volcanic strata.
) y 1

The District of the High Plateaus is a region of extinct volcanism.
The magnitude of the eruptions which have taken place there is small com-
pared with what we know of some other regions, but it is great when com-
pared with what we may see in most of the volcanic districts of Europe.
It is smaller, I presume, than that of Iceland, but greater than that of Autna
or Central France. It is not the magnitude, however, which is so very
striking or suggestive, but the variety of the phenomena and the great
stretch of geological time through which their history ranges. The oldest
eruptions go back to the middle Kocene; the latest cannot be as old as the
Christian era. It is hard to believe that they are as old as the conquest of
Mexico by Cortez. Between the opening and cessation ,of that activity
(if, indeed, it has even yet ceased forever) the eruptions have been inter-
mittent. There have been long periods of repose, but during the pauses
the subterranean forces were only gathering strength and material for fresh
outbreaks.

The highest interest in the region lies in the remarkable variety of the
phenomena presented. It lacks but little of being a complete category of
voleanology, and what it lacks it compensates by presenting something new.
Nearly every form of eruption is exhibited. Every great group of vol-

vv

56 GEOLOGY OF THE HIGH PLATEAUS.

canic rocks, and at least three-fourths of all the important sub-groups have
here their representatives. The clastic derivatives are displayed in variety
and volume truly extraordinary, commanding as much attention as the
massive rocks and presenting some highly interesting problems. It would
be impossible, within the limits of a single chapter, to present a good
synopsis of these facts with a discussion sufficiently extended (and at the
same time precise) to make them intelligible. Since the greater part of
the individual phenomena described in this work consists of those which
belong to the volcanic category, and since no symmetrical grouping of
their entire array has suggested itself to my mind, it will be practicable
to set forth here only those few facts of a high degree of generality which
appear to be applicable to the entire district. In those chapters of this
book which are devoted to the description in detail of the component
members of the High Plateaus, such facts as seem to be instructive will be
adverted to, together with such of their relations as have been satisfactorily
ascertained.

The initial epochs and conditions of the eruptive activity of the High
Plateaus are obscure. The oldest observed rocks having an eruptive origin
are tufas. It is presumable, however, that tufas, especially such as are
here found, are never erupted alone, nor wholly in the fragmentary or pul-
verulent form, but are in part the concomitants of lava floods, and in far
greater part the results of the degradation of volcanic rocks. The tufas of
this district are stratified water-laid rocks of arenaceous texture, sometimes
marly or even shaly; their materials being derived almost entirely from the
decay of lavas. Some of these tufaceous beds are metamorphosed, and the
highly suggestive and interesting fact is there presented that the product of
this metamorphism is a rock having the essential lithologic characters of a
lava.* The rocks from which these ancient tufas were derived are not known.
An abundance of old lavas lie in their vicinity, but always on top of them.
There is, however, one instance in the great gorge near Monroe where a
propylitic mass appears to pass under some of these tufas, but owing to the
complications of faulting there may be a mistake about it. Whether the

lava sheets which yielded by their decay the clastic materials of these

*See Chapter XJ, where this remarkable phenomenon is described and discussed.

EPOCHS OF ERUPTION—PROPYLITES. tl

deposits still remain buried beneath the immense outpourings of middle and
later epochs, or whether they have been wholly dissipated, it is impossible to
affirm. The period during which these tufas were stratified must be referred
to the latter part of the Eocene. They rest everywhere upon beds, which
are either of Bitter Creek or Green River age—are, in fact, the latest strati-
fied masses of the region. Onthe other hand, they must have been depos-
ited before the final desiccation of the great Eocene lake, which appears to
have taken place throughout that part of its expanse now covered by the
High Plateaus after the middle and before the close of the local Eocene.
They are widely distributed, and could not very probably be supposed to
have accumulated in local temporary lakelets. Thus, then, the opening of
the eruptive activity goes back into Eocene time.

The oldest massive rocks of volcanic origin are found in but few places.
The tabular masses which now front the long valleys with escarpments sev-
eral thousands of feet in height have been scored by ravines, which cut
into their innermost recesses. Here, with thousands of feet of more recent
lavas and conglomerates above them, are found large bodies of propylite
and hornblendic andesite, the former clearly the more ancient of the two.
The propylitic masses appear to have been much degraded by erosion
before the eruption of the andesites, for patches of conglomerate with water-
worn propylitic fragments are overlaid by masses of andesite, and the con-
tact of the two is often of such a nature that there can be no doubt that the
massive propylites were water-carved before the andesites were erupted.
It is impossible to say anything concerning the extent of these most ancient
emanations, for the later rocks have completely buried them, and all that
can be seen are the few exposures laid bare by recent faults and excava-
tions. Two centers from which these rocks came have been determined,
and they are also found in two other localities, but under circumstances
which render it quite possible, and perhaps probable, that the two latter are
connected with the two former, the continuity being lost beneath later
accumulations. The two eruptive centers are located, respectively, in the
northern and southern portions of the Sevier Plateau. The two exposures
exhibiting propylitic rocks, which may have been derived from these erup-
tive centers are situated in the grand gorge of the Fish Lake Plateau, and

58 GEOLOGY OF THE HIGH PLATEAUS.

in the deepest ravines of the Awapa, near the Aquarius, where profound
excavations, near the great faults, have disclosed them beneath nearly 3,000
feet of trachytes.

A question has been carefully considered, without reaching a positive
conclusion, whether the tufaceous beds already spoken of may not have
been derived from the waste of these propylites. The tufas are wholly
water-laid beds. Their ordinary aspect is well represented in Heliotypes V
and VI. The stratification has all of the mechanical characters of ordinary
arenaceous beds. In numerous places the tufas are seen to pass horizon-
tally by gradual transition into ordinary arenaceous shales, made up wholly
of materials derived from the decay of non-eruptive rocks. The propy-
lites alone of all the massive rocks seem to have sufficient antiquity to have
supplied the material for these deposits, and the only question seems to be
whether these came from the visible propylites or some unknown volcanics
of still greater age. The tufas have been carefully studied with the micro-
scope in the hope of settling the question, but no solution has been reached.
They contain large quantities of quartz and feldspar, which are often
epigenetic, and the remaining contents are so much decayed that their
original characters are obliterated. But although the antecedence of the
propylites to the tufas cannot be proven, it may at least be said that there
is no fact now known which forbids such a conclusion. More than that,
the inference has some slight preponderance of probability in its favor.

The hornblendic andesites succeeded the propylites with apparently a
long interval between them. They were erupted from the same localities
or from vents in the immediate vicinity. The mass of these rocks now
exposed is greater than that of the propylites, and the lavas are consider-
ably more varied in texture and appearance. ‘Their principal locus seems
to have been in the southern part of the Sevier Plateau, though the masses
revealed in the northern part of the same uplift are but little inferior. The
outbreaks were in massive sheets, which stretched far to the eastward and
southeastward, spreading out over large areas and piling up mountainous
masses. It is not, however, the quantity now exposed which gives us the
real clue to the magnitude of the andesitic extravasations, but rather the

great bulk of the conglomerates derived from their ruins. The andesites,

_

EPOCHS OF ERUPTION—TRACHYTES. 59

considerable as they were, have been chiefly buried by trachytes, but the
conglomerates derived from them are still conspicuously displayed. These
fragmental masses lie around the eruptive centers in beds often more than
a thousand feet thick, and cover areas of which the aggregate extent must
considerably exceed 500 square miles.

The third epoch of activity was by far the grandest of all. It was
marked by the extravasation of trachytic masses, alternating with augitic
andesites and dolerites. A long interval of time separated these eruptions
from the andesitic outbreaks just described, for the andesitic rocks were
extensively degraded by erosion and their fragments gathered into con-
glomeritic masses before the earliest outpours of true trachyte. The area
of activity was greatly extended in the trachytic age, new places cpened
and poured forth immense floods, which at length became so vast that they
overwhelmed and buried the greater part of the district, generating a new
topography. The northern part of the Sevier Plateau, which had given
vent to the propylites and andesites, became a focus of still more extensive
trachytic eruptions. From this center they spread in all directions. Those
which rolled eastward are most conspicuously displayed, and the first
impression is that the larger portion of the trachytes flowed in that direc-
tion. Some of the grander sheets extended more than 20 miles to the
southeast of their origin, and die out near the base of Thousand Lake
Mountain. To the southward they make up the greater part of the bulk of
the Sevier Plateau, reaching nearly 25 miles from the vents, and comimin-
gling with floods poured from median vents in the plateau. To the north-
ward they stretched beyond the locus of Salina Cation, where they have
been much wasted by erosion, but heavy masses are still left to indicate
their former magnitude. 'To the westward the sheets are abruptly cut off
in the face of the escarpment of the west front of the Sevier Plateau,
which reveals more than 3,000 feet of their mass resting upon the andesites
and propylites. Beyond this a great fault throws down Sevier Valley, in
which they are seen in a few places beneath later rhyolites.

It is by no means certain that all the foci of eruption have been ascer-
tained. So great have been the changes produced by erosion, that the
superficial features have been thoroughly remodeled by it. No lofty,

60 GEOLOGY OF THE HIGH PLATEAUS.

AMtna-like summits or craters are visible, and it is doubtful whether the
method of eruption was generally such as would generate mountains
of that character; for the larger deluges appear to have emanated from
fissures located within restricted areas. Yet apparently some piles of
important magnitude were reared by the successive superposition of coulées
around a central vent or pipe, and still bear evidences of their origin, though
they have been reduced to mere remnants by the wear of ages.

In the southern part of the district several foci of eruption are discern-
ible. The most important was just east of the old andesitic center. From
this one emanated the dark trachytic masses which have built up a great
portion of the Aquarius. Another was situated at the southern base of
the Tushar, and disgorged the masses which built the southern portion of
that range. A line of vents stretched southwest from the Tushar along
the western crest of the Markagunt, and sheeted over the greater part of
that plateau. Still another occupied the position of Mount Hilgard, at the
extreme eastern boundary of the High Plateaus, and a chain of vents
stretched southward from it to Thousand Lake Mountain. Around the out-
skirts of the more compact inner district many minor eruptions occurred,
overflowing numerous outlying patches.

The rhyolitic eruptions occur chiefly in the Tushar, the Pavant, and
Markagunt—in a word, belong to the western margin of the district. Their
grandest masses are displayed in the northern portion of the Tushar. They
form the summits of this range, standing in high peaks, which are the
loftiest in Utah, excepting two or three in the Uintas. Here no other erup-
tive rocks are associated with them, except a few small outbreaks of basalt
which overlie them. The platform upon which they lie consists of meta-
morphic Jurassic sandstone, upon the eroded surface of which they were
outpoured. We find here evidence that the eruptions did not occur in
rapid succession, but were separated by intervals of time sufficient to
accomplish much erosion. Old valleys scored in the older lavas were filled
up by later floods, which were, in turn, chasmed with ravines, revealing
the contacts, and this process was repeated again and again.

Two groups of rhyolitic rocks may be discerned in this locality, each

presenting great variety in the texture, as is always the case with rhyolites,

EPOCHS OF ERUPTION—RHYOLITES AND BASALTS. 61

but each preserving certain dominant features. The older of the two has
the character of liparite—a porphyritic texture with conspicuous crystals of
feldspar and quartz, and having a superficial resemblance to some common
trachytes, but more glassy or hyaline. They are usually very dark colored.
The later varieties are nearly white or cream colored—sometimes ashy-gray,
without any apparent crystals even under the microscope, but showing a
reticulated or globulitic ground-mass of great beauty and interest. The
rhyolites of the Markdgunt have a superficial resemblance to trachyte,
being dark gray and porphyritic, with a texture which is decidedly trachy-
tic, but the abundance of free quartz and the fluidal aspect of the ground-
mass under the microscope reveal its true affinities unmistakably. Upon
the western verge of this plateau they have piled up some lofty masses
with broad tabular summits. They are seen in many places to rest upon
older trachytes and in others are overlaid by basalt.

The basaltic eruptions were very numerous throughout the district, but
never attained the magnitudes seen in the other groups. Most of the indi-
vidual coulées are relatively small. The largest masses are seen on the
southwestern flank of the Tushar. Here numerous eruptions from the same
vents have piled up nearly a thousand feet of basalt and spread the lava
confusedly over a considerable area. A large field, with many cones still
standing in a dilapidated condition, is found at the extreme southern portion
of the Markagunt, and a somewhat smaller basaltic area is found in the mid-
dle of that plateau. ;

In every case true basalt is here the youngest of the eruptive rocks,
but much of it still shows considerable antiquity. In the Tushar the larger
vents have been so far obliterated that the cones have vanished and left the
determination of the sources of the lavas to other characters. In the cen-
tral part of the Markigunt the cones have nearly faded away, but are still
recognizable. On the other hand, some of the basalts are strikingly recent,
and a few so fresh that no appreciable change has taken place since their
orifices became silent. Just south of Panquitch Lake, in the Markégunt,
are a number of streams, which, so far as appearance is concerned, might
have been erupted less than a century ago. Half a dozen other streams, in
various localities, might be named of which the antiquity can hardly exceed

62 eS GEOLOGY OF THE HIGH PLATEAUS.

a very few centuries. The cones are perfect, the lava is not faded by time,
and even the spongy, inflated scum of the surface is still black as coal or
faintly tinged by atmospheric reagents. That the basaltic period was a
long one is further manifest by the fact that on the southwestern flank of
the Tushar is a conglomerate composed wholly, or nearly so, of basaltic
materials. These were derived from the degradation of the massive basalts,
which have overflowed that part of the range, and they are well stratified
after the peculiar manner of sub-aérial conglomerates.

The basalts, in choosing localities for eruption, show here a tendency to
abandon those parts of the district which had been the seats of the grander
outbreaks of earlier periods and to find new and independent localities
for their extravasation. It is not always so, however, for the greatest
basaltic floods outpoured hard by one of the most important centers of tra-
chytic eruption. But, on the whole, their situation relative to the older
masses is peripheral. In the Markagunt the greater part of the basalts lie
upon the sedimentary beds. In addition to this, we find many lone vents,
or a small cluster of them, standing far away from the central fields of more
ancient lavas. A large number of basaltic streams have emanated from
the very walls themselves. In truth, no one can fail to be struck with a
peculiar habit which they manifest of seeking strange places from which
to break out. Very many cones are perched upon the brinks of the ter-
raced cliffs or canon walls. In the western wall of the Paunsdgunt the lava
has broken out from the very face of the wall itself. The least common
place for a basaltic crater is at the base of a cliff’ In a great majority of
cases the vents stand near the faults, but the curious part of it is that they
break forth almost always upon the lifted and very rarely upon the thrown
side of the fault.

All of the basalts are of the feldspathic varieties, none of the nephelin
and leucite bearing varieties having been met with.

THE ORDER OF SUCCESSION IN THE ERUPTIVE ROCKS.

The views of F. Baron Richthofen on the succession of eruptions*
have received from American geologists profound attention. Probably no

*A Natural System of Voleanic Rocks. Memoir presented to the California Academy of Sciences
by F. Baron Richthofen, May 6, 1867.

RICATHOFEN’S ORDER OF SUCCESSION OF ERUPTIONS. 63

living observer has studied this problem more carefully nor included in his
observations and generalizations a wider field. His extensive knowledge,
his great acumen, and his ability to generalize brilliantly, though cautiously,
entitle his conclusions to the most earnest consideration. As the result of
his study of voleanic phenomena in many portions of the world, he believes
that the various kinds of eruptive rocks reveal a certain order of succes-
sion in their relative ages of eruption throughout Tertiary time. Arrang-
ing these rocks according to their physical properties and intimate constitu-
tion into five groups, or orders, he finds that they have been erupted in the
following sequence:

1. Propylite.

2. Andesite.

3. Trachyte.

4. Rhyolite.

5. Basalt.

It will seldom happen that more than two or three of these kinds of
rock will be found in direct superposition, the series in any given locality
being always incomplete, and in very many cases a single kind will alone
befound. But wherever two or more are found superposed, the one haying
the prior enumeration in the foregoing list will be the older. The only
exceptions would be where each order of rocks is represented by numerous
individual outbreaks, when the later extravasations of the older order may
occasionally be seen to intercalate with the older extravasations of the later
order. These considerations apply to what are termed ‘‘massive eruptions,”.
where deluges of lava have broken forth from fissures and overwhelmed
the adjoining regions with coulées far exceeding the ordinary emanations
of common volcanoes. They also apply to the history of those grander
vents which have maintained an activity lasting through a considerable
proportion of Tertiary time. But the smaller vents as a rule are of very
brief geological duration, and seldom disgorge more than one kind of lava.
In support of his generalizations he adduces his own extended observations
in Hungary, Germany, and the Sierra Nevada, and those of many colabor-
ers in Armenia, Mexico, Central and South America.

Those geologists who have made a special study of the volcanic rocks

64 GEOLOGY OF THE HIGH PLATEAUS.

of the Rocky Mountain Region from the Great Plains to the Pacific (each
within the limits of his own special field), are almost wholly in accord in the
belief that Richthofen’s law of succession is there sustained. This great
field is indeed not yet fully explored, but a very considerable portion of it
has been examined. The display of the phenomena of extinct volcanism is,
when taken collectively, probably the most extensive and varied in the world.
The magnitude and abundance of the eruptions increase as we proceed
westward. In the Basin Ranges hardly one fails to show important masses
of eruptive rocks, and in many of them such rocks constitute the greater
portion of the visible bulk of the ranges. This is especially true of the
southern Basin Ranges south of the thirty-eighth parallel, and still more
emphatically true of Oregon, Northern California, and the Territories of
Washineton and Idaho.

Of these individualized areas the District of the High Plateaus is a con-
spicuous member, though probably far below some of them in magnitude.
But among those which have hitherto been brought to notice, none, I believe,
present so full and so approximately complete a lithological series. Here
then, if anywhere, we ought to find the means of putting Richthofen’s law to
the test. This was felt after the first season’s work had revealed the ampli-
tude and variety of the materials, and throughout the subsequent study of
the district was never lost sight of* As a result of the study, I am satisfied
that Richthofen’s law is on the whole sustained. Yet there are certain quali-
fications which are required in order to express the exact nature of the
sequence. These do not essentially affect the validity of the law as a whole,
but rather are supplementary to it

There can be no question that the oldest erupted masses now visible
there are propylites. Next in age follow the hornblendic andesites. The
third series of eruptions, which were by far the most extensive, included tra-
chytic rocks, but not trachytes alone. Their associates will be spoken of

*Tt may not be amiss to state here that at the commencement of the study I had no prepossession
in favor of Richthofen’s views—possibly the contrary. I felt rather an intense curiosity. After a year’s
examination I was inclined to the belief that his generalization was not applicable to this district, or
was at most very imperfectly so. It was apparent, however, that there was much complexity, and I
determined to examine the best exposures thoroughly and endeavor to unravel this complexity, if possi-
ble, in order to ascertain whether any real order of succession existed, or whether the sequences were
only accidental or capricious. The result will be seen in the text.

THE OBSERVED ORDER OF SUCCESSION OF ERUPTIONS. 65

presently. The rhyolites as a group are decidedly younger than the
trachytes. Wherever the two are found in contact the priority of the
trachytes is, so far as observed, without an exception. Still, there is little
question in my own mind that some of the more ancient rhyolites of the
Tushar are older than many outbreaks of trachyte in other localities.
Finally, the basalts are clearly the youngest of all eruptions

If this stated the whole case, we should have the essence of Richtho-
fen’s succession almost perfect. The qualification becomes manifest when
we come to the study of the trachytic series. Blended with the heavy
masses of trachyte, we find in all of the greater exposures rocks of a totally
different character. These intercalary sheets belong to the sub-basic or
nearly basic groups, and may be designated, according to their constitution
[augitic trachyte], augitic andesite, or even dolerite. It will be seen at
once that we have here a group of rocks united by certaim common char-
acteristics: First, the possession of notable quantities of augite, sufficient, in
fact, to render that mineral a distinguishing compound; second, a similarity
of habit and facies, which, though distinctly varied, yet vary within quite
moderate limits. The habit and facies are markedly basaltic, being greater
or less degrees of that characterization which is superlative in true basalt.
The older varieties of these intercalary rocks sometimes carry a predomi-
nating amount of orthoclase, which marks them as augitic trachyte; some-
times predominant plagioclase, which relegates them to the augitic andesites.
The later varieties exhibit those peculiar labradoritic feldspars in conspicu-
ous, often “ glassy,” crystals, polarizing in gorgeous bands, with rare sanidin
and copious augite included in a glass-bearing base. They are usually
coarsely crystalline, and have the rough fracture of some typical trachytes,
from which, however, they are separated both chemically and mineralogi-
cally.* Such rocks would be designated by Zirkel augitic andesites, I pre-
sume, but it seems best (with the greatest deference to such an eminent
author) to call them dolerites, and to restrict the designation augitic andes-
ites to less basic varieties.

We have, then, in the age of trachytic eruptions, two series of lavas

*No doubt it was such rocks to which Abich gave the name ‘“ trachydolerite.” Deiters recog-
nized in the Siebengebirge a regular transition from trachyte to dolerite. Zeitschr. d. d. Geol. Ges. 1861.

5 HP

66 GEOLOGY OF THE HIGH PLATEAUS.

intercalating with each other and presenting certain antitheses We have
as the dominant group the true trachytes—rocks having the characteristics
of the sub-acid class, and augitie rocks with the characteristics of the sub-
basic class.* And it is interesting to compare this association with Scrope’s
observations in the Auvergne. It has already been remarked that the vol-
canic phenomena of the High Plateaus reveal a striking similarity to those
of Central France, though upon a much grander scale. Scrope frequently
alludes to the general impression prevalent long before he made his investi-
gations in that region, and held by many at that time, that the basalts were
younger than trachytes, and he frequently contests the correctness of that
opinion. Time and again he cites instances where he finds basalts lying
beneath trachytes as proof that the rule is by no means invariable. It
would be most interesting to know whether he has not included among his
“basalts,” as scores of other most careful observers have done, those iden-
tical rocks which have here been described as augitic trachytes, andesites,
and dolerites, and which a more rigorous classification would separate from
the basalts.

Leaving here these groups to return to them presently, we may ad-
vert for a moment to the relative age of the rhyolites. Instances occur
where it is probable that some of the oldest liparitic outpours are consider-
ably more ancient than some of the youngest trachytes. No infraposition
of rhyolite to trachyte has been observed im situ, but indirect reasoning
leads to the conclusion that the central rhyolitic masses of the Tushar were
erupted long before the effusion of some of the trachytes of the Sevier
Valley. There are many instances in the Markdgunt of rhyolite overlying
trachyte, and the more recent age of the former as a group is perfectly
apparent and incontestible. Lastly, the true basalts everywhere reveal
their greater recency than all other rocks.

“It now becomes of interest to inquire whether this sequence is cor-
related in any regular and progressive manner with the physical properties
or constitution of the rocks themselves ; whether there is with the progress
of the voleanic cycle any regular or systematic method of variation in the

chemical constitution, mineral constituents, specific gravity, texture, or other

* The classification here adopted is fully set forth in the next chapter.

THE OBSERVED ORDER OF SUCCESSION OF ERUPTIONS. 67

properties of the rocks. This inquiry immediately presents itself the instant
we settle upon the conviction that eruptions have an assignable order of
occurrence, and the mind at once springs to the conclusion that there ought
to be such an association. If there be an order of eruption, there must be
a cause for it, and for that cause we look to the properties of the rocks
themselves. But at first glance no such correlation appears. If we arrange
them in a series expressing the great groups in the order of their chemical
constitution, and place in juxtaposition an arrangement according to the
order of eruption, we fail to find at first a clear correlation. Taking Richt-
hofen’s five orders, we have the following comparison :

Arrangement by chemical constitution. Arrangement by order of eruption.
1. Rhyolite. 1. Propylite.
2. Trachyte. 2. Andesite.
3. Propylite. 3. Trachyte.
4. Andesite. 4. Rhyolite.
5. Basalt. 5. Basalt.

With chemical constitution go the other properties, mineral constitu-

ents and specific gravity. No relation here presents itself to the order of

eruption. Yet I think that upon closer inspection a systematic correlation
| may be made to appear by an examination of the sub-groups instead of the
great groups, and the correlation of the sub-groups will reflect itself in the
great groups. Taking the more important sub-groups, those which are most
persistent in their characters, of most frequent occurrence, and of the larg-
est volume, the following succession of eruptions presents itself in the
High Plateaus :* .
. Hornblendic propylite.
. Hornblendic andesite.
. Hornblendic and augitic trachytes (less acid trachytes).
. Augitic andesite (Richthofen).

ou > © DW Ft

. Sanidin trachyte (more acid trachytes).
. Liparite.

. Dolerite.

. Rhyolite (proper).

COM COM SD

. Basalt (proper).

* For classification and exact meaning of terms here employed see next chapter.

te ae

68 GEOLOGY OF THE HIGH PLATEAUS.

Now, let us make the following arrangement. Place at the head of
the series hornblendic propylite. Select from the list in the order given
those rocks which are more acid than propylite. Take next those which
are more basic than propylite, and write them also in the order in which
they occur. We shall then obtain the following grouping:

1. Hornblendic propylite.
3. Hornblendic trachyte. 2. Hornblendie andesite.

5. Sanidin trachyte. 4, Augitic andesite.
6. Liparite. 7. Dolerite.
8. Rhyolite. 9. Basalt.

This resolves the lithologic series into two semi-series, each of which
displays a distinct and unmistakable progression of chemical and physical
properties.. The first includes the acid and sub-acid groups, which increase
in acidity with the process of the volcanic cycle. The second includes the
basic and sub-basic groups, which correlatively decrease in acidity. The
law may be thus expressed in terms of chemical properties to which the
physical properties stand ina relation of dependence: At the commencement
of the volcanic cycle the rocks first erupted are those which belong to the
middle of the lithological scale. As the cycle advances, the rocks resolve
themselves into two semi-series, growing more and more divergent in char-
acter, and when the end of the cycle is neared they become extreme in
their contrast.

Taking Richthofen’s five orders (major groups) and arranging them on
the same plan, we may express the same correlation as follows:

1. Propylite.
3. Trachyte. 2. Andesite.
4. Rhyolite. 5. Basalt.

Possibly it might be thought that this mode of finding a sequence and
a correlation bears a resemblance to some problems in the properties of
numbers, in which, any fortuitous collection of numbers being taken and
treated to certain manipulations, a law of arrangement appears; the real
explanation being a latent petitio principii. But this is not so. Even if we
took Richthofen’s five orders only, the probabilities against a merely fortu-
itous coincidence of orders of eruption with the above double sequence of
physical properties would be as 3 to 1. But if we apply the same treat-

FRAGMENTAL VOLCANIC ROCKS. 69

ment to nine sub-groups and find the law still holding good, the probabili-
ties against a fortuitous coincidence becomes thousands to one; in other
words, a practical certainty. It only remains to discuss the subject as a
question of facts and not of inferences. Do the eruptions follow this law?
There are certain sub-groups which have not been named in the fore-
going arrangement, such as quartz-propylite, dacite, phonolites, &c. As
regards the quartz-propylites, there appears to be a slight departure from
the tenor of the law. Its place is among the earliest effusions, whereas in
chemical constitution it lies not far from the middle of the trachytic series.
But the disagreement is small. Dacite does not occur in the High Plateaus,
and I know too little of its relations to other rocks elsewhere to offer any
discussion.* But all the other sub-groups, so far as observed, harmonize
admirably with the deduced relation, and in truth I can only express sur-
prise at finding not one instance of real anomaly between rocks which occur
in superposition, although such instances have been carefully sought for
during two prolonged and active seasons’ work and were anticipated.

FRAGMENTAL VOLCANIC ROCKS.

Some of the most interesting lithological problems presented by the
volcanic products of the High Plateaus are those relating to the origin and
development of what may be termed the clastic igneous rocks, or rocks
apparently composed of fragmental materials of igneous or volcanic origin,
but now stratified either as so-called tufaceous deposits or as conglomerates.
These are exceedingly abundant in all of the great volcanic districts of the
world, and often enormously voluminous. How those of the High Pla-
teaus would compare, in respect to magnitude, with those of other regions, I
do not accurately know, but absolutely their bulk is a source of utter
astonishment. They cover nearly 2,000 square miles of area, and their
thickness ranges from a few hundred feet to nearly 2,500 feet, the average
being probably more than 1,200 feet. Lavas are frequently intercalated,
but much more frequently no intercalary lavas are seen, and in general
they seldom form any large proportion of the entire bulk when they occur
in conjunction with the clastic masses. The grander displays of these frag-

mental accumulations are seen in the central and southern portions of the

* From present knowledge Lam inclined to inter that dacite is about as anomalous as quartz-propylite.

70 GEOLOGY OF THE HIGH PLATEAUS.

district, though a few important ones are found in the northern part of the
field. The great western wall of the Awapa, the central and southern mass
of the Sevier Plateau, the southern Tushar and northern Markégunt, are.
composed chiefly of such formations. The grand escarpments which wall
the imposing fronts of these plateaus are conglomerates, sometimes capped
with lava, sometimes intercalated, and more frequently without them. Near
the center of Grass Valley we have, on the east, bounding the western
verge of the Awapa, a wall of conglomerate which is more than 2,500 feet
thick ; and directly opposite, to the west, forming the eastern front of the
Sevier Plateau, is an exposure of very nearly equal magnitude, both stretch-
ing southward for 25 miles without interruption, save where erosion has
opened great gorges and ravines, though diminishing in thickness. From
a point a few miles southeast of Marysvale the western front of the Sevier
Plateau exhibits a wall of similar nature, extending south a distance of more
than 40 miles to the terminus of the plateau, with only two brief interrup-
tions. The southward expansion of the Sevier Plateau is made up chiefly
of such masses, and they reappear in the western flank of the Aquarius
beneath its monstrous lava cap. Their thickness will average here much
more than a thousand feet. In the northern part of the Markagunt they
appear to constitute the principal bulk of the area, though no deep expos-
ures are found and their thickness cannot even be conjectured. The south-
ern part of the Tushar rears a wall of similar nature, revealing nearly or
quite 2,000 feet of conglomerate, covering an area of at least 150 square
miles, and probably very much more. The East Fork Canon is cut trans-
versely through the narrowest part of the Sevier Plateau, and exhibits on
either side a series of terraces rising 5,500 to 4,000 feet above the bed of the
stream. The lower 600 to 800 feet consist of “‘tufaceous” sandstones, and
above them are more than 2,500 feet of coarse conglomerate, with a few
massive sheets of intercalary lava. These clastic beds are everywhere seen
throughout the central and southern portions of the district and are built
upon a giant scale.

Equally striking is the remarkable variety presented in their mechani-
cal texture and structure, whether we consider it in the hand specimen or

in the palisade and canon wall. We may consider them under two classes,

FRAGMENTAL VOLCANIC ROCKS—TUFAS. 71

which are, ordinarily, fairly distinguished from each other, though sometimes
we find transition varieties connecting them. The first are the finer clastic
beds, which are usually termed tufas or tuffs; the second are the coarser
beds, generally termed volcanic conglomerates.

I. Turacrous pErosirs.—It has been noted of most of the volcanic
regions of the world, where the period of activity reaches backward well
into Tertiary time, that the earliest material erupted is seen in the present
form of arenaceous or fragmental deposits. The finer or tufaceous beds
have by many geologists been regarded as consisting of material blown out
in a pulverulent form, and which, gathering into the drainage channels, was
swept into neighboring bodies of water or descended there directly, and was
stratified after the manner of sand or silt. Thus they infer that the volcanic
activity in such regions was opened by the discharge of fragmental mate-
rials or ‘volcanic ashes,” which, projected upwards, were wafted by the
winds and precipitated over the adjoining country or waters. This view
will be discussed further on.

There can be no question that the most ancient volcanic materials
hitherto distinguished in the District of the High Plateaus, and of which
the relative age can be assigned, are certain sandstones or beds composed
of exceedingly fine particles of shattered or rounded quartz crystals, feld-
spar, hornblende, and mica commingled in a base of amorphous matter,
which is chiefly argillaceous or kaolinic and charged with oxides of iron.
Wherever the grains are large enough to show their characters or have a
gravelly consistency, they exhibit very clearly minute fragments of volcanic
rock in a decayed or carious condition, resulting from the prolonged action
of water and the atmosphere, and also show extreme mechanical attrition.
This serves to distinguish them from ordinary sandstones, which are usually
composed of rounded quartz-grains. In the tufas quartz-grains occur in
insignificant proportions, and in their place we find granules of the complex
but very massive and obdurate volcanic rocks. Fragments of hornblende
and mica also occur, sometimes in great abundance. The condition of the
ferruginous matter in the tufas is also very different in most cases from its
condition in ordinary sedimentary beds. In the latter rocks it is usually
present as a peroxide, sometimes hvdrated, sometimes not. In the tufas it

ie, GEOLOGY OF THE HIGH PLATEAUS.

usually occurs either as the magnetic oxide or protoxide. In the protoxide
forms it is always in combination in some of the minerals—the undecom-
posed hornblendes and micas or such alteration products as epidote or viri-
dite. These alteration compounds, particularly, are more or less thoroughly
diffused throughout the mass of the rock, impregnating it with a greenish
color, while the unchanged mica, hornblende, and magnetites, disseminated
as black particles, give the rocks a gray color of varying shades from very
dark to very light. Whenever these beds have been subject to metamor-
phic action, as has often happened, the proto-compounds of iron are often
converted into sesquioxide, producing a pinkish color similar to that of
“Scotch granite.” Thus the colors of the tufaceous beds would enable us
to single them out as presumably composed of materials very different from
those constituting ordinary sandstones.

All of these finer beds are stratified after the manner of ordinary
aqueous deposits. That they were water-laid is unquestionable. No rocks
have been observed which could possibly have been accumulated by the
precipitation of volcanic ashes upon the land. The agency of water in
arranging them in their present form is altogether too conspicuous to admit
of any doubt. ‘The origin of these clastic materials, proximately considered,
is in the break up and destruction of older massive volcanic rocks by the
ordinary processes of denudation. It is, indeed, possible that some small
proportion of their ingredients may have been pulverulent material blown
from volcanic orifices and washed into the basins where the strata accumu-
lated, but it seems quite certain that the great bulk of the tufas did not so
reach their present positions. They differ in no other material respect from
the common lacustrine beds than in the sole fact that they are the débris of
voleanic rocks instead of sandstones and gneisses. Inanumber of instances
they are seen to pass, along horizontal exposures, by a gradual transition,
into common lacustrine deposits, the quantity of material derived from the
break up of vocanie rocks becoming gradually less and less, while that
derived from the disintegration of foliated rocks becomes greater and
ereater. Instances of this transition are seen in various parts of the Sevier
Plateau and in the beds beneath the lava-cap of the Markdgunt. Indeed, I

doubt not that those beds, which are apparently most typically “‘ tufaceous,”

“ye. oe

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FRAGMENTAL VOLCANIC ROCKS—TUFAS. 73

in reality hold among their ingredients a notable percentage of intermingled
grains and silt derived from the denudation of sandstones or other quartzif-
erous rocks. Thus, these tufas would seem to be nothing more than sand-
stones and shales of the ordinary kind, so far as their mechanical characters
are concerned, and having the same genesis as any clastic strata, but the
materials of which they are composed being derived from volcanic instead
of from foliated common rocks. .

On this view of the case there is no apparent reason why they should
be sharply distinguished from other strata. It would, indeed, be unjustifia-
ble to proceed to the conclusion that in other parts of the world the so-called
tufas have all had a similar origin, for there is abundant reason for the
belief that considerable deposits of real ‘volcanic ashes” exist elsewhere
But if the tufas of the High Plateaus are similar to those which in other
regions are supposed to be accumulations of ashes, there is reason for believ-
ing that the bulk of strata presumed to consist of materials erupted in a pul-
verulent form has been greatly overestimated, and that such strata, instead
of being common, are on the whole rare and of insignificant magnitude.
Especially I am confident that these beds do not lead at all to the conclu-
sion that the voleanic activity of the High Plateaus was inaugurated by the
ejection of vast bodies of ashes. They seem to point much more logically
to the conclusion that eruptions of lavas not now discernible or identifiable
took place before they were laid down, and were broken up and wholly or
partially dissipated to furnish their materials.

These finer deposits rest upon the Eocene beds, which in the southern
part of the district I have inferred to be of the age of the Bitter Creek
beds of Powell. Whether they are conformable or not is a question 1 can-
not answer. No unconformity has been discovered, both series being very
nearly horizontal wherever they are seen in contact It is not certain that
the tufas are immediately consecutive in age to the Bitter Creek beds, but
at all events I incline to the opinion that no great interval of time separates
them. It is an interesting point whether these tufas were deposited before
the final recession northward of the great Hocene lake, thus representing
the last strata deposited upon this part of its ancient basin, or were accu-
mulated in local lakelets which may have lingered for a period after the

74 GEOLOGY OF THE HIGH PLATEAUS.

great lake had receded. Either view is for the present tenable. The
small extent of the individual beds might argue for local lakelets. There
is no persistent formation subsequent to the Bitter Creek spreading over
the entire area of the district, but merely considerable patches of tufaceous
beds from 100 to 250 feet thick, having no discovered connection with each
other, but occurring in many localities. We find reason for presuming some
to be much more recent than others, for they rest upon volcanic sheets or
conglomerates which can scarcely be so ancient as the middle Miocene.
Those, however, which rest upon sedimentary beds are probably of middle
Eocene age, or thereabout, in the southern part of the district, and a little
more recent in the northern part of it. No distinguishable fossils have yet
been discovered in any of them. On the view that these beds are the
waste of older eruptive rocks, the opening of the volcanic activity of the
district is thus carried back into the middle or early Eocene.

II. Conctomerates.—The coarser clastic formations greatly surpass
the tufaceous beds in bulk. They are also much more variable in their
modes of stratification and mechanical texture and present problems of
great interest.

[st Teaxtwre—Like all conglomerates, they consist of rocky fragments
inclosed in a matrix of finer stuff, and both fragments and matrix are volcanic
material, without any admixture of débris from ordinary sedimentary and
metamorphic rocks. The included fragments range in size from mere
grains to blocks weighing several tons. They are of the same petrographic
characters as the massive rocks of the neighborhood, and side by side lie
pieces derived from widely distinct kinds of lava:—many varieties of rock
may be gathered from a few cubic yards of the same conglomeritic mass.
Cases occur, however, where for considerable distances along a given
stratum the fragments are all of the same variety; in some the varieties
are many; in others they are few. There is no constancy of ratio between
the quantity of rocky fragments and the sandy or impalpable matrix. In
some beds the stony fragments form but a very small proportion of the
bulk; in others, the reverse is true: and there is every possible intermediate
proportion. The individual beds are usually very heavy and thick, the

partings being rare. In many cases the dimensions of the stones are

FRAGMENTAL VOLCANIC ROCKS—CONGLOMERATES. 75)

limited in weight to a few ounces and show a sorting or selection of sizes.
But in most cases the sizes have a much wider range.

Geologists have been in the habit of distinguishing two classes of the
coarser fragmental beds. First, volcanic conglomerates ; second, volcanic
agolomerates or breccias. The conglomerates contain fragments more or
less rounded by attrition, which is held to be an indication that they have
been gathered together and arranged by the action of the water. The
breccias contain fragments which are angular and are presumed to have
been showered down around the vents from which they are supposed to
have been projected. Beds corresponding to both classes are abundant in
the High Plateaus and of very great thickness and area. But I am dis-
posed to accept the conclusion that they have all had a similar origin, and
that the projection of fragments from active vents and their descent in a
mitraille has had very little to do with their accumulation. As a rule, nearly
all of the fragments show comparatively little abrasion. Some, indeed, are
considerably worn; most of them are very little rounded at the angles of
fracture, and a great proportion are in a condition in which it is difficult to
say whether they have been abraded slightly or not at all; for when
detached from the matrix the surfaces are corroded by some action which
may bave been weathering prior to their final burial or the solvent action
of percolating water after their burial and prior to the consolidation of the
stratum. None of the fragments exhibit the sharp edges formed by fresh
surfaces of fracture. Thus, while well rounded fragments (like those of
glacial drift or stream gravel) are uncommon, it is not certain that any
notable proportion have been absolutely free from attrition. The average
amount of attrition is generally small—far less than in conglomerates
usually occurring in a regular system of fossiliferous or stratified rocks.
No sharp distinction can be drawn between those beds of which the
included fragments exhibit a considerable amount of abrasion and those in
which no abrasion can be clearly proven. There is every degree of this
action and every shade of transition Thus it becomes impracticable to
draw any line here between conglomerates and breccias.

It has seemed to me that the small amount of abrasion in the con-

glomerate fragments is susceptible of a partial explanation. The. well-

76 GEOLOGY OF THE HIGH PLATEAUS.

rounded fragments of ordinary conglomerates have been ground and worn
away by the action of sand and grit carried in suspension by the water.
Now the ordinary arenaceous particles are quartz granules, which are
exceedingly hard and much more efficient in effecting abrasion than gran-
ules of softer material would be. But in a volcanic district, where the only
rocks yielding fine detritus are volcanic rocks, quartz sand is a scarce arti-
cle. The mud and fine stuff carried by the streams consist of fragments of
the rocks themselves, particles of feldspar, mica, hornblende, and still more
largely clay stained with iron oxide None of these materials possess the
hardness of quartz and their abrading power is consequently much less.

The great magnitude of these formations is by itself a source of great
perplexity when we inquire as to their origin. Looking up from the val-
leys below to the vast palisades which stretch away into the distance, and
seeing that they are chiefly composed of this fragmental matter, we seem
to be face to face with an insoluble problem. How did all this material get
to its present position and whence came it? That it was blown into the air
in a fragmentary condition and showered down into strata is an explanation
which becomes more and more untenable as our studies progress, and at
length comes to look quite absurd. These conglomerates are often seen
with a thickness of nearly 1,000 feet at distances ranging from 6 to 12
miles from the nearest eruptive focus, and filling all the intermediate space
between their outer boundary and the central eruptive mass to which we
look to find their origin. Prodigious as the projectile force of volcanoes is
known to be, there are no recorded observations which warrant the belief
that this force ever becomes so transcendent as would be necessary to hurl
such enormous quantities of fragments to such distances. The highest
velocity imparted to cannon-shot (over 2,000 feet per second) would be
trifling in comparison, and they would have to rise several times higher
into the atmosphere than the horizontal distances to which they would be
thrown.

But supposing them to be showered down, let us try to imagine them
restored to the places from which the outrushing vapors or gases tore them.
What enormous vacuities we should be required to fill in order to replace

them all! This consideration by itself seems to me sufficient to refute com-

FRAGMENTAL VOLCANIC ROCKS—CONGLOMERATES. 77

pletely the notion that these fragments have been hurled into their present
positions by the explosive energy at the vents.

Scoriaceous or slagery fragments, ‘volcanic bombs,” and the many forms
which lava takes when the blast from the crater carries up portions of the
liquid and scatters them round the surrounding cone, are not found in the
conglomerates—at least I have never observed them. I will except from
this statement, however, one locality in the southern part of the Sevier
Plateau, where a profound gorge (named Sanford Cation) gives a_ brief
exposure of what seems to have been an ancient trachytic vent subse-
quently buried by massive outflows, and which is composed chiefly of cin-
ders. This can hardly be called a conglomerate, however. The fragments
of the true conglomerates are apparently pieces of massive lava, just such as
are riven by the frost and other agencies of secular decay from cold rocks
im situ. Very many of them show more or less weathering or corrosion of
their surfaces, and very many do not indicate a trace of such action beyond
a slight discoloration.. That these fragments have been broken from mass-
ive rocks is too patent to admit of question.

The only explanation of the origin of the conglomerates which does not
involve us in absurdity is that they are derived from the waste of massive
voleanic rocks under the normal processes of degradation manifested in all
mountainous regions. While active vents usually throw out fragmental mat-
ter in great quantities, and while some of the fragments may have been thus
derived, yet I conceive that this process has contributed but an insignificant
portion of the entirety of the conglomerates. In the chapter on the Sevier
Valley and its alluvial conglomerates, I shall describe the process, now in
visible operation, by which beds of a similar nature are accumulating at the
present day upon a scale of magnitude not inferior to that which produced the
colossal formations now seen in the palisades of the plateaus. Throughout
the valleys which intervene between the ranges of plateaus fragmental beds
are accumulating in vast masses High up in the tabular ranges the frosts,
rains, and torrents are gradually breaking up, not only the anciently-out-
poured masses of lava, but also the older. conglomerates, and are bearing
down through the great ravines and gorges the débris torn from the rocks,
and are scattering them over the valley plains in the form of very depressed

78 GEOLOGY OF THE HIGH PLATEAUS. -

alluvial cones, so flat or gently sloped that the conical form is not at first
recognized by the eye. Each cone has its apex at the gateway of some
mountain gorge, while its base is several miles out in the middle of the val-
ley. These cones are so broad and numerous, that they are confluent at
their bases and give the general impression of a very gently undulated
surface of alluvium covering the entire expanse of the valley. Could we
see them in vertical cross-section, we should find them to possess a well-
marked stratification agreeing with the stratification of the older conglom-
erates. A few fortunate exposures have here and there revealed their
internal structure, and a careful comparison leaves little doubt that the val-
ley alluvium and the ancient conglomerates were formed in substantially
the same manner and by the same process.

If it be true that these conglomerates have been derived from the sec-
ular decay of massive eruptive rocks, of which the débris have been carried
down the old mountain slopes by running water and stratified in great
beds of alluvia, then we may expect to find certain correlated facts, of
which the following are examples: (1.) We should expect to find these con-
glomerates grouped around ancient eruptive centers still preserving rem-
nants of the massive rocks which are presumed to have furnished the mate-
rial of the conglomerates. (2.) We should also expect to find that these
remnants consist of rocks of exactly the same varieties as we find in the
fragments of the conglomerates; provided, however, that eruptions from
these centers subsequent to the formation of the conglomerates have not
completely overflowed and hidden the older outbreaks. (3.) We should
expect to find the loftiest portions or crowning summits of the plateaus to
consist not of conglomerates, but of massive rocks; unless, indeed, the rela-
tive altitudes of the two classes of rocks has been reversed or modified by
subsequent upheavals or sinkages.

The general idea here conveyed is that the process which formed the
conglomerates consisted in the transportation of fragmental matter from
high-standing ancient volcanic piles to low-lying plains and valleys around
their bases or along their flanks. These relations, 1 think, are very satis-
factorily shown after a careful analysis of the facts. We may still discern
the more important ancient eruptive centers with the conglomerates grouped

METAMORPHISM OF FRAGMENTAL VOLCANIC ROCKS. 79

around them and the fragments contained in the latter agree with the rocks
remaining in the former. But there is much complication and obscurity in
many instances arising from the fact that these eruptive centers have again
and again been active, the work of one epoch being overflowed and par-
tially masked by the extravasation and still later devastation of subsequent
epochs. Moreover, the loftiest points are composed of massive rocks, and
the positions of the conglomerates are invariably below those of the centers
from which they are presumed to have emanated, except in those cases
where the relative altitudes have been changed by relatively recent dis-
placement. The general problem would have been full of anomalies, how-
ever, were we not in a position to unravel both the complications arising
from vertical movements and those from the recurrence of the volcanic
activity. But being able to restore in imagination the displaced blocks of
country, and in a considerable measure to separate into periods the course
of volcanic activity, we find by so doing that the difficulties vanish and the
facts group themselves into normal relations.

A very striking characteristic of these clastic volcanic rocks, both the
tufas and the conglomerates, is their great susceptibility to metamorphism.
Not only have the beds in many localities been thoroughly consolidated,
but they have undergone crystallization. Those tufas and conglomerates
which are of older date, and which have been buried beneath more recent
accumulations to considerable depths, rarely fail to show conspicuous traces
of alteration, and in many cases have been so profoundly modified, that for
a considerable time there was doubt as to their true character. The gen-
eral tendency of this process is to convert the fragmental strata into rocks
having a petrographic facies and texture very closely resembling certain
groups of igneous rocks. When we examine the beds in situ no doubt can
exist for a moment that they are waterlaid strata. (See heliotypes V
and VI.) The hand specimens taken from beds which are extremely
metamorphosed might readily pass, even upon close inspection, for pieces
of massive eruptive rocks, were it not that the original fragments are still
distinguishable, partly by slight differences of color, partly by slight differ-
ences in the degree of coarseness of texture. But the matrix has become
very similar to the included fragments, holding the same kinds of crystals,

80 GEOLOGY OF THE HIGH PLATEAUS.

and under the microscope it shows a groundmass of the same texture and
composition. Crystals are frequently seen lying partly in the original
pebble, partly in the original matrix, and the surfaces of fracture betray no
inequality of hardness or cleavage, but cut through the pebbles and matrix
indifferently. Microscopic examination discloses a groundmass, differing in
no very important respect from such as are displayed by many eruptive
rocks. The base, however, has, in all the instances which I have examined,
that felsitic aspect which is characteristic of porphyritic rocks, neither glassy
nor strictly microcrystalline, but exhibiting that aggregate polarization
which is not yet satisfactorily explained. There is an entire absence of
glass or fusion products in the groundmass. Free quartz is often found even
in those varieties which consist largely of plagioclase and hornblende or
augite. The fragmental character of the matrix has disappeared; not a
trace of the original clastic condition can be detected, unless it is to be
found in some of the quartzes and feldspars.

I see nothing at all incredible in the idea of metamorphism producing
rocks so closely resembling some eruptive rocks that they cannot be petro-
graphically distinguished from them. It seems rather that we ought to
anticipate just such a result from the alteration and consolidation of pyro-
clastic strata. The materials which compose them consisted originally
of disintegrated feldspar, pyroxene, and the matter which constitutes the
amorphous base of all eruptive rocks. In general they are silicates of
alumina, alkali, lime, magnesia, and iron, from which, no doubt, portions of
the soda, lime, and silica, and to a less extent the iron, potash, and magne-
sia, originally forming the massive iocks from which they came, have been
abstracted by atmospheric decomposition. ‘They still retain portions of all
these constituents, and only require the presence of conditions favorable to
reaction in order to generate feldspar, mica, hornblende, and, perhaps, fresh
quartz. Ordinarily we should anticipate that only small quantities of soda
and lime would be present, and inasmuch as these bases are necessary to
the formation of feldspar (plagioclase), only a partial crystallization would
result. There would be left a considerable quantity of aluminous silicate,
with some magnesia, which might form mica or aluminous hornblende, though

the greater portion of it would ordinarily remain as an amorphous felsite

METAMORPHISM OF FRAGMENTAL VOLCANIC ROCKS. 81

or impure argillite. The obliteration of all traces of granulation in this
residual felsitic base is no more remarkable than it would be in an argilla-
ceous rock. So long as a thorough crystallization of the entire mass
remains impracticable for want of the requisite quantity of alkaline and
earthy bases, much of the groundmass must necessarily remain amorphous ;
and there is no difficulty in believing that this amorphous base may take
those forms and aspects (both microscopic and macroscopic) which are seen
in many forms of porphyroid eruptive rocks.

These rocks, however, never reveal any traces of that igneous fusion
which is displayed by the basalts and augitic andesites on the one hand,
and by the true rhyolites on the other. Glass inclusions, fluidal textures,
fibrolites, or a spherulitic base are never found among them. This absence
of all evidence of igneous action at high temperature is a significant charac-
teristic. Hence the similarity of these metamorphic rocks does not extend
to all igneous or eruptive rocks, but only to limited groups of them, such
as porphyritic trachyte and several other trachytic varieties, to the propy-
lites, and to some varieties of hornblendic andesite.

A detailed description and study of the metamorphic tufas will be found
in the portion of the chapter on the Sevier Plateau, in which the rocks of

the East Fork Cafion are described.
6HP

Csr Ale ah 10 1a DY.

THE CLASSIFICATION OF VOLCANIC ROCKS.

Objects to be gained by a system of classification.—Artificial and natural systems.—The best system
represents with accuracy the existing knowledge.—Progress is from the artificial to the natural
classifications.—All are evanescent and temporary.—Classification of volcanic rocks chiefly with
reference to physical properties.—Transitions to porphyritic racks.—Correlations between physi-
cal properties. —Chemical composition.—Mineral ingredients.—Texture.—Density.—F usibility.—
Wholly crystalline and partly crystalline textures.—Texture as correlated to geological age of
eruptions.—Not universally a true correlation.—Pre-Tertiary lavas common.—Von Cotta’s view
adopted.—View tested by comparison with facts.—Magmas of all ages the same.—Texture due to
conditions of solidification.—Porphyritic texture.—Difiiculty of definition.—No strict demarka-
tion between porphyries and layas.—Crystalline rocks.—Significance of the wholly crystalline
texture.—The two original groups.—Acid and basic rocks.—Subdivision of each.—Andesite.—
Rhyolite.—The four major groups.—Conspectus of minerals characterizing the primary divisions.—
Rhyolites.—Trachytes.—Andesites.—Basalts.—General system.

The objects to be gained by a good system of classification I hold to
be mainly two: first, accuracy of designation; and, second, convenience of
treatment. In speaking of any natural object, it is desirable to indicate by
a single word as much as possible concerning the attributes and relations
of that object, and to avoid as far as possible all confusion with the attributes
and relations of other objects. In order to secure this accuracy and con-
venience it is necessary that a classification should be so constructed as to
express both the differences and community of attributes and relations.
Where the differences of attributes between two or more objects are small
and the community of relations is nearly complete, these objects are grouped
together as to most of their features, and separated only by small distine-
tions, as varieties or species. Where these differences are very great, and
the community very highly generalized, they are separated by much broader
divisions, as in orders or classes. When a category of objects is once clas-
sified and familiarized to the mind, the mention of any one of them will con-
vey not only an idea of the concrete object itself as an individual, but also

<2

:
]

GENERAL CONSIDERATIONS UPON CLASSIFICATION. 83

an idea of its differences and community with other objects of the same
category, so far as those differences and community are understood.

The differences and affinities (that is to say, community of attributes
and relations) between the members of a category are ordinarily not few,
much less single, but numerous and complex; and the value and utility
of a system of classification is about proportional to the number of differ-
ences and affinities which it truthfully expresses. Systems of classification
are spoken of as “artificial” and ‘‘natural.”. My understanding is that an
artificial system is one which takes account of the agreements and disagree-
ments of the clssified objects with respect to only one characteristic or
one very limited set of characteristics. The meaning of the expression
‘natural system of classification” is much more difficult to assign. Most
probably different authors would entertain widely differing conceptions as
to its meaning, none of which would be very definite or precise. They
might, however, agree that a natural system as contradistinguished from an
artificial one takes cognizance of all the characteristics and relations of the
members to each other; the difference and affinity in any case being rated
and valued, therefore, in accordance with the totality of characters and not
dependent upon merely one of them. But it is far easier to say this much
about a system of classification than it is to comprehend it! The truth is,
that a natural system in any such length and breadth is impossible for any
category, unless we know all the members of it and the totality of their
relations ; and there is no reason to believe that human knowledge has ever
reached to that perfection. But as knowledge is ever increasing, we may
at least hope for the time when it shall be sufficient to enable us to find
and designate the greater and more important relations with absolute verity;
and if the systema nature is fitted and keyed together in order and harmony,
as we are fain to believe, the outstanding facts will fall readily into their
places; just as the final parts of a puzzle are quickly placed when the true
arrangement of the other parts is discovered. A purely artificial system
marks the initial stage of generalization of knowledge; a perfect natural
system is for the time being unattainable. The growth of knowledge and
philosophy, however, is marked by a transition, long, laborious and very
gradual, from one to the other; a transition, which is marked by an indefi-

84 GEOLOGY OF THE HIGH PLATEAUS.

nite number of tentative classifications, having less and less of the artificial
character, and approaching nearer and nearer to the natural. Each classi-
fication represents its author’s codrdinated knowledge of the category of
which he treats, and the classifications which are generally accepted at any
time represent the stage of knowledge and induction then prevailing. No
system is permanent and none ought to be permanent, but they ought rather
to change progressively as knowledge and induction progress. Least of all
ought any system to attempt to represent anything more than we actually
know. The best system at any time is that which represents most accu-
rately the state of knowledge and rational induction at that time.

The progress of classification, then, is from the simple or artificial sys-
tems which take account of one set or scale of characters and relations, to the
natural systems which take into account the totality of characters and rela-
tions. Hence the classification is gradually growing more and more com-
plex and difficult. The present conditions of most systems of classifications,
viewed with reference to their respective stages of progress, seem to be
much nearer the artificial than to the natural. Even in those categories of
natural objects which sometimes are claimed to be classified according to
natural systems, the progress from the purely artificial has often been small
and the approach to the natural very distant. Though recognizing that a
natural classification must embrace the totality of characters, naturalists
still employ and are compelled to employ in many cases only a single set
of characters for the grouping of a given category. On the other hand, we
are often able to recognize correlations between the various properties or
characters of a group of natural objects, such that, when we arrange them
according to one set of characters, we find that we have also arranged them
(in consequence of those correlations) in logical harmony with the others.
But this rarely happens except in very small groups with a narrow range
of variation; our knowledge is rarely equal to a full and sufficient recog-
nition of such correlations in large groups. Most of the later classifications,
however, assume the existence of such correlations while using a single
character as a criterion. Although this course is far from being wholly
satisfactory, it appears to be the only practicable one. Sometimes this
assumption holds true to a remarkable extent; much more frequently the

BASIS OF THE CLASSIFICATION OF VOLCANIC ROCKS. 85

assumed correlations are, so far as we can discern them, seen to be only
very partial and imperfect. S.ill we may hold that, for the time being, the
best classification is the one which expresses the largest number of facts
and relations hitherto ascertained, and we may advantageously adopt such
a classification in preference to any other, though conscious that it fails to
bring into recognizable order some outstanding facts and relations which
we are compelled for the present to look upon as anomalies.

In proposing a system of classification of volcanic rocks, I shall endeavor
to conform to the foregoing conceptions as to the purposes and scope of
any or all classifications. Strictly speaking, I can pretend to nothing more
than the most convenient and accurate expression which the nature of the
case may admit, of the state of my own knowledge and convictions con-
cerning the properties and relations of volcanic rocks. Holding that all
classifications are ephemeral, merely indicating the instantaneous phases
of advancing knowledge, it is fully admitted to be an artificial one for the
most part, and is natural only so far as nature has been truly discerned
and expressed. The object in presenting a new classification instead of
selecting and adopting an old one is to give precision to the terms employed,
and to lay down from the beginning a systematic statement of the views
entertained regarding the affinities of the various kinds of eruptive rocks
so far as known and understood by the individual writer. Not only does
there seem to be no impropriety in any or every writer expressing as accu-
rately and systematically as possible his own views of such relations and
affinities, but it is rather incumbent on him to do so, and in no way can
this be accomplished so compendiously as by a scheme of classification.*

In a classification of voleanic rocks, the facts which it is desirable to
formulate and arrange are, first, those having reference to the physical con-

*I may advert here to a malpractice of some writers, who take advantage of slight pretexts to
coin new names for slightly-altered divisions of old groups. A new name is always an inconvenience,
even though it may be necessary ; unless, indeed, it be a purely descriptive one, conveying at once its
significance or giving some conception of its meaning to one who hears it for the first time. Thus, the
introduction of such names as protogene, elvanite, nevadite, miascite, &c., entails the necessity of
much labor and effort to fix in the memory their meaning, all of which might have been avoided and
every useful purpose subserved by using the terms hornblendic granite, quartz porphyry, granitoid
rhyolite, nephelin syenite, &c. Irrelevant terms like the first may be very convenient to the writer or
speaker, but they are very inconvenient to the reader or hearer. Inasmuch as all classifications are
evanescent and constantly shifting, it is manifestly desirable to make them as easily intelligible as
possible.

86 GEOLOGY OF THE HIGH PLATEAUS.

stitution of the numerous kinds and to their degrees of affinity; second,
those having reference to their genesis. In other words, we desire a
formula which shall express what the rocks are and the causes which made
them what they are. It may be said at once that we have no knowledge
of the genesis of volcanic rocks sufficient to make a coherent formula, or
out of which we can construct a system of causation, however crude. We
know that they came up out of the earth in a molten condition, and that
is all we can confidently say of their origin. Our classification, therefore,
must, from the necessities of the case, be confined to an expression of what
we know concerning their physical constitution. In this direction our
knowledge is sufficient to justify an attempt to formulate it.

Let us look first at those physical properties which are common to all
voleanic rocks, and which, therefore, serve to distinguish them as a cate-
gory from all other categories; if, indeed, such a distinction really exists.

1. All volcanic rocks have been in a state of fusion at a high tem-
perature.

2. All volcanic rocks have been displaced from unknown depths in
the earth, and have risen in a fiery, liquid condition, either to the surface,
where they have outflowed as lavas, or have intruded themselves, part-way
up, among colder overlying rocks, where they have quietly solidified.

3. They consist of aluminous silicate, combined with lime, magnesia,
soda, and potash; iron is very rarely absent—perhaps never wholly want-
ing.
have tolerably narrow ranges of variation. Thus the silica never materi-

Moreover, the quantities of these several oxides, though varying,

ally exceeds 80 per cent. nor falls sensibly below 45 per cent.; the alumina
ranges from 10 to 20 per cent., the lime from 1 to 10 per cent., &e.

4, All voleanic rocks consist of an amorphous base, holding crystals,
except, however, some intrusive rocks, which appear to be wholly erystal-
line. In some obsidians, on the other hand, crystals are exceeding rare,
though probably no great mass of obsidian is wholly without them.

Although it seems as if there ought never to be any difficulty in dis-
tinguishing a volcanic rock from any belonging to other categories, yet
this difficulty sometimes arises. A rock may have been fused and dis-

placed from its seat; it may have the chemical constitution and “ half-

PHYSICAL PROPERTIES OF VOLCANIC ROCKS. 87

crystalline” texture of ordinary lavas, and yet it may not have been
erupted or subjected to that mechanical action which is the most con-
spicuous feature of volcanism. It may have been intruded into a dike, or
between strata, and only brought to daylight after the lapse of many
geological periods by the agency of denudation. Many of the quartz
porphyries and the intrusive or “laccolitic” trachytes of the West, and
many basalts or dolerites, are of this character. Are these truly volcanic
rocks? Before attempting to answer this inquiry let us advert to the
wholly crystalline rocks, such as granite, syenite, diorite, diabase, &e.
These are not usually accounted to be volcanic rocks; yet they have been
heated and rendered plastic, and they have been intruded into narrow
dikes and veins and between strata, though they have never been erupted,
so far as we know. Between the intrusive rocks of a wholly crystalline
texture and the intrusive rocks of a half-crystalline texture there may be
found a true transition of varieties, and a hard and fast line cannot be drawn
between them. Chemically, the two classes are sensibly exact counterparts
of each other, and are very nearly so in respect to their constituent min-
erals. But the failure to find a boundary is no bar to classification, which
takes account not only of differences but also of affinities; and hence, while
speaking of volcanic and granitoid rocks as distinct classes, we must still
keep in mind the reservation that there is a border country between them.

Having indicated the characters which belong to all voleanic rocks as a
class, and which at the same time serve to distinguish them from other classes,
we may next proceed to consider how they differ among themselves, and
what affinities exist between the different groups. It may be repeated here
that considerations relating to the genesis of rocks—the causes and pro-
cesses which have made them what they are—should not be directly or
primarily taken into the account. We know too little about their genesis,
and any attempt to include such considerations would merely lead us to
embody what we conjecture rather than what we know, and would almost
certainly mislead us. We can take account only of well-known facts, and
these are to be found chiefly in those chemical and physical characters
which have been extensively studied and compared. These are chiefly as

88 GEOLOGY OF THE HIGH PLATEAUS.

follows: 1. Chemical composition. 2. Mineral ingredients. 38. Texture.
4, Density. 5. Fusibility.

Of these characters the most important surely is the chemical composi-
tion. In truth, differences of chemical constitution apparently lie at the foun-
dation of most of the other varying characters. It is the primary determi-
nant of the minerals which are formed in the lavas and certainly also of the
specific gravity and fusibility. The texture, also, is to a considerable extent
dependent upon it, though in this respect the rock is influenced more by
other conditions. But on the whole there is a well-marked correlation
among the physical properties of volcanic rocks, and we may easily recog-
nize the important fact that variations in the chemical composition carry
with them tolerably definite and dependent variations in the other physical
properties.

Correlation between chemical composition and mineral ingredients—The
minerals which are formed in volcanic rocks are to a very important extent
determined by the chemical composition of the magma. The most abundant
constituent of volcanic rocks is silica; its quantity ranging from 45 to 80
per cent. Those rocks which possess the higher percentages of silica have
on the whole more acid minerals than those which possess lower percentages
of silica. The minerals of the more acid rocks are quartz and potash-soda
feldspars, while those of the more basic rocks are lime-soda feldspars, augite,
and olivin. Rocks of intermediate constitution contain both kinds or inter-
mediate kinds of feldspar, with abundant hornblende or equivalent augite.
We may discern the principle of selection, which determines the minerals
by studying each chemical constituent in detail. It might be readily antici-
pated that free quartz would be segregated and crystallized in a rock con-
taining a very large percentage of silica. Indeed, the law of definite pro-
portions regulating the combinations of all substances requires us to believe
that in all ordinary volcanic rocks holding more than 65 to 68 per cent. of
silica this excess of silica must be present uncombined, whether as free
quartz conspicuous to the eye or as an intimate mixture of the groundmass.
There is no fixed percentage at which silica becomes excessive, since that
will depend largely upon the atomic weights and affinities of the other sub-

stances present. But, in a general way, those rocks which contain large

PHYSICAL PROPERTIES OF VOLCANIC ROCKS. 89

quantities of alkali (soda and potash) may have a larger percentage of
silica without excess, than rocks containing more of lime, magnesia, and
iron and less of alkali. Thus trachytes, which have a comparatively large
proportion of soda and potash, and very little lime and iron, seldom show
any evidence of excess of silica unless the percentage exceeds 68 per cent.,
and then, as the silica increases, they graduate into rhyolites. On the other
hand, such rocks as propylite and andesite, which contain an abundance of
lime and iron, begin to show evidence of an excess of silica when the percent-
age of it exceeds 62 per cent. or sometimes even 60 per cent. The reason for
this is not far to seek. The alkalies are capable of forming definite combi-
nations with a much higher percentage of silica than are lime, magnesia,
and iron. The alkalies give rise to the acid feldspars, albite, and orthoclase,
while the lime gives rise to the basic feldspar, anorthite, and iron and mag-
nesia to the equally basic minerals of the pyroxenic, hornblendic, and olivin
groups.

On the other hand, the alkalies sometimes form basic minerals, such as
leucite and nephelin. This happens whenever these bases are present in
quantities in excess of those required to form feldspar, or, what amounts to
the same thing, when the ratio of silicate of alumina to soda or potash is
less than that required to form albite or orthoclase. Hence, in basic rocks
rich in potash, we find leucite, and when they are rich in soda, nephelin,
either or both replacing feldspar.

Turning now to the magnesian minerals, the same kind of correlation
is seen. Where the quantity of magnesia relatively to the silica is very
great olivin isformed abundantly. This is the most basic mineral occurring
in eruptive rocks, and is found only in rocks which are least siliceous.
Where the quantity of magnesia is less, augite and hornblende are
formed. In the two latter minerals it appears that lime, magnesia, and
iron protoxide largely replace each other, lime predominating in augite,
and magnesia in hornblende. They are moderately basic, but less so
than olivin. In the more acid rocks magnesia takes frequently the form
of mica (biotite), in which the quantity of protoxide base is still less than
in hornblende. .

With regard to alumina, it is somewhat remarkable that although the

90 GEOLOGY OF THE HIGH PLATEAUS.

quantity of this constituent is second only to that of silica, it varies less
than any other. It rarely falls below 14 per cent. and rarely exceeds 19
per cent. of the entire rock. There is a tendency to a slight excess of *
alumina above the quantity required to form feldspar in the acid rocks and
a tendency to a slight deficiency for the formation of feldspar in the basic
rocks.* Hence the slight excess of alumina of the acid rocks may readily
be taken up by the aluminous micas and aluminous hornblende; and in the
basic rocks, on account of the deficiency of alumina, the lime cannot all
take the form of feldspar, and a considerable portion of it appears in the
very abundant augite.

Thus we find that basic rocks have basic minerals and acid rocks have
acid minerals, and that the mineral ingredients stand in correlation to the
chemical composition of the magma, and that the nature of the latter is a
determinant of the former. Perhaps the most striking example is to be found
in the varying conditions*which determine the formation of augite and
hornblende. These two minerals differ but little in chemical constitution,
and yet their slight differences are distinctly correlated to differences in the
composition of the magmas from which they crystallize. In augite, lime
and iron are found in greater quantity and alumina in less quantity than in
hornblende. Although the differences in these respects are rather small,
they appear to be strictly proportional to correlative differences in the gen-
eral groundmass in which they respectively occur.

Correlation between chemical composition and specific gravity—The exist-
ence of such a correlation is perhaps too well known and too obvious to
require any discussion. In general the density holds an inverse ratio to
the acidity.

Correlation between the chemical composition and fusibility—The fusibility
of volcanic rocks has not been investigated so fully as other properties, and
neither lithologists nor geologists appear to have attached any very great

*The percentage of alumina, however, is less in the acid than in the basic rocks, and yet the
excess above the quantity required to form soda and potash feldspars is usually greater in the former
rocks than in the latter, on account of the great acidity of the alkali feldspars; indeed, there is rarely
any notable excess of alumina in the basic rocks above what is required for the basic lime-feldspar.
Thus the rocks which have the smaller percentage of alumina curiously enough have an excess above
the requirements of feldspar, and it appears in the accessory minerals, while the rocks which have the
higher percentage are rather deficient in it.

CORRELATION OF COMPOSITION AND TEXTURE. 91

importance to the differences in this respect which may exist between the
various groups. Still, we have the investigations of Daubeny, Deville, and
Mallet, which are so far concordant that they indicate decisively the exist-
ence of a true relation. The acid rocks have decidedly higher melting tem-
peratures than the basic rocks. Many blast-furnace slags approach tlie vol-
canic rocks in constitution, and the great amount of experience gathered in
iron-smelting amply confirms the same relation so far as the cases are fairly
comparable. We may, with considerable confidence, state as an approximate
truth that the melting temperatures of volcanic rocks have a direct ratio to
their acidity.

The textures of volcanic rocks are no doubt due in part to peculiar-
ities of chemical constitution. The vitreous character of the rhyolites, the
coarse, harsh texture of the trachytes, the compact, fine-grained texture
and peculiar fracture of the andesites and basalts are surely in due a
great measure to their constitution, but how or why we do not know.
There is, however, another sense in which texture is ordinarily spoken of,
and to which high importance is attached, and this sense takes account
of the degree or extent to which the groundmass of a rock is crystallized.
By far the most important difference between a volcanic and a non-erup-
tive plutonic rock, so far as pure petrographic considerations are concerned,
consists in the fact that the plutonic non-eruptive rock is wholly crystal-
line, while the volcanic rock is only partially so. Otherwise the two kinds
might be quite indistinguishable—might consist of the same constituents.
This distinction, depending upon the extent of crystallization, however, is
of great importance, since it arises in all probability from causes associated
with the genesis and geological evolution of the rocks themselves. The
nature and properties of the silicates are such, that under the conditions
ordinarily existing their crystallization is attended with difficulty and pro-
ceeds very slowly. An indispensable requisite for crystallization is mobility
of molecules inter se, and for this mobility a liquid condition of the magma
is essential. But the silicates possess the following peculiarity: at a tem-
perature sufficiently high to render them very liquid crystallization is im-
possible; at a temperature just low enough for crystallization, they are

exceedingly viscous and the mobility very much impeded. The crystals,

92 GEOLOGY OF THE HIGH PLATEAUS.

_ therefore, form very slowly, and time becomes an important element in
determining the whole amount of crystallization. It is easy to see that an
eruptive lava, rapidly cooling under the sky, may remain but a short time
at the temperatures at which crystals can form. On the other hand, an
injected or plutonic mass may long retain its high temperature. In the
former case the rock finally becomes half-crystalline, in the latter case
wholly crystalline. That this is the explanation of the textural differentia-
tion of the plutonic and erupted rocks seems very probable, and thus tex-
ture becomes associated with the genesis of the rock and the causes which
have made it what it is.

There is a very respectable school of German lithologists who make the
geological age of igneous rocks a primary criterion of classification. They
place all igneous rocks, whose intrusion or eruption occurred prior to Ter-
tiary time, among the granitoid or porphyroid classes, and all Tertiary or
Quaternary eruptives among the true volcanics. For example, all augitie
plagioclase rocks of Pre-Tertiary origin are regarded as diabases, mela-
phyres, or augitic porphyries, &c., while all of Post-Cretaceous origin are
regarded as basalts, ‘‘ trachydolerites,” &c. Such a classification most as-
suredly could be defended only upon the assumption or ascertained fact
that certain characters are found in the more ancient eruptives which are
wanting in the more recent ones and vice versa. Is this assumption uni-
versally true? JI hold that it is not. That in a great majority of cases the
Pre-Tertiary igneous, as we now see them, are granitoid or porphyroid,
while those of later epochs are volcanic, thus presenting textural differences,
is undeniable. But exceptions exist, and they are highly important ones.
It is possible, not to say probable, that many more exceptions might be
looked for than can at present be specifically named if there were not a
certain looseness in the use of names, by which rocks of the volcanic tex-
ture are classified with the granitic groups. This is especially observable in
the augitic divisions. The augitic rocks of the Paleeozoic system, notably
those of Carboniferous age, are frequently classed as diabase, when more
properly they might be in many instances placed among the dolerites or
basalts. Indeed, some intelligent observers, who are not committed in any

way to the foregoing generalization, do not scruple to call the intruded and

PRE-TERTIARY VOLCANIC ROCKS. 93

contemporaneous rocks of the Carboniferous in England and Scotland
basalt, while others who desire to be non-committal call them traps, which
may mean either diabase, basalt, or dolerite, or even augite-andesite. Pro-
fessor Geike* specially mentions basalt and dolerite as among the inter-
bedded and contemporaneous Carboniferous traps of Great Britain, and so
eminent a geologist is certainly not liable to confuse his technical terms.
Mr. Jukes also mentions the basalts of the South Staffordshire coal-fields
(Rowley Rag) as being of Carboniferous age. Still more ancient are cer-
tain basalts of the northern peninsula of Michigan, of which the fragments
are found abundantly in the drifts of Wisconsin and Illinois. These were
all erupted prior to the Potsdam period; and though they are usually called
ereenstones, many of them are certainly basalt. Sir W. Logan and T.
Sterry Hunt mention doleritest of Archazan age in Canada (Grenville),
much of it very fine-grained and sometimes amygdaloidal, and Sir Will-
iam pronounced it to have been erupted prior to the Silurian, which is
seen to overlap the denuded dikes in which it occurs. Prof. J. W. Daw-
son speaks of basalts{ of Triassic age extensively developed along the
eastern shore of the Bay of Fundy, especially in the vicinity of Cape
Blomidon. The oldest volcanic rocks from the Rocky Mountain Region
of which I have any knowledge, are found in rounded pebbles of the
Shinarump conglomerate, which lies at the top of the series to which Pro-
fessor Powell has given that name, and which is supposed to be of Tri-
assic or Permian age. ‘These are fragments of a very fine-grained basalt,
quite indistinguishable from the water-worn pebbles of the latest Tertiary
basalts. Numerous cases might be cited of the occurrence of augitic rocks
with a volcanic texture erupted prior to Tertiary time, and far back, indeed,
into the Archean, though unquestionably the augitic rocks of earlier epochs
possess in the great majority of cases the granitic texture—in short, may
very properly be called diabase. It is difficult to resist the conclusion
resulting from the various accounts of these rocks that their textures
depend chiefly upon the conditions of cooling. Where this has been rapid,
as, for instance, in cases of contact with dike-walls, the magmas have been

* Address British Association, Dundee meeting, 1867.
¢ Geology of Canada, 1868, pp. 36, 653.
t Acadian Geology, pp. 94, 98.

94 GEOLOGY OF THE HIGH PLATEAUS.

even vitrified (tachylite), and where it has been protracted, the resulting
rock has taken the granitoid texture—become, in short, diabase.
Furthermore, instances of Palzeozoic trachyte are not wanting. In the
Laurentian rocks of Canada they are, according to Dr. T. Sterry Hunt,*
very abundant and extensively displayed. At Brome and Shefford they
occupy two areas of twenty, and nine, square miles, respectively, and their
period of eruption must have been soon after the Quebec epochs At
Yamaska a micaceous trachyte occurs differing from the foregoing, and at
Chambly and Regaud, a porphyritic trachyte. The island of Montreal
offers a great variety of trachytic rocks, some of which, according to Dr.
Hunt, cannot readily be distinguished from the trachyte of Puys de Dome.
At Lachine a phonolite is also mentioned as associated with trachytic dikes.
Thus we do find among Pre-Tertiary eruptives rocks which pos-
sess all the essential characters of true lavas. The. occurrence of Ter-
tiary granitoid rocks is probably less common. Still they do sometimes
occur. True porphyries of Tertiary age are much more frequent. Those
intrusive masses, to which Mr. G. K. Gilbert has given the name of
laccolites, are in every sense porphyries. Most of them, however, belong
to the non-quartziferous division of felsitic porphyry, and are distinct
from the common elvanite or quartz-porphyry. But in the Elk Mount-
ains of Colorado we find laccolitic masses of quartz-porphyry graduat-
ing into granite porphyry and porphyritic granite. The age of these in-
trusions is not accurately known, though it is certain that they are Post-
Cretaceous. Laccolitic rocks of trachytic and rhyolitic constitution seem
to be tolerably abundant throughout the mountain regions of the West.
Nevertheless, the fact remains that the Pre-Tertiary eruptives are on the
whole preéminently granitoid or porphyroid in texture, while the Tertiaries
are as decidedly volcanic. It seems, therefore, at first as if a correlation
existed between age and texture. Forthwith arises the inquiry, what is
the significance of that relation? To this question it seems to me that Von
Cotta has given a very satisfactory answer, which may be summarized as
follows. The eruptive magmas of Tertiary time did not differ at the time of
eruption in any material respect from those of older epochs, any more than

*Geology of Canada, 1863, p. 656.

AGE OF THE GRANITOID AND PORPHYRITIC ROCKS. 95

two eruptions of the same epoch may differ from each other without calling
for a distinction in their classification; but the textural differences which
we now observe are due to the different conditions under which similar or
sensibly identical magmas have solidified. The granites have solidified
probably at great depths in the earth and under enormous statical pressure,
while volcanic rocks have solidified at the surface. Porphyries, which
usually occur in dikes or in intrusive masses, have solidified at intermedi-
ate horizons, though under conditions probably more nearly approaching
those of volcanic than of granitoid rocks. The Paleeozoic and Archean
ages may have had their voleanic rocks, differing in no assignable respect
from those of recent date, and upon a scale as grand and equally varied,
but denudation has dissipated them. The granitoid rocks now exposed
to our view have been brought to the light of day only by an enormous
erosion, which has removed the thousands of feet of strata beneath which
they received their present texture.

This explanation is fortunately capable of a test by comparison with
the facts presented by the rocks themselves, and though all the facts have
not been collected and studied in this light, yet our knowledge of their
general scope and bearing is considerable, and my belief is that they fairly
sustain the theory. The granites and syenites are almost invariably found
in localities where denudation has proceeded through a long series of
epochs and has been vast in amount.* They are usually associated with
metamorphic rocks which have been laid bare by the removal of great
masses of superincumbent strata. They are not often found as interjected
beds in unaltered or little altered Palaeozoic or Mesozoic strata; much less
as contemporaneous flows. The eruptive syenites and granites, therefore,
harmonize with the theory.

The diorites and diabases have a different mode of occurrence. The
diorites, so far as known, are believed to be almost invariably intrusive,t
either in the form of dikes or intercalary between sedimentary beds. The
same also appears to be true of those diabases which possess an unquestion-
able granitoid texture. There are, indeed, many rocks to which the name

*It would be impracticable here to enter into a full discussion of particular cases without pro-
tracting the discussion indefinitely. The statement will, I think, be generally admitted.
tJukes and Geike, Manual of Geology.

96 GEOLOGY OF THE HIGH PLATEAUS.

of diabase is given by some lithologists, but which are really dolerites and
basalts, bearing indications of a volcanic origin, and these are found as
contemporary or interbedded coulées. They differ notably, however, from
the intrusive diabases, though they are sometimes confounded with them.
In short, the ancient eruptives which remain as coulées have the voleanic
textures, and those which remain as intrusives have the granitic or some-
times the porphyritic texture, and the diorites and diabases equally with the
syenites and granites present no obstacle to Von Cotta’s hypothesis, but
are to all appearances in full accord with it.

It is as certain as anything in geological science can well be that the
texture of the granitoid eruptive rocks could not have been derived (at
least directly) from any special conditions existing prior to their eruption.
Every theory must presuppose that during their eruption or intrusion they
were plastic, and that a portion of their groundmass, if not the whole of it,
was amorphous and in a condition of igneous or aqueo-igneous fusion, and
in such a condition it is little less than absurd to suppose that any texture
at all resembling granite could have prevailed. The closely interlocked
crystals of such a groundmass are as antithetical to the very idea of plas-
ticity as it is possible to conceive. The crystalline texture must surely
have been a development altogether subsequent to plastic movement.*
There is, therefore, a lurking fallacy in the statement that granitoid rocks
had their periods of eruption in the earlier ages, while the volcanics’ had
theirs in Tertiary time. The true and rational mode of stating the case
may be this: that through all the ages igneous magmas have been erupted,
which have, according to their final resting-places and the conditions there
existing, consolidated either into granitoid or half-crystalline rocks. The
magmas themselves have been the same in all ages, each to each within its
own group, and so too have the resulting rocks each to each under equiva-
lent conditions of consolidation. We find in the Tertiaries only volcanic
rocks, because the corresponding granitoids are far beneath them and not yet
laid bare by secular erosion. We find among Pre-Tertiary eruptions chiefly
granitoids, because the corresponding volcanics have been swept away.

*It is of course intelligible that some crystals may have existed in an amorphous fluent paste
during the eruption.

POSITION OF THE PORPHYRIES IN CLASSIFICATION. 97

Texture, then, if the foregoing views be true, is associated with the
genesis of rocks and is determined by the conditions under which the rocks
have solidified. Although it may seem to be a trivial character, in reality
it is a very important one, since it is an index of conditions and occur-
rences of vital importance to the genesis of the rocks and their geological
relations. For it is of the highest geological importance to know whether
certain rocks have been erupted or have been formed in situ; whether they
are indigenous or exotic. The indications given by texture may be uncer-
tain at times, and occasionally even misleading; but on the whole, so far
as they are now understood, they may be relied upon. The differences of
texture have heretofore been employed chiefly to distinguish the eruptive
from the non-eruptive igneous rocks. The wholly crystalline are non-
eruptive; the partially crystalline are eruptive. But, although the wholly
crystalline rocks are not commonly found in the form of lava sheets or
coulées, they are occasionally found in the form of intrusions, and so, also,
are the partially crystalline rocks. The intrusive condition is, therefore, a
kind of intermediate stage between the eruptive and non-eruptive condi-
tion, representing an abortive attempt at eruption, sometimes resulting in a
slight displacement of the magma, sometimes almost accomplishing an out-
pour. In very many cases—probably in many more than we are now jus-
tified in affirming—this qualified eruption is associated with a texture which
seems to be characteristic of it, the porphyritic texture.

A satisfactory definition of “porphyry” is almost impossible to find.
The most general conception is that it applies to a rock consisting of crys-
tals, usually feldspar and quartz, imbedded in an “‘unindividualized” paste
or base; but forty-nine-fiftieths of all intrusive and eruptive rocks come
fully within such a definition. Except an insignificant quantity of obsid-
ians and aphanitic rocks, all volcanics are decidedly porphyritic. And
yet lithologists employ the term to designate a group of rocks different
from volcanics, not only in their geological relations, but in their appear-
ance as dependent upon texture. There are certainly some rocks which
we do not hesitate to call porphyry, and regard them as being quite distinct
from the common lavas; the distinction, moreover, being a textural and
not a chemical one. As nearly as we can reach a description of the spe-

7HP

98 GEOLOGY OF THE HIGH PLATEAUS.

cialized porphyritic texture, it apparently amounts to this: The ground-
mass consists not only of crystals embodied in a base of matter which is
not visibly crystalline, but both crystals and base have certain distinctive
features; the crystals of quartz are more perfectly defined in their outlines
and possess more distinctly the perfect forms, edges, and angles of their
species, the predominant occurrences being the double hexagonal pyramids.
The feldspar crystals are also usually distinguished by their perfect forms,
especially at the terminations of the prisms, by their large size and by their
many and rare angles. In the volcanics the quartzes are not only fragmental,
poorly developed, and of uncertain boundaries, but are often rounded and
imperfect at the positions of the edges and angles, while the feldspars are
exceedingly irregular and indefinite in shape, not often presenting the well-
defined edges and angles distinctive of their species. The base of porphyry
is, to a great extent, mysterious and inexplicable. Usually it is (macro-
scopically) exceedingly fine-grained, homogeneous, and compact, with no
visible trace of crystallization. Under the microscope it presents certain
appearances which have puzzled for many years all investigators. With
polarized light it exhibits a behavior which is characteristic of erystalliza-
tion, and yet no individual crystals can be detected. It is homogeneous in
oue sense, and yet seems to be minutely granular, as if with greater mag-
nifying power and better definition it would resolve into minute crystalline
points; but the latter expectation generally proves a delusion. Not always,
however, for sometimes a moderate power resolves the base into a mosaic
of crystals, like the groundmass of granite, reproduced upon a microscopic
scale. The base of voleanic rocks is usually more or less glassy or fluidal
in texture, full of microlites, and even when granular is not nearly so much
affected by polarized light.

Many minute characters might be pointed out, but it is needless here.
There is no hard and fast line between the porphyritic and volcanic texture,
for the latter often simulates the former to a greater or less extent, and
even the differences already indicated sometimes vanish or become so
poorly pronounced that we fail to apprehend them with confidence. Still,
in the long run and in the great mass of cases, we are able to make a
distinction, and we find the differences associated with modes of occur-

CLASSIFICATION OF THE ERUPTIVE ROCKS. 99

rence of the rock masses. The true porphyries are eminently intrusive
rocks.

Into the detailed classification of the granitoid or wholly crystalline
rocks it is not intended to enter. It will suffice to say that they have been
regarded by almost all geologists and petrographers as separated from the
volcanics by wide barriers, resting upon wide differences in their geologi-
cal relations, in their modes of occurrence, their genesis, and geological
history. I have endeavored to show that the distinction is well founded.
It seems right that they should be placed in different classes, not because
the mere lithological fact that they differ in respect to their degrees of erys-
tallization is such a great thing in itself, but rather because it implies a
totally distinct category of relations. Whether a third class should be
admitted, viz, the porphyritic rocks, is not so clear. For my own part, I
incline to the admission of only two classes of igneous rocks, the volcanic
and plutonic—the former eruptive, the latter non-eruptive. I recognize,
however, that those who-are disposed to regard the porphyries as coérdi-
nate in value with the granitoids or eruptives, may have much to say in
support of their tenets.

Passing now to the consideration of the volcanic rocks as a class, the
principles upon which it is believed they ought to be subdivided have, in
general terms, already been indicated. We ought not to endeavor to take
account of anything more than their chemical and physical properties,
since we should otherwise run the risk of serious error. And it has been
pointed out that a decided correlation exists among these properties; so that
if we take a rational system, based upon one set of properties, we shall at
the same time express the other properties. The broader basis I believe to
be the chemical one, and I regard it also as the most convenient.

It has long been recognized that lavas are easily distinguished into
two principal groups, contrasting with each other not only in the superfi-
cial aspects and in the minerals they contain, but also in their composition.
One of these groups was ordinarily a coarse-grained, light-colored rock, of
rather low specific gravity. It contained crystals of monoclinic feldspar,
sometimes abundant free quartz, and also hornblende and mica. The other
group was usually fine-grained, compact, very dark colored, and very

100 . GEOLOGY OF THE HIGH PLATEAUS.

heavy, holding triclinic feldspar, augite, and magnetite. Upon analysis,
the two groups were found to differ greatly in chemical composition; the
lighter orthoclase rocks were found to be much richer in silica and much
poorer in iron, lime, and magnesia, than the others. This led to the divis-
ion into the two well-known groups of acidic and basic rocks. To the
former the name of trachytes was usually applied, while the latter were
termed basalts. As knowledge of volcanic rocks increased and became
more detailed, it was at length recognized (by Beudant) that the basic rocks
were susceptible of further division. The study of the South American
voleanoes convinced him that two types of basic rocks could be distin-
guished—one the typical basalts, characterized by an abundance of augite,
magnetite, and usually olivin commingled with lime-feldspar; the other
apparently a less basic rock, containing hornblende rather than augite, very
little magnetite, and never olivin. The two types differed in appearance,
the more basic being nearly black, the less basic being usually greenish,
and certain tolerably constant differences of texture being easily recog-
nized, though hard to describe; the name basalt being preserved for the
more basic variety. Beudant called the other type Andesite.

The name trachyte for a long time was used very vaguely, and it is now
somewhat surprising to find what a vast range of variety it was made to
cover. It was applied not only to the light-colored orthose and quartzose
rocks, but was extended over varieties belonging well within the basic
division, including Beudant’s andesites, and hardly stopped short of any-
thing except the extremely basic olivinitic basalts. The general sense of
the more acute lithologists, however, was against such a sweeping use of
the name, and in favor of confining it to the orthoclase-bearing varieties.
Although in this restricted use of the name trachyte a considerable number
of varieties had been noted by various writers, Richthofen appears to have
been the first to have clearly discerned that the trachytic group resolved
itself into two members. Of these the most acidic division was charac-
terized by the presence of free quartz and a general poverty in all min-
erals except quartz and orthoclase (sanidin); also by peculiarities of texture.
The less acidie division rarely contained free quartz, and never in nota-
ble quantity ; was richer in sanidin as well as in the accessory or subordi-

CLASSIFICATION OF THE ERUPRTIVE ROCKS. 101

nate minerals, hornblende, mica, magnetite, &c. It also possessed in
nearly all varieties that coarse, rough texture from which the term trachyte
originated. The validity of this distinction has been well established by
later investigators, and in Germany and America it is universally accepted.
To the more acidic division Richthofen gave the name Rhyolite, and pre-
served the name trachyte for the remainder of the older acidic semi-class.

Thus far we are able to subdivide the volcanic rocks into four parts or
groups instead of two, as was usually done in the time of Durocher. The
older acidic semi-class may be resolved into two groups, the Rhyolites and
Trachytes, while the basic semi-class may be resolved into two, the Ande-
sites and Basalts. Now, these four groups represent in a very decided
manner a progression in the chemical constitution, and also correlative pro-
gressions in mineral constitution, in specific gravity, &e. The rhyolites are
at the acidic end of the scale of progression and the basalts at the basic end.
The trachytes may be called sub-acid rocks and the andesites sub-basic
rocks, thus:

Acid rocks—RHYOLITES.

Sub-acid rocks—TRACHYTES.

Sub-basic rocks—ANDESITES.

Basic rocks—BASALTS.

We shall find further on that this progression is not perfectly rigorous
and exact, but presents certain apparent anomalies; that some rocks, for
instance, which ought to be and are rationally called andesite are more acid
than some rocks which are with equal reason called trachytes. Yet, on the
whole, the progression is strongly pronounced and unmistakable, and the
seeming anomalies do not invalidate the general law.

If we considered chemical constitution alone, however, we should be
unable to determine the relative position of any rock in the lithological
scale without a chemical analysis. ‘The patent evidence of its position and
character is found in the minerals it contains. These, it has already been
asserted, are determined by the chemical constitution, and in return indicate
that constitution. Each group of rocks has its characteristic group of min-
erals, of which some may be regarded as essential to the diagnosis of the

rock, while others are merely “accessory,” being generally present, but

102 GEOLOGY OF THE HIGH PLATEAUS.

sometimes wanting. The accessory minerals are, with rare exceptions, far
inferior to the essential ones in respect to quantity. The following con-
spectus exhibits these minerals :

CONSPECTUS OF MINERALS CHARACTERISTIC OF THE PRIMARY DIVISIONS OF VOLCANIC ROCKS.

Groups. Essential minerals. Accessory minerals.
Group I.
Acid rocks—Rhyolites-......----. Orthoclase (usually as sanidin) | Hornblende, biotite, plagioclase.
and free quartz.
Group II.

Sub-acid rocks—Trachytes -..-.-. Orthoclase (usually as sanidin).| Hornblende, biotite, augite, pla-
gioclase (the latter seldom
wanting), nephelin (in pho-
nolite), magnetite.

Group III.
Sub-acid rocks—Andesites (in- | Plagioclase ....-........---..--- Hornblende, augite, biotite ortho-
cluding propylite). clase (in subordinate quantity
and seldom wholly absent),
magnetite.
Group IV.
Basic rocks—Basalts........-.... Plagioclase (in some cases re- | Olivin, magnetite.
placed by leucite or nephelin),
augite.

In addition to the minerals presented in the foregoing scheme, there
remain several others of considerable importance. These are chiefly leucite
and nephelin. Leucite is found in some basalts replacing the feldspar, and
is treated in the classification precisely as if it were plagioclase. Though
widely distinct from that group of minerals in its crystallographic forms, it
closely approaches them, in chemical constitution, differing in this respect
mainly in containing a little higher percentage of potash than normal ortho-
clase. Nephelin holds exactly the same relations and presents the same
distinctions, but holds a high percentage of soda instead of potash. It is
found not only in the basalts, but also in phonolite, and is generally held
to be the most characteristic mineral of the latter rock. If now we treat
these two minerals as just so much triclinic feldspar, we shall find no diffi-

CLASSIFICATION OF ERUPTIVE ROCKS—RHYOLITES. 103

culty in assigning them to their places in accordance with all their natural
affinities. Leucite rocks will fall readily among the basalts. Nephelin,
when associated with other minerals common to the basic rocks, may be
considered as replacing labradorite, and the rock containing it may be
assigned to the basaltic group. When associated with orthoclase, as in
phonolite, the rock will fall among those trachytes which contain notable
percentages of plagioclase.

It yet remains to speak of those lavas which contain no distinct min-
erals, but which are wholly glassy or amorphous, like obsidian, pumice, &c.
Here chemical constitution becomes the sole criterion, and although the
external or macroscopic facies may often indicate to the trained eye the
approximate constitution, the only safe guide to determination is a chemical
analysis.

I. RHYOLITES. The rhyolites are distinguished by their high per-
centage of silica and by the presence of orthoclase and free quartz. The
number of varieties of texture found in this group is immense. We find
some which have an outward semblance to granite; others containing large,
beautiful, and perfect crystals of glassy feldspar an inch or more in length,
and large grains of quartz imbedded in a compact matrix; others having
the coarse, irregularly granular aspect of trachyte; very many with a
groundmass full of elongated vesicles like drawn-out glass and holding
small crystals; very many which are so vitreous or slag-like that the crys-
tals are discernible only with the microscope, and many which exhibit no
determinable crystals. So protean are the forms, that the lithologist may
well feel discouraged in attempting to resolve the group into intelligible or
rational subdivisions. Richthofen has attempted it, however, but it seems
to me with very partial success. While he has no doubt divided the more
prominent sub-groups, cases are often encountered which neither of them
appear to satisfy, and microscopic research indicates that many of the
characters he has seized upon are less distinctive than the external appear-
ances might at first suggest, and brings to light many others which are of
high importance, and which the external appearance does not suggest at all.
Considering external characters alone, however, his subdivisions may repre-
sent a convenient temporary grouping of the greater part of the rhyolites.

104 GEOLOGY OF THE HIGH PLATEAUS.

It will be noted that while chemical constitution and mineralogical com-
ponents are the basis of the larger and broader divisions, the texture may
here be employed to distinguish the secondary characters.

Group I.—RHYOLITES.

Sub-groups. Characteristics.
Sub-group 1.
NEVADITE or granitoid rhyo- | Having a superficial resemblance to granite; highly crystalline, with
lite. conspicuous quartz and feldspar; the crystals rounded, cracked, and
irregular in contour. Base resembling some of the coarser varieties
of trachyte.
Sub-group 2.
LiPaRITE or porphyritic rhyo- | Having a decided porphyritic texture; compact base; crystals perfect
lite. or nearly so, often of large size; not conspicuously vitreous.
Sub-group 3.

RHYOLITE proper or hyaline | Having a fluent groundmass, sometimes wholly without crystals, but
rhyolite. more frequently with them, but crystals less perfectly developed;
vesicular, with vesicles much elongated and drawn out; or not vesicu-
lar, but with lines of flow suggesting a vitreous or candy-like mass.

Foliated or structureless. Generally fibrolitic or spherolitic.

The microscopic characters of the hyaline rhyolites and some of the
liparites have been studied and analyzed in a most admirable manner by
Professor Zirkel, and described by him in the volume on Microscopic Pe-
trography in the series of Reports of the Survey of the Fortieth Parallel, to
which volume the reader is referred.

Il. TRACHYTES. The trachytic group is characterized chemically
by a high degree of acidity, but inferior in that respect to the rhyolites. Its
dominant minerals are orthoclase, with a subordinate amount of plagioclase.
It is distinguished mineralogically from rhyolite by the absence of free
quartz, by the greater abundance of plagioclase, and of the subordinate
minerals hornblende, magnetite, augite, and biotite. In its texture and
physical characters it is also well separated in most cases, showing a
tendency to develop the coarsely granular and porphyritic habitudes rather
than the hyaline and vitreous, though the latter are not wanting, nor even

extremely uncommon. This group is nearly as varied in character as the

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CLASSIFICATION OF ERUPTIVE ROCKS—TRACHYTES. 105

rhyolites, and the same difficulty is experienced in finding a suitable system
of subdivision. In attempting to divide them, Richthofen has given two
subdivisions, sanidin-trachyte and oligoclase-trachyte. ‘The admission of an
oligoclase-trachyte involves a dilemma. If (as appears from his language)
he contemplates a rock in which oligoclase is the dominant feldspar, it can-
not, according to ordinary conceptions and definitions, be a trachyte at all,
but rather an andesite. If it means that it is abundant, though subordinate
to orthoclase, then the same is true of by far the greater portion of the whole
trachytic group. Again, sanidin-trachyte also seems objectionable as a
characteristic name of a subdivision of the trachytes, since sanidin is the
predominant mineral of the entire trachytic group.

And yet my own limited studies have led me to the conviction that
Richthofen, with his rare insight into the real nature of the subjects he has
investigated, has hit upon a valid distinction, which we may safely follow.
Among the older trachytic eruptions we find rocks into which plagioclase
largely enters; indeed, to such an extent that we are often doubtful whether
it may not preponderate over the sanidin, or at least be very nearly equal
to it. In these same rocks we also find an abundance of hornblende and mag-
netite, giving them the dark iron-gray aspect which is presented by many
andesites. These hornblendic trachytes, however, are usually coarser and
rougher in fracture than the andesites, and the hornblende crystals are
rarely found in such perfection and full development as in the andesites,
and macroscopic inspection will generally enable us to form a very good
opinion as to which of the two we are dealing with, though sometimes we
are deceived. It is evident that such trachytes are not far removed from
the andesites, both in chemical and mineral constitution, and they sometimes
blend with them.

On the other hand, we encounter among the later trachytes a different
series of macroscopic characters. They are very deficient in hornblende,
and more often contain mica (biotite). They are usually light-colored,
pale-gray, or red, or light brown, and almost never dark gray. In texture
they vary widely, but in no case do they ever suggest any affinity to
andesite, but rather to rhyolite. Some of the varieties, indeed, approach
rhyolite so closely that we often have still greater difficulty in separat-

106 GEOLOGY OF THE HIGH PLATEAUS.

ing them from it than we encounter in separating extremely hornblendic
trachytes from andesites. In these trachytes sanidin is the only important
mineral, and though plagioclase and hornblende are not uncommon, they
are never conspicuous, and never seem to exert any notable effect upon the
character or aspect of the rock.

In seeking for purely descriptive names, it seems to me that the older
trachytes will be sufficiently discriminated if we call them simply horn-
blendic trachytes. It occasionally happens that the other group requires
to be spoken of collectively, and I shall in such cases employ the term
sanidin trachytes, rather than coin a new name. But for precision it may
be necessary to subdivide them rather more minutely, since these so-called
sanidin-trachytes embrace very wide variations of lithological aspect. The
time has not yet come to divide the immense trachytic group according to
definite and final principles. To accomplish that will require the careful
study of an enormous range of materials. Although my own observation
is far too limited to encourage the hope of finding a complete and satis-
factory arrangement, I am tempted to give provisionally and tentatively a
subdivision embodying such a grouping as will embrace the facts within
my knowledge.

Group IL—TRACHYTES OR SUB-ACID ROCKS.

Sub-Group A.—SANIDIN TRACHYTES.

Characteristics.

1. GRANITOID TRACHYTES. -.-- Trachytes having a superficial resemblance to granitic rocks; holding
much orthoclase and less plagioclase, with few other minerals; a
very little biotite and hornblende; crystals conspicuous; a some-
what porous base, containing little ferritic matter. Usually very
light-colored rocks ; seldom dark gray.

2. PORPHYRITIC TRACHYTE....| A base resembling that of porphyrite, with very conspicuous and per-
fect crystals of orthoclase (usually the turbid or milky variety), often
large. The base very fine, compact, and non-vesicular; more or less
ferritic, sometimes showing a feeble aggregate polarization. The

groundmass shows none of that coarse, rough texture so common in
| other trachytes.

TRACHYTES OR SUB-ACID ROCKS. 107

Group Il.—TRACHYTES OR SUB-ACID ROCKS—Continued.

Characteristics.

3. ARGILLOID TRACHYTE ...-.. A rock of very clayey or earthy aspect, suggestive of thick slate; very
highly charged with ferritic matter, rendering it opaque in the thin-
nest sections; holding crystals of feldspar (orthoclase) and grains of
magnetite, and seldom any other macroscopic mineral. The fracture
is highly characteristic, there being no cleavage; but the rock crum-
bles rather than splits. It is impossible to strike off thin flakes.
The fracture is very angular and irregular, though the ordinary
coarseness of trachytes is not exhibited. It is a very voluminous
rock in the plateaus and well distinguished.

4, HYALINE TRACHYTE........ Trachytes having a fluidal texture, indicative of flowing in a viscous
state, with very small, and sometimes few, and always poorly-devel-
oped crystals of feldspar. Mostly reddish or purplish; often with a
brick-like texture; sometimes foliated and resonant (clink-stone) ;
moderately vesicular. Often slightly quartziferous and approaching
the rhyolites.

SuB-GROUP B.—HORNBLENDIC TRACHYTES.

5. HORNBLENDIC TRACHYTE...| This comprises most of those dark-colored varieties of coarse, harsh
texture, exceedingly rough, though many are less so. Hornblende
and magnetite are abundant, the former in well-developed prisms.
The feldspars are less conspicuous than in the preceding varieties, but
are really present in greater quantity, as shown by the microscope.
Plagioclase very abundant. Iron gray is the usual color.

6. AUGITIC TRACHYTE.....----| It seems doubtful whether this rock should be considered as anything
more than a variety of the hornblendic sub-group. It is character-
ized by the presence of augite in place of hornblende. The varieties
are usually finer grained than the hornblendic, and resemble more the
augitie andesites, to which, indeed, they are so closely related that it
is sometimes difficult to distinguish them. Magnetite abundant and
some biotite.

7 PHONOLITE Re eesteceeecieeee A rock in which nephelin takes the place of triclinic feldspar. Usually
contains also orthoclase and some hornblende; resonant, foliated, and
in the rockmass is generally laminated in a very peculiar and strik-
ing manner.

8. TRACHYTIC OBSIDIAN ..---.. A wholly glassy or vitreous rock, having the normal constitution of
trachyte.

108 GEOLOGY OF THE HIGH PLATEAUS.

Ill. PROPYLITE AND ANDESITE. Richthofen has made two
distinct orders of these rocks, each of equal taxonomic value with the other
great groups, é. g., trachyte and basalt. There is no question that a
tolerably sharp definition can be drawn between them, and that they are
as readily distinguished in most cases by the unaided eye as by the micro-.
scope. The microscopic characters have been analyzed and described most
thoroughly by Zirkel. But though the distinctions are well-drawn, and
once mastered can seldom be confounded, the question arises, are they of
sufficiently radical importance to warrant their separation into groups of
such high rank as the trachytes and basalts? It seems to me that we can-
not do so without a violation of those fundamental principles which have
gradually become almost universal in fixing primary characters. On purely
chemical grounds so wide a distinction seems untenable, because the
chemical difference is very small, and often so indefinite that it cannot be
formulated. On mineralogical grounds the distinction is essentially no
ereater. Both of them are characterized by the predominance of plagio-
clase, with accessory hornblende or augite and sometimes free quartz. The
real difference is found in the respective textures, and in slight though con-
stant differences in the modes of occurrence of the accessory minerals, and
in some of the minor characteristics of the feldspars. But these distinguish-
ing characters are precisely the same in their general nature and equivalent
in degree to distinctions which are used in the trachytes, rhyolites, and
basalts for separating the sub-groups, and which in other rocks have never
risen to higher taxonomic values. If we follow the same methods and
valuations in these rocks which we adopt in the other groups, it seems to
me that we can only assign them to the rank of subdivisions of one prin-
cipal group.

With regard to the augitic andesites, Richthofen has placed them in
the same major group as the hornblendic andesites. Zirkel, on the other |
hand, has placed them among the basalts. In deciding which of these two
authorities it is best to adopt, the following considerations may be pre-
sented. It is not obvious that they use the term in precisely the same
scope, nor embrace within their respective meanings quite the same rocks.

We have certain rocks containing plagioclase, with abundant though sub-

PROPYLITE AND ANDESITE. 109

ordinate orthoclase, and with proportions of augite and magnetite very
much smaller than is usual in the basaltic group. We have also vari-
eties in which the orthoclase is much less though still notable, and the
augite and magnetite, accompanied with glassy or slaggy material included
in the groundmass, are very copious; and there are many intermediate
varieties. It seems probable that Richthofen may have contemplated only
the former in his expression of the characters of augitic andesite, while
Zirkel, taking the entire range of variety as one sub-group, with the more
augitic and vitreous ones as the type, did not find reasons for separating
them, and, therefore, placed them together among the basalts, to which his
types certainly most nearly approach. It must be admitted that a hard
and fast line cannot be drawn within this range, nor can it be satisfactorily
drawn between the more acid augitic andesites and the augitic trachytes.
Nevertheless, it seems advisable to draw one arbitrarily, and place the more
acid varieties among the andesites and the more basic among the basalts
(dolerite), thus following Richthofen rather than Zirkel.

Group II—SUB-BASIC ROCKS—PROPYLITE AND ANDESITE.

Sub-groups. f Characteristics.

1. HORNBLENDIC PROPYLITE -.| Consisting of predominant plagioclase and subordinate orthoclase, the
former especially, in large, well-formed crystals, abundantly dissem-
inated throughout a compact, homogeneous base. The fracture is
superficially like diorite or other medium-grained granitoid rocks.
The varieties usually are olive or tawny green color, sometimes red-
dish, or the green and red are banded, the former greatly predominat-
ing. Hornblende is rarely conspicuous to the eye, but in the micro-
scope is seen in abundance in small fragments, disseminated dust-
like, or in spangles. It is pale green and with sharply-defined edges.
Biotite and brown hornblende sparingly occur. The facies of the rock
suggests that it has been more or less altered and the microscope and
chemical analysis confirm it.

2. AUGITIC PROPYLITE (?)...-- This rock is mentioned by Richthofen, but has not been recognized in
the High Plateaus.

3. QUARTZ PROPYLITE ......-- A rock haying the essential characters of hornblendic propylite, but
with the addition of a notable amount of free quartz. It is generally

more siliceous rock than the latter and in most occurrences is
fresher in appearance.

110 GEOLOGY OF THE HIGH PLATEAUS.

Group IIIl.—SUB-BASIC ROCKS—PROPYLITE AND ANDESITE—Continued.

Sub-groups. Characteristics.
4, HORNBLENDIC ANDESITE --.| Consists of plagioclase, either wholly or with subordinate orthoclase

and with hornblende; the latter usually conspicuous; the crystals
imbedded in a base which is usually moderately fine, sometimes a lit-
tle coarse. The color is almost always green, from light to very dark.
The fracture is peculiar, splintery or conchoidal, radiating from the
point of impact. The hornblendes are mostly of the dark-brown
variety ; in the thin section with a black, shaded border. The base
shows fluidal structure, but not always.

5. AUGITIC ANDESITE .........| Usually a more basic rock than the foregoing; feldspar almost wholly
plagioclase ; augite taking the place of hornblende; either gray or
nearly black in color, never with greenish cast unless much altered ;
the more basic varieties merge into the dolerites and the less basic
into the augitic trachytes by transition. Resemblances to dolerite
most frequent.

6. DACITE OR QUARTZ ANDE- | Containing predominant plagioclase feldspar, with free quartz and al-
SITE. | most always abundant hornblende. It has a somewhat rhyolitic tex-
ture and habit. Sometimes biotite replaces the hornblende.

IV. BASALTS. The classification and subdivision of the basalts pre-
sent some difficulty.. In the basic lavas we have occurrences in which the
minerals leucite and nephelin replace wholly or in part the feldspars, and a
question arises as to the importance which is to be attached to this substitu-
tion. In theother great groups the subdivisions have rested upon texture and
general habitus of the sub-groups as well as upon the occurrence of accessory
and subordinate minerals in conspicuous quantity. In the acid and sub-
acid rocks accessory minerals are relatively in small proportions and varia-
tions of texture and habit very strongly pronounced. In the basic rocks
the reverse is true—the accessory minerals are more numerous, almost
rivaling the primary ones, while the texture, though considerably varied,
is far less so than in the acid rocks. These considerations would lead us
to rest the subdivisions rather upon a mineralogical basis than upon a tex-
tural one. Some authors separate dolerite from the so-called “true basalts”
on textural grounds, the former being macroscopically crystalline while the
basalts proper exhibit distinct crystals only under the microscope. Even

BASALTIC GROUP. Tai

an intermediate variety of texture (anamesite) has been named in which the
crystallization is recognizable but not conspicuous. I fail to discover sufli-
cient reasons for a subdivision on textural characters alone, but differences
of habitude which are tolerably constant may, I think, be founded upon
the mineralogical constitution. The basalts almost invariably contain
olivin in abundance, while in the dolerites it is far less common though
sometimes found. The dolerites are as a group more siliceous, though the
true basalts sometimes have more than the normal percentage of that consti-
tuent. In the true basalts such minerals as augite, magnetite, olivin, leucite,
and nephelin reach the extremes of their proportions; in the dolerites the
same minerals are on the whole less abundant, and the predominance of the
feldspathic ingredient is more emphatic. It has seemed to me, therefore,
that the name dolerite should be fully recognized as applicable to a sub-
group of the basalts, including those coarser-grained varieties in which the
proportion of silica is notably higher than in the typical basalts, and also
including the more basic of those rocks which Zirkel has called augitic

andesites.
Group IV.—_BASIC ROCKS—BASALTS.

Sub-groups. Characteristics.

eb OLERITMeseeaeiasecesiaie ace Distinctly crystalline; plagioclase feldspar with (usually) subordinate
orthoclase; augite always conspicuous and in large amount; much
magnetite; a glassy base with pronounced fluidal texture; formless
clots of black ferruginous material usually considered as amorphous
augite. Color, dark gray to nearly black.

2. NEPHELIN-DOLERITE ..----- Similar to the above but with nephelin replacing a part of the plagio-

clase.
Bh LAG Al ooded ose0 coaced ances Fine-grained; feldspar crystals distinguishable only by the microscope.

Abundant augite and a glassy base; olivin usually present. Very
dark colored, nearly black.

4, LEUCITE-BASALT....-...-.. With leucite replacing a part of the feldspar and sometimes the whole
of it.
5. NEPHELIN-BASALT.... .---- With nephelin replacing feldspar.

(, NAGE Bd) G80) pS 556 aoceK00 anos A vitreous obsidian-like lava, having the basaltic constitution.
|

112 GEOLOGY OF THE HIGH PLATEAUS.

The foregoing scheme of classification is in the following conspectus
given as a whole. Of the various sub-groups the following have not yet
been detected among the eruptives of the High Plateaus: nevadite, por-
phyritic trachyte, augitic propylite, dacite, nephelin-dolerite, leucite-basalt,
nephelin-basalt. All of the others are well represented. The trachytic

group, however, very far overshadows all the others in volume and variety.
Group I.—AcIpD ROCKS. RHYOLITES.
. Sub-groups.
1. Nevadite. 2. Liparite. 3. Rhyolite (proper).
Group II.—SuB-ACID ROCKS. TRACHYTES.

Sub-group A.—Sanidin trachytes. Sub-group B.—Hornblendic trachytes.

1. Granitoid trachyte. 5. Hornblendic trachyte.
2. Porphyritic trachyte. 6. Augitic trachyte.

3. Argilloid trachyte. 7. Phonolite.

4. Hyaline trachyte. 8. Trachytic obsidian.

GrovupPp IIJ.—SuB-BASIG ROCKS. ANDESITES.

Sub-groups.

1. Hornblendie propylite. 4. Hornblendic andesite.
2. Augitic propylite (?). 5. Augitic andesite.
3. Quartz propylite. 6. Dacite.

Group IV.—BASIC RocKS. BASALTS.

—

. Dolerite.
. Nephelin dolerite.
. Basalt (proper).

oo bo

Sub-groups.

4. Leucite basalt.
5. Nephelin basalt.
6. Tachylite.

CHAPTER V.

SPECULATIONS CONCERNING THE CAUSES OF VOLCANIC ACTION.

The cause of the succession of rocks apparently a single phase of the more general cause of voleanism.—
The probable subterranean locus of volcanic activity.—Notion of an all-liquid interior.—Not asso-
ciated with volcanicity, and gives no explanation.—Large vesicles not tenable.—Localization of
volcanic phenomena.—Independence of vents.—Growth and decay of action.—Lavas not primor-
dial liquids.—Comparison of lavas with metamorphic rocks: First, with reference to chemical
constitution; second, mineral components; third, texture.—Possibility that lavas are remelted
metamorphic rocks.—All lavas cannot so originate.—Average composition of eruptive and sedi-
mentary rocks compared.—Agreement in composition between basalts and sedimentary rocks.—
Mr. King’s hypothesis of segregation of crystals.—Primitive magma.—Conjectured source of
lavas.—Dynamical cause of eruptions.—Cyclical character of voleanism.—Elastic energy of erup-
tions.—Real nature of the dynamical problem.—The origin of the energy.—Increase of local sub-
terranean temperatures-—Relief of pressure.—Access of water.—Linear arrangement.—Mechanics
of eruptions.—Penetrating power of lavas.—Expelling power.—Not effervescence, but pressure of
denser rocks overlying their reservoirs.—A simple application of hydrostatic laws.—Explanation
of the sequence of eruptions.—A compound function of density and fusibilty.—Graphical repre-
sentation.—Discussion of the hypothesis and objections to it.—Exceptions and anomalies.

Ihave doubted the propriety of embodying in a work devoted to a
statement of observed facts any views of a speculative nature. But the
representations of my director and associates have encouraged me to do so,
inasmuch as the subject is quite germane to the observations, and the ob-
servations are such as have stimulated great curiosity as to their causes. I
shall, therefore, present a trial hypothesis, which seems to me to explain the
sequence in the eruptive rocks now testified to prevail generally through-
out the Rocky Mountain Region.

It seems as if the explanation of such an order of facts could only be a
phase of the more general cause of volcanism itself. But the origin of vol-
canic energy is one of the blankest mysteries of science, and it is strange
indeed, that a class of phenomena so long familiar to the human race and
so zealously studied through all the ages should be so utterly without ex-
planation. Nothing could be further from my intention than propounding

3 HP 113

114 GEOLOGY OF THE HIGH PLATEAUS.

a general theory of volcanism, for neither the facts nor the antecedent gen-
eralizations are ready for it. Such a theory must be the work of several
generations to come, and must gradually grow into form and coherence as
all great theories have done heretofore. Yet there are a few conceptions
of a high degree of generality which, perhaps, contain the germs of a theory,
though in their present condition they are vague and formless. They may
be said to resemble stones in the quarry, rough and unhewn, but which may
some time become corner-stones, columns, and entablatures in the future edi-
fice. I shall propose some of these considerations, not in the form of a con-
nected theory of volcanism, but as partial constituents of a theory in a
highly generalized form, taking care to proceed no further than existing
knowledge may afford at least some justification in proceeding.

I. The first consideration has reference to the probable subterranean
locus of wolcanic activity. In the present stage of our knowledge it
seems little credible that the sources of eruptive materials can be located
at very great depths. It is almost impossible that they could have
emanated from a general liquid interior. Taking the common notion that
the earth has formed, by cooling, an external rocky shell, enveloping a
nucleus which was once an intensely heated liquid, and which may still be

So, either partially or wholly, the ordinary principles of hydrostatics lead us
to conclude that all the primordial volcanic energy ought to have been
exhausted even before a stable crust could have been first formed. We
are in the habit of regarding the earth as hot within, but gradually dissipat-
ing its heat by conduction through the crust and by radiation into space,
and if this conception have any truth, or even verisimilitude, then the erup-
tion of portions of its primordial liquid masses ought to become more and
more difficult with the process of ages—nay, ought to have ceased at a
period long anterior to the most ancient of any of which systematic geology
can take direct cognizance; for secular cooling can only strengthen the
rigid envelope and continually abstract from the heated magmas below the
heat which renders them liquid and eruptible. We cannot in this connec-
tion ignore the plainest consequences of hydrostatic laws. A solid crust
covering a fluid nucleus, or a portion of that crust covering a large liquid

vesicle, could not remain stable for an hour unless the liquid were denser

LOCAL CHARACTER OF VOLCANIC PHENOMENA. 115

than the crust. If the liquid were lighter an eruption would be inevitable,
and once started would continue until the lighter liquid had all found its way
to the surface. If the liquid were heavier, it could no more be erupted
than a frozen lake could erupt its waters and pour them over its icy
covering.

Lest these considerations should seem too purely speculative to author-
ize us to conclude that lavas cannot be emanations from a general liquid
interior or from vesicles holding primordial liquid magma, we may turn to
other considerations more concrete and bearing more directly upon the
point. Volcanic eruptions are very local phenomena. At any given epoch
they are confined to a few localities of very small relative extent. They
have no general distribution in the sense of a widely-extended and con-
nected system. Hach volcano is an independent machine—nay, each vent
and monticule is for the time being engaged in its own peculiar business,
cooking as it were its special dish, which in due time is to be separately
served We have instances of vents within hailing distance of each other
pouring out totally different kinds of lava, neither sympathizing with the
other in any discernible manner nor influencing the other in any apprecia-
ble degree. Again, we find vents at high levels and at low levels in close
proximity with each other, and both delivering the same kind of lava. The
great craters of the Sandwich Islands are remarkable instances of this kind,
and indicate that each crater derives its lavas from a distinct reservoir. It
is inconceivable that a liquid from a common reservoir could rise and out-
flow from the loftier vent while the lower vent remained open. ‘The same
phenomenon is exhibited at AZtna and in Iceland and other active volcanoes.
Then, too, we have the outpouring of widely distinct kinds of lava from
the same orifice at. successive epochs, and as a general rule the grander
volcanoes present a succession of eruptions marked by different kinds
of lava; and it should be noted that these varieties of ejecta are not
intermixed nor formed by the commingling of two or more magmas, nor
do they present intermediate and transition types, but each coulée has a
well-defined character, which serves to distinguish it and assign it to its
proper place in the classification. All these subordinate phenomena, and
many others which it is needless to mention here, are apparently incon-

116 GEOLOGY OF THE HIGH PLATEAUS.

sistent with the assumption that lavas are portions of a primordial, uncon-
gealed earth-liquid, forming either a general fluid nucleus or extensive iso-
lated vesicles. They point rather to many small reservoirs, situated at no
very great depths, each of which contains, not a primordial liquid, but a
liquid secreted, so to speak, from surrounding rocks, or generated by a sec-
ondary and progressive fusion of solidified matter occurring in macule within
the layers of the rocky envelope of the earth. The whole tenor of volcanic
phenomena bespeaks a process which is extremely local—a process which
has an inception, a growth, a culmination, a decadence, and a final cessa-
tion, all within a limited and rather small area and determined by some
local cause.

But we find the strongest evidence against the hypothesis that lavas
are primordial liquids when we come to the study of their physical, chemi-
eal, and mineralogical characters. We do not, indeed, have any very deci-
sive grounds for asserting what the primordial liquids might consist of or
what would be their petrographic characters if any of them were erupted

to the surface, and so far we might not be justified in saying that the lavas

from volcanoes are distinct from them. But there are some eruptive masses
which are very plainly not primordial. For instance, a decidedly conspicu-
ous mass of these products are not fused rocks, but hot mud holding large
quantities of rocky fragments, which have unmistakably formed the clastic
components of strata. The volcanoes of Central America and the Andes
and of the Batavian Islands have within the last century disgorged astound-
ing masses of hot mud—material which has not been fused at all, but
rendered plastic and capable of flow by the combined action of heat and
watery solution. It cannot be admitted that such erupta can have come
from primordial materials. And the indications are no less distinct that the
greater part of the true lavas have originated from other sources.

The careful and systematic study of the petrographic characters of all
rocks, whether sedimentary, metamorphic, or eruptive, has enabled us to
compare them intelligently, and to form some conclusions as to the homolo-
gies on the one hand and the distinctions on the other which exist between
them. The great generalization that the foliated crystalline rocks are
altered sediments has long since passed into geological science as a fully

COMPARISON OF ERUPTIVE WITH METAMORPHIC ROCKS. 117

accepted theory. But the relations between the metamorphic and eruptive
rocks constitute a pending question.

It will be unnecessary here to enter very minutely into a discussion
of these relations, and, indeed, a full discussion would require a very long
and copious review of the existing state of lithological science. It will be
sufficient to state in a summary manner those points of comparison which
immediately concern the subject in hand. The conclusion to which this
comparison tends is that a large proportion of the igneous rocks have the
petrographic characters which we ought to expect would result from the
fusion of certain groups of metamorphic stratified rocks. There are three
points of view from which the comparison may be made; these are with
reference, first, to chemical constitution; second, to mineral components ;
third, to mechanical texture.

1st. Metamorphic and igneous rocks compared with respect to chemical
constitution—The eruptive rocks are highly complex compounds, and always
contain certain constituents which may be called essential constituents.
These are silica, alumina, lime, soda, potash, and magnesia—six in number.
Jron in the form of some oxide is almost always present, but since it is
occasionally absent, or found in exceedingly small quantity, it cannot be
regarded as a universal and essential constituent. Silica is always the
dominant ingredient, and though the quantity of it varies greatly, yet the
variation is within tolerably definite limits, almost never exceeding 890 per
cent., and almost never falling below 45 per cent. The remaining five
constituents likewise vary, but always within tolerably narrow limits.
Thus alumina rarely falls below 13 per cent. and rarely exceeds 26 per
cent. Lime rarely exceeds 14 per cent. magnesia 10 per cent., soda 9 per
cent., and potash 8 per cent. The variations in the relative proportions of
these constituents is sufficiently wide to give well-marked specific or even
generic differences in the kinds of volcanic products; but the variations
are so limited and the relative proportions subject to such moderate depart-
ures from normal ratios, that the whole category of eruptive rocks possess
at least ordinal if not family likenesses. Turning now to the metamorphics
we find a far wider range of chemical constitution. Thus we have quartzites
which are almost pure silica; we have crystalline limestones and dolomites

118 GEOLOGY OF THE HIGH PLATEAUS.

which are nearly pure calcic and magnesian carbonates; we have clay-
slates, serpentines, chloritic, and mica schists, which have a composition
not at all similar to that of eruptive rocks. But while a large proportion
of the metamorphic rocks have no chemical correspondence to the eruptive
rocks, there is another large proportion of them in which the constituents
correspond almost exactly to those of the eruptives. These are the gneisses,
the hornblendie and the augitic schists. The greater part of the true
gneissic rocks yield by analysis practically the same results as granite,
syenite, rhyolite, and acid trachyte. The hornblendic schists have about
the same constituents as the diorites, propylites, and hornblendic trachytes,
while the more basic hornblendic (sometimes augitic) schists hold the same
relation to diabase, dolerite, and augitic andesite This, then, we find that
the eruptive masses have their representatives (chemically considered)
among certain groups of metamorphic rocks.

2d. Metamorphic and igneous rocks compared with respect to mineral com-
ponents—Chemical identity or similarity implies no necessary and exact
correspondence in mineral constituents, for the minerals which may be
formed in a rockmass under varying conditions of temperature and environ-
ment cannot be determined solely by the chemical composition of the
magma. ‘The crystals of the metamorphic rocks are formed according to
the commonly accepted theory of metamorphism, at rather low or very
moderate temperatures, while the crystals of igneous rocks are in part at
least, and perhaps wholly, generated at high temperatures. Hence it is
not surprising that metamorphic rocks should contain some crystalline
forms which are seldom or never found in the igneous except as alteration
products, or should contain some forms in abundance which the latter con-
tain very sparingly. There are, however, some minerals which may be
formed indifferently at high or low temperatures, and the most important of
these are undoubtedly feldspar and hornblende. Those which form with
great facility at low temperatures are certain forms of mica, quartz, chlorite,
and the zeolites, and those which seem to be associated with higher tem-
peratures are leucite, nephelin, olivin, and less decidedly augite. By a
comparison of the two classes of rocks, therefore, we find an agreement in

respect to those minerals which are indifferent to variations of conditions;

COMPARISON OF ERUPTIVE WITH METAMORPHIC ROCKS. 119

and disagreement only in those minerals which are decidedly dependent
upon variations of condition. ‘The metamorphics abound in low tempera-
ture minerals, the eruptives in high temperature minerals. Both classes
contain abundant feldspar, mica, and hornblende, which seem to be but little
affected by temperature, so far as concerns the facility with which they are
formed. .

3d. Metamorphic and igneous rocks compared with respect to mechanical
texture—In the modes of aggregation of the rock-forming materials, the
two classes of rocks differ radically. Nor could we anticipate any agree-
ment here. The metamorphics have not been melted down, but retain with
greater or less distinctness their original foliation. The changes have been
purely molecular. Where the metamorphism is complete the rock is ordi-
narily made up of purely crystalline matter, each crystal being a definite
mineral species, with definite optical and crystallographic properties pecu-
liar to its kind, the whole interlocked into a mosaic of great beauty, which
is revealed to the eye by a polished surface, or still more clearly by a thin
section under the microscope. But the volcanic rocks have a totally differ-
ent texture, of which the distinguishing characteristic is the presence of a
non-crystalline or amorphous base in which crystals are disseminated.
Sometimes the crystals are wholly absent, and the amorphous base-consti-
tutes the entire rock, as in pitchstone and obsidian. The distinction, then,
between the texture of a thoroughly metamorphic rock and an extravasated
mass is that the former is wholly crystalline, while the latter is either par-
tially or wholly amorphous. And yet we have rocks which present every
shade of transition between the two textures. The gneisses, for instance,
lose their foliation and become indistinguishable from granites. The
granites present varieties which have larger and more perfect crystals
imbedded in a maze of smaller ones. We may select a series in which, the
mosaic of surrounding crystals becomes finer and finer and the inclosed
crystals more perfect and contrasted, and such a group is called porphy-
vitic granite or granite porphyry. Following this chain of varieties, the
crystalline base gradually passes into one in which the utmost power of the
microscope fails to detect any individualized crystals, but merely indicates
by indirection that the base has been in some -way influenced by the crys-

120 GEOLOGY OF THE HIGH PLATEAUS.

tallogenic force, for it continues to polarize light. This is the case with
typical porphyries and with many trachytes and rhyolites. In the extreme
varieties all traces of crystalline arrangement in the base have disappeared,
and the inclosing matter is very similar to common glass, while the inclosed
crystals are sharply defined within it.

But while there is a sufficiently close agreement between the eruptive
rocks on the one hand and some of the metamorphics on the other, there are
many metamorphics which have very little in common with the eruptives.
Such rocks as quartzite, limestone, dolomite, and argillite are never found in
the eruptive condition. Here it is necessary to anticipate, in part, the course
of the argument. The hypothesis to be invoked will consist in the assump-
tion that the proximate cause of eruptions is a local increment of subter-
ranean temperature, whereby segregated masses of rocks, formerly solid,
are liquefied. Since a state of fusion is necessary to an eruption, we may
throw out of consideration all those materials which are so refractory that
they cannot be liquefied by temperatures within the highest range of vol-
canic heat. But the most refractory metamorphic or sedimentary strata
are the very ones which have no correlatives among the eruptives; and,
conversely, those strata which are most fusible have rocks of correlative
constitution among the eruptives. Hence we may in part clear the way
for the proposition that quartzites, limestones, &c., are never erupted, be-
cause they are infusible at the highest volcanic temperature. We have not,
indeed, the means of directly measuring volcanic heat, but we may infer
that it is never in excess of that required to melt the most refractory rhyo-
lites, since these lavas bear no evidence of being heated beyond a tempera-
ture just sufficient to liquefy them. Rhyolites and trachytes bear strong
internal and external evidence that at the time of eruption they were just
fused and no more, while basalts often betray evidence of superfusion.
Thus, in the comparison of the two classes of rocks, we may discard from
consideration those of simpler constitution, like quartzites, dolomites, argil-
lites, limestones, &c., and confine our discussion to those more complex,
stratified masses which alone are fusible and, therefore, alone eruptible.

Our comparison of the metamorphic and igneous rocks, therefore, indi-

cates in many ways and argues strongly for a common parentage. The

NOT ALL LAVAS ARE PRODUCT OF REFUSION. 121

approximate identity of chemical constitution is what we should anticipate
on that assumption. We should expect to find some minerals common to
both classes of rocks, while other minerals are found in one class alone.
We should look for nothing but contrast in the respective mechanical tex-
tures; and we find the anticipated agreements and contrasts.

But there is an important consideration which will not permit us to
conclude that all eruptive rocks are derived from the fusion of metamor-
phics; for whence came the materials of the metamorphic rocks them-
selves? Accepted theories declare that their ultimate origin was in the
primordial materials of the earth-mass, which were broken up, decom-
posed, and the several components sorted out and arranged in the form of
sediments; and these sedimentary formations gradually accumulated until
they completely buried the primordial mass, so that no portion of it is
anywhere exposed, so far as has yet been discovered. But when the prim-
itive mass was finally buried, from what sources could the materials have
been derived which could add fresh layers to the covering? To this there
is but one possible answer. After the greater portion of the original sur-
face had been covered, additional sediments must have been derived from
the extravasation of primordial matter. This conclusion seems to be logi-
cally perfect. In the past epochs these primitive materials must have been
continually extravasated, though, as the body of sedimentary formations
increased, it is possible that they too began to be erupted by secondary
fusion, and with the lapse of time formed an increasing proportion of the
total extravasation, while the proportion of primitive matter as gradually
diminished. Now, have we any reason for supposing that the evolution of
the earth has so far advanced that primitive matter has ceased to erupt, and
that modern outbreaks consist wholly of materials which had once before
in the world’s history been poured out, broken up, decomposed, stratified,
metamorphosed, and again erupted? If so, then the body of stratified rocks
is no longer increasing, but the revolutions of time are simply working
over the stratified rocks again and again. But this is improbable in a high
degree. There is no warrant whatever for such a belief, and therefore no
justification for the inference that all eruptive rocks are derived from the
secondary fusion of the metamorphics. But if it is probable that some of

22, GEOLOGY OF THE HIGH PLATEAUS.

the lavas have emanated from. primordial rocks, what are they? There is
one great, group of lavas which quickly furnish ground for suspicion.
Recurring here to the generalization that the materials composing the
stratified rocks have been ultimately derived from primordial matter, it is
but an identical proposition to say that. the chemical constitution of that.
primordial matter ought inferentially to be such as would yield the mate-
rials of the sedimentary rocks. It ought to possess the same constituents,
and ought also to contain them in substantially the same proportions as the
average constitution of the stratified rocks taken as a whole category. In
a word, it should be what some biologists might call a synthetic or compre-
hensive type of rock, from which the stratified materials might be differ-
entiated by the known processes of sub-aerial decomposition and selection.
Secondly, it ought not to conform in composition to any one variety of
stratified rock, unless, perchance, in some rare exceptional cases. Thirdly,
it ought to be a very abundant and voluminous rock, erupted at almost any
geological age or period, from the present as far back into the past as we
are able to discriminate the age of an eruption. Among the several groups
or sub-groups of volcanic rocks do we find any one of them answering to
this ideal type? This question does not admit of a very brief and decisive
answer. We have no very accurate knowledge of the mean constitution
of the stratified rocks. There is a statement, handed down, I believe, from
Bischof, and passing current in the text-books, that silica constitutes very
nearly 50 per cent. of the mass of all known rocks, and the estimate seems
to be a very fair one. Its probable error is certainly small if the impres-
sions of the geologists who have given much attention to lithology are to
be trusted. This percentage of silica is substantially the same as that
found in the basalts, and if there be a synthetic type of eruptive rocks
this fact fastens suspicion at once upon the basaltic group. . Probably no
lithologist will hesitate to say that next to silica the most abundant con-
stituent of the stratified rocks is alumina; but the exact proportions we do
not know. Alumina is, however, known to be the second in quantity in
the constitution of average basalt. But the third constituent of basalt in
respect to quantity is iron oxide; in the foliated rocks it is unquestionably

lime. Here is a discrepancy, and a well-marked one, which we cannot

SYNTHETIC CHARACTER OF BASALT. 123

explain away without resorting to doubtful postulates and conjectures.
Iron oxide forms at least 10 to 12 per cent. of normal basalt, and, while it
is found abundantly in almost all foliated rocks, it cannot be admitted that
it forms so large a percentage of their average constitution. With regard
to lime, however, which forms about 8 or 9 per cent. of the basalts, the
percentage is apparently in harmony with what we know of the constitu-
tion of the foliated rocks. With regard to the remaining important com-
ponents—magnesia, soda, and potash—the same relative correspondence is
found; but whether the correspondence be exact or not, we have not the
data for determining.

Relative order of abundance of the oxides constituting basalts and the foliated

rocks.

Basalts. Foliated rocks.
Silica. Silica. ( Silica.
Alumina. Alumina. Alumina.
Tron oxide. Lime. Lime.
Lime. Magnesia. or J Iron oxide.
Magnesia. Tron oxide. ; Magnesia.
Soda. Soda. | Soda.
Potash. Potash. Potash.

With the single exception of iron oxide, therefore, the basalts, as
nearly as we have the means of ascertaining, have a constitution repre-
senting approximately the average composition and proportions of the
foliated rocks. There is no other known volcanic rock which approaches
that relation so nearly; all others contain too much silica and alkali and
too little lime. But so long as the iron oxide remains an outstanding
anomaly we cannot be justified in pronouncing the basalts to be the exact
syuthetic type. It remains to be added that the basalts alone fail to show
that agreement in chemical constitution with any known and abundant
metamorphic rock which we find in all other voleanic groups In truth, its
whole range of characters is indicative of an origin among magmas which
have never passed through the reactions and mechanical processes which
prepared and arranged the materials of the sedimentary strata. Lastly,
the basalts are among the most abundant of eruptive rocks, and if we
reckon with them the more ancient dolerites or diabases, they have always

been abundant in all ages as far back as our knowledge extends.

124 GEOLOGY OF THE HIGH PLATEAUS.

But not only should we infer that the primordial masses of the earth
(or “primitive crust”) were basic like the basalts or dolerites, but that
they were very nearly homogeneous. If we are at liberty to speculate at
all upon the physical condition of an all-liquid planet, its molten surface
exposed to radiation and to the action of its immense -atmosphere, we
should be led to infer that it would be agitated by disturbances similar
in nature, though inferior in magnitude, to those affecting the sun, thus
producing a thorough and homogeneous mixture of the compounds of
silica with alumina, the earths, and alkalies. This admixture once formed
would, so far as we can now see, remain unaltered until it cooled suffi-
ciently for the reactions of the atmosphere. We know of no natural
processes capable of separating the more acid parts of such a magma
except the chemistry of the atmosphere acting at temperatures far below
the melting-points of the silicates. We have the results of that process in
the quartzites, granites, gneisses, and syenites among the siliceous rocks;
and the limestones and dolomites among the basic rocks; with argillaceous
rocks as the residuum of the decomposition. Yet if these rocks could be
remelted together they would form one homogeneous magma. Every iron-
smelting furnace is an experimental demonstration of the tendency of silica
to take up and hold at fusion-temperature alumina, lime, magnesia, potash,
and soda in proportions exceeding those which occur in nature. No facts
are known to me which justify the conclusion that segregation into two
magmas could occur in such a state of fusion. Nor would it be of any
service in this connection to establish the possibility of such a segregation.*
It is suggested by Mr. King that erystals might form in the liquid and sink
by reason of their superior specific gravity. Although I hold it to be
extremely doubtful whether any crystals are formed while the rocks are
melted, and very probable that the greater part of them are formed during the
viscous stage of cooling (especially the hornblendes and pyroxenes), there
is one consideration which would prevent us from using this view to predi-
cate a theory of a single magma separating into two or more of very differ-
ent degrees of acidity. The low percentage of silica in basalt is due not

“Tron, however, might separate from such a compound, either as a regulus or as magnetic oxide,
if the conditions were favorable and the oxide in excess.

GENERAL RESULTS OF COMPARISON. 125

only to the low percentage in the feldspar and augite, but also to an equally
low percentage in the base. The high percentage in rhyolite and trachyte
is due not only to the feldspar, but still more to the even higher percentage
of silica in the base. If there has been segregation, it must, therefore, have
affected not only the crystals, but the base even more than the crystals.
Such a separation, therefore, does not seem explicable by supposing a pre-
cipitation of crystals.

Gathering together now the threads of this comparison, we are led to
the conclusion that the constitution of the eruptive rocks forbids the belief
that the acid varieties, or even the intermediate varieties, can be primordial
masses from vesicles which separated in a liquid condition from the original
earthmass and remained liquid up to the time of their eruption. Chemical
considerations of a cogent character lead up to the inference that primordial
magma ought to possess a constitution similar to rocks of the basaltic group,
though perhaps somewhat less ferruginous (?), and that it should be nearly
homogeneous. And in generalour inference from the nature and constitution
of the volcanic rocks, from their great variety, from the localization of
eruptive phenomena, from the intermittent character of volcanic action,
from the independence of the several vents, is that the lavas do not emanate
from an earth-nucleus wholly liquid, nor from great subterranean reservoirs
still left in a liquid condition “from the foundations of the world,” but from
the secondary fusion of rocks, a part of which may have formed the primi-
tive crust, while the remaining part consisted of deeply-buried and meta-
morphosed sedimentary strata. No doubt some cautious philosophers may
regard this inference as specifying a little too minutely the locus of volcanic
activity—more minutely than a rigorous deduction from known facts will
permit us to regard as positively proven. But at all events there is one
proposition which may be laid down with no small degree of confidence,
and it is this: We must at least admit that te source of lavas is among segre-
gated masses of heterogeneous materials. This arrangement would be well
satisfied by a succession of metamorphic strata resting upon a supposed
primitive crust of magma having a constitution approximating that of the
basaltic group of rocks.

II. The second general consideration has reference to the dynamical

126 GEOLOGY OF THE HIGH PLATEAUS.

cause of volcanic eruptions, or the force which has brought them to the
surface.

Not only are volcanic phenomena very local in respect to area, but the
period of activity in any given spot is very limited in respect to duration.
No region has always been eruptive, and we may be reasonably confident
that none will continue to be eruptive indefinitely. Volcanicity has its
inception, passes through its cycle, and lapses into final repose. We do,
indeed, find localities which have twice been the scene of such devastations
during the entire period of which systematic geology takes cognizance, just
as battles have more than once been fought on the same plain with cen-
turies between; but the intervals separating such visitations are so vast
when measured even by the geological standard of time, that there is no
obvious relation between them. It is not strange that a process which
shifts its arena throughout the ages should occasionally revisit the scenes of
former operations. This migratory character suggests to us that the normal
condition of the nether regions is not one of unrest, but rather of quietude.
What is the disturbing element which invades their secular calm, convulses
them with earthquakes and explosions, and causes them to pour forth their
fiery humors? With this problem geologists and physicists have wrestled
in vain. Here speculation seems to be peculiarly unfruitful. To-day it
looks promising; to-morrow turns it into ridicule. We do not know the
determining cause of volcanic eruptions. Yet there are a few facts of a
high degree of generality, around which we linger with inquiring, anxious
minds, hopefully promising ourselves that light will shine out of them at
some future day, and to these it may be proper to briefly advert.

We may contrast the explosive condition of volcanic products during
an eruptive cycle with their. quiet and inert condition before the cycle
began. These same materials lay quietly in the earth for long periods,
some of them, perhaps, since that imagined primordial epoch when a crust
began to form. Some change has come over them, converting them into
energetic explosive mixtures. The problem is to find an adequate cause
for such a change and the nature of its operation. This statement of the
conditions of the problem is in strong contrast with the view which regards
lavas as primordial liquids charged with volcanic energy waiting for a con-

THE PROXIMATE CAUSE OF ERUPTIONS. WA

venient season to explode. It presents the case as a problem of energy
acquired by some secondary forces, of which we are at present ignorant.

There is one general assumption which satisfies all the main requisites
of voleanism. It is this: Volcanic phenomena are brought about by a local
increase of temperature within certain subterranean horizons. This, indeed, is
not a solution of the problem, for it throws us back instantly upon the ulte-
rior question, What has caused the increase of temperature? All my efforts
to find an answer to this ulterior question have utterly failed. But the
proximate idea is suggested on every hand, and its reality takes deeper root
in conviction the more itis contemplated. Around it the broader facts take
form and coherence. It explains their secondary character as contradis-
tinguished from the primordial. It explains the cyclical phases of volcan-
ism; their beginning in a recent epoch of the world’s secular history; their
erowth, decay, and extinction. It explains their intermittent character—
why eruptions are repetitive instead of continuous. It explains the explo-
sive and energetic character of the phenomena ; and, lastly, it explains the
lithological order of the eruptions, as will presently be shown.

But there is another and alternative assumption. We may suppose
the deeply-seated rocks in regions of high temperature to undergo changes,
one result of which is to lower their melting-points. This is not so strange
as it might at first seem, for its accomplishment is conceivably within
known physical laws. A relief of pressure is one conceivable mode.
Probably another would be the absorption of water under great pressure
and at high temperature. It can hardly be doubted that a rock charged
with water and so confined that the water cannot readily escape is more
fusible than the same rock in an anhydrous condition. The fact that
lavas bring to the surface considerable quantities of water may be held to
be evidence that water does find access to them from above. The only
alternative view is that water formed a part of their original constitution.
This is undoubtedly the case on the view that lavas are remelted metamor-
phic rocks; for the metamorphies all contain water, partly mechanically
held and partly as water of combination in hydrous minerals. The amount
of contained water is variable, but ordinarily more than one per cent. and
sometimes much more This quantity, however, probably falls far below

128 GEOLOGY OF THE HIGH PLATEAUS.

the volume of steam ordinarily given off by voleanoes. Unless the esti-
mates of observers are altogether deceptive, the quantity of water blown out
of voleanic vents must beara far greater ratio to their lavas than one or two
per cent., and we seem to be compelled to assume that the lavas derive their
water from extraneous sources, and the penetration of surface water to
regions of volcanic energy is by far the easiest explanation. The penetra-
tion of water, then, is a consideration of importance, but the precise nature
of its effects we have no means of determining, and any attempt to follow
them would lead us into discussions too purely speculative to be of value.
The relief of pressure is another possible mode of liquefying rock. It
is postulated by Mr. Clarence King as a basis of his theory of volcanic
eruptions. This relief is effected through the removal of superincumbent
strata by the process of denudation. Such removals have taken place upon
a vast scale, and though geologists have possibly been suspected by other
scientists of helping themselves very liberally to a supply of cause and
effect of this kind, yet the surveys of our western domain have proven that
they have been very modest and abstemious. But that such a process
could have played a very important, much less a fundamental, part in caus-
ing volcanic eruptions seems to be negatived by facts. We do not find that
eruptions always occur in localities which have suffered great denudation.
We do not find even that they occur in such localities predominantly. Most
of the existing volcanoes and most of those which have recently become
extinct are situated in regions which have suffered very little denudation in
recent geological periods, and many of them in regions of recent deposi-
tion. Aitna is built upon a platform of Post-Tertiary beds and Vesuvius
stands upon late formations. The same is true, according to Dr. Junghuhn,
of the voleanoes of Java, and this fact is repeated in the great volcanoes of
the Cape de Verde and Canary Islands. The High Plateaus of Utah,
which have been the theater of volcanic activity since the Middle Eocene,
are localities of minimum erosion, while the denudation of the non-voleanic
regions around them has been stupendous. It can hardly be supposed that
the volcanoes of the Pacific have broken forth from denuded localities,
unless the denudation took place at a considerable period of past time.

But whatever may be the effects of the relief of pressure, and how-

THE MECHANICAL ASPECT OF ERUPTIONS. 129

ever essential the presence of water may be to the total process of erup-
tivity, something’ more is obviously needed, and this additional want is
apparently well satisfied by a local rise of temperature in the rocks to be
erupted. For it cannot be insisted upon too strenuously that from a
dynamical standpoint the problem to be explained is the passage of lava-
forming materials from a dormant to an energetic condition. And when
we resolve this very general statement into a more special and definite one,
we find that it means the passage of solid materials into the liquid condi-
tion and (as will be indicated further on) a decrease of density. Whatever
may be the ulterior cause of volcanicity, a rise of temperature in the
erupting masses seems to be an indispensable condition, and in assuming
it we are apparently doing nothing more than taking the most obvious facts
and giving them the plainest and simplest interpretation.

III. The third general consideration has reference to the mechanics
of eruptions. The fact that lavas are generated at the depth of several
miles below the surface being given, how do they reach the surface? A
study of the geological relations of eruptive masses furnishes a decisive
answer to this question. The power of lava to penetrate and burrow into
solid rock would never have been credited or even suspected had we not
the proof of it in the rock exposures. The opening of fissures and the
rise of lava into the gaps is one of the commonest and most intelligible
methods. All volcanic areas are traversed by dikes, and near the centers
of eruption they are exceedingly numerous. But what is most suggestive
is the fact that many lavas, after rising part-way to the surface, suddenly
tear open the strata and diffuse themselves between the beds, forming sub-
terranean lakes at levels far above their original source. These intrusive
lavas are exceedingly common, so much so, that they appear to have con-
stituted in all ages a notable proportion of volcanic movements.

But when a vent is established through which lavas can find escape,
we have still to consider the propelling force which urges them onwards or
upwards. A very common view, long entertained by many geologists, is
that the escape of lavas is analogous to what takes place when a bottle of
warm champagne is suddenly uncorked. So comprehensible and plausible

is this explanation that its wide acceptance is not surprising. In some
9m P

130 GEOLOGY OF THE HIGH PLATEAUS.

cases, for want of ability to show the contrary, it may be accounted a suf-
ficient explanation, and in general it cannot be questioned, that in most
volcanoes this identical action plays a more or less important part. Scoria,
pumice, and volcanic dust have unquestionably this origin; but the whole of
the extravasation is not so accomplished. The outpour of lava is a very
different matter. It is comparatively calmand quiet in its flow, like water
welling forth from a spring; sometimes boiling, bubbling, and spurting a
little, but never boisterous or obstreperous. It continues its flow for days
and sometimes weeks, but at length ceases and comes to rest.

A careful examination of the details of volcanic eruptions leaves*the
impression that they are pressed up by the weight of rocks which overlie

their reservoirs, and that their extravasation is merely a hydrostatic prob--

lem of the simplest order. The conception of a liquid inclosed in a cavity
beneath the surface and opening to the outer air through a stand-pipe
requires some discussion when we come to apply it to volcanic eruptions.
Our conceptions of the constrained motion of liquids are derived from
experiments upon small quantities of them in small vessels; but when we
come to such enormous volumes as are disgorged by volcanoes, a consider-
ation arising from mere magnitude enters into the scheme—a consideration
which has no bearing in relation to small volumes. This is the strength
of the receptacle. It is a well-known principle in mechanics that the
relative strength of a body is inversely proportional to its size. Thus,
where we have similar bodies subject to forces which are proportional to
their own masses, the resistance to detrusion is proportional only to the
square of their linear dimensions. It is this relation which limits the span
of an arch or the length of a truss. Now, if we could conceive the contents
of one of these subterranean lava reservoirs to be suddenly annihilated,
so great must be their dimensions that the rocks above would instantly
sink into the cavity, just as the rocks above a coal-mine do on small provo-
cation. A small cavity, on the other hand, might persist. Now, the point
I wish to illustrate is that the strength of the retaining-walls of a lava
reservoir are relatively so weak, in consequence of the large dimensions,
that their effect is very nearly the same as it would be if the lava were
overlaid by another liquid wifh which it could not commingle. It is the

THE EXPLANATION OF THE SEQUENCE. 131

gross weight of this overlying cover of solid rocks, I conceive, which
presses the lava upward through any passage where it can find vent.

It will follow, then, as a corollary, that the lava will rise to the sur-
face or not according to its density. If it be lighter than the mean density
of the rock above its reservoir, it will reach the surface and nothing can
keep it in; if it be heavier than the overlying rock, it will never reach the
surface.

IV. We come now to the explanation of the sequence of volcanic
rocks. In order that any eruption of lava may take place two preliminary

-conditions are requisite: First. The rocks must be fused. Second. The
density of the lavas must be less than that of the overlying rocks. Having
shown from independent considerations that the proximate cause of vol-
canic activity may be a local rise of temperature in the deeply-seated rocks,
it only remains to follow the obvious phases of the process. We know
that the volcanic rocks vary within tolerably ample limits as to their chem-
ical constitution, and that associated with these chemical differences are
notable differences of physical properties. Some are more fusible than
others and some are heavier than others. We also presume that prior to
eruption these different rocks were within the earth separated as if in strata
or in macule. Imagining, then, a rise of temperature in a nether region
where the constitution of the magma is variable—here very siliceous, there
very basic, with many intermediate varieties, all arranged in any arbitrary
manner and in each other’s neighborhood—it is quite certain that not all of
these magmas would be both fused and sufficiently expanded by heat to be
ready for eruption at the same time. The more refractory rocks might not
be melted or the heavier ones might not be sufficiently expanded. There
would, therefore, be some selection as to the order in which they would
become eruptible. But upon what principle would the selection be made ?
The acid rocks are known to have the highest melting temperature, but the
basic rocks in the cold state have the highest specific gravity. It is just
possible that the acid rocks may be light enough to erupt at an early stage
of the process but are not yet melted, and that the basic rocks may be
melted but must await a further expansion in order to reach the surface.
The first selection would then fall upon some intermediate rock. Let us

132 GEOLOGY OF THE HIGH PLATEAUS.

see if there be anything in the physical properties of the rocks to justify
such a hypothesis. We can represent this best by a graphic expression of
their physical properties regarded as functions of temperature and acidity.

Let the axis of abscissas, Plate 4, represent the proportions of silica
characteristic of the various groups of volcanic rocks, the figures along that
axis representing percentages from 40 to 80. Let the ordinates represent,
first, the density of the rocks in the cold state. Considering now any one
variety of rock, take the point on the axis of abscissas corresponding to its
percentage of silica, and erect an ordinate proportional to its density. For
all the varieties of rocks construct ordinates in the same manner and join
their upper extremities. On the assumption that the density is rigorously
correlated to the percentage of silica, a curve would be constructed repre-
senting the density as a definite function of the silica. This assumption,
however, is not strictly true, being subject, indeed, to notable variations ;
yet in a general way it is more or less an approximation to the truth. The
anomalies will be adverted to in the sequel.

It is known that the rocks of the basic and sub-basie groups are when
cold considerably more dense than the average of the foliated rocks, and
the same is true of some of the sub-acid rocks, and according to the doc-
trine heretofore laid down such rocks could not be erupted at all were it
not for the fact that when intensely heated and liquefied, their density is
notably diminished and reduced below that of the strata which overlie
them. Hence the more basic the rock, the more it must be heated to
reach an eruptible density. The ordinates, then, may be used to represent
the relative increase of temperature which must supervene in order to ren-
der the rocks light enough to reach the surface, and as these increments of
temperature are directly proportional to the density of the rock, the same
curve may (in the absence of fundamental constants) be used to express
the increments of temperature required by the various rocks to reach an
eruptive density.

Again, let the ordinates represent the relative melting temperatures of
the various sub-groups, the assumption still being that the fusibility is a
definite function of the proportion of silica. This assumption is probably
subject to still wider variations than that which postulates a dependence of

PLATE II.

2.90
DOE ASO) CL ee EE eee Seen

(SAM OO NEO OB sty BAS ERO E

T Lipartte- # ease = Sa nn--- == 555
ORIOLE CE ae ies ae

5 Santidin Trachyte. Sager :
USSUL ECC BALI OLE S OCC Ses ee ag ae ey ES
3.Hornblendic Trachyte._.|-. *

2.Hornblendic ANWMESLLE 9.09 :
VLAD LOO NY DOL Le eR ge aN (ee es a eee

|
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rr
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f
|
1
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|
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1 —

2.60

go

50 55 05 7

1R. HBL. ANDES/ITE
3. HBL. TRACHYTE

14. AUG. ANDESITE 5. SANIDIN TRACH:

|

6. DOLE RITE Y.LIPARITE
I, BASALT 8. RHYOLITE

}
|
Y PROPYLITE |
|

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1
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1
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THE EXPLANATION OF THE SEQUENCE. 133

density upon silica, but it is still known that there exists an approximation
to such a dependence. ‘This will also be subsequently alluded to. A curve
may be constructed, as before, representing this dependence, which may be
called the curve of fusion. Since both density and fusion have approxi-
mate relations to the quantity of silica present (and for present purposes
such relations are assumed to be exact), they are functions of each other.
We know that with increasing percentages of silica the density diminishes,
while the melting temperature increases, and hence the two curves if in-
definitely prolonged will somewhere intersect. It remains to determine,
if possible, the point of intersection. Let us for the present arbitrarily
assume that the point of intersection is such that both curves have a com-
mon ordinate erected from a point on the axis of abscissas corresponding to
60 per cent. of silica, which is very nearly the normal percentage of horn-
blendic propylite. I shall hereafter adduce reasons for believing that this
arbitrary assumption is very nearly or quite true.

We have now (ex hypothese) two curves, one representing the tempera-
ture required to render the rocks light enough to rise hydrostatically to
the surface, the other representing the temperature required to fuse them.
Conceiving, then, a general rise of temperature to occur among subterra-
nean groups of rocks, no eruption could take place at any temperature less
than that represented by the ordinate drawn at 60. For the basic rocks
would still be too dense, while the acid rocks would be unmelted. But
when that temperature is reached, the propylite would be in an eruptible
condition. By a further increase of temperature hornblendic andesite and
trachyte would become eruptible, the former having passed the fusion point
and the latter having passed the density point of eruption. And in gen-
eral as the temperature increases the line of eruptive temperature cuts the
two curves at points further and further from the lowest point of eruptivity,
and these points correspond to rocks which become more and more diverg-
ent in their degrees of acidity ; one set progressing to the acid extreme, the
other to the basic extreme. If now our fundamental assumptions are true,
or in essential respects conform approximately to the truth, then the se-
quence of eruptions which those assumed conditions would give rise to con-

forms to the sequence which we find in nature. Let us, then, examine these

134 GEOLOGY OF THE HIGH PLATEAUS.

assumptions, with a view to ascertaining, as well as we are able, how
neatly they approach the truth.

1st. It is assumed that the density is some approximately definite func-
tion of the percentage of silica. There are indeed considerable variations
from exactness in this respect, and we may select two or more species of
rock having the same silica contents, but which differ conspicuously in
density. Yet nothing is more certain than the fact that as a general rule
the assumption is very near the truth. This is so well known that further
discussion is probably unnecessary.

2d. It is assumed that melting temperatures also bear an approximately
definite ratio to the silica. Here the variations from exactness are no
doubt somewhat greater than in the case of density. Still, we know that
on the whole the law strongly prevails, and that the melting temperature
diminishes with the acidity of the rock.* The blast-furnace slags present
often very close approximations to many of the volcanic rocks, and these
approximations are not infrequently so close as to be fairly comparable.
In such cases it is familiar to those who are acquainted with the practical
working of furnaces that the more basic slags are much more easily fused
than the more acid ones. The absolute melting temperatures, however,
are not accurately known.

3d. The assumption that the two curves (density and fusion) will ordi-
narily cut each other at the ordinate of 60 per cent. of silica is one which
presents greater difficulty. Translating graphical terms into concrete lan-
guage, the meaning of it is this: It assumes that rocks having a normal
percentage of about 60 per cent. of silica, and corresponding lithologically
to the hornblendic propylites are fused and rendered light enough to
erupt at one and the same temperature; while rocks more basic are fused
at a lower temperature, but require a higher one to be sufficiently ex-
panded; and rocks more acid are sufficiently expanded at a lower tem-
perature, but require a higher one to fuse them. Is there any independent
evidence of the verity of this assumption? The point is a very important
one; indeed, vital. For if the intersection of the two curves be elsewhere,

*See observations of Bischof on fusion of igneous rack, D’Archiac, vol. iii, and results of Deville
and Delesse, Bul. Soc. Geol. France, 2d ser. iv. D. Forbes Chem. News, xviii.

THE EXPLANATION OF THE SEQUENCE. 135

the theory is fatally impaired. In the absence of evidence fixing the inter-
section here, we might have arbitrarily taken it to be at some other point—
at a point, too, outside of the scale of acidity within which volcanic rocks
are always confined, as in Figs. 1 and 2. In either of these cases the

Fig. 1. Fia. 2

260 | ert R260 pus
C10)

“0) = GO Go YO CO avi WG) 7O. co

rocks would have been, according to the terms of the theory, erupted
strictly in the direct or inverse order of their densities throughout. But I
believe we do possess some distinct evidence that the point of intersection
is rightly chosen, and that this evidence may be read in the petrographic
and mechanical characters of the rocks themselves. A very striking
characteristic of the basaltic lavas is their perfect liquidity at the time of
eruption and their power to flow in comparatively narrow and shallow
streams to great distances. It is in the basalts that this property is most
marked and conspicuous. Coulées only two or three hundred feet wide
and only twenty or thirty feet thick are usually found flowing mile after
mile with facility, and larger streams reach from thirty to fifty miles from
their orifices. Very thin sheets of basalt flow on to great distances. No
other rocks in streams of such small cross-sections reach distances so far
from their origin. And when we recall the circumstances which favor a
rapid cooling and solidification, this preservation of fluidity is remarkable.
The experiments of Bischof and Deville agree in indicating that the latent
heat of fusion is less in the basalts than in other rocks. The larger amount
of surface which these thin streams or sheets expose, the disappearance of
heat which is consumed in expelling in the form of vapor the included
water, all combine to dissipate or render latent the contained heat of the

136 GEOLOGY OF THE HIGH PLATEAUS.

lava with extreme rapidity. In the basaltic rocks we have thus, as I
believe, most satisfactory evidence that when they reach the surface they
are heated to a temperature much above that of mere fusion. In no other
way are we able to account so satisfactorily for the persistency with which
they retain their extreme liquidity and flow to such great distances. The
same fact appears in the study of the minuter textural characters of the
basalts. Under the microscope everything indicates an intense degree of
ignition. The presence of glass particles and the absence of water cavities,
the isotrope base, the exceeding compactness of the rock, its vitreous
character, and (in the massive portions) the absence of all traces of
viscosity or ropy condition, point to the same conclusion. All this is in
strong contrast with rocks of the sub-acid group. The trachytes and pro-
pylites appear to have been erupted, in many cases, in a viscous condition,
or in one which was not by any means thoroughly liquid. They are found
in thick, cumbersome masses, and, unless the outpour was of excessive vol-
ume and mass, do not appear to have flowed far from their orifices. The
trachytes, however, vary much in this respect; some appear to have been
quite liquid, others exceedingly tough and pasty, with all intermediate con-
sistencies, though in the most fluent ones-there is no evidence of excess of
temperature above the point of complete fusion. As a general rule their
sluggish character is well pronounced. In the rhyolites there is evidence
of intense ignition and thorough fusion; but the banded, ropy, and fibro-
litic character is suggestive of a temperature just sufficient to melt them to
a vitreous consistency, but without that perfect limpid liquidity of the
basalts in which the rhyolitic texture would certainly be completely oblit-
erated.

Now, the pyroxenic divisions—the basalts, dolerites, augitic andesites—
all betray evidence of superfusion, or a temperature much in excess of
that required to melt them. In the hornblendic andesites the same appear-
ances are seen, though less in degree. In the propylites they have van-
ished, and are not discernible in the trachytes and rhyolites. This is in
accordance with the assumption contained in the theory. All rocks more
basic than propylite betray evidence of superfusion, and hence it is at

propylite in the ascending scale of acidity that superfusion is presumed

CONSIDERATION OF APPARENT EXCEPTIONS. ax7/

to cease.* If, then, these facts will bear the interpretation which I have
placed upon them, we have in the rocks themselves the evidence required
to show that propylite is a rock which at a certain temperature is just suf-
ficiently fused and just sufficiently expanded to fulfill the mechanical con-
ditions requisite for eruption.

It still remains to look at some points in the application of this theory
to the succession of eruptions, which would at first sight appear anomalous
if not inconsistent with it.

We do not always find the order of succession heretofore described to
have been strictly followed; we find exceptional cases. Instances are not
wanting where true basalts have outflowed prior to the eruption of rhyo-
lites, and are even known to be overlaid by trachytes in the Auvergne
district of France, or as Lyell has found to be the case in the Madeira
Islands. These, however, seem to be exceptional instances. Even in the
Auvergne and Madeiras the great preponderance of occurrences conform to
the observed law of Richthofen, and so far as our knowledge of other
regions extends the departures from this law are not-common. But it may
be asked whether a single unequivocal exception is not sufficient to seri-
ously impair, if not wholly break down, the explanation of the sequence
here given. So far are they from impairing it, that I think a little exam-
ination will show that not only ought we to look for exceptions, but we
may even be surprised that exceptions have not been found more numer-
ous than they appear to be. In the brief explanation given it has been
assumed tacitly, that the rise of temperature has been uniform or followed
some definite law of variation throughout the entire field of subterranean
magmas. In its simplest or typical form the proposition assumes that in
all typical or normal cases the rise of temperature affects all parts of this
field alike. But this we could not expect. Itis not probable that a uniform
rise of temperature would take place in all parts of the field, but may vary

*Tt was when I was contemplating the great distances traversed by slender basalt streams in
Southern Utah that this theory suggested itself to me. I could not doubt that such lavas must have
been ejected at a temperature much more than sufficient to melt them. This seemed to contrast pow-
erfully with the habits of trachytic masses. It occurred to me then that this high temperature might
be absolutely essential to the eruption of so dense a rock as basalt, while a considerably lower one
would suffice for lighter rocks. Immediately the higher melting temperature of the rhyolites and
trachytes suggested itself, and almost as quickly as I write it the theory took form in my mind and the
double function of density and fusibility associated itself with the double sequence.

138 GEOLOGY OF THE HIGH PLATEAUS.

horizontally in the amount of rise as we pass from point to point. It may
also rise more rapidly in the lower part of the field than in the upper; and
as between many fields, local circumstances may accelerate beyond the
mean rate the fusion and expansion of one class of rocks or retard the
same effects in others. Thus, while there is a normal or typical order of
eruptions, it may become liable to not infrequent exceptions arising from
want of exact homogeneity of conditions.

There are several sub-groups of rocks which present difficulties some-
what greater and have the appearance at present of being somewhat anom-
alous. These are principally quartz-propylite and quartz-andesite or
dacite. These rocks are much more siliceous than the other members
of the groups to which they are mineralogically most nearly allied, being
about as siliceous as the more acid trachytes. They have apparently had
their epochs of eruption coevally with the hornblendic members of their
respective major groups, while according to the theory their epochs should
have fallen much later. I am unable to harmonize these apparent anomo-
lies with the main theory upon any considerations which at once carry with
them a conviction of intrinsie probability and an obvious reason for their
exceptional relations. They are comparatively rare rocks, and do not
occur in very extensive masses; their physical constitution and properties
are much less known than their chemical and mineralogical. Their infe-
rior bulk, however, does not break the force of the anomaly if it be real.
Considerations like the following, suggest themselves: The theory assumes
that the physical properties (density and fusibility) have a definite rela-
tion and dependence upon the proportion of silica which a rock contains.
Although this is approximately true, it is in all probability not rigorously
so, and indeed the probabilities, so far as fusibility is concerned, are that
the variations from definiteness in the dependence of fusibility upon the
percentage of silica are in some cases very notable, though these varia-
tions may not impair the general law as an approximate expression of the
truth. In spite of their high percentage of silica, therefore, these rocks
may turn out to be exceptional in having a degree of fusibility correspond-
ing very closely to that of the hormblendic members of the major groups

to which they belong. While, therefore, we cannot claim the dacites and

~a

IMPERFECT CHARACTER OF THE PREMISES. 159

quartz-propylites as contributing their quota of support to the theory, we
may still hold that they are not necessarily in conflict with it.

There is another conceivable mode in which the law here propounded
theoretically may be modified in a manner which would yield results dif-
fering from the standard sequence to which it has been applied and give a
somewhat different but still a definite succession. It might be affected by
the depth at which the seat of volcanic activity is located, and also by the
value of the mean density of the overlying rocks. Assuming our theory
to be correct, let us call the depth at which Richthofen’s succession becomes
the normal one, unity. Suppose the depth to be considerably greater than
unity, the melting temperature of the acid rocks would then be greater on
account of the increased pressure. Recurring to the graphic diagram, the
effect of this modification would be to transfer the intersection of the fusion
and density curves to the left or toward the basic end of the scale, and
rocks more basic than propylite would be first erupted and the succession
would be more or less modified. The nature of the modification will
readily appear by treating the modified diagram in the same manner as has
been employed already. Or suppose the depth of eruptive activity to be
less than the assumed unity: the intersection of the two curves would
be transferred to the right and an inverse series of modifications would
result. On the assumption that the secular cooling of the earth is gradu-
ally sinking the seat of voleanicity to lower horizons, it would follow that
a corresponding modification is secularly proceeding in the normal order
of succession in volcanic eruptions.

This theory has one important element of weakness which it is neces-
sary to point out. The assumption that the proximate cause of volcanic
activity is an increase of temperature is to a great extent an arbitrary one.
Conclusive proof of it does not seem to be obtainable at present. There
are numerous indications of it, many facts which seem to point to it; yet
that strong, convincing evidence which can entitle such a proposition to
absolute confidence is wanting. Hence the theory should be called rather
a trial hypothesis, in which there is an important premise which remains to
be proven. It is a frequent resort, however, in all sciences to adopt such
premises provisionally, and they gain strength or the contrary in proportion

140 GEOLOGY OF THE HIGH PLATEAUS.

as they are useful or otherwise in explaining a wider and wider range of
facts. This was true of the hypothesis of a luminiferous ether and of
gravitation. Neither of these postulates could be proven @ priori, and have
gained acceptance because they explain all facts to which they stand re-
lated. Following these precedents, we may inquire whether a rise of sub-
terranean temperature is consistent with other categories of facts besides a
succession in the order of eruptions and explains other phenomena.

I have endeavored to show that the whole tenor and purport of the phe-
nomena of volcanicity point to the conclusion that lavas are not primordial
liquids but secondary products derived from the liquefaction of solid matter
situated below the surface in layers or macule. Of this statement of the case
in its grosser aspect I believe the circumstantial evidence sufficient to con-
vince a scientific and impartial jury. Taking a generalized view of the sub-
ject, the objections against primordial liquids are insuperable. If the whole
interior of the earth below a crust a few miles in thickness is liquid, the sta-
bility of that crust is intelligible only on the assumption that the crust is less
dense than the liquid, and if the reverse is true it seems inevitable that the
crust would be speedily submerged. The same reasoning would be appli-
cable to residuary vesicles or primordial reservoirs of great extent under-
lying states and empires. If we adopt the conception of a multitude of
small vesicles left by the secular consolidation of the globe gradually
squeezed out one after another, other difficulties equally palpable arise.
These vesicles should, in the process of ages, become fewer and fewer, and
show signs of exhaustion. But observation teaches us that the eruptions of
Tertiary time are apparently as numerous, as varied, and as grand as any
which have occurred in anterior ages. But, above all, the intermittent
pulsating character of the eruptions in any volcanic cycle is at variance
with such an assumption. If this primordial liquid has lain in its receptacle,
possessing, from the beginning of the world, all the essential requisites of
eruptibility except that it is waiting for some accident to open a vent for
it, yet, when the vent is once opened, why does it not pour forth at one
mighty belch all its lavas and then close up forever? Why should it re-
quire some hundreds or even thousands of eructations with intervals of

years to completely exhaust it? Why, in the course of the cycle covering

INTERMITTENT CHARACTER OF ERUPTIONS EXPLAINED. 141

hundreds of thousands and even millions of years, should the same vent or
cluster of vents yield so many different kinds of lava? So completely do
the facts of voleanology antagonize the primordial character of lavas, that
we seem driven to seek an opposite theory of their origin.

These difficulties cease to be such and become normal phenomena
when we take the postulate of local increments of temperature. The re-
fusion of rocks becomes a slow and very gradual process. But when the
melted rock is ready for issue, it does not follow that a steady stream ot
lava would keep flowing as long as the temperature continues to rise. We
must now take into consideration the mechanism by which the expulsion is
effected. This has already been suggested as the weight of overlying rocks
_ crowding in upon the reservoir, and as these rocks are rigid relatively to
small reservoirs, there is a limit to the smallness of the eruption. As the
quantity of melted rock increases, this rigidity relatively diminishes until
rupture takes place and all the lava hitherto accumulated is expelled. The
overlying masses are then soldered up for a time, during which more lava
is melted, and when the quantity is sufficient a second eruption occurs, and
so the intermittent character is established and for a long period maintained.

This assumption also explains the co-existence of vents at different
levels, the presumption being that each vent derives its lavas from inde-
pendent layers or macule, and that several macule or layers can suc-
cessively find issue through the same vent when the magmas which they
contain reach the eruptive condition.

There is, however, one comprehensive or generalized fact connected
with volcanoes which this assumption does not explain by itself, though it
is not in any obvious respect inconsistent with it. This is the geographical
distribution of voleanoes. It is well known that existing and recently extinct
vents stand in the vicinity of the ocean and large bodies of inland water; a
few exceptions, however, being known. But it has been repeatedly re-
marked that the postulated rise of temperature is asserted to be a proximate
cause, itself requiring explanation by the production of some ulterior excit-
ing cause. If we were able to find this ulterior cause, we should then know
why volcanoes have their present distribution. It may be proper to remark
here that this distribution would lead us to look for that cause in occur-

142 GEOLOGY OF THE HIGH PLATEAUS.

rences which take place in waters and in their vicinity. It has long been
held that water plays an essential part in volcanic eruptions, and it is quite
natural that we should infer from the association that the penetration of
water to the internal fires is after all the determinant; but, on the other
hand, we cannot leave out of view the fact that there is water on the land
as well as in the sea, and that every year from 30 to 50 inches of rain are
ordinarily poured over the surface and the underground water-ways and
fissures are kept full. An abundant penetration may, therefore, take place
on land as well as under the sea. It does not seem justifiable, therefore,
to conclude that the mere presence of water is the sole determinant. There
is, however, one class of processes peculiar to bodies of water. It is be-
neath their surfaces that sediments are accumulated, often to the thick-
ness of thousands of feet, until by their gross weight they subside. It may
be that the ultimate cause of volcanism will eventually be traced to the
shifting of vast loads of matter from place to place upon the earth’s sur-
face, but at present this subject has not been investigated from a mechan-
ical standpoint with sufficient method and system to admit of safe generali-
zation or even of legitimate speculation.

The assumption that a rise of temperature is the proximate cause of
volcanic energy, then, is not a wholly arbitrary postulate, but is consistent
with a wide range of facts; brings into order not only the broader but also
the subordinate facts of volcanology, and apparently affords a working
hypothesis.

CHAPTER VI.

STRATIGRAPHY OF THE DISTRICT.

Palxozoic formations.—The Shinérump,—Its strong lithological characters.—Constaney over wide
extent of country.—Coloring.—Architectural forms.—Age of the Shinérump, either Permian or
Lower Triassic.—Continuity with Red-beds of Colorado, New Mexico, and Arizona.—Triassic forma-
tion.—Vermilion Cliffs.—Cliff forms of the Triassic.—The Jurassic series.—Comparison of sec-
tions.—White sandstone.—Remarkable cross-bedding.—White Cliffs.—Architecture.—Jurassic
shales.x—The Cretaceous.—Alternations of sandstone and iron-gray shales.—Dakota Group.—
Laramie Group.—Intervening formations not correlated.—Lignitic character of the Cretaceous.—
Close of the Laramie period.—Unconformities.—Post-Cretaceous disturbances and erosion.—Ter-
tiary formations.—Attenuation southward.—Pink Cliffs.—Tertiary lignites.

The study of the stratigraphy of the District of the High Plateaus and
of the regions adjacent thereto has been chiefly the work of Messrs. Powell,
Howell, and Gilbert. I have had little to do with it, except to take their
results as starting points and add my own testimony in the way of elabora-
tion. Mr. Howell rapidly traversed the district in 1874 and seized the
salient features with remarkable rapidity and acumen. The geological hori-
zons of the larger groups were determined by him, and all that was left to

me was to ascertain their extent and distribution in greater detail.
PALHOZOIC FORMATIONS.

The oldest strata of the district belong to the closing epochs of Palzeo-
zoic time; except, however, that upon the northwestern flank of the Tushar
some crystalline rocks, supposed to be of Archzean age, are revealed in
momentary exposures in the ravines where the overmantling rhyolite has
been deeply scored by the mountain streams. On the northeastern flank
of the Aquarius Plateau the summit of the Carboniferous is laid bare, the
exposed area being about eighteen miles in length by six miles in width at
the widest part. A remarkable dislocation, forming a part of the Hurricane
fault, turns up a brief exposure of the same horizons southwest of the Mar-

kagunt Plateau. The western side and summit of the Privant Range is
143

144 GEOLOGY OF THE HIGH PLATEAUS

composed almost wholly of Carboniferous strata, bent and faulted after the
manner peculiar to the Basin Ranges. Although yielding characteristic
fossils, none of these Carboniferous exposures present sufficient materials for
special study. The great fields of Carboniferous rocks are found in the
Kaibabs to the southward and in the basin to the westward.

THE SHINARUMP.

Resting everywhere upon the Carboniferous of the Plateau Country is
a series of sandy shales, which in some respects are the most extraordinary
eroup of strata in the West, and perhaps the most extraordinary in the
world. To the eye they are a never-failing source of wonder. There are
especially three characteristics, either one of which would render them in the
highest degree conspicuous, curious, and entertaining. First may be men-
tioned the constancy with which the component members of the series pre-
serve their characters throughout the entire province. Wherever their proper
horizon is exposed they are always disclosed, and the same well-known fea-
tures are presented in Southwestern Utah, in Central Utah, around the junc-
tion of the Grand and Green, in the San Rafael Swell, and at the base of the
Uinta Mountains. As we pass from one of these localities to another, not a
line seems to have disappeared nor a color to have deepened or paled. So
strongly emphasized are the superficial aspects of the beds and so persist-
ently are they maintained, that only careful measurement and inspection of
each constituent seam can impair the prima facie conviction that these
widely-separated exposures are absolutely identical. Detailed examination,
however, does show some variation in thickness and slight changes in the
constituent members; but, on the whole, the constancy is, so far as known
to me, without a parallel in any formation in any other region. The sculp-
tured cliffs of the Shindrump reveal the edges of the component layers as
rigorously parallel as if a skillful stonemason had laid them down, and nar-
row bands can be followed for miles without any visible change in their
aspect.

A second striking feature is the powerful coloring of some of the beds.
With the exception of the dark, iron-gray shales of the Cretaceous, the tints
of the other formations are usually bright, lively, and often extremely deli-

THE SHINARUMP. 145

cate. In the Shindrump they are mostly strong, deep, and so rich as to
become cloying. Maroon, slate; chocolate, purple, and especially a dark
brownish-red (nitrous-acid color), are the prevailing hues, while one heavy
sandstone bed is yellowish brown. At the base of the series is a thick
mass of perishable shale not so conspicuous in its colors; it is in the mid-
dle members that they are so resplendent. Alternating horizontal belts of
varying tones and shades, not merging into each other by gradation, but
like ribbons joined at their edges, are seen wherever the formation is ex-
posed in the same general vertical succession, and give the Shinarump Cliffs
an aspect most constant, peculiar, and wholly unlike any others Here
and there a thin line of white trenchantly separates the dark layers, em-
phasizing the distinctions, while the brown sandstone above heightens the
contrasts. The effect upon the mind is impressive and oppressive.

Probably the most striking characteristic of this formation—one which
is destined to make it one of the most notable of the freaks of nature in
the popular estimation—is to be found in the architectural forms which
have been carved out of it by the process of erosion. A common style
of sculpture is represented by heliotype XI, taken from the southeastern
flank of Thousand Lake Mountain. Probably the most striking forms are
the buttes, which are often seen fringing the long lines of cliff bounding
the Shindrump terraces in the San Rafael Swell, and again near the junc-
tion of the Grand and Green. These last have been described in glowing
terms by Dr. J. S. Newberry and by Professor Powell.

The age of the Shindrump is either Permian or Lower Triassic. To
which of the two periods it should be assigned is not yet free from doubt.
Within the limits of the Plateau Country no fossils have yet been discov-
ered which give a satisfactory solution to this question. Mr. E. E. Howell
found in the shales south of Kanab, lying at the base of the formation, a
small number of fossils which were so poorly preserved that only generic
characters could be asserted with confidence. If any conclusion were to
be drawn from them it would be that their general aspect is Jurassic. But
the whole Triassic series, and most of the Shinarump itself, overlie the hori-
zon from which they came, and, moreover, the types are well known to have
a great vertical range.

W580 IP

146 GEOLOGY OF THE HIGH PLATEAUS.

Thoughout the region lying between the Great Plains of Colorado
and Wyoming and the Basin area, wherever the horizons from the summit
of the Carboniferous to the base of the Jurassic are exposed, there are usu-
ally found sandstones and arenaceous shales, distinguished by their rich red
coloring, their tolerably constant texture and appearance, and the absence
of fossils of distinctive character. In many places they may be imperfectly
resolved into two groups, though ordinarily they show no well-marked
plane of division between them; the distinction being somewhat vague and
uncertain. The Triassic age of the upper portion is pretty well ascertained.
Mr. Clarence King has found fossils in the lower portion which he believes
to be sufficient to justify him in calling it Permo-Carboniferous. But the
want of a clear boundary between the two divisions of these ‘‘ Red-beds”
has led many geologists to regard them provisionally as one formation,
under the name of Trias. In the Plateau Country these beds appear to
be conformable with each other, while the contact with the Carboniferous
below is in several places distinctly unconformable. They gradually pass
into the Trias above, and if a divisional plane is to be drawn, it is impossi-
ble to locate it within a belt of 500 feet of monotonous shales, and hence
the tendency has been to regard the whole series as one group, and to use
the names Upper and Lower Trias for the designation of different portions
which, in reality, are not at present distinctly and precisely separable.
Perhaps, also, some hesitation arises from the importance which must ottach
to a full recognition of the Permian age of these lower beds. The identity
of the Shinarump of Utah and Arizona with the lower Red-beds of Colo-
rado and Wyoming is unquestionable, and the formation, therefore, covers
an area probably exceeding 250,000 square miles, with many exposures,
and there is no intrinsic improbability that it is buried beneath a still
greater area. If its age be Permian, then the Permian becomes a forma-
tion, ranking in importance stratigraphically with the Trias and Jura, and
can no longer be considered as a merely local deposit coming in here and
there to round off the majestic proportions of the Carboniferous. While the
Permian age of these beds, therefore, is quite possible, there is good reason
for laying a heavy burden of proof upon the advocates of that view.

The thickness of the Shinarump formation is difficult to determine,

THE SHINARUMP. 147

owing to the gradual transition into the Vermilion Cliff series above. Dis-
regarding the doubtful horizons, the thickness along the Hurricane ledge is
not far from 1,300 feet, and somewhat less at Kanab; and, in general, it
attenuates very slowly and gradually as we recede southeastward, though
it never sinks to small proportions anywhere within the limits of the Pla-
teau Country. Besides the transitional shales above, there are three sub-
divisions. Commencing at the base, they are as follows:

i Silico-accillaceous sShalesie essere eee eee ele eit etree erie 450 to 650 feet.
2. Belted, highly-colored arenaceous and siliceous shales ....-.....-. 400 to 500 feet.
oy BTOWM SANG StONG). 2, sre net Sho ats See a ey ee Oe er GA eeee ey eerste 150 to 250 feet.

The thickness of the transitional shales up to the base of the Vermilion
Cliff sandstone may be reckoned from 550 to 750 feet. With these shales
there often appears a singular conglomerate. It consists of fragments of
silicified wood imbedded in a matrix of sand and gravel. Sometimes
trunks of trees of considerable size, thoroughly silicified, are found, to
which the Piute Indians have given the name ‘“‘Shindrump,” meaning ‘“ the
weapons of Shinav,” the wolf-god. The conglomerate is found in many
widely-separated localities, with a thickness rarely exceeding 50 feet. It
occasionally thins out and disappears, but usually recurs if the outcrop be
traced onwards, resembling the mode of occurrence common to the coal-
seams of the Carboniferous coal measures. It is the most variable member
of the Shindrump thus far observed. It is found on the west flank of the
Markdgunt and throughout the great circuit of cliffs south of the High Pla-
teaus; it is seen at Paria, and again at the Red Gate between the Aqua-
rius and Thousand Lake Mountain, the characters of the formation being
quite the same in all these localities. The conditions under which it was
accumulated would seem to have been remarkably uniform, and may have
been similar in some respects to those attending the formation of coal. The
subsequent silicification of the wood upon a scale so extensive and even
universal is certainly a very striking phenomenon, and one for which no
explanation suggests itself. It may be of interest to mention that at Leeds,
in Southwestern Utah, the fragments of silicified wood were found to be
strongly impregnated with horn-silver. Subsequent prospecting, which had
been stimulated by this curious discovery, led to the finding of horn-silver

148 GEOLOGY OF THE HIGH PLATEAUS.

impregnating the sandstones and shales in sufficient quantity to attract both
miners and capital to the locality.

The Shindrump has but a few exposures within the District of the
High Plateaus. The best example is seen at the Red Gate, at the foot of
Rabbit Valley, where the Fremont River passes out into the desert waste in
the heart of the Plateau Province. <A belt of this formation is seen near
the summit of the Water-Pocket flexure, flanking the northeastern part of
the Aquarius a few miles from its base. Itis brought up to daylight south-
west of the Markagunt by the Hurricane fault, and the beds are there
sharply flexed in the vicinity of the fault-plane, but quickly smooth out to
the eastward and southward. The principal area of the Shindrump is south
of the Vermilion Cliffs, in the northern part of the Kaibab District, around
the junction of the Grand and Green and in the San Rafael Swell. Gen-
erally speakin

2, it is usually found as the first terrace above the Carbonif-

erous in the areas of maximum erosion.

THE TRIAS.

Next above the Shindrump shales is found an extensive series of sand-
stones constituting the Trias. Probably no formation in Southern Utah is
better exposed, but notwithstanding this, it has not in this part of the Pla-
teau Province hitherto yielded a solitary fossil of any kind. Still we are
not in doubt about the correlative age of the group for its continuity with
beds found by Newberry in New Mexico, and yielding a distinctly Triassic
flora; its further continuity and identity with Red-beds in the Uintas having
a Jurassic fauna above and the unmistakable Shindrump shales below; and,
lastly, its identity with the beds of Idaho, which furnished Dr. Peale a well-
marked Triassic fauna, are sufficiently certain.

The contact with the shales below is usually conformable, ‘but in the
vicinity of the Hurricane fault, where the whole Triassic series is. displayed,
the junction is often unconformable. The separation, however, of the
Trias into an upper and lower series, so far as Southern Utah is concerned,
is based upon lithological grounds chiefly. It is also a matter of great
convenience to effect this separation, since each division has its own topog-
raphy, and their distributions differ notably. There is, also, a decided con-

THE TRIAS—VERMILION CLIFFS. 149

trast in their respective aspects, and the geologist who studies them in the
field is constantly reminded of the distinctions. The Upper Trias consists
of many beds of sandstone with shaly partings. Usually the component
members do not attain great thickness, but a few of them occasionally have
a thickness exceeding 200 feet. Very many of them are cross-bedded in
a beautiful manner, and although this feature is not so strongly marked
as in the Jurassic sandstone, it is almost always conspicuous enough to
attract attention. The whole formation is brilliantly colored, the predomi-
nant hue being a bright lively red, often inclining to orange. Occasionally,
however, this color gives place to a strong yellow or bright brown. ‘These
are very distinct from the deep crimson, chocolate and purple of the Shina-
rump, and, furthermore, change from red to brown along the course of a
single layer or bed, while in the Shindrump every layer preserves its color
without a trace of change through many miles of exposure. ‘The predomi-
nant red, approximating to vermilion, induced Professor Powell to give the
local name of Verminion Cuirrs to their grandest and most typical
exposure.

The Upper Trias is in truth the great cliff-forming series of the Plateau
Country. No other formation equals it in the extent and variety of cliff
exposures. The Vermilion Cliffs extend from the Hurricane fault to Paria,
more than a hundred miles in a straight line, and more than twice that dis-
tance if we follow the sinuosities of their escarpment. Throughout this
distance they front the south with a succession of superposed ledges, rarely
less than 1,000 feet in height and often exceeding 1,500 feet; throwing out
great promontories, and deeply notched by estuaries and bays. Wherever
exposed in more easterly regions the same tendency to form cliffs may be
observed. These escarpments have their distinctive architecture and a
structure quite as peculiar to the formation as those of the Shinarump
below and the Jurassic above. Let us recall here that the series is com-
posed of manifold layers of sandstone, with many shaly layers intervening.
Usually three or four members are massive beds of very homogeneous
sand rock, with a thickness of 100 to 250 feet. Recall, also, that the most
effective attack of erosion is made primarily against these yielding shales,
while the overlying and more obdurate sand rock is thereby undermined

150 GEOLOGY OF THE HIGH PLATEAUS.

and cleaves off by its vertical joints. Take now a series of these alternat-
ing massive layers and softer shales, the long process of erosion gives a
series of perpendicular walls, alternating with sloping taluses. This com-
posite architecture is one of the- most persistent features of the formation.
Something like it is seen in the Carboniferous strata forming the walls of
the Marble Canon of the Colorado, but there are also many wide differences
both of detail and ensemble.

Taina ean es 2 ST Ge eel ae eae Ie

ene ee lease ala

o 1 D je Jopes fon iieces [mer]

rap ey
forces 5 Sears, aE Awan. leg sata el eee

Fic. 3.—Generalized profile of Vermilion Cliff.

The thickness of the Upper Trias is from 1,100 to 1,800 feet, being
greatest in the vicinity of the old shore line, and very slowly attenuating
to the eastward.

THE JURASSIC.

The Jurassic series consists of two members, the lower being a massive
sandstone of great thickness, the upper a series of calcareous and gypsifer-
ous shales from 200 to 400 feet thick. Underneath the sandstone is a small
group of shaly beds, which are presumed to be of Jurassic age, but no deter-
minable fossils have been taken from them. It has been a long-standing
and difficult question whether the Jurassic sandstone is not, after all, a
mere upward continuation of the Vermilion Cliff beneath. Much color
was given to this supposition by the fact that no unconformity between
them has been detected in this vicinity, and still more by the fact that as
we travel eastward and southeastward from the High Plateaus the distinc-

THE JURASSIC SERIES. TS

tion between them gradually fades, and the two seem to merge into one.
Neither of them have yielded any determinable fossils. | Nevertheless,
Tam convinced that the probable plane between the Jura and the Trias
lies between these two sandstones. In the Uinta Mountains the Triassic
sandstones have the same general features as they exhibit upon the south-
ern flanks of the High Plateaus. Comparing the Jura-Tyias section of the
Uintas with that of the High Plateaus and Kaibabs, we find a concordance
in the several members.

Urnta SECTION. | IKANAB SECTION.
Feet. | Feet.

Caleareous, shales, limestone and gypsifer- Calcareous shales, limestone and gypsifer- |

Ousishalestensecoerccc acc ne toes sees el O00, OwsyShales asec necimctessisesteeeeerseeeeels 500
|

Massive, cross-bedded white sandstone... .| 1,100 | Massive, cross-bedded white sandstone----| 1, 400
Mhinkcalcareousishalowseees eee e oreo LOON|ELbingcalcareousishaleieese =o rea | 50

| i
Vermilion Cliff series.-....-.-..--. Sees ae 1,100 | Vermilion Cliff series................-----|] 1,500
Upper Shindrump shales and conglomerate.) 1,000 | Upper Shindrump shales and conglomerate -| 750
Beltedyshalestee= oases seen sesaecen Ce Sees A00u | eBeltedishalesiy ees mass cc se estes 400
Lower Shingérump shales. ...........:.....| 300 | Lower Shinérump shales....-....-...-.-- 500
A comparison of these two sections will lead to the conviction that the
white sandstone of the Kanab region is identical with that of the Uintas.

But the latter has Jurassic fossils above and below it, and hence we may
conclude that the former is also Jurassic, although fossils of that age are
found only above it, and none of any kind either in the sandstone itself or
in the thin shales below.

Starting from the village of Cedar, west of the Markdégunt, we find the
sandstone in great force, and may trace it southward around the flank of
that plateau, and thence eastward around the Paunsdgunt, and beyond the
Paria River. In the Kaiparowits it is wholly lost beneath the Cretaceous,
but east of the Kaiparowits it reappears. It skirts the southern and east-
ern slopes of the Aquarius, and is grandly displayed in the Water-Pocket

152 GEOLOGY OF THE HIGH PLATEAUS.

flexure. It forms one of the terraces which lie west and north of the San
Rafael Swell, but north of that area it dips beneath later formations, and is
buried thousands of feet beneath the Cretaceous-Hocene deposits. A hun-
dred miles north of the San Rafael it is turned up again upon the southern
slopes of the Uintas with the same characteristics which it shows elsewhere.
The line of outcrop with the intervals of concealment thus traced is nearly
500 miles. Wherever exposed along this belt the lithological characters
are preserved without material change. But, on the other hand, if we trace
the sandstone across this general line of strike and follow it southeastward
into northeastern Arizona and New Mexico, its thickness slowly diminishes,
its features lose force and individuality, and it seems to blend gradually
with the Vermilion Cliff sandstones below. It is not certainly known at
present whether the whole formation thins out in this direction or whether it
forms a part of the beds which have been assigned by Newberry to the
Upper Trias. Most probably it thins out altogether. A little way beyond
the Glen Cation in New Mexico the fossiliferous Upper Jurassic shales are
seen to rest directly upon sandstones which are believed to be Triassic, and
the Jurassic white sandstone of the High Plateaus is nowhere seen. A
little farther on the Jurassic shales also disappear, and the Cretaceous
touches the Trias. Thus the Jurassic sandstone appears to have been a
littoral or off-shore ‘formation thrown down along the coast of the Mesozoic
mainland, which occupied the region now forming the Great Basin. Some
doubt still attaches to the origin of those portions which flank the Uintas,
but our ideas of a geography so ancient are very vague and our knowledge
very fragmentary.

The lithological characters of the Jurassic white sandstone render it a
very conspicuous formation. Through a thickness of more than a thousand
feet, sometimes of nearly two thousand feet, it is one solid stratum, with-
out a single heterogeneous layer or shaly parting. A few horizontal cracks
are seen here and there, but inspection shows that they are merely the
seams where two systems of cross-bedding are cemented together. In gen-
eral, it is one indivisible stratum. This massive character has had its effect
upon the clifi-forms that have been sculptured out of it. These forms are

bold headlands and gigantic domes, usually without any minor details, but

HeLioryere VILL,

Heliotype Printing Co., 220 Devonshire St., Boston.

Cross BEDDED JURASSIC SANDSTONE.

JURASSIC WHITE SANDSTONE—CROSS-BEDDING. 153

simple in the extreme, and majestic by reason of their simplicity. The
color of the rock is almost always gray, verging towards white. Occasion-
ally it is a very pale cream color, and again pale red. The red becomes
more common as we recede from the old shore line towards the east.
But of all the features of this rock the most striking is the cross-bedding.
It is hard to find a single rock-face which is not lined off with rich tracery
produced by the action of weathering upon the cross-lamination. The
massive cliff-fronts are etched from summit to base with a filagree as intri-
cate and delicate as frost-work. The same phenomenon is seen in the Ver-
milion Cliff sandstones below, often so rich and complex that it excites
constant admiration. Dr. Newberry speaks of it with enthusiasm as pre-
sented in the Triassic sandstones of New Mexico. But it is far less won-
derful than the cross-bedding which the Jurassic presents at every exposure.
In the Colob Terrace, south of the Markagunt, the rock weathers into many
cones and pyramids, and the details produced by the action of the weather
upon the cross-bedding are grotesque and often ludicrous. A journey down
the Upper Kanab Canon is enlivened by ever-recurring displays of this
phenomenon, presented with a profuseness and variety which extort excla-
mations of delight from the beholder. The Jurassic sandstone was de-
posited over an area which cannot fall much short of 35,000 square miles,
and the average thickness exceeds 1,000 feet. The imagination is utterly
baffled in the endeavor to conceive how a mass so vast and at the same time
so homogeneous and intricately cross-bedded throughout its entire extent
could have been accumulated.

Overlying the white sandstone is aseries of beds which may be called
shales with some reservation, and here we find for the first time an abun-
dance of distinctive fossils. They are clearly of Jurassic genera and species,
and enable us to correlate the horizon with confidence. They belong to a
well-marked formation, which is represented not only throughout the greater
part of the Plateau Province, but also in Colorado, Wyoming, and Northern
New Mexico. From many large areas, indeed, it has been denuded, but
throughout Utah it is never wanting from those exposures where its pres-
ence could be looked for.

That constancy of lithological character which is so conspicuous in

154 GEOLOGY OF THE HIGH PLATEAUS.

older Mesozoic members does not prevail in this one, for it is highly varia-
ble not only in the mass, but also in the constitution of the beds. In some
exposures it is more than a thousand feet thick; in others, it is less than two
hundred. Where its volume is greatest it is more arenaceous, and where
the volume is less the beds are shaly, marly, and calcareous. Usually sey-
eral seams of limestone occur, and in these the fossils are found often
abundantly. One notable feature is the small amount of cement in the
arenaceous layers, which are, therefore, very poorly consolidated, and the
rock weathers and wastes away with extreme facility. Gypsum and sele-
nite occur abundantly in these beds, and especially noticeable is the latter
mineral, which is seen sparkling and glittering in the sunlight in the bad-
lands to which the decay of the strata gives rise,

THE CRETACEOUS.

Throughout the District of the High Plateaus and the broad terraces
which flank it upon the south and east the Cretaceous system has the same
relative magnitude and importance which distinguish it in other portions
of the West. In absolute mass it is inferior only to the Carboniferous ;
but as the latter formation is usually covered by later ones over the greater
part of the West, and especially of the Plateau Country, the Cretaceous
exposures are everywhere the dominant ones and most conspicuous. The
series consists of many beds of sandstone and argillaceous shale, the latter
decidedly predominating. The number of beds is very great, but they
show a tendency to form groups, here a series of sandstones with a few
shales, there a series of shales with a few thin seams of sandstone. Two
conditions, however, have combined to render the group a difficult one to
study and to correlate with coeval groups in other regions. The first is the
want of sharp and persistent divisional horizons; the second is the great
variation of the lithological characters along the outcrops, and the changes
which almost all the strata undergo as we trace them from place to place.
No two sections show any close agreement in the bedding. Since the fos-
sils are generally confined to a few of the many layers, it is frequently dif-
ficult to find a valid separation, and even when we discover one we cannot

apply it to every locality. But while we are often at a loss to decide to

CRETACEOUS SYSTEM. 155

what part of the Cretaceous system a particular exposure should be assigned,
we are rarely in doubt about its Cretaceous age, for each member of the
system possesses lithological characteristics only a little less emphatic and
distinctive than those of the Trias and Jura. They consist of very heavy
alternating masses of iron-gray argillaceous shales and bright yellowish-
brown sandstones, which the observer will seldom be in danger of con-
founding with the members of any other group. ‘The iron-gray shale some-
times gradually passes into a bluish-gray or light dove-color, especially to the
eastward of the High Plateaus. At the base, or near the base of the Cre-
taceous system, is a conglomerate, the age of which is doubtful, since the
horizon separating the Upper Jurassic has not yet been accurately deter-
mined, and the conglomerate may ultimately prove to be a part of the latter
group.

The upper and lower divisions of the Cretaceous can be correlated
with a very high degree of probability with the Laramie and Dakota
groups of Colorado, respectively. Our inability litherto to subdivide the
intervening members prevents us for the present from asserting any exact
correlations with the middle Cretaceous divisions of that State. The sand-
stone near the base of the system, with a few underlying shales, is without
much doubt the extension of similar strata found in Southwestern Colorado
and Northwestern New Mexico by Messrs. Holmes and Peale, and referred
by them to the Dakota Group. ‘The fossils found in this group are Ostrea
prudentia (White), Gryphea Pitcheri, Exogyra laeviuscula, I. ponderosa, Pli-
catula hydrotheca (White), Avicula linguiformis (Shumard), Camptonectes pla-
tessa (White), Callista Deweyi (Meek and Hayden). In these lower Creta-
ceous beds are also found a good workable seam of coal and numerous
Carbonaceous shales. The coal outcrops near Upper Kanab, south of the
Paunsagunt Plateau, and also in Potato Valley, south of the Aquarius.*

The equivalence of the Upper Cretaceous shales with the Laramie
beds is founded upon their known continuity with strata of that age in
Western Colorado and along the course of the Green River south of the

Uintas. This continuity can be traced very clearly in the great cliffs west

*A good workable coal is found at several places on the southwest flank of the Markdégunt,
but I am not quite sure that it belongs to this horizon.

156 GEOLOGY OF THE HIGH PLATEAUS.

of Castle Valley, which swing around the north end of the San Rafael
Swell and merge into the broad Upper Cretaceous mesas east of it. The
fossils which are found in these shales are of brackish-water habits, and
although the species are in many cases new or peculiar to the locality, yet
their general facies and generic forms are clearly such as harmonize with
this correlation. The mass of the Laramie beds is here very considerable,
averaging about 1,800 feet. They contain many Carbonaceous shales, and
workable seams of coal have also been observed which apparently lie near
the base of the group.

Between the summit of the Dakota and the base of the Laramie beds
lie from 2,000 to 8,000 feet of sandstones and shales which must represent
the middle Cretaceous divisions. These are as yet not subdivided nor cor-
related with the divisions of Colorado and Wyoming.

The whole Cretaceous system of the High Plateaus and their encir-
cling terraces is lignitic, and coal is found at many horizons. Nor does
one portion of the series seem to abound in coal more than another. Car-
bonaceous shales are found along the great escarpments in many localities,
and a considerable number of workable beds of coal are also known.

At the close of the Laramie period we come to a physical break in
the course of the deposition. Prior to that epoch the disturbances and
resulting unconformities appear to have been few and inconsiderable. The
continuity of deposition from the base of the Trias to the summit of the
Cretaceous appears to have been unbroken, and the only unconformities
seen are local and usually slight. But at the close of the Laramie period
extensive disturbances took place along the old Mesozoic shore line which
now marks the boundary of the Great Basin. Considerable areas have
been found from which the Cretaceous strata were extensively denuded
before the deposition of the earliest Tertiary beds began, and where the
lower Eocene is seen to lie across the upturned and beveled edges of the
Cretaceous. - In the locality now occupied by the Aquarius Plateau and
Thousand Lake Mountain the lower Eocene rests directly upon the Juras-
sic, and the Cretaceous series is wholly wanting over a large part of the
area. A great monoclinal flexure runs under the Aquarius from the south,

and where it disappears beneath the great lava cap of that plateau the his-

UNCONFORMITY OF TERTIARY AND CRETACEOUS. 15

“|

tory of the unconformity is clearly revealed. The monoclinal involves the
whole Cretaceous system, but not the overlying Tertiary, and fixes the age
of the disturbance between the close of the Laramie and the beginning of
the Tertiary. The northern extension of the Water-Pocket flexure indi-
cates a precisely similar movement coeval with the one already recited.
This flexure disappears beneath volcanic accumulations at Thousand Lake
Mountain. The summit of that mass consists of lava-capped Tertiary
strata resting upon the Jurassic, while to the northeast of the mountain the
Cretaceous beds are rolled up towards it monoclinally, with patches of
level Eocene beds lying unconformably across their edges. An uncon-
formity of Tertiary and Cretaceous is also laid open to view in Salina
Canon. Around the flanks of the Markégunt Plateau many exposures of
this unconformity are also seen. In truth, there appears to have been at
this epoch a series of displacements having a north and south trend, break-
ing up the Mesozoic system into long blocks by well-defined monoclinal
flexures, and the uplifted portions everywhere suffered denudation prior to
the deposition of the Tertiary beds. On the other hand, very many of the
contacts of the Eocene and Laramie beds are apparently conformable.
This occurs wherever the older series escaped distortion, and throughout
the central parts of the Plateau Province they usually did escape it. The
great disturbances were for the most part localized in the vicinity of the old
shore line, and only now and then extended far away from it. The dis-
turbances, being also chiefly monoclinal flexures and faults, did not disturb
very noticeably the horizontality of the strata except along the very narrow
locus of the flexure itself.

The existence of these unconformities indicates a lapse of time between
the close of the period of deposition of the Laramie beds and the begin-
ning of the local Eocene. Nor could this period have been of very trifling
duration, for there are instances of extensive erosion of the Upper Cre-
taceous prior to the deposition of the earliest Tertiary. In the Aquarius
Plateau and in Thousand Lake Mountain the Lower Eocene rests upon the
Jurassic, and in the southern amphitheaters of the Aquarius the Tertiary
lies across the beveled edges of the whole Cretaceous system. Whether
such an occurrence may be construed as meaning a temporary emergence

158 GEOLOGY OF THE HIGH PLATEAUS.

of land from the water, or whether it merely indicates a local exposure to
denudation, it is not possible at present to say.

TERTIARY LACUSTRINE FORMATIONS

The history of the Plateau Country which is at present best known is
the history of its Tertiary formations. This remains to be written; but
materials for it have been widely collated, and are in the possession of Pro-
fessor Powell, who will, it is believed, discuss the subject at an early day.
A more promising and instructive one probably is not to be found in the
entire range of North American geology. Nothing more is needed here
than a mere summary, which may serve as a guide and index to the mean-
ing of the terms employed in this monograph.

The Tertiary system of the Plateau Country is lacustrine throughout,
with the exception of a few layers near the base of the series, which have
yielded estuarine fossils. The widely varying strata were accumulated
upon the bottom of a lake of vast dimensions, and were derived from the
waste of mainlands and mountain platforms, some of which are still dis-
cernible. The region of maximum deposit was in the vicinity of the
Wasatch and Uintas, where in the course of Eocene time more than 8,000
feet of beds were laid down. As we proceed southward, these heavy de-
posits attenuate, partly by a diminution in the thickness of the individual
members and partly because the period of deposition ceased earlier the
farther southward we go, until in the southern part of the province only
the lower Eocene is found, or, indeed, was ever deposited. The High
Plateaus occupy the belt through which this diminishing bulk and successive
elimination of upper members is well seen. In the Wasatch Plateau, at the
extreme northern part of the district, we find the two lower divisions of the
Eocene present in great volume; and in the valley of the Sevier and San
Pete we find what is undoubtedly a still higher division. At the southern
portion of the district only the lower division can be clearly made out,
though some of the upper beds may prove to belong to a later period. The
present weight of evidence, however, seems to me to place them in one divis-
ion, the “ Bitter Creek” of Powell.

In the southern plateaus, the Markagunt and Paunsdgunt, we find

TERTIARY LACUSTRINE SERIES. 159

the following members of the Bitter Creek, which are much the same in all

exposures :
SOUTHERN BITLER CREEK. Heat:
1. Upper white limestone and calcareous marl (summit of series) .---.--------- 300
2p ins cal CALCOUSUSADUS LONG br seyret a eee ede, ee fea ee vereie er etter ete ee 800
om einiaconelomeracel (baseOlatheyseries) meee eeeiaeee aaa eeeeenee 500
1, 650

In the northern part of the district we have a larger development of
the Bitter Creek series, and resting upon it some heavy masses of the Lower
Green River shales, and probably a considerable portion of the Upper Green
River Group is also represented. There is, however, no exact correspond-
ence in the lithological or stratigraphical succession of the component mem-
bers of the Bitter Creek when the northern and southern portions of the
district are compared. A series of sections from the northern part is given
in the following chapter.

The Pink Cliffs, which form such a striking feature in the scenery of
the southern terraces, are exposures of the fine-grained calcareous sand-
stone forming the middle member of the Bitter Creek. The same expos-
ures are exhibited in the southern and southwestern flanks of the Marka-
gunt around the entire promontory of the Paunsigunt and in the circuit
of the Table Cliff. In the Aquarius Plateau the Lower Eocene is found,
but in smaller volume than elsewhere, and it is decidedly diminished in
mass upon the summit of Thousand Lake Mountain. But it resumes
its normal thickness farther north, and then grows more and more massive
throughout the extent of the Wasatch Plateau.

In their general characteristics these Tertiary strata are similar to the
Laramie beds upon which they generally rest, being shaly and marly and
sometimes lignitic. It * noteworthy, however, that in the southern part of
the district of the High Plateaus no lignite or carbonaceous material has
yet been discovered in the Tertiary beds. But in the northern part of the
district the lignites are found abundantly not only in the Lower Eocene
(Bitter Creek), but even in the Lower and Upper Green River (?) beds.
In the San Pete Valley coal has been mined for local use for several years,
and taken from what appear to be seams of Green River age. A more
detailed description of the Northern Tertiaries will be given in the next
chapter.

CHASE dave Vell

THE WASATCH PLATEAU.

Situation and structure of the Wasatch Plateau.—Of what strata composed.—The great monoclinal.—
The Cretaceous platform south of it.—Salina Cafion.—The Jurassic Wedge.—East and West Gun-
nison faults.—San Pete Plateau.—Sedimentary beds composing the Wasateh Plateau; Bitter Creek,
Lower Green River, and Upper Green River beds.

The name of Wasatch Plateau has been given to the northernmost of
those highlands of tabular form which are the subject of the present mono-
graph. It is in some sense an outlier of the group, and presents features
peculiarly its own, though sharing with them a common history and many
similar features. It slightly overlaps at its northern end the main range
of the Wasatch Mountains, and stands en échelon to the southeast of Mount
Nebo, the last great mountain of that beautiful chain. The interval between
Nebo and the plateau is about 15 miles, and is filled partly by a medley of
low hills and partly by a depression called San Pete Valley, which lies
along the base of the table. The western flank of the uplift is a mono-
clinal flexure of the grandest proportions. Along a base line nearly 50
miles in length the Tertiary strata bend upward to the summit in a single
sweep, diversified by minor inequalities arising partly from minor fractures,
partly from erosion, but never of such magnitude as to mask the general
plan of the uplift, nor even to greatly disfigure its symmetry. The minor
features, though elsewhere they might seem of considerable moment, are
mere ripples upon the great wave. At the summit the strata suddenly flex
back to horizontality, and when we reach it we find ourselves upon a long
narrow platform, nowhere more than 6 miles in width, usually much nar-
rower, and here and there reduced to a knife-edge or even eaten through
by erosion. To the eastward the profile at once drops down, often by a
great cliff, always abruptly, by a succession of leaps across the edges of
the sensibly horizontal strata, to lower terraces, succeeding each other at

intervals of 3 to 6 miles, and consisting of older and older formations.
160

EAI:

Rarteel Srre@l

igs aay.
7

ee ee te eae er <e Nee =: > ates ae oe Bete 4 Sina ss : = = oe Bien
tl NEE \ OPS CSN EO
1 ; me A ZA
| rest > SS Se Se

an Rateel Srrell

A ae

Legend.
Tertlory
Laramie Opper Trias
Lover Trias,§ =

Jurassic Wii™ Carbonife Err]

Cretaceous |i

the San Rafael Swell.

» PLATE I.

San Ratcel Srretl

Ae HAT MT UAT
Fan ue i ae AT 1
ui \ i it

ni t maar) a
413 LA A

; Musinia Nesotch Plateau

San Rafael SrrelZ

0

Tertlary es

Laramie OpperTrias SEF
Seve Geel k tre
Gee Cretaceous EA Lower Trias.

Sdlina Caviorn

Turassic WA Carhoni#e Ty

Sections from San Pete and Sevier Valleys across the Wasatch Monoclinal to the San Rafael Swell.

Scale 4 Miles : Linch

——+ > —_

GENERAL STRUCTURE OF THE WASATCH PLATEAU. 161

The eastern front of the plateau is simply a wall left standing by the erosion
of the region which it faces. The Tertiary beds upon the summit, as well
as the Cretaceous beneath, once spread, unbroken and undisturbed, as far
to the eastward as the eye can reach, and thence far beyond the limits of
vision. From the strange land which that summit now overlooks at an
altitude of 11,500 feet, more than 8,000 feet of Tertiary and Mesozoic
strata have been swept away, and the region which has been thus devas-
tated is large enough for a great kingdom. The Wasatch Plateau is a
mere remnant of that protracted process, and, so far as it extends, is a
mere rim standing along a portion of the western boundary of the Plateau
Province. .

The western front of the plateau, then, is a great monoclinal flexure,
and its eastern front is a wall of erosion. To the northward the beds which
compose it stretch far up toward the Uinta Mountains, still ending in lines
of great cliffs or bold slopes gradually swinging to the eastward until, after
a course of nearly a hundred miles, they cross the Green River, where
Powell named the Tertiaries the Roan Cliffs, and the Upper Cretaceous
the Book Cliffs. Southward the Tertiaries forming the summit of the
plateau end abruptly in a precipice extending east and west, while the
underlying Cretaceous beds continue, forming a lower terrace overlooking
the still lower level of Castle Valley. The average altitude of the table is
about 11,000 feet, and it stands from 5,500 to 6,000 feet above San Pete
Valley on the west and about the same height above Castle Valley on the
east. To gain an adequate conception of the great monoclinal, which forms
the western flank, we must recur to the consideration that the upward
curvature and reflection to horizontality leaves the Lower Tertiary beds full
5,500 feet above still more recent ones in the valley below. If the latter
were now continuous across the summit, as they once probably were, the
altitude would be from 1,500 to 2,000 feet greater than at present. Thus
the total rise of the monoclinal appears to have been more than 7,000 feet,
and the uplift has occurred with a near approach to equality along a line
of strike of 50 miles. The transverse structure will be seen by referring
to Plate 3, sections 6 and 7.

The platform of the summit is rugged, the irregularities being due

11 HP

162 GEOLOGY OF THE HIGH PLATEAUS.

mainly to erosion, the degradation of 1,500 to 2,000 feet of beds having
proceeded unequally, although the stratification still retains its sensible
horizontality. Upon the southwestern shoulder there is considerable com-
plication of the displacement. ‘Two or three sharp faults, running north
and south, include between them a long block from 2 to 3 miles in width,
which has dropped, the amount of the fall varying from 600 to 1,700
feet. The length of this block is at least 27 miles and may be greater.
It is much complicated by minor fractures, and a portion of its southern
extension into the Cretaceous terrace south of the Wasatch Plateau has
been described and illustrated by Mr. G. K. Gilbert* as an instance of a
“zone of diverse displacement.” The general appearance and relations of
this complicated downthrow suggest that the upper recurving branch of the
great monoclinal was subject to tension during the uplift, and the beds,
being unable to stretch, were rent apart, allowing the block to sink.

The Cretaceous terrace, upon which we may look down while standing
upon the southern terminus of the Wasatch Plateau, is no doubt, from a
structural point of view, a part of that plateau; but the loss of its Tertiary
beds by erosion has reduced its altitude to a level 1,500 to 2,000 feet lower.
It continues the structural features southward to plateaus next in order,
forming a kind of connecting-link between the northern and southern
uplifts. Its chief deformation is due to the sunken block already described.
The two faults between which it has fallen increase for a time their throw
as they continue southward, reaching a maximum of nearly 3,000 feet, and
then decreasing to zero at points about 18 and 20 miles, respectively, south
of the Wasatch Plateau. The structural depression thus produced has been
called Gunnison Valley, but, this name being preoccupied, it should be used
provisionally. It contains abundant evidence of its origin, for the Tertiary
beds are seen to abut against the Cretaceous along the lines of faulting,
and the latter beds tower far above them. The drainage of this valley is
to the westward, through a deep canon called Salina Canon, which is a
clearly defined, but by no means uncommon example of a general fact,

which is repeated so frequently throughout the entire Plateau Country that

*Amer. Jour. Science; also, Geol. Uinta Mountains, J. W. Powell. The minor fractures are too
small to appear effectively upon the stereogram, and have been omitted, but the main faults are intro-
duced.

SALINA CANON—THE JURASSIC WEDGE. 163

it has now become a generalization of great importance. Its formula is
exceedingly brief. The principal drainage channels are older than the dis-
placements.

Salina Canon cuts through the southern continuation of the great
monoclinal at a point where its rise is a minimum, and nearly midway
between the Wasatch Plateau on the north and the Sevier and Fish Lake
Plateaus on the south. Even here it plunges into a wall forming the
uplifted side of a great fault of which the shear could not have been much
less than 3,000 feet, though fully 2,000 feet of upper beds have been re-
moved from the uplift by erosion. After a course of about 25 miles the
canon opens into the Sevier Valley. It carries a fine stream, whose waters
join the Sevier at the town of Salina. Along the descent of this stream
the beds dip more rapidly than the stream descends. This relation between
the course of a drainage channel and the inclination of the strata is not the
usual one in the Plateau Country; on the contrary, the strata much more
frequently dip upstream, and rivers usually emerge from cliffs instead of
entering them. In this respect Salina Canon is an exception, though not
an isolated one. :

A remarkable displacement is found along the eastern side of the Sevier
Valley, between Gunnison and Salina. A narrow belt of rocks of Jurassic
age is thrust up, forming a chain of foot-hills and bad lands, and the later
Tertiaries are seen to flex upward against their western sides and terminate
in a “hog-back,” while they abut almost horizontally against their eastern
sides. A small remnant of Tertiary beds is here and there found as a thin
capping lying upon the Jurassic beds unconformably, and patches of vol-
canic rock farther southward are also seen to cover them. The belt of
Jurassic rocks nowhere exceeds two miles and a half in width, but its
length is nearly 40 miles, extending from a point about 7 miles south
of Manti along the base of the great monoclinal and the throw of the Sevier
fault as far as Monroe, where it ends, to all appearances, somewhat ab-
ruptly, or perhaps disappears under the great mass of volcanic rocks which
form the loftiest part of the Sevier Plateau. These older beds dip east-
ward, always at a high angle, which sometimes passes the vertical. This
inclination was attained, without doubt, in part before the commencement

164 GEOLOGY OF THE HIGH PLATEAUS.

of Tertiary time, and probably during the Cretaceous epoch. It may
belong to a class of flexures produced near the close of the Cretaceous, of
which several instances are found in the district, chiefly in its southeastern
portions. They all involve the Cretaceous beds in the displacements when-
ever they are present, but not the Tertiaries, which, when found in contact,
overlie them unconformably. After the upturning of this flexure it may
have stood as a long narrow ridge near the western shore line of the great
Cretaceous-Eocene lake and been subject to a considerable amount of
degradation, which removed the Cretaceous beds and finally planed down
the whole mass until it stood but little above the common level. In the
oscillations of the shore line during the Green River epoch it would seem
to have been overflowed by the waters of the lake during the last stages
of its existence, receiving a thin deposit of the beds of that period, which
have since been nearly all removed, though just enough traces of them are
left to render it certain that they once extended over it in a sheet which is
locally very thin. At some epoch subsequent to that of the latest deposi-
tion a fault occurred, cutting along these Jurassic beds, throwing up the
western side into a great ‘‘hog-back.” By the subsequent denudation of
the overlying Tertiaries the highly-inclined Jurassic beds are left project-
ing above them and also above the continuation of these Tertiaries on the
eastern or thrown side of the fault. Thus they form a narrow belt between
the interrupted Tertiary formations. The fault is directly in the prolonga-
tion of the Sevier fault, but the throw is reversed relatively to it. It is
designated on the stereogram as the East Gunnison fault, and its northern
continuation is found on the west side of San Pete Valley, extending nearly
and perhaps quite to the base of Mount Nebo, though its details have not
been examined in that vicinity. ‘The sections across this Jurassic Wedge,
as I have termed it, will be found in Mr. Howell’s delineations (Plate 3),
sections 1 to 13.

On the west side of the Sevier Valley runs another fault parallel to
the foregoing and presenting similar and even homologous features, but
with the throw on the opposite side. Both in linear and vertical extent the
dimensions of this displacement (termed the West Gunnison fault) are less

te Parti

JLOGICAL SECTIONS

FUE ViltCiiNitiye@ lr

AND SALINA, UTAH

IN E. HOWELL.
Ce

mbhered in order from north to south

a that points in the same longitude
rézcal line. Their geographic

onamap included in part 2 of this plate.
section ts the level of the sea.

rtical scales are the same.

nnagn)] ajadung

wosmuuny

Wasatch Plateau

E A
East >
se
N wy
AY a9 - Wasatch Plateaw
ae
Ree eh
QE PES Bent es = lessees
Sec. 7
East >
= y
| ) 10

| |

Horizontal scale; Miles

nvaynn eduogy

uornuny

Gunnison Plateau

> of B f
we
>
AY 4 . a
os G Plalt Oe
y Cc UnUsor eau B x
of
= &

Gunnison Plateau

Plate Parti

GENERAL GEOLOGICAL SECTIONS

IN- THE VICINITY OF
GUNNISON ano SALINA, UTAH

EDWIN E. HOWELL.
——

The sections are numbered in order from north to south
and areso arranged that potnts in the same longitude
are in the same vertical line. Their geographic

positions are given ona map included in part 2 of this pale.
The base line of each section ts the level of the ava

The horizontal and vertical scales are the same

Wasatch Plateau

Wasatch Plateau
———

fs —— ed

Wasatch Plateau

ee =

Wasatch Plateau

0 S000 1000 Axo.

Verlica/ scale; Feer
0 5
Lol eee

Horizontal scale; Miles

WAP

SHOWING THE

POSITION of tHe CEOLOCICAL SECTIONS

IN THE VICINITY OF

CUNNISON ano SALINA, UTAH

=.=

Mu-si-nié-2

Peak

Coal Hortzow

Plate Part 2

GENERAL GEOLOGICAL SECTIONS

IN THE VICINITY OF
GUNNISON ann SALINA, UTAH

EDWIN E,HOWELL.
The seclions are numbered in order from north lo south, and are 40 arranged
that pots inthe same longitude are in the same vertical line.
The base line of each section ws the level of the sea.
The horizontal and vertical acales are the eame.

Wasatch Monoclinal

Sao oom

Verfical scale; Feer

t * () 1 2 3 + 5
S658} Lit |

MAP

SHOWING THE

POSITION or tHe CEOLOCICAL SECTIONS

IN THE VICINITY OF

CUNNISON ano SALINA, UTAH

‘CUNNISON

Mu-si-ni-x

l !
Horizontal scale;Miles

4 OO eI iON *

Nis W | S808:

SEDIMENTARY BEDS OF THE WASATCH PLATEAU. 165

than those of the East Gunnison fault. Its position and relations are shown
in the stereogram and in the sections above referred to.

Between the East and West Gunnison faults is an uplift, qualifiedly
tabular in form, which may be called the San Pete Plateau. Its northern
end is separated from the base of Mount Nebo only by a canon, which
emerges near the town of Nephi. Eastward it looks down upon San Pete
Valley, westward upon Juab Valley, which may be regarded as the north-
ern continuation of Sevier Valley. Southward the plateau slopes slowly
as far as the town of Gunnison, where it becomes the floor of the Sevier
Valley. Its altitude is insufficient to warrant its admission as a member
of the group of High Plateaus. Its general form may be illustrated as
follows: If from a point situated about six miles south of Gunnison we
travel north 30° east, our course would lead us up into San Pete Valley;
if we travel north 30° west, it would lead us down the Juab Valley; if we
travel due north, we shall ascend the easy slope of the plateau to its sum-
mit at its northern end. Its transverse structure is shown in the sections.
Plate 3; sections 1, 2, and 3.

SEDIMENTARY BEDS COMPOSING THE WASATCH PLATEAU.

The Wasatch Plateau consists of beds of Upper Cretaceous and early
Tertiary age, the latter being correlated, as well as any lacustrine beds of
the Rocky Mountain region can be, with the Lower Eocene. In the low-
lands immediately adjoining are found, on the east the Lower Cretaceous,
and on the west a singular occurrence of the Upper Jurassic. There is
found also in the Sevier and San Pete Valleys, and in the low uplift between
them, a series of strata of later age than the Tertiaries of the plateau,
though from many considerations it appears that their age is with great
probability early Tertiary and immediately subsequent to that of the strata
upon which they rest. They are believed to be local deposits only, and to
have accumulated here and there after the commencement of the general
disturbance and uplifting which resulted in the drainage of the great
Eocene lake.

The principal Tertiary series is provisionally divided into two; the
lower can be referred with confidence to the same horizons as those oceu-

166 GEOLOGY OF THE HIGH PLATEAUS.

pied by the beds which Powell has called Bitter Creek, lying upon the

- southern slopes of the Uinta Mountains. This determination does not rest
upon identical fossils, for the two localities do not yield the same species;
but upon the most decisive of all evidence, the known continuity of the
beds. Between the Bitter Creek beds of the Uintas and those here assigned
to the same epoch is an unbroken exposure along which the identity can
be traced. The fossils found are Viviparus trochiformis (White), Hydrobia
Utahensis (White), several undetermined species of Physa, Planorbis, and
Linnea, and some plant remains. The total thickness of this series is
about 2,200 feet, but varies a little in different sections. The following sec-
tion was measured by Mr. E. E. Howell at the southwest angle of the plateau,
and very well represents the general character of the whole formation.

Feet

a) Shaly limestone, containing Physa, Limnea, and Planorbis .....-...--..--- 250

{b) Gray and cream-colored limestone with Physa.........-..-----------+----- 400

(e)iePealespinksarenaceous limestones =r -.eeeee sa eee er eee eee eee ree 250
(d) Gray limestone, shaly and green at base, with Hydrobia, Physa, and Vivi-

(MUP moaccs Sods! aoe bos Das0ad ano eee aahbooroaHsbououKcn O67 eanenosede 350

(e) Cream-colored calcareous sandstone.. ...............---------fs0--- 23s: 350

(AiGrayslmestonemwitheVavip anus yee eet imiec ei sae iete eee ee erect 600

2, 200

This series has been designated No. 3 in the various sections, and
though it has not been connected with the Lower Tertiary beds in the
southernmost of the High Plateaus its identity is probable in a high degree,
so much so that it is taken for granted. The beds which overlie it are
separated by a distinct plane of demarkation in the principal sections and
by lithological characters. They are much more variable in their constitu-
tion and in their bedding. Its members are designated as series No. 2, and
the following sections by Mr. Howell illustrate their characters :

Series No. 2 (Tertiary), section No.7 A: Feet.
(a) Cream to gray shaly limestone, with fishes, Planorbis, Viviparus, and indistinct

Plant (remains s<2 5.5 See Seesicie Sarco ona Ee Or eee ee OEE Eee 350
(})RGreenishicalcareous'shale=- == ses see eee seen a eee a eee eee eee 750

(c) Pale red, purple, and slate-colored marls, Hii occasional bands of caleareosu
gray sandstone, fish-scales being found in some of the more calcareous
MOMDELS 2 aje 215 sie oie weejswie = Sine Bue eieeeae eie eee Re REE RE ECC EEE EEE 400

1, 500

SEDIMENTARY BEDS OF THE WASATCH PLATEAU. 167

Series No. 2 (Tertiary), section No. 7 B: Feet.

(a) Cream and gray limestone, containing a few fish-scales; bed of chert at top... 300
(b) Greenish calcareous shale ......--.-.------- sdbadeoancsoxe sedodone dusnos 300
(C)pBaleired marlyjshalereeerer eee eee ee eee e eer eater ee 300
900

These beds are assigned provisionally to the Lower Green River epoch.
Unlike the series below them, they cannot be directly connected with the
strata lying at the base of the Uintas, nor are their fossils a satisfactory
guide to a decisive correlation, though the presence of fishes resembling
those of the Green River beds might be regarded as indicating such a rela-
tion. They have not, however, been identified as belonging to the same
species as those of the latter formations. The beds in question are found
only in the Sevier and San Pete Valleys, in the uplift between them, and
extending a short distance up the great monoclinal flanking the west side
of the Wasatch Plateau. That they formerly extended over that plateau,
and for an indefinite distance eastward, is very probable. In this portion
of Utah they are the last lingering remnants of a series which was nearly
and in many large areas quite the last to be deposited and the first to be
attacked by the general process of degradation which has swept away such
vast masses of strata. From the summit of the Wasatch Plateau this whole
group of beds has been eroded and about 300 feet of the Bitter Creek beds
immediately beneath, and this amount of denudation is probably the mini-
mum of the whole Southern Plateau Province, except where the sediment-
ary beds have been protected by volcanic rock or have enjoyed unin-
terrupted protection in gravel-covered valleys between great uplifts.

The uppermost series of Tertiary beds has been alluded to as consist-
ing probably of a series of local deposits accumulated after the general
upward movement of the whole Plateau Province had commenced, though
it seems probable that this movement was then in its earlier stages. The
beds contain fossils very similar and perhaps in some cases identical with
the species of Planorbis Physa Helix (2), and Viviparus, which are found in
the series upon which they rest. Lithologically they are much more
variable. Some of them are conglomerates, which are apparently of allu-
vial origin, and none of them are found to be continuous over a large area.

168 GEOLOGY OF THE HIGH PLATEAUS.

They all lie near the ancient shore line of the great Eocene lake, and cases
of unconformity, not only with the underlying series, but among themselves,
are not uncommon. Their physical characters are, in general, indicative of
an epoch of gradual displacement in the several tracts which they occupy.
It would be obviously extremely difficult to correlate such a group with
any such formations as those which are found on both flanks of the Uintas,
forming the comparatively regular and systematic strata of the Upper Green
River series, though general considerations may warrant a provisional
reference of these local deposits to that period.

The unconformities just spoken of are probably in some cases apparent
rather than real. It is easy to see that while deposits are accumulating
along the slope of a flexure which is in process of formation, the two going
on pari passu, there may result a want of parallelism in successive layers
as well as other irregularities which produce collectively the appearance of
unconformity. This differs, however, from that type of real unconformity
which is usually relied upon as proof of an interval of time between con-
tiguous formations in which the record is interrupted by a blank of unknown
duration. Where the exposures are satisfactory the apparent and real occur-
rences may be distinguished, but in a majority of cases the distinction is
not easy to find.

The thickness of the formation is highly variable, ranging from 300 to
750 feet. It consists of alternating marls and sandstones, the latter being
sometimes coarse-grained, with here and there a patch of conglomerate.

CHAPTER VElhie

THE TUSHAR.

Sevier Valley from Gunnison southward.—The Pdvant.—Salina.—Grandeur of the plateau fronts.—
The northern end of the Tushar.—General structure of the northern part of the range.—Its inter-
mediate character between the plateau and basin types.—Rugged and mountainous aspect of the
higher parts.—Mounts Belknap and Baldy.—Eastern front.—Bullion Cation.—The Tushar fault.—
Rhyolites and their numerous varieties.—Basalt upon the summit.—Succession of eruptions and
the intermissions.—Southern portion of the Tushar.—The great conglomerate.—Progressive
growth of the range.—Alternations of volcanic activity and repose.—Southern termination of the
Tushar.—Midget’s Crest.—Dog Valley.—Succession of eruptions in the southern part of the
range.—General history of the Tushar.

The road leading southward from Gunnison up the valley of the Sevier
River lies along a smooth plain between the Pavant Range on the west and
the great monoclinal on the east.. The interval separating these uplifts is
about 30 miles from summit to summit and about 8 miles from base to base
(see Plate 3, sections 4 to 13). To the east and northeast from Gunni-
son is seen the Wasatch Plateau, just distant enough to afford a fine view
of its grand proportions. Its southwestern angle is decorated with a huge
butte perched upon a lofty pedestal and crowned with a flat, ashlar-like
block, which is a conspicuous land-mark from every lofty point to the south-
ward. This mass is called Musinia, and at once arrests the attention by its
peculiar form, whether seen from far or near. Southward, at a distance of
nearly 30 miles, loom up the high volcanic plateaus. The Fish Lake and
northern portion of the Sevier tables present their transverse profiles
towards us, and are seen to be separated by a depression called Grass
Valley. Far to the south-southwest is seen a portion of the Tushar, the
main mass being hidden by a very obtuse salient of the Pavant. The
absence of Alpine forms and the predominance of the long and slightly--
inclined profiles of the plateau type rob these great masses of their
grandeur and beauty; for they produce an optical deception which carries
the horizon up near their summits, while in reality it is far below. Yet

some sense of the reality is awakened when from the plain below, in the
169

170 GEOLOGY OF THE HIGH PLATEAUS.

torrid heat of July, we see the fields of lingering snow light up their
gloomy crests. To the westward rises the Pévant, its eastern flank ascend-
ing with a smooth swell to a crest line which looks down into Round Val-
ley; and beyond that rise to still greater altitudes the mildly sierra-like
summits of the range. The broad valley of the Sevier is treeless, and sup-
ports but scantily even the desert-loving Artemisia. It is floored with fine
loam, which, under the scorching sun, is like ashes, except where the fields
are made to yield their crops of grain by irrigation. As we ascend the
valley to the southward the scenery is impressive, for every object is
molded upon a grand scale; though it is only by long study and familiarity
that the huge proportions are realized. The absence of details, the smooth-
ness of crests and profiles, at first deceive the eye and always tend to
belittle the component masses. <A stretch of 10 miles from Gunnison
throws to the westward the salient of the Pavant and reveals the south-
ward extension of the valley for 85 miles, beyond which rise the summits
of the Tushar in full view. Right opposite this point the Pavant has now
changed its aspect to one contrasting strongly with the view we had of it
from Gunnison. There we saw a dull, monotonous slope; here we behold a
splendid array of cliffs, showing the edges of Tertiary strata gently slop-
ing towards us, carved and broken after the usual fashion. of the Plateau
Country, and lit up with flaring colors—red, white, and yellow. The indi-
vidual cliffs and crags are neither very high nor very long, but rise above
each other terrace-like, after the manner of a rambling series of fortifica-
tions, with tier upon tier and with numberless salients and curtain walls.
To one viewing plateau scenery for the first time this portion of the Pavant
would be a source of surprise and enthusiasm; to one familiar with the
colossal walls in the heart of the Plateau Province it is tame and almost
insignificant.

Fourteen miles south of Gunnison is the little Mormon village Salina,
a wretched hamlet, whose inhabitants earn a scanty subsistence by lixiviat-
ing salt from the red clay which underlies the Tertiary beds in the vicinity.
Around and beyond this village is a dismal array of bad lands of great
extent, presenting a striking picture of desolation and the wreck of strata,
while beyond and above them rise the northern volcanic sheets of the

SEVIER VALLEY—THE PAVANT. 171

Sevier Plateau. The lava, the desolation, and the salt strongly suggest
recollections of Sodom and Gomorrah. At this point Salina Creek emerges
from its cation through the great monoclinal—a fine, large stream. To the
south-southwest the valley of the Sevier becomes considerably narrower
and the Pavant lower, but the slope of that range gives place to an abrupt
wall, due to a fault. A few miles south of Salina commences the great
Sevier Plateau on the east side of the valley, its northern end gradually
and steadily sloping upwards as we proceed south and its western wall
becoming more and more abrupt, until it becomes a cliff of grand dimen-
sions. From the town of Richfield, 18 miles south of Salina, we may
behold it in all its grandeur, rising 5,800 feet above the plain below; its
upper third a sheer precipice, the lower two-thirds plunging down in steep
buttresses which thrust their bases beneath the level floor. Its aspect is
dark and gloomy from the dark gray dolerites and trachytes which make
up its whole mass Right at our backs are the lively tints of the Tertiaries
in the Pavant; beds of pink, carmine, and cream, alternating with almost
pure white, and with a rigorously even stratification. A stronger contrast
it is difficult to imagine. Yet a mile or two beyond Richfield these rain-
bow beds suddenly give place to a black rhyolite,* which has spread from
some unknown vent and covered the Tertiaries.

Moving still southwards along the flank of the Pavant, which slowly
but steadily diminishes in altitude, we reach its junction with the Tushar
about 16 miles southwest of Richfield. Here a lateral valley from the
west joins the Sevier Valley, the upward continuation of the latter being
due south between the towe:ing heights of the Sevier Plateau on the east
and the Tushar on the west. The separation of the Pavant from the
Tushar is mereiy a low divide or saddle, or, if the idea is more acceptable,
the former may be regarded as the northern continuation of the latter at a
lower altitude. The lateral valley, as we ascend, narrows rapidly to a mere
canon, and from is southern brink rise the great spurs of the Tushar.

The northern portion of this uplift is crowned by volcanic peaks,

* This is a somewhat exceptional rock ; very little feldspar, much free quartz, and the vesicular
specimens have the elongated, wiry, and fluctuated vesicles which are eminently characteristic of rhy-
olite. The black color, almost equal to that of basalt, is apparently due to the presence of an unusual
quantity of magnetite.

172 GEOLOGY OF THE HIGH PLATEAUS.

which reach higher altitudes than any other summits in Utah excepting a
few in the Uintas. There are three points which reach above 12,000 feet,
viz: Delano, 12,160 feet; Belknap, 12,080 feet, and Baldy, 12,000 (?) feet.
There is nothing in the aspect of this portion of the Tushar mass to sug-
gest to the eye a plateau structure; on the contrary, the appearance is in a
high degree sierra-like, and it is quite possible that this portion of it should
be regarded as belonging rather to the basin than to the plateau type of
uplift. But so far as the structure depends upon vertical displacement, it
is much obscured by the enormous series of volcanic floods which have
been poured over it by numberless eruptions. Frequent indications, how-
ever, are seen of a general and moderate dip of the whole series to the
west, leading to a presumption that the whole Tushar mass has a tilt in
that direction. But while the exposures are numerous, there are no ex-
tended ones among them. ‘The process of erosion has here wrought out a
sculpture differing utterly from that presented by the sedimentaries, and
one calculated to conceal the frame-work of the mountains instead of lay-
ing it bare. The degradation has here been very great; greater certainly
than in some of the other volcanic plateaus. Instead of great cliffs, we
find only slopes covered with débris and soil, with here and there a pro-
jecting ledge, which is soon lost beneath a talus. The best exposures are
seen along the eastern front, facing the Sevier Valley, and in the deep
gorges opening into it and heading far back in the heart of the range.
These all concur in indicating a general slope of the beds to the westward,
which is strongest near the eastern flank and smaller in the central portions
and western flank. The northern portion is also deeply scored with grand
ravines, well calculated to kindle the enthusiasm of the mountaineer and
task his energy. The exposures which they-contain, so far as they have
been examined, accord with those in the eastern gorges in presenting a
westward inclination. It is quite possible that many faults complicating
the structure have escaped detection, but it is not probable that any sub-
ordinate displacements yet to be discovered will seriously impair the con-
clusion adopted regarding the general structure.

But while the plan of the entire uplift seems to be most nearly allied
to the plateau type, it is equally apparent that there is a strong tendency

GENERAL STRUCTURE OF THE TUSHAR. 173

toward the basin type. ‘The latter may be represented by conceiving the
strata forming the platform of a given tract to be rent by a long fault, and
upon one side of it to be lifted and tilted at a considerable angle. This
inclined mass is usually further fractured by smaller faults rudely parallel
to the principal one, and complicated by more or less warping. The
plateaus also are usually tilted, the Aquarius and Kaibab being most nearly
horizontal. But there is a marked difference between the two types in the
amount of inclination. In the plateaus it seldom exceeds three degrees; in
the basin it is rarely less than ten. In the plateaus the warping and minor
displacements are seldom important, and the whole aspect is calm and even;
in the basin they are extensive, and the aspect is wild and distorted. In
the plateaus, the obvious characteristic features are the broad platforms of
the tables, the gently sloping terraces and the majestic repose of the mighty
cliffs which bound them; in the basin, they are the sharp ridges, cusp-like
teeth, and tumultuous slopes of sierras. Probably the correct view to be
drawn from a comparison of the two structures is that the basin type repre-
sents an advanced stage of an action which has been imperfectly developed
in the plateaus. Had the tables been pushed up higher, they might have
been as much inclined as the sierras and as much comminuted and distorted.

The Tushar is in some portions at least, and so far as observed in most
portions, more inclined than any other of the High Plateaus, but so far as
can now be discerned it approaches more nearly to the tabular than to the
sierra type. Lying within the geographical limits of the Great Basin, it is
not surprising that it should show an approach to the structure of the latter
province. It may be regarded as indicating a transition between the two
forms, though more nearly allied to those peculiar to the Plateau Province.

It is difficult, however, to realize this conclusion as being a true one
when we stand upon the southern termination of the Paévant, and look at
the cluster of peaks which crown the summit of the Tushar. Two noble
cones ending in sharp cusps stand pre-eminent, while behind them numer-
ous dome-like masses rise to nearly the same altitudes. The two peaks are
Belknap and Baldy, which reach above the timber-line, and are very strik-
ing on account of the light cream-color of their steep slopes and the ashy-
gray tips of the apices. These pyramids are not apparently the remains

174 GEOLOGY OF THE HIGH PLATEAUS.

of craters, but mere remnants of the uppermost sheets, which have been
almost wholly removed by erosion. From their bases radiate profound
gorges separated by huge buttresses, which extend to the lowest valleys
and plains, while beyond them rough crags and shattered domes rear their
bald summits to the clouds. But all this grand detail of mountain form
has been carved out of the vast block of the tabular mass by the ordinary
process of erosion. The lavas accumulated sheet upon sheet, the subter-
ranean forces uplifted the block and tilted it, and the rains and torrents have
done the rest.

The eastern front of the Tushar is far more rugged and mountainous
than the western, and the explanation is obvious. The western slope is along
the dip of the strata, which, though considerable near the crest, is slight as
we recede from it westward. The eastern slope is across the upturned
edges, and from. the nature of the case is very abrupt. The power of water
to corrade and carve rapidly increases with the slope, and the resultant
sculptural forms are correspondingly bold and craggy.

The loftiest, boldest, and most diversified portion of the Tushar fronts
the Sevier Valley in the vicinity of a little hamlet called Marysvale, situ-
ated about 27 miles south of Richfield. The great mountain wall leaps at
once from the narrow platform of the valley to nearly its greatest altitude.
Immense ravines, rivaling those of the Wasatch in depth, but narrower and
with steeper sides, have deeply cleft the great tabular mass, and subdivided it
into huge pediments, which from below appear like individual mountains.
The finest gorge is named Bullion Canon, in the jaws of which the little
village of Marysvale is situated. Ascending it, we may gain some informa-
tion concerning the structure of this portion of the Tushar mass. The
lowest beds forming the base courses of the uplift are quartzites resulting
from the metamorphism of sedimentary strata, which are believed to be of
Jurassic age. They are considerably disturbed, yet not excessively so.
The prevailing dip is to the west, though it is by no means uniform. The
main fault, which has thrown down the platform of the Sevier Valley,
runs north and south along the base of the mountains, but the whole dis-
placement is probably by a series of parallel repetitive faults. I have seen
but one of the faults west of the principal displacement, but have inferred

BULLION CANON. 175

their existence by the recurrence of beds, which seem to be identical both
individually and serially, at higher and higher levels up the cation.

As we ascend Bullion Canon from Marysvale we observe on either side
a hard quartzitic rock well bedded in massive layers, exhibiting consider-
able metamorphism. It is also somewhat variable in the dip. The strata
incline upward at first, but soon flex easily back until the dip is westward.
The thickness of the series seems to be very considerable, though the ap-
parent thickness may be partly due to repetitive faults of small shear. At
a distance of about 3 miles from Marysvale and 2,600 feet above that vil-
lage, we come upon the volcanic series. A mass of dark-colored liparite
rests upon the quartzite, having a thickness of about 450 feet. About 500
feet higher the quartzite reappears, being probably the same bed as below,
but thrown up by a minor displacement, and it is covered by the same or
a similar sheet of liparite The quartzite, however, is more altered than
the portion of it below, and in general as we ascend from Marysvale
through the quartzitic beds the signs of increasing alteration are unmistak-
able. From this point upwards eruptive rocks alone are seen. The lower
masses are dark liparites, with abundant quartz and monoclinic feldspar and
decomposed hornblende. Still higher rocks of a porphyritic texture and a
dark purplish hue lie in great volume. They have a striking resemblance
superficially to the argilloid trachytes of the central and eastern plateaus,
but contain abundant quartz, and the microscope confirms their rhyolitic
character. These two groups of eruptions are separated by local conglom-
erates derived from the older of them, and the surface of the latter is seen
to have been much eroded, indicating a considerable interval of time
between the periods of activity. The summit of the series consists of a
group of rhyolites (proper rhyolite), which contrast strongly with those
beneath. They are very light colored, without crystals, and yet not hyaline.
They are highly siliceous, and exhibit in the thin sections a fibrolitic or
spherolitic groundmass of beautiful texture and very interesting. Some
of the specimens are exceedingly siliceous, and are resolved under the
microscope into an aggregation resembling very fine-grained quartzite and
appear to be quite abnormal. The light-colored masses are generally true
rhyolites of no uncommon kind. This rock forms the lofty peaks crown-

176 GEOLOGY OF THE HIGH PLATEAUS.

ing the northern summit of the Tushar mass, and occurs in several out-
lying knobs and small crests to the éast and northeast of Belknap. But the
northwestern slope of the range has been mantled by great floods of it,
which have poured in massive sheets from summit to base, burying the
antecedent topography of the mountain and generating a new one. The
individual eruptions making up this rhyolitic mass appear to have been
numerous, some very voluminous, others very small. The smaller ones
are seen to fill up old ravines and to mold themselves upon uneven pre-
existing surfaces, while the grander floods pour over everything and spread
out over great expanses of mountain side. Although this lava is, with the
exception of a few minor basaltic streams around the western base of the
Tushar, the most recent of all the outbreaks, yet absolutely it is of con-
siderable antiquity. Since the extinction of the vents from which it was
emitted there has been a long period of erosion. Belknap and Baldy,
together with the eastern outliers, are mere remnants of piled-up sheets,
which were perhaps once continuous, but are now separated by profound
ravines, which have been excavated by erosion.

The indications are abundant that the period separating the earliest
from the latest eruptions was a very long one. The contact of the earliest
liparites with the Jurassic quartzites shows heavy floods of lava pouring
over a very uneven surface and piled up in layers by successive eruptions
to a thickness of more than 2,000 feet. These, in their turn, show a sub-
sequent degradation by erosion not only in the sculpturing and carving of
the beds, producing an unconformity in some of the contacts, but also in
the existence of local conglomerates composed of the water-worn fragments
of the dilapidated rocks cemented by finer detritus derived from the decom-
position of the feldspathic materials.

These earlier eruptions appear to have been followed by a long period
of calm, during which they were attacked by the degrading force and
slowly wasted by decay. In many places the beds were cut through down
to the quartzite and a fresh topography was carved out by erosion. After-
wards the activity was reopened with fresh eruptions of a different charac-
ter. These second eruptions were grander than the first, some of the beds
being many hundreds of feet in thickness, spreading over great areas, and

Ilenroryer Ll,

220 Devonshire St., Boston.

Heliotype Printing Co.,

VOLCANIC CONGLOMERATE. THE TUSHAR.

RHYOLITES SOUTHERN PORTION OF THE TUSHAR. idea

extending far to the westward, expanding as they extend. Of this rock, a
dark purplish porphyritic rhyolite, the great central mass of the Tushar is
composed.

The second period of activity was followed by another interval of
repose. During this interval the greater part of the uplifting of the range
took place. The faults traverse and dislocate both the first and the second
series of eruptions. It was also a period of great erosion, during which the
turmoil of mountain peaks, domes, and spurs were carved on the eastern
flank, and that side of the range devastated in a striking manner by the
slow ravage of time. The third epoch of eruption was the least of all and
most local, being confined to the portion around Belknap and Baldy, and
furnishing the cream-colored rhyolite and a few small outbreaks of basalt.

The southern portion of the Tushar contrasts with the northern por-
tion in many respects. It exhibits a totally different group of eruptive rocks.
In the northern part the extravasated rocks are rhyolites; in the southern
part they are trachytes, augitic andesites, dolerites, and basalts. The form
of the southern part of the uplift is distinctly tabular or plateau-like, while
the northern part has the sierra aspect. .

About 3 miles south of Belknap, standing upon the brink of an old
coulée, we look southward over a broad expanse of comparative calm lying
at a slightly lower level. In this expanse the tabular form of the Tushar
mass is no longer doubtful. A lofty plain diversified by ridges of erosion
is spread out before the gaze, clad with spruce and aspen and opening in
grassy parks. ‘The abundant streams have carved gently-sloping ravines
and pleasant knolis, where the dark lavas may occasionally be detected
dipping very gently to the west, but near the eastern rim rising more
boldly to the timber line (11,500 to 12,000 feet), where they are suddenly
cut off and present their truncated edges to the eastward in the boldest of
mountain slopes. This part of the plateau summit is about 22 miles in
length, 8 to 10 miles in width, and the mean altitude about 10,000 feet.
Erosion has given to this lofty watershed a surface very similar to that
which may be observed in any well-watered country, and which is in
strong contrast with the peculiar forms observable at lower levels where the

precipitation is much smaller
12HP

178 GEOLOGY OF THE HIGH PLATEAUS.

The eastern front of the Tushar preserves that rugged mountainous
aspect already described throughout two-thirds of its extent. The southern
third is a wall of imposing grandeur, presenting to the eye the effect of a
perpendicular escarpment, though really it is inclined at a slope of 60° or
more. It is a magnificent object as seen from Circle Valley, rising nearly
2,000 feet above its base, and its base standing at the summit of a long
slope which rises 2,000 feet above the valley bottom. This great cliff is a
conglomerate composed of the ruins of older volcanic rocks. It is stratified,
but not so conspicuously as most of the similar formations so abundant
throughout the district. The finer material which incloses the rocky frag-
ments is a light-gray pulverulent detritus, evidently resulting from the
decomposition of feldspathic materials and highly aluminous. Some of the
members of this series of heavy beds consist chiefly of this finer material,
holding comparatively few fragments; in others the fragments are much
more abundant, constituting the greater part of the mass. The fragments
are usually somewhat rounded at the edges, but in most cases the amount
of attrition is small, though seldom wholly unrecognizable. The mode of
origin of this and similar conglomerates will be discussed in detail in a sub-
sequent chapter. It is a sub-aérial formation throughout, and the mode of
accumulation may be seen and studied hard by in all the valleys of the
district. (See Heliotype IL.)

These beds are of ancient origin, having been formed prior to the
great displacements which have given the Tushar its present structural
features. The inclosed fragments are wholly variable in character. None
of the rhyolitic, trachytic, and basaltic rocks of later age are seen among
them, and the inference is irresistible that its formation was completed before
these last-named masses were erupted. The source of these materials seems
to have been the adjoining mass of the present Tushar table to the north-
ward. To realize how this may have been we are obliged to go back in
time to the later Eocene or early Miocene, when, in all probability, these
great outbreaks occurred, and endeavor to reconstruct the country. At
that time the centers or loci of eruption were doubtless in the very heart
of the range, and stood considerably higher than the adjoining part of the

country, just as they do now, though more recent movements on a grand

DEGRADATION OF THE TUSHAR—CONGLOMERATES. 179

scale have produced new features by uplifting the range en masse. But as
these recent movements apply to the whole uplift, the relative altitudes of
the loftier portion, which furnished the débris, and the less lofty portion,
which has received it, have not been much, if at all, changed with respect
to each other. But erosion has apparently effected what displacement has
not; it has nearly equalized the levels of the two portions. The volcanic
masses near the foci must have been very voluminous, for the conglomer-
ates derived from them extend with great thickness over a large area, rival-
ing in bulk, if indeed they do not surpass, the enormous masses yet
remaining. Wherever we find strata composed of clastic materials, the
present methods of reasoning in geological science compel us to acknowl-
edge that they have been derived from the degradation of masses of even
greater magnitude.* _ In the case of a great sub-aerial conglomerate, formed
under conditions which are still existing and a process still operating, we
naturally look to the vicinity or border of the conglomerate itself for the
source of the materials. We find a very obvious source to the northward.
The structure of the great uplift of which the conglomerate forms a part
and large masses of eruptive strata in situ, composed of materials agree-
ing with those found in the clastic beds, confirm this view so strongly, that
there seems no room for question. But the mass of the conglomerate argues
an enormous degradation. ‘To supply so vast an accumulation the older
eruptive area in the central part of the Tushar must have been piled thou-
sands of feet high with successive sheets no longer visible, or have been
the theater of eruptions separated by long intervals of erosion, which in
the long run removed the lavas as fast as they were erupted. A view which
is a compromise between these two I regard as decidedly preferable, and
most fully sustained by the general tenor of the evidence throughout the
entire district. We may look back to a period somewhat earlier than Mid-
dle Tertiary, when the volcanic eruptions built up Adtna-like highlands of
eruptive materials, not by rapidly succeeding outpours, but by alternating
emission and quiescence. Between the outbreaks many years or centuries
may have elapsed, but the accumulation was much more rapid for a time than

*Except in cases where pulverulent and fragmentary materials have been ejected and scattered,
which is not the case in the present instance.

180 GEOLOGY OF THE HIGH PLATEAUS.

degradation, and the altitudes of the eruptive centers increased. Now and
then came a long interval of repose, indicated by the quiet accumulation of
considerable, though very local, masses of stratified conglomerate here and
there. Again the energy was renewed and fresh outbreaks occurred, fol-
lowed by a long rest. After a protracted series of alternating eruptions and
unequal intervals of rest there came a very long period of repose to be
reckoned by a geological standard of time, during which these massive
conglomerates accumulated and the huge volcanic piles were razeed—a
period in which there may have been eruptions, but in which, on the whole,
the ceaseless erosion leveled down the highlands and leveled up the low-
lands.

But the building of the conglomerate beds did not close the volcanic
cycle. After they had acquired their enormous bulk there came another
period of outbreaks, some of them in the old localities, others in new ones,
pouring fresh sheets over the wasted centers and over their scattered and
stratified débris, piling up fresh mountains of lava and generating a new
topography. This second series of eruptions differed strikingly in litho-
logical character from the first. The earliest series in the Tushar, so far as
known, is andesitic and trachytic ; the second is rhyolitic and basaltic. In.
the northern part of the range the dominant rock of the second series is
rhyolite, with a limited occurrence of basalt. In the southern part of the
range the relative abundance of the two groups is reversed, rhyolite being
uncommon, and in most areas being replaced by true trachyte. These
beds cover both the central part of the Tushar and the conglomerates at
the southern end. They lie upon the eroded surface, filling old ravines and
spread out in broad sheets over the tabular summit, obliterating upon the
surface the definition between the conglomerate and the degraded mass
which furnished its materials, though the junction is exposed in the eastern
front of the range by the great fault which at a later epoch was formed by
the general uplifting of the whole mass.

The southern termination of the Tushar is marked by a group of lofty
summits a few hundred feet lower than Belknap at the northern end and
Delano near the center, but full 1,600 feet higher than the wall and tabular
summit which connects them with the central part of the table. They are

SOUTHERN TERMINATION—MIDGET’S CREST 181

superposed masses of volcanic beds resting upon the great conglomerate.
Here the faulted wall of the range swings around to the southwestward and
rapidly dies out. (See stereogram.)

The lofty crest at the southern end of the Tushar has been named
Midget’s Crest, and it presents to the southeast three bold salients, standing
about 5,600 feet above Circle Valley, which lies at the base of its great
spurs east-northeast. Its absolute altitude is about 11,600 feet. It is a
volcanic mass, built by the accumulation of andesitic, trachytic, and basaltic
sheets. The three salients are from 1,400 to 1,600 feet higher than the
summit of the conglomerate cliff to the north of them and their superior
eminence is due to this accumulation of lavas. The conglomerate passes
beneath them though its outcrop is masked by the talus.

The sheets which compose Midget’s Crest belong to a later period than
those which occupy the central part of the Tushar range, and which were
broken down to form the great conglomerate. Coulées of the same period
are found north of this crest, upon the summit of thé tabular part of the
Tushar, where they are mainly trachytic. Upon the extreme summit of the
southern crest lies a true basalt, highly vesicular upon its surface, and the
first impression is that it is a comparatively recent eruption—Post-Pliocene
or Quaternary—the rocks on which it rests being certainly very much older.
It is of small expanse and thickness and is abruptly cut off at the crest-line
of the ridge. Its origin cannot easily be conjectured. There are no indi-
cations of a vent in the vicinity and, notwithstanding the freshness of its
appearance, it may be as old as early Pliocene. But the beds on which it
lies are less doubtful. They face southeastwardly, forming the salients
already mentioned, and have been wasted greatly by the general degrada-
tion. When the period of dislocation and uplifting set, in they extended
as far to southeast as the principal fault which runs around this angle of the
plateau with a throw of about 3,500 to 4,000 feet, and the. entire mass
between the crest-line and the fault has been denuded to a corresponding
depth. The origin of the lavas I believe to have been to the southeast
and east of the ridge in the vicinity of the faults, where evidences of great
contortion and considerable chaos are still visible, and where rocks appar-
ently identical with those upon the summit of the table and near the

182 GEOLOGY OF THE HIGH PLATEAUS.

summit of the crest are still discernible, though now they lie at least 3,000
feet below them.

Immediately south of Midget’s Crest lies Dog Valley—a pleasant
moderately diversified platform—with an absolute altitude of about 7,500
feet or 1,500 feet above the Sevier at Circle Valley. It is a part of the
last-mentioned focus of eruptions of the second or middle epoch, but erosion
has leveled down most of the ancient irregularities, and left it a field of
rolling hills, well covered with soil, loam, and sharp gravels. Its real history
might not have been suspected, were it not for the vast floods of lava which
spread out from it in all directions for many miles, growing thinner and
broader as they recede.

Southwest of Midget’s Crest the altitude of the plateau gradually
diminishes until its summit at last is lost in the next region. The fault
which originated the escarpment of the plateau suddenly becomes a mono-
clinal which dies out in the space of about 6 miles. This monoclinal is
composed of conglomerate of unknown thickness, but not less than 1,500
feet in the vicinity of the flexure. It turns up at an angle of 28° to 30°
against the diminishing wall of the plateau, but soon straightens out towards
the south and decreases rapidly in thickness. It is composed of basaltic
(doleritic) materials chiefly, quite similar to, and perhaps identical in part
with, the remnants of that kind of rock forming the extreme summits of the
salients on Midget’s Crest.

The western base of the Tushar I have seen in part only, and have
given that part merely a cursory examination. It is possible that there
exists a fault of about 1,200 feet along this base with a throw to the west;
a continuation of the Hurricane fault, which appears in great force about
15 miles south of the southwest slope of the Tushar. But I have not
verified the existence of such a fault in this locality, and such an occurrence
may not be necessary to explain the features presented, so far as observed-
The summit of the table, after maintaining for about 10 miles an easy slope
to the west, suddenly increases the descent of the profile to the broad plain
below. The surface contour here cuts across the ends of the lava sheets,
which are seen to be considerably attenuated when compared with" the

huge masses exposed upon the upturned eastern flank of the range.

THE BUILDING OF THE TUSHAR. 183

Whether the somewhat abrupt western boundary is due to the faulting
suggested above or to the termination of the old coulées it is not possible to
say with confidence, but the former view seems to furnish the easiest
explanation.

At the western base of the Tushar, near the town of Beaver, is seen a
very recent basaltic crater in a very perfect state of preservation. Farther
northward are others, some of them so recent that we may easily suppose
that their eruptive activity has ceased within a few hundred years. Many
of the basaltic craters throughout the Plateau Country seem to be equally
recent, though many others have considerable antiquity. On the whole,
however, the true basalts are the most recent of all eruptions. They are
seldom found in the heart of the older eruptions—indeed, I am able to
recall but few such instances—but they occur around the outskirts of
older voleanie districts, and often at a considerable distance from them. In
respect to magnitude of eruptive mass, the basalts are here decidedly
inferior to every other class of rocks.

THE BUILDING OF THE TUSHAR.

To go back to the commencement of the series of events and pro-
cesses which have combined to rear this majestic range to its present alti-
tude and proportions and give it its present details is no easy task. But
while there is much room for conjecture, there are many facts which appear,
after careful analysis, and which are sufficient, when properly arranged, to
give a connected history, even though it be but a faint outline.

It is necessary to find, in the first place, some initial epoch marking
the beginning of the train of events which have been directly concerned
in the construction of the range, and this is the same epoch which forms
the starting-point probably of the processes which have built all of the
High Plateaus. This is the close of the Upper Green River epoch. The
direct evidence that the Tushar had its birth-throes at this period is not so
clear as in the others, but the cumulative indirect evidence is very strong
and will become apparent as the discussion proceeds. It may be suflicient
to remark just here that this view harmonizes with all known facts and all

observations, and is in conflict with none.

184 GEOLOGY OF THE HIGH PLATEAUS.

The Tushar stands upon the course of the western shore line of the
great Eocene lake. This shore line may be traced, with a very close ap-
proach to exactitude, from the southern base of Nebo across Juab Valley
to the Pavant, and through that range longitudinally as far as the northern
flank of the Tushar. For the whole series of lacustrine beds may be seen
abutting sharply against the disturbed beds of Carboniferous and early
Mesozoic age along this line, excepting where their junction is concealed
for a short distance by the alluvia of the Juab Valley. Through a portion
of its extent this fragment of the coast was rockbound; for in the Pavant,
at least, plicated and contorted Carboniferous rocks still overlook the Ter-
tiary beds, with every indication that this relation has remained unaltered
throughout Tertiary time, though general movements of displacement in-
volving the entire range have otherwise modified its topography. Like all
rockbound coasts it had its sinuosities—here an estuary, there a peninsula;
here a bight, there an outward swing of the shore. This coast line strikes
the Tushar near its northwestern angle and is instantly lost beneath floods
of rhyolite. Nothing is seen of it until nearly 50 miles south-southwest it
is revealed in the Iron Mountains by Tertiary beds cut off against the
Trias. If we suppose a straight line joining the broken ends to represent
the mean position of the coast line, the whole of the Tushar would stand
within the Eocene lake; but this supposition is not tenable. On the east-
ern flank of the range, near Marysvale, and thence southward for 10 miles,
we find the base of it to be composed of metamorphosed quartzites, upon
which a few patches of limestone rest, holding Pentacrinus asteriscus, a
highly characteristic Jurassic fossil, and upon this quartzite and limestone
immediately rest the lavas. No trace of a Tertiary or even Cretaceous
stratified rock is to be seen. The uneven eroded surface of these beds,
with hills and valleys and rocky eminences, was thus sealed up at the very
epoch of which we speak and broken open at an epoch long subsequent
by the shearing of a great fault and by the cutting of ravines, thus reveal-
ing ina manner which cannot be mistaken the existence of a land area.
It lies at least 15 miles to the eastward of the straight line joining the
broken ends of the lake coast. Either, then, we have a peninsula or an
island in the lake to mark the nucleus of the future Tushar. The Tertia-

THE BUILDING OF THE TUSHAR. 185

ries are seen lapping around both the northern and southern extremities
of the range, and it is probable that they are concealed not far from its
eastern base.

Such was the relation of the area to its surroundings when the earliest
eruptions (so far as they have been observed) took place. They broke
forth at first along the course of the present eastern front, a little east of
the main divide as it now stands, and along a line nearly 30 miles in length,
having a general trend north and south. They were not continuous along
this line, but were massed in at least three places: one near the northern end
of the Tushar, one (and this the principal one) near the central part of the
front, and the other near the southern end, but a few miles southeast of it.
The location of this latter center of eruption cannot be fixed at present
with exactitude, and may have been more remote than I was at first led to
suppose. ‘The interval between the southern and middle sources is greater
than that between the middle and northern, and it is not certain that this
second or northern interval was well marked, though the southern interval is
very distinctly so. What other vents existed, or even whether any others
existed at all, it is not now possible to determine, on account of subsequent
accumulations which have buried the surrounding country. This period
of eruptive activity was certainly a long one; for between the outbreaks
erosion went on, leaving traces of its action in the eroded surfaces of its
sheets and in the many small local conglomerates formed out of their decay.
But the accumulation by successive outpours was far more rapid than the
waste, until there came a long period during which these vents were sealed
up and degradation proceeded. At the commencement of this period of
repose the eruptive masses must have been piled up to a great altitude and
covered an extensive area, for the conglomerates which were formed by
their dilapidation are of immense extent and thickness and sufficient in
mass to build a goodly range of mountains. The southern interval was
almost wholly filled up by the fragments washed into it and stratified, and
the conglomerate thus formed stretches far to the southwest, always main-
taining a great thickness. At least 2,000 feet of it oceupy the southern
interval, and it is still many hundreds of feet thick 8 or 10 miles away.

In many respects the relations of the eruptive masses to the country

186 GEOLOGY OF THE HIGH PLATEAUS.

they occupied at the close of the earliest volcanic period presents a very
strong analogy to those of Central France, as described by Sir G. Poulett
Scrope in his work upon that region.* In point of magnitude the earliest
eruptions of the Tushar were probably comparable to those of the Cantal,
covering perhaps a larger area but with a greater thickness.

After a long period of comparative quiet, during which the greater
portion of the mass of these earlier eruptions was broken up by erosion
and scattered over the adjoining lowlands and intervening valleys, came
the second period of eruption, upon a scale grander than the first. The
foci of activity were in close proximity to those of the first period. The
outpours at the northern portion still remain in great bulk and are chiefly
rhyolitic. But the grandest floods of all are in the center of the range,
where they are laid open by several deep gorges, the largest of which is
Bullion Canon. The course of the streams was here to the westward
chiefly, where they widened out and grew thin as they receded from their
origin. The total thickness remaining of these rhyolitic masses probably
exceeds 2,000 feet, and there is good evidence that a considerable amount
has been lost by erosion. What floods may be hidden beneath the floor of
the Sevier Valley at the eastern base it is impossible to say or even to con-
jecture. Thus for the second time the Tushar was built up by extravasated
materials and to an altitude greater probably than at first.

A second period of comparative calm now followed, during which
erosion was at work cutting deep gorges, carving out pediments, and leay-
ing a rugged series of peaks and domes along the eastern flank. But
another agency in mountain structure also intervened. This was an exten-
sive vertical movement of the whole mass. At what precise epoch the
faults which now separate it from the platform of the Sevier Valley were
started it is impossible to say with precision. It is clear, however, that the
commencement of the displacement was subsequent to the deposition of
the great conglomerates which were formed by the destruction of the older
Tushar, and it is almost certain that the displacements had not attained any
great magnitude or a magnitude comparable to the present during the
second eruptive period. The principal part of the uplifting has apparently

*The Geology and Extinct Volcanoes of Central France, by G. Poulett Scrope, 1858.

THE BUILDING OF THE TUSHAR. 187

been accomplished since the close of this second activity, though some of
the movement may, in the absence of evidence to the contrary, be assigned
to this period.

The second period of cessation in the eruptions was broken at a com-
paratively late epoch by a third outbreak at the northern end and at sey-
eral localities on the eastern flank in the vicinity of the faults. To this
third eruptive period belong the whitish rhyolite and the basalts, together
with several masses in the Sevier Valley which have emanated from the
foot of the range, and which will be discussed when we reach in regular
order the description of that valley.

The history of the Tushar, therefore, comprises five tolerably distinct
periods since the commencement of the various activities which have
brought it to its present stage.

1st. An older eruptive epoch, building up an ancient volcanic mass.

2d. A period of decay, in which the mass thus built was nearly leveled
down, and its fragments scattered far and wide and reconstructed in the
form of conglomerates and alluvial beds.

3d. A second eruptive period, more extensive than the first, rebuilding
the dilapidated mass.

4th. A second cessation of eruptions and the introduction and progress
of extensive uplifting and faulting, accompanied by considerable erosion.

5th. A third series of minor outbreaks of much smaller extent than
either of the others, some of which (around the bases of the range) are
very recent.

In this history we perceive the combination of most of the important
forces and agencies of geology: eruption, displacement, erosion, and accu-
mulation; all performing their parts in the general work, and yielding an
intelligible result in the erection of a grand uplift.

CAPAC Eve Lexes

THE MARKAGUNT PLATEAU.

Description of its general features and relations.—Dog Valley.—One of the principal eruptive centers of
trachytic masses.—Characters of the lavas.—Basaltic eruptions and conglomerates.—Bear Val-
ley.—Little Creek Peak and Bear Peak.—Tufaceous beds.—Overlying lavas.—Degradation of the
plateau.—View from the summit of Little Creek Peak.—Journey over the Markdgunt.—Succes-
sion of eruptions, andesites, trachytes, rhyolites, basalts.—Central group of ancient basaltic
cones.—Their dilapidated condition.—Panquitch Lake.—Exposures of contact between the lavas
and sedimentaries.—Modern basaltic outpours.—Other basaltic fields.—Relative recency of the
basalts.—Surface changes since the eruptions.—Connection of the Markégunt basalts with those
of more southern regions.—Sedimentary formations of the Western and Southern Markégunt.—
Tufaceous deposits.—Pink Cliff beds.—Correlation of local Tertiaries with those of the Wasatch
Plateau.—The Cretaceous.—Jurassic and Triassic formations.—The Shindrump.—Tho Southern
Cliffs of the Markaégunt.—Outlook to the far southward.

The Markagunt Plateau lies southwest of the Tushar. From the
southern salient of Midget’s Crest a considerable portion of its expanse
may be seen, though the view is not a very good one. In truth there is
nowhere to be obtained a good panoramic overlook of the Markaégunt, for
there is no stand-point sufficiently lofty. The observer on this summit,
standing more than a mile above the neighboring lowlands, will find it diffi-
cult to realize that the most distant verge visible along the southwestern
horizon has an altitude about equal to his own. With the exception of
two respectable masses shooting up in the middle-ground of the picture,
there are no peaks nor strongly individualized summits; nothing, in fact,
to suggest mountains. It is a broad expanse of rolling hills and ridges,
rarely exceeding 600 feet in altitude. The whole platform has a slight dip
to the eastward; being, however, not an inclined plane, but dish-shaped.
The eastern base of the plateau lies at the foot of the southern Sevier
Plateau, being the thrown side of the great Sevier fault. From this line it
rises by a very slow ascent, not exceeding 24°, westward to its summit.
The character of the gradients will be understood by a reference to the
stereogram. (Atlas sheet, No.5.) The general relations of this plateau

~

183

DOG VALLEY. 189

to the country at large may be comprised in the statement that it is an
excellent illustration of what Powell has called the Kaibab structure.
The length from north to south cannot be definitely given until we can fix
its northern boundary, which, if done at all, must be done arbitrarily, for
it fades out so gradually that no real demarkation exists. The same may
be said of its eastern boundary. But assuming the plateau to extend
northward to the base of the Tushar and eastward to the Sevier Plateau,
the length would be about 50 miles and the breadth about 28 miles.

The greater part of this area is covered with ancient eruptions resting
upon Tertiary lacustrine beds. Around the southern and western sides of
the plateau the sedimentary strata project several miles beyond the volcanic
sheets and end abruptly in giant cliffs, facing the south and west, and
deeply scored by erosion. The western wall of the plateau is formed by
the northward prolongation of the Hurricane fault, while the southern wall
consists of cliffs of erosion without any known dislocation of great magni-
tude. These southern cliffs are the lingering remnants of Tertiary and
Cretaceous beds, which once extended over the entire region to the south-
ward beyond the Colorado, but have throughout Tertiary time receded
by waste to their present boundary.

The detailed description will begin at the northern portion. At the
foot of the lofty summits which crown the southern end of the Tushar
lies Dog Valley, inclosed south and west by rolling and somewhat rugged
voleanic hills and by remnants of a great volcanic conglomerate. Similar
hills are found to the eastward, and the whole tract is a center or focus of
eruptions of the trachytic epoch. The cones and craters which may once
have existed are no longer visible, having been wasted to a medley of hills
by a period of decay which stretches far back towards middle Tertiary
time. Soil and gravel, with a rich growth of wild grass and shrubbery,
now mantle these degraded remnants, giving them a rather pleasant and
gentle aspect. Yet the outcrops of volcanic sheets around the borders and
away from the valley betray its history in spite of the effort of nature to
hide it. East, west, and south the old floods are seen to radiate away for
many miles from this center, spreading out and growing thinner as they
were poured along over the ancient inequalities of the land. They also

190 GEOLOGY OF THE HIGH PLATEAUS.

flowed northward in great volume, but since their eruption the eastern
Tushar fault, swinging westwardly, has uplifted full 3,000 feet the extension
of the sheets in that direction. The lavas which flowed eastward are all
trachytic, but represent two groups of trachytic rock, one being highly
hornblendic, the other being almost pure feldspar and granitoid in appear-
ance, with a very few small but well-defined crystals of biotite. The horn-
blendic variety is exhibited in much greater quantity than the other, is very
coarse-grained in texture, and lies in masses of great thickness. In sey-
eral places single floods are seen between 300 and 400 feet thick, as if
erupted in a highly viscous state, and appearing to have moved with great
slowness and much internal resistance. This appearance is not only com-
mon, but is highly characteristic of the most typical trachytes, and gives
rise to the exceeding coarseness and roughness which the etymology of the
name implies.

Upon the western side of Dog Valley many masses of coarse dolerites
and some basalts are found. Being among the latest outbreaks of the
locality, they have suffered most from erosion, and their débris are widely
distributed in the form of conglomerates over the surrounding regions.
These conglomerates are well stratified, and when the exposures are viewed
at a distance great enough to render the rocky fragments no longer dis-
tinguishable, they reveal a lamination quite as conspicuous as a succession
of sedimentary strata. These conglomerates lie in the heaviest masses in
the northwestern portion of the valley, and turn up against the southern
end of the Tushar at an angle of 22°, showing a thickness exceeding 1,500
feet, without exposing its entire extent. No individual mass of conglomerate
has been observed to extend over any large area, but they seem rather to
have filled up depressions. They increase and diminish rapidly in thick-
ness, and obviously represent many local accumulations, which are not
continuous among themselves. This arrangement is to be expected upon
the theory that their origin is alluvial, a theory which (if it needs any
special support) will appear to be abundantly sustained when we come to
the examination of their formation at the present time in the larger valleys
of the district.

The elevation of this valley above that of the Sevier on the east is

BEAR VALLEY. 191

about 1,400 feet. It cannot be regarded as a part of the Markégunt, but
occupies an intermediate position between that plateau and the Tushar. It
is interesting chiefly as being the locality from which emanated a large
portion of the lavas of the trachytic eruptive epoch. Probably it was the
scene of eruptions of the first epoch also, though the lavas which it may
have there poured forth are deeply buried beneath the great extravasated
masses of the second period, and are revealed only in the fragments of
andesite which are seen in the older conglomerates and by the lower beds
at the base of the Tushar, which are brought up to daylight by the fault at
its base.

Crossing the southern rim of Dog Valley we descend into another
valley of a little lower altitude, called Bear Valley. The divide between
the two consists of a low range of hills, which are the degraded remnants
of old volcanic piles which were once, no doubt, of imposing magnitude,
giving vent to the huge sheets of lava which diverge from them, but are
now reduced to mere hills and discrete masses of dolerite and basalt.
Reaching the bottom of Bear Valley, we find a smooth, park-like inclosure
of ample dimensions, with high hills of trachyte on the east and the bril-
liant rosy red of the Kocene (Bitter Creek) on the west. It has already
been stated that the Markagunt has a fringe or border of sedimentary rocks
upon its western and southern sides, and this border is from 2 to 6
miles in width. In other words, the voleanic beds which cover its central
and eastern portions do not extend to the western and southern margins of
the uplift. Bear Valley lies at the foot of a broken crest which is formed
by the sudden termination of these eruptive masses. This boundary is a
very irregular one, having westward projections and eastward recesses.
But it is necessary to keep in mind one important relation. The vents stood
near this western margin. The main flow of the erupted materials was
towards the east, in which direction they extended probably as far as the
Sevier Plateau, or until they are lost beneath more recent sub-aérial accu-
mulations. ‘Towards the west their progress was arrested by the rising
slope of the country, and they do not appear to have extended more than a
very few miles in that direction. Then, as now, the face of the country
sloped downward from west to east, though the gradient was considerably

192 GEOLOGY OF THE HIGH PLATEAUS.

smaller than at present. A few large eruptions, however, reach out west-
ward, producing the sinuous course of the boundary which marks their
termination. One of these westerly projecting masses separates Bear Val-
ley into two portions, connected by a narrow gorge cut through it by
erosion. :

Overlooking Upper Bear Valley from the eastward stand two con-
spicuous mountain masses called Bear Peak and Little Creek Peak, of
which the respective elevations are 9,870 and 10,040 feet. Although
of moderate altitudes, they present, in consequence of their isolation, a
very commanding appearance and attract the attention from every point
of view in the surrounding country. They are also interesting on account
of their structure and the masses which constitute their bulk. The beds
which lie at their foundations merit some description.

Wherever we examine the contact of the volcanics with the sediment-
ary beds along the western verge of the eruptive rocks of the Markagunt,
we usually find a series of strata composed of finely comminuted volcanic
materials. Sometimes it is a fine sandstone; sometimes an argillaceous
rock with minute fragments of feldspar and mica; sometimes a calcareous
or marly deposit. Often rolled and rounded fragments of notable size are
included, and the beds have then a coarse or gravelly texture, the grains
being fragments cf some eruptive mass so much decomposed that it is dif-
ficult to determine its exact variety. These beds are always well stratified
and have clearly been deposited by water, and do not differ from ordinary
sedimentary beds, except in the fact that the materials which make up their
mass have been derived from eruptive rocks. The individual beds are
usually of small superficial extent and small thickness, and are often
seen running out with “feather-edges.”. They always overlie the system-
atic lacustrine Tertiaries of early Eocene age. Similar formations are
found at the northern and southern extremities of the Sevier Plateau and
in the East Fork Canon, where they have been more or less metamor-
phosed. They are exhibited on the west side of Bear Valley and again
along the base of the great trachytic wall of the Markdégunt in considera-
ble variety. Wherever found they seem to constitute a group by them-
selves of more recent age than the uppermost Tertiaries of the Wasatch

BEAR VALLEY. 193

Plateau and Lower Sevier Valley. As these last-mentioned formations
have been inferred provisionally to be of Green River age, the beds of
voleanic sand, &c., may form an upward continuation of the same group,
or may even be considerably more recent, though many circumstances
seem to indicate that they were deposited in immediate succession to the
definite Green River beds without any protracted interval to separate them.
Their significance is purely local. They indicate that the eruptive activity
had commenced and had given vent to large masses of lava before the
extravasation of the older volcanic masses now remaining, and that these
most ancient ejections had been wasted and either utterly swept away or
buried where they have not up to the present time been laid bare. These
beds are seen in considerable mass on both sides of Upper Bear Valley,
and on the southeast side they constitute the lower courses of the two
mountains which tower above it and the long curtain wall which connects
them. Resting upon them is a sheet of lava of very interesting character.
It is identical in constitution with a sheet exposed in Kast Fork Canon, and
which will be described in detail in the chapter on the Sevier Plateau.
Upon this lava rests a layer of coarse rhyolite, which is evidently much
more recent in age, and forms the summit wall of the west side of Upper
Bear Valley. This layer is not seen on the eastern side, but in place of it
numerous trachytic beds are found alternating with conglomerate.

At the bases of the two mountains these same beds of volcanic sand
are seen and the succession of trachytes and conglomerates. The upper
masses of the mountains are mostly trachytic, though between the flows.
there is one prominent conglomeritic mass. The stratification is remarka-
bly even throughout, considering the volcanic nature of the components,
but it is not horizontal. In both mountains there is an east or east-south-
east dip, and they present the general aspect of great buttes left by the
denudation of the surrounding country, though the similitude is not exact.
A portion of their eminence, however, is due to a fault of about 800 feet
displacement which runs along their western bases, and the remainder of
their relative altitude is probably. due to the denudation of the general
platform to the east of them and to the dip of the beds. ‘These eruptions

are all very ancient (Miocene?), and since their extravasation they have
1G} dat 3p

194 GEOLOGY OF THE HIGH PLATEAUS.

been uninterruptedly exposed to erosion, and it is by no means surprising
that the average degradation should have been many hundreds or more
than a thousand feet. There is no evidence that they are old cones piled
up of eruptive matter around local vents, but are unmistakably carved out
of a mass of interstratified lava sheets and bedded fragments, like great
cameos, and their altitudes notably augmented by local uplifting.

The summit of Little Creek Peak gives a fine view of the surround-
ing country, though the altitude is insufficient to command the great expanse
of the Markégunt to the southward, which is higher than the peak itself.
But north and east the prospect is excellent. As soon as the firs and
spruces are cleared the Tushar is in full view to the northward, the grand
pyramids of Belknap and Baldy stand out in splendid relief against the
horizon, and the inclined plateau, whose summit they crown, is seen in
detail. It may be recalled that this plateau slopes to the west, while the
Markdégunt slopes to the east. The Hurricane fault bounds the western
front of the Markagunt, while the Tushar has a great fault upon its east-
ern front. The two plateaus gradually merge into each other through the
intervening area of Dog Valley. The shitting of the displacement from
the west front of the Markigunt to the east side of the Tushar is an inter-
esting structural feature and worthy of a careful study, for it is often
repeated in the Basin ranges, and constitutes one of the most important
modifications of that type of structure. We may for present purposes
regard the Tushar and Markdgunt as a single block, of which the length is
nearly 80 miles and the width a little more than 20. The southern por-
tion is tilted eastward (Markdgunt) and the northern portion is tilted
westward, while the intervening or middle part is warped and otherwise
flexed. Now if this great block were a simple warped surface, the middle
portion would be synclinal. In reality it is an anticlinal area. An anti-
clinal axis leaves the Hurricane fault at a very acute angle, and crosses the
block obliquely to the commencement of the Tushar fault. These structu-
ral features may be discerned distinctly from the summit of Little Creek
Peak.

Looking westward from the same point we behold in the foreground
a scene eminently characteristic of the western border of the Markagunt.

JOURNEY OVER THE MARKAGUNT. 195

It is a valley of erosion carved into the plateau by a plexus of streams.
The proportions are grand, and the abrupt slopes which wall it about on
every side are very impressive. It is a vast Coliseum, opening to the west-
ward by a deep and narrow canon leading to the floor of the Great Basin
near Parowan. The walls west, south, and north are all Tertiary (Bitter
Creek) and luminous with colors, which are all the more conspicuous from
contrast with the dark trachytic beds which overlook them from the east-
ern side. Several great valleys of similar aspect and excavated in the same
manner occur elsewhere in the sedimentary belt which borders the western
portion of the Markagunt. The plateau is there yielding slowly to the
destroying agents, and the continuance of the process through indefinite
time will at last destroy its eminence. It taxes the credulity to think that
this work has been gradually accomplished by the feeble action now in prog-
ress; but the results here witnessed sink into insignificance when compared
with those which are forced upon the conviction when we look upon the
regions drained by the Colorado.

Eastward from the foot of the mountain the plateau slopes almost
insensibly to the base of the Sevier Plateau, which rises against the eastern
sky. The country is rough with hills and rocky valleys, though these ine-
qualities upon so vast an expanse as the back of the Markégunt are as mere
ripples or waves upon the bosom of a great lake. In this direction none
but old volcanic rocks and conglomerates are visible. To the southward
the view is not extensive. The plateau slowly increases in altitude in that
direction until it becomes more lofty than the peak. So much of it as is
visible presents a pleasant but rather monotonous appearance, with rolling
hills and ridges, grassy slopes and scattered groves of pines.

A journey over this broad surface is a pleasure excursion, but not
remarkably instructive to the geologist. The explorer will enjoy the lus-
cious camps beneath the shade of century-old pines, beside sparkling streams
of the purest water, and will see with pleasure the keen relish with which
the animals devour the luxuriant wild grass. Nature is here in her gentle
mood, neither wild nor inanimate, neither grand nor trivial, but genial, tem-
perate, and mildly suggestive. A few cations which it is a pleasure to cross;
long grassy slopes which seem to ask to be climbed; hill tops giving charm-

196 GEOLOGY OF THE HIGH PLATEAUS.

ing pictures of shaded dells and sloping banks, with distant views of the
Tushar and the mighty wall of the Sevier Plateau, combine to produce a
medley of pleasant scenes and experiences which will always be looked
back to with refreshment. As a field of geological study it is in great part
meager. Now and then a bit of local curiosity is excited by a curious
result of rain sculpture, by remains of small lake deposits, by the curious
weathering of rocks, by some strange freak of the old lava flows, none
of which will find places here. Broad facts are comparatively few.

Among the most noteworthy is the succession of eruptions. In the
central part of the Markagunt the oldest eruptions observed were andesitic.
These are displayed in a disconnected way in the deeper ravines of the cen-
tral and northern portions, but are elsewhere so masked by subsequent
floods that their extent and the circumstances of their extravasation are not
fully intelligible. Whether they were generally distributed over the face
of the plateau or represent a number of local eruptions it is not possible to
say with certainty. Wherever deep canons are found in the central part of
the area they lay open great masses of dark andesitic lava, and areas are
occasionally found where surface erosion has removed the later rocks and
laid the andesite bare. In any event, whether generally or discontinu-
ously distributed, the mass of this rock is very great. No propylitic erup-
tions have been observed in the Markdgunt.

Next in order are found great masses of trachyte. Over the greater por-
tion of the expanse of the Markigunt these are the surface rocks. In reality
their volume may not exceed that of the andesites, which they usually cover,
but being more frequently seen they appear to be the dominant rock, and
I incline to the opinion that they are so. On the whole, the varieties of
trachyte are less numerous in the Markagunt than in the more eastern pla-
teaus of the district; but their number is still very great. The least com-
mon variety is the hornblendic; but the augitic trachytes are abundant,
and the commonest of all is a highly porphyritic argilloid variety. The
latter consists of a reddish or purplish fine base, resembling a rather rough
argillite, holding crystals of white opaque orthoclase. One of its most per-
sistent characteristics is its fracture, which is very peculiar. Most voleanic

rocks, when broken, present a tolerably even or gently rounded though

Heuioryere IIT

Heliotype Printing Co.; 220 Devonshire St., Boston.

TUFA. MARKAGUNT PLATEAU.

SUCCESSION OF ERUPTIONS IN THE MARKAGUNT. JES)?

rough surface ; but this trachyte breaks with an exceedingly jagged, angu-
lar, and irregular fracture, so that it is impossible to hammer out a neat and
shapely specimen. The grandest masses of trachyte, not only in the Marké-
gunt but in the other plateaus, consist of this variety. It lies in immense
beds, often two or three hundred feet in thickness, spreading out over many
square miles with remarkable regularity and homogeneity. In the Marka-
eunt it forms mesa-like platforms, ending in low precipices, where the shal-
low canons and ravines have cut into it. It breaks up or rather crumbles
with unusual facility for an eruptive rock, producing a coarse gravel, which
floors the ravines below. This rock is so distinct in its characters that it
seems almost to justify a separate name, but I shall content myself with a
purely descriptive designation, and call it argilloid trachyte.

The augitic varieties of trachyte are found in sheets, which are usually
much thinner and cover smaller areas, though the number of them is much
ereater. The total bulk is less than that of the argilloid variety, though
absolutely it is very great.

The rhyolites are the third group of eruptives found in the Markdgunt.
They are seen in large masses along the very highest part of the plateau,
from the crest of which they poured out in massive sheets. They are
probably as ancient as the older liparitic masses of the Tushar, but always
overlie the trachytes whenever they are in contact with them. They belong
altogether to the liparitic sub-group, with an abundance of porphyritic crys-
tals of feldspar and quartz. None of those hyaline fluent rhyolites which
characterize the northern Tushar are seen here. Although their volume is
very great, it is far less than that of the trachytes, and the areas which they
cover are much smaller.

The fourth group is the basaltic. Among the High Plateaus the Mar-
kaégunt and Tushar alone present extensive outpours of rocks of this class.
A few small eruptions are found in the eastern plateaus and notably in the
intervening valleys, but they are not comparable in extent to those of the
Markagunt. Here they are confined to the southern half of the plateau.
A little south of the center is a large tract in which are still preserved
remnants of a considerable number of basaltic craters, though so much
degraded that they are not immediately recognized. ‘They form a large

198 GEOLOGY OF THE HIGH PLATEAUS.

cluster of rolling hills, rarely exceeding 300 feet in altitude above the
platform on which they stand, covered with soil mingled with decayed
vesicular cinders. ‘Their true nature is disclosed by the scoriaceous char-
acter of the fragments which constitute the greater portion of their mass.
It will be remembered that basaltic craters, when well preserved, are rather’
symmetrical truncated cones, with conical or funnel-shaped depressions at
the summit, and the entire mass is composed of vesicular fragments blown
out by the escaping steam and gases and falling with approximate uniformity
around the orifice. The spongy character of these fragments renders them
an easy prey to the chemical forces of the atmosphere, and they are readily
decomposed. After thousands of years of weathering these cones are
literally dissolved, losing their lime, iron, and alkali, while the alumina and
silica remain, and the cone gradually loses its form and is reduced to a
shapeless heap of soil with commingled cinders in every stage of decay.
Around the bases of these ancient cones we find half-revealed sheets of
basaltic lava. Any eruption may be followed by the building of a cinder-
cone, and most basaltic outbreaks are so supplemented (at least in this dis-
trict); but it isnot always so. A considerable number of the basaltic sheets
have been disgorged where no trace of a cone remains, and some of these
are so recent that the last thousand years may have witnessed the catas-
trophes.* It is notable that the most extensive outpours are most frequently
without them. Among the basalts of the locality of which we are speaking
are many cinder-cones in an advanced stage of decay. The floods of basalt
which have emanated from them lie in many sheets, none of which indi-
vidually present great thickness, but by superposition have built up this
part of the plateau from 500 to 800 feet above the normal platform. They
are for the most part concealed by their own ruins, but numerous ravines
have been cut into them, showing in many places their edges and giying a
general idea of their mass and distribution. They rest upon older trachytes
and occasionally andesites which had been scored by ravines’ before the
basaltic outbreaks, and in a number of places the uneven surfaces of contact

are clearly revealed.

*Tam speaking in general terms of the basalts. Those of the locality just spoken of are all
probably older than the Quaternary.

PANQUITCH LAKE—MODERN BASALT. 199

A few miles southeast of this basaltic field is a picturesque lakelet,
occupying a depression in the plateau, called the Panquitch Lake—a sheet
of water about a mile and a half in length and a mile in width It is a
delightful locality, both for the tourist and the geologist. Around it stand
forests of pine (P. ponderosa), while farther up the slopes of the plateau are
thickets of spruce and aspen. Broad and stately ravines, bearing sparkling
streams from the higher levels open near its margin, and the traveler, weary
of the desert wastes below, revels in the rank vegetation which clothes their
rocky slopes. Through the brief summer the longest and richest grass
carpets their floors and every knoll and sloping bank is a parterre of the
gayest flowers.

Around this lake the voleanic strata are seen resting upon the sedi-
mentaries; in short, it is a locality where the eruptive rocks have diminished
in thickness, and they gradually disappear southward and southeastward.
To the west and southwest they continue still in immense bulk, with greater
variety and stronger contrasts than in the northern part of the plateau.
Here the oldest eruptives are trachytic. They are finely displayed upon
the northern side of the lake, where they form low cliffs or steep slopes, and
an abrupt canon entering from the northwest still more clearly lays them
open to view. As we approach the lake from the northeast (the usual
route), the instant we reach the summit of the hill from which we first see
the expanse of its surface, a most conspicuous object upon the south side of
the lake immediately attracts the attention. It is a flood of basalt so recent
and so fresh in its aspect that we wonder why there is no record or tradi-
tion of its eruption. It is dense black, and its ominous shade is rendered
still more conspicuous by the lively colors of the sedimentary rocks and
soil around it. We see at first only the end of a grand coulée, but beyond
it rise rough, angry knolls and mountainous waves as black as midnight,
telling of more beyond. Riding to the base of it, we find it to be com-
posed of numberless fragments, ranging in size from a cubic foot to many
cubic yards, piled up in strange confusion. A continuous bed or sheet is
nowhere to be seen; nothing but this coarse rubble, looking like an exag-
gerated pile of anthracite dumped from the cars at the terminus of a great

coal railway. A close inspection confirms this impression of recency
Wi | i

200 GEOLOGY OF THE HIGH PLATEAUS.

given by the first view. The surfaces of the fragments are not affected
by weathering to any notable extent, and it is only by comparison with
surfaces fractured by the hammer that we can find an assurance of
an exceedingly slight impairment of its original freshness. No doubt
this is largely due to the fact that this portion of the mass is not in the
slightest degree vesicular. In other parts of the coulée highly vesicular
fragments were encountered; but where I first approached it every stone
was as compact as a dike. But even the vesicular specimens show so
little weathering, that it is hard to believe that this eruption is as old as
the discovery of America. Such appearances, however, may be very de-
ceptive. Iam not aware that there is any authentic record of a volcanic
eruption within the present limits of the United States, though it is quite
possible that a number of them have occurred since the conquest of Mexico
by Cortez. In this region it may have easily escaped the chronicles of the
Spanish priests, even if such a dire event had occurred only a hundred
years ago, and two hundred years would have destroyed all reliable tradi-
tion of it among the Indians.* This basalt came from a vent situated about
3 miles southwest of Panquitch Lake, and from the same source flowed a
considerable number of large streams all presenting the same appearance
of recency. An attempt was made to reach the crater, but the climbing
over the rough angular blocks piled up in the worst conceivable confusion
proved to be so perilous, that after several misadventures it was abandoned.
From surrounding eminences several overlooks were obtained, from which
it was inferred that there are several vents clustered near each other, and
from three of them at least there have been a number of eruptions. Noth-
ing like a cinder-cone, however, was distinguishable. The lavas appear to
have reached the surface and overflowed like water from a spring, spread-
ing out immediately and deluging a broad surface around the orifice, and
sending off into surrounding valleys and ravines deep rivers of molten rock.
One flood rolled northeast towards Panquitch Lake, but came to rest before
reaching it. A second flowed eastward down a broad ravine situated about
3 miles from the lake. The largest streams went to the southeast into
; * There is said to be Sinmlinon among the Mohave Indians that their ancestors were driven out

of Central Arizona by volcanic eruptions, and though very recent basalts are found there, many cir-
cumstances combine to oppose such a tradition even if there be one.

BASALT FIELDS OF THE MARKAGUNT. 201

the tributary ravines of Mammoth Creek (the main fork of the Sevier
River), and reach a point about 6 miles from their origin.

Besides this field of very recent basalt, remains of much more ancient
basalt are found in the vicinity and in much larger amount. In truth, the
basaltic eruptions go back to a period sufficiently remote to have permitted
important changes in the configuration of the country to take place in the
interval separating the present from the earliest eruptions of this class.
During that interval a considerable number of outbreaks, separated by
many centuries (probably hundreds of centuries), have occurred. Basalt
fields of different ages are readily distinguished. Among the oldest, proba-
bly, are the first basalts spoken of in this chapter. Of an antiquity which
may be quite as great are two large masses, lying respectively southeast
and southwest of Panquitch Lake. The southwest field is much eroded,
and consists of a tabular mountainous mass immediately overlooking the
very recent basalt field just spoken of. 'The edges of the sheets composing
this tabular mass project in bold cliffs around its flat summit in the same
manner as is frequently seen in lower regions, where buttes of sedimentary
rocks owe their origin and preservation to a protecting mantle of lava. On
all sides it is girt about by a talus of blocks, which have fallen by the sap-
ping of the foundations of the mass through untold ages. Since this lava
was disgorged broad valleys and deep ravines have been scored in the plat-
form of the Markdégunt, and the minor details of topography arising from
the general process of surface sculpture have been carved out, and an
older topography has been swept away or so completely remodeled that
it cannot now be reconstructed.

Southeast of the lake a wide expanse of country has been covered
with ancient basalt, but only remnants are now left, covering mesas and
buttes of sedimentary rocks and overlying fields of still older trachytes and
volcanic conglomerates. Ravines of considerable magnitude and broad
valleys have been cut into the country which they once covered, and these
excavations have in several instances given passage to more recent floods
of basalt, some of which extend as far east as the Sevier River. These
later basalt fields are in an excellent state of preservation, but soil has
accumulated upon them, and the face of the rocks shows deep weathering.

202 : GEOLOGY OF THE HIGH PLATEAUS.

The different stages of the decay are readily discerned, and it is easy to see
that the various basaltic eruptions, though they may, in a certain geological
sense, be considered as belonging to one epoch, and that a very recent one,
have occurred at intervals which, measured by a historical standard of
time, have been very long. The lithological characters also vary to some
extent; the more ancient floods being less heavily charged with magnitite,
and on the whole less basic and a little lighter-colored, also less finely tex-
tured, than the most recent ones, and of a little lower specific gravity.

Finally, the largest basalt field of all and, with the exception of that
one nearest to Panquitch Lake the most recent, is found near the south-
west. margin of the plateau, covering about 25 square miles, with a con-
siderable number of cones, from which a large number of eruptions have
issued. This field I have had no opportunity to examine in detail, and it
is not easily accessible on account of the exceedingly rough character of
its surface. Much of it is clothed with dense forests of spruce, which alone
render it almost impenetrable, and prevent the observer from obtaining a
satisfactory view of it. Its mean altitude is more than 10,000 feet.

The basaltic eruptions of the Markagunt are a portion of a belt of
such eruptions, which extends along the course of the Hurricane fault and
the country adjacent to it far southward across the Colorado River into
Arizona. Eruptive rocks older than basalt within this belt are very few
and of small magnitude. The volume and number of basaltic eruptions
increase as we proceed southward, and reach a great development near the
Grand Canon, where more than a thousand square miles are covered with
it and more than a hundred cones are still standing. South of the Colorado
many large basalt fields are known to exist, but they have not been thor-
oughly studied. Throughout the Hurricane belt they occur in patches,
often small, but frequently extensive. It is a notable fact that by far the
ereater portion of them occur upon the uplifted side of this great displace-
ment; indeed those upon the thrown side are comparatively trivial. This
fact seems to be generally true throughout the District of the High Pla-
teaus and also throughout the country to the south of it. It is, moreover,
ests the possibility of a correlation

oD

so strongly emphasized, that it suge

between these basaltic eruptions and the greater upward displacements.

SEDIMENTARY BEDS—TUFAS. 203

On the other hand, an equally striking fact is the apparent independence of
basaltic eruptions of the minor or local inequalities of a country. They
have broken out, with seeming indifference, upon hill-tops and slopes, in
valley bottoms, upon the brinks of great cliffs of erosion, upon buttes, and
upon broad mesas. ‘The only localities where I have not seen them are in
canons and at the bases of cliffs of erosion.*

SEDIMENTARY FORMATIONS OF THE MARKAGUNT.

Around the western and southern borders of the MarkAégunt extends a
broad belt of sedimentary formations almost wholly unencumbered with
volcanic emanations. The volcanic cap ends always abruptly upon the
highest part of the plateau several miles from the plateau limits, and usually
presents to the westward a line of cliffs looking down into the great valleys
and amphitheaters where the ravines and canons of the sedimentary belt
begin. The destroying agents have wrought terrible havoc in the strata,
cutting chasms which have laid bare in grand sections the series of sedi-
mentary strata from the Eocene to the base of the Trias inclusive.

The most recent deposits are those local accumulations first encoun-
tered in Bear Valley, consisting of the sands and marls derived from the
decay of volcanic rocks. We seldom miss them from their proper place at
the base of the voleanic cap, and they attain considerable thickness (200 to
350 feet) in numerous exposures along the western margin of the trachyte.
From what rocks they were derived it is impossible to say; no lavas older
than themselves have been detected. They rest everywhere upon the
Kocene limestones, frequently shading downwards into sandstones undis-

* Perhaps I ought to qualify this assertion of seeming indifference to minor topographical features
by saying that basaltic vents occur very often upon the brink of cliffs of erosion, and never (within my
own observation) at the base of one; often upon the top of the wall of a cation and never within the
canon itself, though the stream of lava often runs into the canton. So numerous, indeed, are the in-
stances of cones upon the verge of a cliff of erosion or caiion-wall, that I was at one time led to suspect
that it was a favorite locality. This is very conspicuous in the large basaltic field near the Grand Canon
in the vicinity of Mount Trumbull, where 10 large cones stand upon the very brink of the great abyss
and have sent their lavas down into it. Away from the cafon a considerable number of craters are
seen upon the various cliffs near the Hurricane Ledge, and far to the northeastward half a dozen are
found upon the crests of the White Cliffs. Out of rather more than 300 basaltic cones of this region, I
have noted 33, or nearly 11 per cent., occupying such positions. Whether this is accidental it is diffi-
cult to say, but when it is remembered that they do not occur at the bases of such cliffs, nor in the
canons (so far as I have observed), the fact is certainly aremarkable one. In our present ignorance
concerning the nature of the forces and chain of causation which lead up to and precipitate voleanic
phenomena, it would be vain to speculate upon the reasons for this apparent preference of locality.

204 GEOLOGY OF THE HIGH PLATEAUS.

tinguishable in composition and texture from ordinary sediments derived
from ordinary materials. Nor is their exact age assignable, since they have
yielded no fossils, but the probabilities are great that they are not far from
middle Eocene age.

Beneath them lies what is called the Pink Cliff series, which is known
to be Lower Eocene.* At the base brackish-water fossils are found, which
give place as we ascend to a fresh-water fauna. The upper members are —
limestones, which are usually more or less siliceous, and the silica in-
creases in the lower members, where gravelly beds, layers of sandstone,
and even conglomerate are found. The highly calcareous members strongly
predominate. The coloring is always striking and vies in brilliancy with
the Triassic beds. The highest member is frequently almost snow-white,
with a band of strong orange-yellow beneath it. But the great mass of
color is a pale rosy-pink. When the sun is low and sends his nearly
level beams of reddish light against the towering fronts and mazes of
buttresses, alcoves, and pinnacles, they seem to glow with a rare color,
intensely rich and beautiful—flesh-of-watermelon color is the nearest hue I
can suggest. Some of the beds do not naturally possess this color, but
have been painted superficially by the wash from the beds above them, or
possibly have taken on the color through exposure, while they are yellow
within.

The identity of these beds with the Bitter Creek of the Wasatch Pla-
teau and of the Uintas seems clear. The connection by actual continuity is,
indeed, wanting, but the fossils, though few, are convincing, and the rela-
tions to the Cretaceous beneath are strictly homologous to those which pre-
vail farther north. Some doubt arises whether the white limestone which
caps the series should be referred to the Bitter Creek or to the Green River
beds. Mr. Howell, whose opinions are of great weight, inclined to the lat-
ter view, and thought that one of the members of the Wasatch Plateau
(No. 2), which I have referred to the Lower Green River period, was want-
ing, and that the white limestone should be correlated to those beds which
I have referred doubtfully to the Upper Green River. It is true that two

*I use the term Eocene in its local sense. It may or may not be coéval with the European
Eocene. Probably it is very nearly so.

TERTIARY FORMATIONS. 205

or three species of fresh-water mollusca seem to sustain his view, but the
fresh-water forms of the Plateau Province so frequently have a very great
vertical range, that they are apt to mislead in just such cases, and require
collateral evidence to justify such a conclusion. On the other hand, there
is no indication in the appearance of the rocks of such a break of the con-
tinuity, and the whole of the Tertiary here exposed seems to belong to one
series without unconformity and without any break in the conditions nec-
essary to continuous deposition. It has, therefore, seemed to me unadvis-
able to intercalate a vacant horizon in a series which to all appearances is
continuous.

The white limestone at the summit of the formation is a very con-
spicuous member and forms the surface of the plateau for a considerable
distance south of Panquitch Lake, where it is laid open by ravines and
exposed in buttes capped by basalt. It reaches a thickness of rather more
than 300 feet in some places, but is usually much less. It is very impure;
‘sometimes very siliceous, holding agate or chalcedony, and is also some-
times marly. The total thickness of the Eocene beds is from 1,100 to
1,200 feet.

The epoch of final emergence from the lacustrine condition seems to
have been earlier here in the southwestern part of the Plateau Province
than in the middle or northern portions. This is indicated by the earlier
age of the most recent lacustrine beds; for as we proceed northward later
and later members gradually make their appearance. In the south, not
more than the lower third of the Eocene is present; in the middle district,
barely more than one-half; while around the southern slopes of the
Uintas nearly or perhaps quite the whole of it is revealed. It may be con-
jectured that the Lower Green River beds once existed here and were
eroded and wholly removed before the volcanic eruptions began. This
cannot be wholly disproven, but the view is extremely improbable; for in
the epoch immediately following the final emergence the conditions were
not favorable to a rapid erosion; the region was not at that time an elevated
one; it could scarcely have exceeded a few hundred feet in altitude above
sea level, and there were no important displacements nor dislocations.
The Bitter Creek heds cover many hundred square miles of continuous

206 GEOLOGY OF THE HIGH PLATEAUS.

territory with splendid exposure, and have in many places been thoroughly
protected from destruction since early Miocene time at least, but nowhere
have they been seen to be covered with any more recent sedimentary
formations, excepting the local beds of volcanic sand. It is not probable
that every vestige of such a formation, had it existed, should have been so
completely destroyed, nor that an erosion of such magnitude should have
been withal so uniform as to stop everywhere at the summit of the very
perishable limestone which forms the uppermost member of the Bitter
Creek.

Here, as elsewhere, the volume of Cretaceous beds is very great,
probably attaining more than 4,000 feet. The valleys and gorges which
reveal them descend to the westward, while the rocks dip at varying angles
to the eastward; thus in the course of 5 or 6 miles the water-courses pass
through the entire series. ‘The Cretaceous mass is composed of alternating
sandstones and dark-gray shales, which are usually very heavily bedded,
uniformly stratified, and have strong and persistent lithological characters. |

The subdivision of the Cretaceous rocks and their correlation with
those of the Plateau Province at large I have not attempted; the study of
them has been too superficial and the number of fossils collected is much
too small, while the series itself is enormous and highly variable. It is
evident at once that, though the series as a whole possesses the same general
characteristics as prevail elsewhere, it is very inconstant in details, and
comparatively few of the subordinate members can be strictly correlated
over extended intervals. The great beds of shale are the most striking
members, attaining many hundreds of feet of thickness, with slight inter-
ruptions of arenaceous layers, which hardly mar the uniformity of their
aspect. Coal of good quality is found in workable beds in the lower half
of the series. There is a strong family likeness in all the Cretaceous ex-
posures of the Plateau Province, and their features are as characteristic
of the formation as the peculiarities of the Trias; but the wonderful per-
sistence over great areas which marks the Triassic members cannot be
affirmed of the Cretaceous.

No series of rocks can be more strongly marked by their lithological
characteristics than the Mesozoic formations which here underlie the Creta-

LITHOLOGICAL CHARACTERS OF THE MESOZOIC. 207

ceous. Quite as strongly individualized are the topographical features
which have been sculptured out of them. The great marvels of surface
sculpture found throughout the lower Plateau Province, the grand cliffs
with strange carvings and elaborate ornamentation, the wonderful buttes
and towering domes, the numberless shapes which startle us by their
erotesqueness owe their peculiarities as much to the nature of the rocks
themselves as to the abnormal meteoric conditions under which they were
produced. Each formation has its own fashions—its own school of natural
architecture. The Gray Cliffs, the Vermilion Cliffs, the Shindrump (Lower
Trias)—each has its own topography, and they are as distinctly individu-
alized as the modes of building and ornamentation found among distinct
races of men.

The uppermost member of the Jurassic series is fossiliferous, and has
yielded a fauna which, though not very abundant, is still highly characteristic
and sufficient to fix its age with certainty as Upper Jurassic. Immediately
below it is the Gray Cliff sandstone, so wonderful for its cross-bedding, for
the massiveness and homogeneity of its stratification, and for its persistence
without any notable change of character over great areas. This formation
has been assigned to the Jurassic solely on the ground of its infra-position
to the fossiliferous member just mentioned. The Gray Cliffs have not
yielded a solitary fossil hitherto of any kind. Next below is the Vermilion
Cliff series, characterized by beds of sandstone built up in many layers,
with a tendency towards shaly characters, though seldom or never a true
shale. It is as persistent as the Gray Cliffs above, and in color it contrasts
powerfully with it. The Gray Cliffs are nearly white, and are merely
toned with gray; the Vermilion Cliffs are intensely, gorgeously red. ‘The
latter also is destitute of fossils, except a few obscure fish-scales, though
great search has been made for them. Beneath lies the Shindrump. It
consists of a very remarkable conglomerate above and a series of shales
below. The conglomerate is made up chiefly of fragments of silicified wood,
cemented by a light-colored matrix of sand, lime, and clay, out of which
the woody fragments weather and are scattered over the plains below.
The shales below consist of a succession of layers, each a few feet or a
very few yards in thickness, preserving that thickness with remarkable

208 GEOLOGY OF THE HIGH PLATEAUS.

uniformity over miles of exposure and contrasting with each other by their
varying shades of chocolate, dark red, and purple, producing an effect of
colored bands of small thickness individually but great collectively, and
with a perfect regularity or parallelism. (See Heliotype No. XI.)

The Lower Mesozoic series (Jura and Trias) is found in the MarkAégunt
only in the immediate vicinity of the great Hurricane displacement, which
defines the western boundary of the structure, and is only seen there along
the southern portion of the west flank. I have not visited them, but Mr.
Howell has examined them somewhat cursorily, and the results of his
observations, in the form of notes, are before me. There is a general agree-
ment of the sections he there found with the general section of the Plateau
Country to the eastward, though there are minor differences which might
be worthy of future study. All of the notable Mesozoic groups and beds
are present and seem to be on the whole somewhat thicker than they are
to the eastward, but the thickness is more variable and the deposition
generally more unequal. In close proximity to the great fault, the beds
are in some places flexed abruptly upwards on the uplifted side of the fault,
but in passing eastward they speedily recur to the general east or east-
northeast dip of 1° to 2° which prevails throughout the plateau. Nowhere
in this vicinity does the Carboniferous seem to be exposed, though in
several localities it must be very near the surface in the immediate line of
the fault. Where these upward flexures occur, the plane of denudation
between the summit of the plateau and the fault cuts across the entire
series of Mesozoic and Cenozoic formations more than 10,000 feet in thick-
ness.

From the southwest salient of the Markagunt we behold one of those
sublime spectacles which characterize the loftiest standpoints of the Pla-
teau Province. Even to the mere tourist there are few panoramas so
broad and grand; but to the geologist there comes with all the visible
grandeur a deep significance. The radius of vision is from 80 to 100
miles. We stand upon the great cliff of Tertiary beds which meanders to
the eastward till lost in the distance, sculptured into strange and even
startling forms, and lit up with colors so rich and glowing that they awaken

enthusiasm in the most apathetic. To the southward the profile of the

‘ANOLSGNVS oIssvunr daadaaq ssoUg

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‘nojsog “75 at1ysuonag o¢%

‘XT AdALOMIATT

VIEW TO THE SOUTHWARD. 209

country drops down by a succession of terraces formed by lower and lower
formations which come to the daylight as those which overlie them are suc-
cessively terminated in lines of cliffs, each formation rising gently to
the southward to recover a portion of the lost altitude until it is cut
off by its own escarpment. Thirty miles away the last descent falls
upon the Carboniferous, which slowly rises with an unbroken slope to the
brink of the Grand Canton. But the great abyss is not discernible, for the
curvature of the earth hides it from sight. Standing among evergreens,
knee-deep in succulent grass and a wealth of Alpine blossoms, fanned by
chill, moist breezes, we look over terraces decked with towers and tem-
ples and gashed with canons to the desert which stretches away beyond
the southern horizon, blank, lifeless, and glowing with torrid heat. To the
southwestward the Basin Ranges toss up their angry waves in characteristic
confusion, sierra behind sierra, till the hazy distance hides them as with a
vail. Due south Mount Trumbull is well in view, with its throng of black
basaltic cones looking down into the Grand Canon. 'To the southeast the
Kaibab rears its noble palisade and smooth crest line, stretching southward
until it dips below the horizon more than a hundred miles away. In the
terraces which occupy the middle ground and foreground of the picture
we recognize the characteristic work of erosion, Numberless masses of rock,
carved in the strangest fashion out of the Jurassic and Triassic strata, start
up from the terraced platforms. The great cliffs—perhaps the grandest of

all the features in this region of grandeur—are turned away from us, and
only now and then are seen in profile in the flank of some salient. Among
the most marvelous things to be found in these terraces are the cafions;
such canons as exist nowhere else even in the Plateau Country. Right
beneath us are the springs of the Rio Virgin, whose filaments have cut
narrow clefts, rather than cafions, into the sandstones of the Jura and Trias
more than 2,000 feet deep; and as the streamlets sank their narrow beds
they oscillated from side to side, so that now bulges of the walls project
over the clefts and shut out the sky. They are by far the narrowest
chasms, in proportion to their depth, of which I have any knowledge.

All the Tertiary strata of the Markégunt, together with the entire

Mesozoic series, with the possible exception of the Gray Cliff sandstone,
14 0P

210 GEOLOGY OF THE HIGH PLATEAUS.

once extended over the vast expanse before us and far beyond the limits
of vision to the south and southeast. One after another they have been
swept away by the ordimary process of erosion, and the great expanse
of desert around the Colorado has been denuded down to the Carbonifer-
ous. Here and there an insulated patch of the Trias remains, fading
remnants of formations which were once continuous and without a break;
but the whole of the vast Cretaceous system and the heavy Eocene beds
have not left a single butte upon the denuded portion. Sixty to eighty
miles to the east of us the Cretaceous still extends uninterruptedly from
the southern slope of the Aquarius Plateau to the Colorado and thence
into Arizona. A little farther westward and the Upper Trias similarly
stretches across the interval. But from the eastern wall of the Kaibab to
the mouth of the Grand Cafion the Carboniferous forms the floor of the
country, and no later beds are found within 50 miles of the river except a
few outliers of the Shindrump.

CHAPTER X.

SEVIER VALLEY AND ITS ALLUVIAL CONGLOMERATES.

The headwaters of Sevier River.—Upper Sevier, or Panquitch Valley.—Panquitch Cation.—Circle Val-
ley.—Origin of the Sevier Valley.—Conglomerates.—Their various kinds.—Sources of the mate-
rials.—Transportation of coarse débris and the natural laws governing it.—Action of rivers upon
transported materials.—Action of the sea.—Alluvial conglomerates.—Formation of alluvial cones
at the openings of mountain gorges.—Their structure.—Alluvial cones now forming in the val-
leys of the district.—A comparison between the modern alluvial formations and the ancient con-
glomerates.—Identity of the process which formed both.

The South Fork of the Sevier River heads in the Markdgunt near its
southwestern crest, the springs being scattered among the basalt fields,
which cover a considerable area in that vicinity. Two fine creeks flow
eastward in broad valleys, meandering down the slopes of the plateau until
they meet the opposite slopes which descend from the western wall of the
Paunsigunt. Here the southernmost creek (Asa’s Creek) is deflected north-
ward, and 6 miles below, Mammoth Creek joins it, the two forming the South
Fork of the Sevier. Thence northward the stream flows for more than 50
iniles, receiving a few insignificant tributaries, until at the foot of Circle Val-
ley it is joined by the East Fork issuing from a mighty chasm, which cuts
from top to bottom the great Sevier Plateau. Still northward it pursues
its course nearly a hundred miles more, receiving one important affluent at
Salina and another at Gunnison, until it suddenly springs westward at the
Paévant and cuts a chasm through it; then turning south-southwest, it mean-
ders through a forlorn desert for about 60 miles, and ends at Sevier Lake, a
large, nauseous bittern of the Great Basin. The site of this lake was at
a recent epoch covered by a southward extension of Lake Bonneville. It
is interesting to reflect that as late as Post-Glacial time the waters which
fell upon the crests of the Pink Cliffs of Southern Utah were there divided;

a part to flow southward into the Grand Canon, the remainder to flow north-
211

Pale GEOLOGY OF THE HIGH PLATEAUS.

ward into Lake Bonneville,* and thence through the Snake River into the
Columbia.

Where the upper tributaries of the South Fork reach the foot of the
Markégunt slope the altitude is about 7,000 feet. At the junction of
the East Fork it is 6,000 feet, and where the river enters the PAvant it is
5,000 feet.t In any ordinary region the Sevier would not be dignified by
the name of a river. In the early part of July its flow is a little less than
1,000 cubic feet per second, and this volume diminishes to about half that
in September. Nevertheless it is the largest stream between Great Salt
Lake and the Colorado.

The name Sevier Valley might with propriety be given to the entire
trough of the stream, but local names have been given to different portions
of it which are well separated by transverse barriers through which the
river has cut narrow passages. The most important of these is encountered
by the Southern Fork, about 17 miles north of (below) the town of Pan-
quitch. The great outbursts of trachytic lava which flowed eastward
from Dog Valley here stretch athwart the course of the stream and wall
against still more ancient coul¢es, which broke forth from vents situated in
the southern half of the Sevier Plateau, and over them have accumulated
large masses of conglomerate derived from their ruins. There has also
been local uplifting of a few hundred feet transversely to the greater
structure-lines, so that now the confused masses of trachyte and conglom-
erate form a barrier from 800 to 1,000 féet high and 10 miles in width
across the valley. Through this mass thé fork has cut a noble canon,
called Panquitch Canon. Above this barrier (southward) lies a large valley-
plain, having on the east long alluvial slopes, which rise gently to the base

of the Sevier Plateau, and on the west the still longer and gentler slope of

* Although all American geologists are well aware of it, it may not be generally known that the
name ‘‘Lake Bonneville” has been given to a vast body of fresh water which during the Glacial and
Post-Glacial periods, occupied the eastern part of the Great Basin. This lake had an area about three-
fourths as great as that of Lake Superior, and its greatest depth was about 1,000 feet. This lake out-
flowed to the north into the Snake River and thence into the Columbia. The increasing aridity of the
climate since the close of the Glacial epoch has dried up most of the sources of the lake and evaporated
the waters of the lake itself, so that now only a few remnants are left. Of these, Great Salt Lake is by
far the most important. Utah Lake is a body of fresh water, and has an outlet through the Jordan
River into Great Salt Lake. Sevier Lake is another remnant of Lake Bonneville.

t These altitudes are probably within 50 feet of the exact truth.

CIRCLE VALLEY—RHYOLITES AT MARYSVALE. 215

the Northern MarkAgunt, crowned by the Bear Peak and Little Creek
Peak in the background. From Panquitch Canon the stream emerges into
Circle Valley, which is much smaller in area but far grander in scenery—
indeed, the grandest of the High Plateaus. On the east rises the long pali-
sade of the Sevier Plateau 4,300 feet above the river; on the west the
wall of the Southern Tushar, which opposite the valley is 4,200 feet above
it, and from 5,000 to 6,000 feet above it in its northern and southern exten-
sions. The Tushar shows rugged peaks and domes planted upon a colossal
wall; the Sevier Plateau shows a blank wall without the peaks. Very
grand and majestic are these mural fronts, stretching away into the dim dis-
tance calm, stern, and restful. Yet they fail to impress the beholder with
a full realization of their magnitude. This is true of mountains in general,
but pre-eminently so of great cliffs. If one-third of the stuff in the Sevier
Plateau, east of Circle Valley, had been used to build a range of
lively mountains, they would have seemed grander and possessed what no
palisade can ever possess—beauty and animation. It is otherwise with the
Tushar. There the great wall has magnified the mountains by giving them
a noble sub-structure on which to stand, and the mountains have magnified
the wall by giving it something to support.

Twenty miles south of Circle Valley and just below the hamlet of
Marysvale another considerable barrier lies across the valley of the Sevier.
It consists of a mass of rhyolitic lavas, which broke out in the valley bot-
tom in many eruptions, and now remain as a chaos of tangled sheets
stretching from wall to wall. The river has maintained a canon through
the mass right at the base of the spurs of the Tushar, whose front here is
not mural but mountainous. Emerging from this barrier the river flows
unobstructed through its main lower valley between the PAvant and Sevier
Plateau until it darts into the former 70 miles to the northward.

The valley of the Sevier is due to structure, and owes to erosion only
the canons which are cut through the two barriers of volcanic rocks which
have poured across it. The upper valley (Panquitch Valley) lies along the
great displacement which has lifted the wall of the Sevier Plateau. Below
Panquitch Canon, from Circle Valley to the mouth of Marysvale Canon,
the valley platform is a block between two faults, with the Sevier Plateau

914 GEOLOGY OF THE HIGH PLATEAUS.

on the east and the Tushar on the -west. Farther northward to the Juab
Valley a similar relation prevails. So far is the entire trough of the Sevier,
except at the barriers, from being due to erosion, that its floor has been
built up by the growth of alluvial formations of considerable magnitude.
They are of special interest because of the light they throw upon an inter-
esting problem in dynamical geology.

THE FORMATION OF CONGLOMERATES.

There are several kinds of conglomerate, formed by processes which,
though they may have some features in common, are on the whole strik-
ingly different. Glacial drift, though it undoubtedly falls within the usual
conception of a conglomerate, has an origin wholly different from that of a
littoral or alluvial conglomerate. Yet in respect to the source from which
its materials are derived—the disintegration of the harder rocks by water
and frost—the distinction is not well marked. The great difference is in
the methods and agents of transportation and final distribution. Alluvial
conglomerates agree with the littoral in having the same origin for their
materials, and the same transporting agent, moving water, but the two dif-
fer in respect to the conditions under which the transporting power is exer-
cised and the materials distributed. Thus these three kinds have some-
thing in common and each has some features peculiar to itself.

Sources of materials—The stones and pebbles included in these forma-
tions are derived from the break-up of the hardest classes of rocks, which
are usually metamorphic or volcanic. Ordinary sandstones, limestones,
and clays, and shaly rocks in general seldom contribute to the mass of
fragments found in conglomerates. Attrition, weathering, and solution
utterly destroy them before they reach a resting-place. A few remnants
of rock not usually reckoned as metamorphic nor volcanic are some-

times inclosed, but they come from sedimentary strata as hard and endur-

ing as the others, and such strata are rare. Hard masses, originally con- |

tained in softer beds, are sometimes found, but they owe their preservation
to their excessive durability, such as the flints of chalk, the chert, and many
forms of amorphous silica occurring in limestones. The localities from

which the stones come are no doubt very near those where they are

TRANSPORTATION OF DEBRIS. 215

deposited, as compared with the distances traveled by finer detritus. In-
stances where stones weighing from two to five pounds have traveled 50
miles are common. Where ice is the vehicle, the distance may be almost
indefinitely great. It would seem to require extraordinary circumstances
to justify the belief that a conglomerate could be formed as far as 50 miles
from the sources of its fragments, and it is probable that most of the strati-
fied beds are formed in the very neighborhood of those sources, though
beds of small gravel, graduating into coarse and then into fine sandstone,
may extend away much farther.
Transportation —Transportation by ice, whether floating, or moving
upon the land, forms a subject by itself, and has no analogy to the agency
of water in moving débris. It will therefore be passed over, since it takes
no part in the operations which are the object of this discussion. The
movements of the coarse materials which build up conglomerates differ
from those of the finer sediments, though they have something in common.
The greater portion of the fine silt, much of the fine sand, and the whole
of the chemical and organic precipitates are carried by moving waters in
suspension, and are thrown down when the waters come to rest. The
coarser materials are impelled along the bottoms of rivers and the shelving
floors of the ocean and lakes near the beaches. Here the want of habitual
observation and common experience is apt to mislead us and render dif_i-
cult the obtaining a just apprehension of the nature and magnitude of this
impulsion. Any day we may see the rivers turbid with earthy matter, and
it is an easy step from this observation to the great generalization that the
land is wasting away and heavy strata accumulating beneath the ocean.
But it is not so easy to see what goes on beneath the water. The’times
when the processions of stones are on the move are times of high water,
and flooding rains, when geologists are as prone as other people to seek the
kindly welcome of roofs and closed doors; times when the deep and murky
‘waters prevent us from seeing and the roar of the torrent from hearing the
movement, even if we ventured out to watch it. Thus, the process is not
a matter of common and direct experience; nay, experience might seem at
first to lead us to a contrary conclusion. When a stream is low and clear

216 GEOLOGY OF THE HIGH PLATEAUS.

we may note the stones which pave its bed, and after a flood has passed
and the stream again is clear we may find that there has been little change
in them; but to conclude that no stones have passed in the interval would
be a mistake. Those which retain their places have lodged there and been
fastened to the bottom by a packing of sand or wedged together like the
cobbles of a pavement. If the sources of the materials continue to furnish
them, doubtless many stones have been hurried along over this pavement
during the flood, a few finding a resting place, but more of them passing
on to be ground into silt or to find resting-places in deeper waters below.

But there is another method quite different from this precipitate one,
and by which it is very probable that much larger movements are effected,
though much more slowly. It never happens that the materials to be
moved are of uniform grain. Mud, sand, gravel, shingle, and cobble-
stones always accompany coarser débris in varying proportions, and form a
matrix in which the larger fragments are imbedded. An acceleration of
the current removes the finer stuff and retardation replaces it with fresh.
The washing out of the matrix of sand and grit which holds a pebble in
its place leaves the pebble to the unobstructed energy of the current. If
that energy is sufficient it will be carried along until the current slackens
or until it finds a lodgment. If the energy is too small, the pebble will
remain until the ceaseless wear of attrition reduces it and brings it within
the power of the stream to move it. Nor are these movements dependent
solely upon periodical floods. _ Any cause which alternately accelerates the
movement of water may produce them, and these causes are many. © Every
stream and every shore current is affected by numerous rhythmical move-
ments which produce these alternations in many ways and many degrees.
The waves and surf, the undertow, the tides, the shifting of shore cur-
rents, the storms and monsoons, the ripples of the brook, the numberless
surgings and waverings of rivers, the shifting of channels, the building and
destruction of sand bars, the freshets—all are causes by virtue of which
any spot at the bottom of the water is subject to alternate maxima and
minima in the velocity of the water which passes over it. Sooner or later,

then, the pebble must move on, provided any maximum of velocity in

TRANSPORTATION OF DEBRIS. PAT

the water is sufficient to move it when subject to no other resistance than
its own weight.*

Thus whatever a stream receives it carries along, whether it be water
or solid rock. Certainly much of the matter rolled into it is in the form of
coarse fragments, but it urges them onwards, grinding them to silt as they
move. Nothing which it receives does it retain, except in places here and
there where its current is suddenly checked, and here for a time coarser
materials accumulate. But in the secular life of the river even these local
accumulations may in turn be removed by subsequent changes of relative
level along different portions of its course.

The distance which a fragment may ultimately travel is independent of
its original size. Large stones, being moved with difficulty, are detained at
numerous halting places and subjected to long attrition until they are suffi-
ciently reduced to be within the power of the current, and at length become
no bigger than those which were originally smaller. In truth, all frag-
ments, in a certain sense, travel the same distance ultimately, for they all
pass the mouth of the river in the form of silt and dissolved constituents.
Viewed in another aspect, however, the size of the fragment determines in
a general way its amount of progress. The larger ones have at any given
stage moved a shorter distance and the smaller ones a greater distance—
on the average.

The action of a current upon rocky fragments, then, is to sweep them
along and to grind them to powder as it sweeps. It never accumulates
them except in a limited way and under circumstances which will be here-
after described at some length. Whether the detritus which a river dis-
charges shall be in the form of pebbles, gravel, or silt, depends upon the
length of the stream and the power of its current. A long stream with a
low slope and sluggish current along its lower course, but with more rapid
tributaries above, will have dissipated its fragments and discharge nothing
but silt. A short stream with a rapid descent may readily discharge coarse

*Where a sudden retardation of the velocity of a stream occurs, as by the sudden widening or
deepening of a channel, and where this change predominates over all other changes from maxima to
minima, there will occur a persistent accumulation of coarser débris without any great admixture of
finer. * * * Concerning the power of water to move pebbles, it will be merely necessary to refer
to Dr. Hopkins’s well-known theorem.

218 GEOLOGY OF THE HIGH PLATEAUS.

fragments, shingle, and gravel. The latter may build up a conglomerate at
its outlet; the former never.

The action of the sea upon coarse materials has a very close analogy
to that of rivers. Currents are generated by the tides and winds along
coasts. The surface-waters are rolled in waves upon the shore and flow
outwards along the bottom. But their directions are frequently vacillating,
trending both ways along the coast with varying obliquity. These cur-
rents are usually fast enough to move gravel, shingle, and pebbles as large
as those ordinarily seen in marine conglomerates, and may transport them
several miles. The general effect of the agitation produced in littoral waters
by tides and winds is to seize upon the loose materials of the shore within
reach and distribute them over the bottom with an approach to uniformity,
and this distributive action prevails wherever the influence of that disturb-
ance exists.

The distribution of the materials —It is sometimes a little difficult to real-
ize the agency which has, in the stratification of conglomerates, scattered
the fragments over considerable areas and arranged them harmoniously in
beds. The stratification of conglomerates is often as conspicuous as that
of finer strata, though in general it is less so In the case of marine con-
glomerates, which are usually formed in the vicinity of the shores, and at
no great distance from the sources of their materials, the problem is not
difficult. Currents of no mean intensity are perpetually generated along
the bottom, near the coast, by tides and the outward flow of water, which
has been blown landwards at the surface by winds. These currents, though
having at any given locality an average direction, in the long run are never
constant in direction from hour to hour, nor from day to day, but sweep
hither and thither. But the average flow at the surface is generally land-
wards, while at the bottom it is seawards. In any case, however, the gen-
eral trend is oblique, with reference to any given portion of a coast, and
never, or at least very seldom, normal to it. These vacillating movements
are highly conducive to a harmonious and definite arrangement of the
materials upon which the currents act, ever tending to sift and to sort them,
and finally to stratify them. The power of these currents to transport is

perhaps greater than we are apt toimagine. The drift of sand along coasts

ALLUVIAL OR TORRENTIAL CONGLOMERATES. 219

is a process which has often awakened the surprise of engineers who are
called for the first time to deal with the problems of harbor protection and
is ever revealing wonderful things. Not only does the finer loose material
move in grand procession under the influence of unseen, though still com-
prehensible, agencies, but very coarse detritus is carried slowly with it.
The tendency of the process, however, is not towards an indiscriminate
mixing of all sorts and sizes, but towards the grouping into layers, here of
coarser, there of finer, stuff, according to the variations in the power of the
moving water.

But there is another class of conglomerates which claims our special
attention. These are of alluvial origin, formed, not beneath the surface of
the sea nor of lakes, but on the land itself. They do not seem to have
received from investigators all the attention and study which they merit.
They are usually called gravels—perhaps are sometimes or even frequently
mistaken for glacial drift—but their homology to the ordinary stratified
conglomerates of the systematic strata is not always recognized. Through-
out great portions of the Rocky Mountain region they are accumulating
to-day upon a grand scale and have accumulated very extensively in the
past.

The processes of degradation are far more energetic and effective in
mountains than upon plains. The agents which disintegrate rocks—frost,
rain, chemical solution—have the greatest freedom of action upon the steep
slopes of the numberless ravines, and are continuously breaking off frag-
ments and reducing them to sand, gravel, and clay. Not only is the greater
part of the finer mold gathered up by the swift rills and torrents, but frag-
ments of considerable size, attaining, under favorable circumstances, the
weight of several tons, are caught and urged downward in rushing rapids
with an energy which must be seen in order to be realized. The many
streamlets and filaments of a mountain amphitheater gradually unite, as we
descend from the crest of the mountains, generating a creek, which attains
its greatest flood near the mountain base, and when the snows melt in the
spring its swollen current sweeps onward a mass of clastic material of every
description from impalpable clay to bowlders. Within the mountain masses
the descents are rapid and the streams are torrents. Reaching the valleys

220 GEOLOGY OF THE HIGH PLATEAUS.

or plains, their velocity is at once checked by the diminished slope and the
coarser débris comes to rest. These streams lie (within the mountains) in
ravines usually profound, with steep flaring sides, and opening upon the
valley bottoms or plains through magnificent gateways, and every long
range or ridge has usually many such gateways opening at intervals of a
very few miles along its flank. At the gateway the stream begins to

surrender a part of its freight and to build up its channel. The check’

given to the velocity of the stream here is marked, indeed, but less incisive
than might at first be supposed. The profile of the bed of the stream does
not have an angle at this point, but is curved very gently, and is concave
upward. Indeed, it is so throughout the entire course of the stream out-
side the gate and generally for a considerable distance inside the gate.
Thus the velocity of the stream slows down gradually and not suddenly.
As the velocity gradually diminishes so the stream gives up more and
more of its load. But the stuff which it drops along any small part of its
course is by no means of the same size; that is to say, there is no rigorous
sifting of the material in such a manner that the stones or particles at any
given place are of uniform size, while finer ones are carried on to be scrupu-
lously selected where the slope and velocity are less. On the contrary, all
sorts are deposited everywhere. Nevertheless there is a tendency to sort-
ing. Higher up the slope there is a greater proportion of coarser deposit;
lower down there is a larger proportion of finer deposit; but everywhere
the coarse and the fine are commingled.

Where the stream is progressively building up its bed outside of the
eate, it is obvious that it cannot long occupy one position; for if it persisted
in running for a very long time in one place it would begin to build an
embankment. Its position soon becomes unstable, and the slightest cause
will divert it to a new bed which it builds up in turn, and which in turn
becomes unstable and is also abandoned. The frequent repetition of
these shiftings causes the course of the stream to vibrate radially around
the gate as a center, and in the lapse of ages it builds up a half-cone, the
apex of which is at the gate. The vibration is not regular, but vacillat-

ing, like a needle in a magnetic storm; but in the long run, and after very

Limbioryer LV.

Heliotype Printing Co., Y

220 Devonshire St., Boston.

VOLCANIC ALLUVIAL CONGLOMERATE ON TRACHYTE, PANQUITCH CANON.

ALLUVIAL CONES—THEIR STRUCTURE. 221

many shiftings, the stream will have swept over a whole semicircle with
approximately equal and uniform results.

The formation thus built up is an ‘‘Alluvial Cone.” As we travel over
these cones their forms are usually recognized by the eye, though some-
times with difficulty. The slant of the cone (of which more will be said
hereafter) is usually quite small, though sometimes very conspicuous. It
varies greatly but not capriciously, depending much upon the nature of the
materials of which it is composed. Most frequently these cones are so large
and so flat, that it is only by very close scrutiny and comparison with sur-
rounding objects that their forms are optically recognized, and many cases
occur where we become aware of their true figures and relations only by the
use of our pocket instruments. There is one feature which the eye seldom
recognizes or even suspects. The profiles are not (even typically) truly con-
ical, but are slightly curved instead of having a rectilinear slope. They are
concave upwards, the slope being a little greater near the apex and slightly
or sometimes notably diminishing towards the periphery. The slopes near
the circumference usually lie between 1° and 2°; those near the apex
between 2° and 34°. The lengths of the radii of the bases often exceed
3 miles, sometimes exceed 4 miles, and seldom fall below 2 miles. Per-
haps 3 miles would be a fair average for those found in the valleys of the
District of the High Plateaus. So nearly together are the gateways along
the mountain and plateau flanks, each having its own alluvial cone, that
the cones are confluent laterally ; giving rise to a continuous marginal belt
along the base of the plateau flanks consisting of alluvial slopes which are
sensibly nearly uniform.

The conical form of these accumulations is ordinarily tolerably accu-
rate and often remarkably perfect. It is a surprisingly harmonious result
of a process which in its elements is apparently irregular, but becomes
regular only by averaging the results of its constituents. Not only is the
regularity seen in the external form of the cone, but it is found whenever
an opportunity occurs to examine its interior structure. This is sometimes
revealed to us. In the vicissitudes to which a stream so conditioned is
subject it occasionally happens that indirect causes have set it at work
cutting into its cone; dissecting it, so to speak, by a deep cut and laying

222, GEOLOGY OF THE HIGH PLATEAUS.

bare its anatomy. Our surprise is often great at finding the cone wonder-
fully well stratified, but in a peculiar way. ‘The most perfect stratification
is presented when the dissecting cut is made radially. But when a cut
transverse to the radius is made by excavations of another stream, the strati-
fication, though still conspicuous, is much less uniform and harmonious.
The cone appears to be built up of long radial or sectoral slabs superposed
like a series of shingles or thatches.

There are marked differences between the cones formed by streams
which have their entire descent within unaltered sedimentary strata and those
running among volcanic and metamorphic rocks. The fragments resulting
from the decay of sandstones, limestones, and shales are much more sus-
ceptible to the influence of weathering and are more readily worn-out by
the abrasion of travel. Even when they escape destruction by the wear
of the torrent and reach a resting-place upon the surface of the cone,
the gentler but more insidious action of meteoric forces gradually crum-
bles them to sand or dissolves them, and they at length disappear. But
the compact volcanic and metamorphic rocks are much more durable
and do not yield so readily either to mechanical or chemical forces; more
of them reach the cones, where they survive long enough to be buried
beneath later accumulations and thus receive final protection from dissolu-
tion. Hence the cones derived from the waste of sedimentary strata sel-
dom contain much coarse débris, while those from harder rocks are largely
composed of it. This difference in texture im turn produces some difference
in the proportions of cones. The sedimentary cones are usually very
slightly flatter and broader. The difference in this respect is on the whole
quite small, but the measurement of a considerable number of both kinds
seems to indicate that it really exists.

In consequence of the flatness of the cones and their lateral confluence,
the general result of their serial aggregation is a long and thick stratum
made up of many subordinate folia. In process of time it may also
become consolidated and hardened into a rock mass resembling in all
essential respects the stratified conglomerates usually reckoned among the
members of a stratigraphic series. That distinctions between such a con-
glomerate and one deposited littorally would be readily detected after close

ALLUVIAL CONGLOMERATES. 223

inspection of favorable exposures we may well believe; yet it is highly
probable that the two kinds would be confounded on a hasty examination,
and the distinction would be difficult to verify even by careful study, unless
the exposures were extensive and conspicuous enough to display very fully
and clearly their respective characters. These doubts generally would
prevail in those cases where a decision would have to turn only upon the
intimate structures of the deposits. Collateral circumstances, however,
may often decide the question.

Throughout the volcanic portions of the District of the High Plateaus
the conglomerates are present in prodigious masses. They constitute a
large proportion of the rock masses of the plateaus, and form many miles of
escarpment more than a thousand—sometimes more than 2,000—feet in
thickness. In the central and southern portions of the plateaus they can-
not fall much short of one-half of the masses now open to observation, and
taking the volcanic portion of the entire district, a rough estimate would
place their volume at least at a third of the whole eruptive material. They
are well stratified, and though the distinctness of the bedding is somewhat
variable, the stratification never becomes obscure. Indeed, on the whole,
these conglomerates seem to be about as well stratified as the average of
those which are attributed to sub-aqueous deposition. The individual beds
are not so thick and massive and show partings more frequently or at
shorter intervals.

The occurrence of large stratified accumulations of pyroclastic mate-
rials in regions or districts which have been the theaters of protracted vol-
canic activity is a fact of common observation. They abound throughout
the State of Colorado and along the more or less volcanic ranges of North-
ern Wyoming, Montana, and Idaho. They excited the admiration of
Scrope in Central France, and are conspicuous in Sicily and around Vesu-
vius. Indeed, every volcanic region will doubtless be found to display
them to a greater or less extent. Where large bodies of water wash the
flanks of volcanic mountains and ranges we may expect to find large
bodies of sub-aqueous conglomerate formed from their débris. Voleanic
tuffs are formed by the mechanical projection of dust, ash, rapilli, and

small fragments from vents blowing out gases and steam, and falling

224 GEOLOGY OF THE HIGH PLATEAUS.

at considerable distances from the orifices. Want of opportunities for ob-
serving such formations of unquestionable origin prevents me from having
any just conception of the nature, extent, and texture of such accumula-
tions. But it seems sufficiently clear that there could be no difficulty in
distinguishing them from such as are with equal certainty attributable to
sub-aqueous or alluvial deposition. I have observed but few exposures
which I can attribute to such an origin. That the great mass of conglom-
erates of the High Plateaus were accumulated from the d¢ebris derived from
the erosive destruction of volcanic beds cannot be doubted. The only
question is whether they are alluvial or sub-aqueous, and of the former
origin I entertain no doubt. The fragments seldom fail to reveal traces of
attrition and weathering, never preserving sharp angles like those pro-
duced by fresh fracture. But, on the other hand, the attrition is not
ordinarily extreme. In most cases there is enough of it to indicate dis-
tinctly that the fragments have really been abraded, though with no
ereat loss of substance. The stones of sub-aqueous conglomerates, on
the contrary, are almost always much worn and rounded. Again, the
sizes of the stones range from a fraction of a cubic inch to several cubic
feet ; in rare instances to more than a cubic yard.

In whatsoever manner we compare the great conglomerates now form-
ing solid rock masses and uplifted as plateaus with the alluvial conglom-
erates now forming in the valleys, we cannot fail to be impressed with the
evidence that both were formed by essentially the same process. The only
differences of any appreciable moment which are now discoverable arise
from the fact that the older conglomerates have been consolidated into rock-

oD

masses, while the later ones have not.

CHAPTER XI.
SEVIER AND PAUNSAGUNT PLATEAUS.

General structure and form of the Sevier Plateau.—Sculpture.—Ravines.—Superposed features and
details. —Northern portion of the plateau.—A gigantic cliff—Monroe Amphitheater.—Lava beds
exposed within it.—The Gate of Monroe.—Propylitic masses.—Clastic voleanic beds at the base
of the series.—Hornblendic andesites.—Intervening period of erosion of the propylites.—Horn-
blendic trachytes and augitic andesites.—Argilloid and granitoid trachytes.—General succession
of the eruptions.—Comparison with the succession found in the Auvergne.—Eastern side of the
Sevier Plateau and Blue Mountain.—Great extent of the emanations from the principal volcanic
centers of the northern part of the plateau.—Eroded lava-capped mesas around Salina Cation,.—
The Black Cap.—Augitic trachytes.—Lava sheets south of Monroe Amphitheater.—Central vents
of the Sevier Plateau.—Volcanic conglomerates.—An ancient cone, buried in lava and exhumed
by erosion.—Conglomerates south of the central vents.—Southern focus of eruptions.—Andesitic
conglomerates.—Southern termination of the Sevier Plateau.—General succession of eruptive
sheets.—Sections.—East Fork Cation.—Effect of the Sevier fault.—Tufaceous deposits exposed in
East Fork Catton.—Their transitional characters.—Their metamorphism and the resemblance of
the metamorphs to lava sheets.—Phonolite hill.—Grass Valley, its structure and origin.—Exist-
ence of an ancient lake in Grass Valley.—The causes which produced it.—Tufaceous deposits of
Mesa Creek.—Their recent formation.—Their transitional characters—Alluvial cones of Grass
Valley.—The Paunségunt.—Lower Eocene beds.—Faults.—The southern terraces.—Paria Valley.
—A grand erosion.—The scenery of Paria Valley.—Table Cliff and Kaiparowits Peak.—The Pink
Cliffs and architectural forms sculptured from them.—A recent basaltic cone.—Scattered basaltic
craters of the southern terraces.

The Sevier Prateau is next to be described. It is a long and rather
narrow uplift, having a fault along its western base and inclining to the
eastward; at first very gently, then with a stronger slope, which grades
rapidly down into Grass Valley. The length of this table is about 70
miles, and its width varies from 10 to 20 miles. It is, therefore, lone
and narrow like the general ground-plan of a mountain range. But its
structure has very little analogy to ordinary mountain uplifts. It has no
sharply upturned strata upon its flanks reclining against a core of meta-
morphic rocks—no summit ridge marking the axis along which granitoid
and schistose rocks have been protruded, nor even the monoclinal ridge
which characterizes the Wasatch and Basin Ranges. It is a tabular mass
very like the inclined blocks of the Kaibab region to the southward. The

inclination is very small, seldom exceeding three or four degrees upon the
15 HP 295

226 GEOLOGY OF THE HIGH PLATEAUS.

summit, though reaching a considerably greater slope upon the eastern flank.
The eastern side, indeed, suggests a monoclinal flexure, but the bending of
the profiles is so small and their sweep is so gradual that we may forbear
to call it such. It is hardly pronounced enough to justify such a designa-
tion.

Standing in the Sevier Valley and looking at this barrier there are
many stretches along its western front which appear quite like a common
mountain range. Profound gorges, V-shaped, heading far back in its mass,
have cut the table from summit to base and open through magnificent
gateways into the valley. The residual masses between these gorges pre-
sent their gable-ends to the spectator, who cannot see what is behind them,
and they look exactly like so many individual mountains, while in reality
they are merely pediments carved by erosion out of a gigantic palisade.
Other long stretches of the western front are unbroken and present to the
valley of the Sevier a wall of vast proportions. The summit of the plateau
is not smooth, but carved into rolling ridges and vales, deepening eastward
into canons, while at several places volcanic ridges cross it transversely.
These last are the remnants of old volcanic piles worn down and half
obliterated by long ages of decay, for they belong to the middle epoch of
volcanic activity, which may be as old as the Middle Miocene. They
present from a structural point of view a peculiar relation to the table on
which they now stand. In almost every great mountain range of ordinary
type the axes of those minor ridges or superimposed features which had
their origin in general causes which built the entire range lie roughly
parallel to the main uplift in the relation of superimposed waves of displace-
ment. But here it is otherwise. The volcanic ridges which are planted
upon the Sevier Plateau run not along its major axis, but across the table
from side to side. The movement which hoisted the plateau en masse was
not sensibly embarrassed by such trifles as a few ridges of volcanic piles.
The features impressed by erosion, on the contrary, conform to the usual
law which prevails in mountain ranges. The streams pour down from the
summit along whatever slopes may haye been generated by the details of
the uplift, and have carved their vales, gorges, and canons accordingly.

Since these run across the table or perpendicular to its major axis they

GENERAL VIEW OF THE SEVIER PLATEAU. DPT

have sculptured ridges of erosion which trend that way. If we view the
Sevier Plateau from the north, its transverse profile is alone seen, and the
tabular summit slightly inclined is conspicuous to the eye. But if we
view it from the east or west, its long summit is seen in many places to be
somewhat rumpled and even serrated by the ridges of erosion and by the
old voleanic remnants viewed endwise.

The northern end of the Sevier Plateau is not well defined. A long,
gentle ramp, deeply scarred and much wasted by erosion, begins a little south
of Salina and ascends southward to the summit. It is best appreciated as
we journey up the Sevier Valley from Salina to Richfield. We then
observe the whole platform of the country to the east of us gradually
gaining in altitude through a distance of 20 miles, until from being a
thousand feet above us at Salina it becomes 5,800 feet above us opposite
Richfield, and there presents to the west a stupendous battlement of nearly
vertical wall above and abrupt spur-like slopes below, thrusting their but-
tresses beneath the valley plain. For nearly 10 miles this tremendous
escarpment is quite massive and unbroken, simple in form and more than
a mile in height. Opposite Monroe a large amphitheater has been exca-
vated in the plateau by a plexus of streams, and may be likened to a huge
bowl filled with mountains. From this point southward the plateau wall is
notched repeatedly by profound ravines heading far back in the table,
until, at a distance of about 32 miles south of Monroe, the plateau is cut
completely in twain by the East Fork Canon. From this gap southward
30 miles the southern division of the plateau presents a very few incon-
spicuous breaks, and terminates in a low wall at a rather lofty and broad
transverse valley known as the Panquitch Hayfield. The eastern front of
the table looks down into Grass Valley, but from a much smaller eminence,
both because the eastern front is absolutely lower than the western, and
because Grass Valley is absolutely higher than Sevier Valley. ~The
descent into Grass Valley along the northern and central parts of the pla-
teau is rather abrupt, frequently precipitous; but along the southern part
_ it is very gradual.

The Sevier Plateau is composed chiefly of volcanic sheets of grand

dimensions and enormous cumulative thickness, and of immense beds of

228 GEOLOGY OF THE HIGH PLATEAUS.

alluvial conglomerate derived from their degradation. Only at the north-
ern and southern ends are the sedimentaries clearly seen in mass lying
beneath the old lavas. Ata few intermediate points, however, and espe-
cially in East Fork Canon, some metamorphosed beds of peculiarly inter-
esting character are exposed, and these will receive special attention in the
latter part of this chapter. ;

The eruptions which compose the plateau mass belong to several well-
separated periods, which for the most part had their locations at the same
centers or axes. Of these centers or axes there are in the Sevier Plateau
three—one at the loftiest part of the table at the summit of its northern
slope, the second about 20 miles farther south, the third in the southern
section of the plateau, right abreast of Panquitch Canon and about 30
miles south of the second. They may be distinguished as the northern,
central, and southern eruptive centers respectively. Of these the largest
and most voluminous is the northern one; in truth it is apparently the
most important one of the entire district.

Immediately opposite the Mormon town Monroe the great wall of the
plateau rises more than a mile above the valley plain, presenting the edges
of the volcanic beds, which appear to be very nearly horizontal and more
than 4,000 feet in thickness. How much more is impossible to say, for the
lowest sheets are concealed. Upon the summit of the wall a transverse
ridge runs across the table to the eastern side and ends in a high knob
overlooking Grass Valley and named the Blue Mountain. It was in the
vicinity of this ridge that the grander eruptions had their origin.

The great amphitheater near Monroe has laid open the table to its
foundation, but the promise of information conveyed by such a section is
not fulfilled. It has revealed a bewildering maze of earlier rocks lying in
all possible positions and having but few intelligible relations to each other.
Upon them rest later floods in rather regular bedding, which succeed each
other to the summit. I have revisited this locality repeatedly, but have:
generally found at each visit more questions than answers. ‘The confusion
among the lower rocks is indescribable, and the exposures of any given
bed so fragmentary that I have been compelled to abandon the effort to
unravel the knot, and can give an account of only the most general rela-

MONROE AMPHITHEATER—PROPYLITES. 229

tions presented. The most conspicuous rock of the oldest series is a ridge
of hornblendic propylite extending across the opening of the amphithea-
ter. he stream which drains the amphitheater has cut a cleft 20 or 30
feet wide and more than 500 feet deep through this barrier (Heliotype 1),
and the gorge has received the name of Gate of Monroe. The length of
this chasm between propylitic walls is about half a mile. Following it
downstream the massive propylite gives place suddenly to beds of con-
glomerate and clay, baked and altered by heat, which abut in the natural
section against the propylite. They are probably younger than the vol-
canic rock and may have been derived from its waste. At the upper end
of the gorge the propylitic mass ends suddenly—a lateral ravine parallel
to its precipitous face hiding its mode of exit. On the other side of the
ravine is a mass of andesite succeeded by trachyte, both apparently younger
than the propylite. The propylitic mass may have been erupted at as early
a period as Middle or Late Eocene, for the stratified beds which abut against
its western flank have evidently been water-laid, and there is no evidence
of the existence of any considerable body of water in this locality later
than the epoch referred to. Moreover, beds of similar nature, sometimes
altered, sometimes not, are found around the eruptive centers in many
localities, and have been derived from the destruction of some unknown
volcanic rocks. Fragments of similar altered rocks are brought down by
the stream from some of the forks above, showing that on both sides of the
propylitic mass these peculiar sediments were deposited. Very partial
exposures of propylitie rock are also found elsewhere in the deepest part
of the ramifying gorges, cut by the many streams that unite in the creek
which cuts the cleft in the larger barrier of the amphitheater.

These propylitic rocks are interesting, inasmuch as they furnish another
instance of that priority in time among Tertiary eruptions which Richt-
hofen has claimed for them. Here they are not only older than all other
eruptives, but they appear to speak of an epoch in which they alone were
erupted, and that epoch probably goes as far back as the Middle Eocene.
They certainly do no appear among the later or the middle eruptions. A
period of rest from volcanic disturbance succeeded their extravasation, and

during that quiescent period they were much ravaged by erosion. Patches

230 GEOLOGY OF THE HIGH PLATEAUS.

of conglomerate, formed of their fragments, were accumulated and are here
and there brought to light where erosion has deeply excavated the still
erander masses of subsequent lavas overlying them So completely were
these most ancient rocks overwhelmed, that erosion has only revealed a
very small portion of them and left us to conjecture what may be the
extent of those portions now concealed. It is not improbable that the
clastic beds, formed of the waste of volcanic rocks, and which underlie the
great lava caps of the plateaus and in turn rest upon the Bitter Creek and
Green River beds, may have derived their sands and clays from the decom-
position of some of these propylitic masses.

These ancient eruptions are succeeded by those of a middle epoch,
lying across the surface of an eroded country, which they overwhelmed.
These second lavas are much less chaotic in their arrangement and much
less affected by erosion during the intervals between the eruption of succes-
sive floods. ‘lhey are, therefore, more intelligible, and some idea of their
sequences has been obtained, though less definite than is desirable, because
the exposures are so partial and so much obscured by débris and soil.
These outpours were upon a very large scale, the masses being often several
hundred feet in thickness and spreading out over large areas. The lower
masses are andesitic and show but little variety. They all belong to the
hornblendic group and are characterized by triclinic feldspar, with a mode-
rate proportion of hornblende, with some augite and magnetite, and are
very compact and rather fine-grained. Higher up, these give place to
coarse-grained trachytes, with both monoclinic and triclinic feldspars and
abundant hornblende. These occasionally intercalate with sheets of doler-
ite. Still higher, a totally distinct group of trachytes is found. They con-
sist largely of the argilloid variety—a fine-grained, highly ferritic, reddish
paste, holding porphyritic crystals of opaque monoclinic feldspar. There is
probably no eruptive rock within the district more abundant. It forms the
summit of the series of middle-aged eruptions in many localities. Very
nearly coeval with it is a group of trachytes, having an appearance faintly
resembling a fine-grained syenite, though not by any means wholly erys-
talline. It varies in color from iron gray to light gray. It shows a tendency

to break up into slabs or tiles from an inch to four or five inches thick, the

MONROE AMPHITHEATER—TRACHYTES AND DOLERITES. 231

cleavage being sometimes parallel with the bedding, sometimes making a
large angle with it, like slate. Hornblende, augite, and black mica, in very
small crystals, are sparingly disseminated through it. Associated with
these are masses of doleritic lava. I use this designation to indicate a rock
more basic than andesite, but less so than basalt; and though more nearly
approaching the latter, is distinguished from it both in mode of occurrence
and in aspect. It is associated with the middle eruptions and I believe
never with the later. Its feldspars are triclinic (Labradorite), frequently
in large crystals, which have a conspicuous glassy luster, resembling sani-
din. It never contains olivin. Usually it is blackish and nearly as dark as
basalt, but in some cases it is red, even in compact specimens.

We have, then, in this great amphitheater more than 4,000 feet of
volcanic rocks, belonging to at least two periods, and possibly more, separ-
ated by long intervals of erosion—the oldest going back into the latter part
of the Eocene, the younger belonging to I know not what period exactly,
but from general considerations, am disposed to regard them as Miocene or
early Pliocene, covering a long period in their totality, which may extend
throughout the entire range of Miocene and Pliocene time. At the base of
the series we find large bodies of rock, consisting of plagioclase, with con-
siderable quantities of accessory hornblende, and also having the habit of
hornblendic propylite and hornblendic andesite. These were much eroded
after their eruption and before the extravasation of the later coulées. They
are succeeded by heavy masses of rather fine-grained augitic andesite in
great sheets, reaching a thickness of 300 and even 400 feet, and are followed
by equally heavy masses of trachyte, sometimes augitic, sometimes with
no great or notable amount of any accessory mineral. With these last
doleritic eruptions intercalate

Scrope, in his work on the ‘‘ Volcanoes of Central France,” repeatedly
mentions the occurrence of “ basalts” intercalating with the trachytic masses
of Mont Dore and the Cantal. He was particular to call attention to the
fact that in that region no confirmation was found of the view which had
been entertained by some geologists that the basalts were erupted at a later
period than the trachytes, and notes many instances where “ basalt” was
overlaid by trachyte. It is clear, however, that Scrope included under

232 GEOLOGY OF THE HIGH PLATEAUS.

the name basalt nearly, if not quite, the whole category of dark-gray and
black augitic rocks of rather fine-grained texture, high specific gravity, and
more or less conchoidal fracture. To the range of variation which is now
known to extend through this class both in respect to chemical and min-
eralogical constitution he appears to have attached little importance, and,
indeed, was unacquainted with such distinctions as have been established by
later researches. It has seemed to me possible that the earlier rocks which
he has called basalt may prove to be augitic andesite, while the most recent
ones are the most basic of their class, and therefore identical with the rocks
now assigned by more recent classification to basalt in the more restricted
sense of the term, and finally that intermediate varieties may there exist,
which are equivalent to those rocks which I have here designated as doler-
ite. At all events, there is this correspondence—both localities present the
intercalation of augitic-plagioclase rocks with trachytes.

Let us now examine the east side of the plateau directly across from
the great amphitheater. Another grand exposure is presented here. There

is no fault on this side of the table—at least, none has been observed—

but a large valley has been excavated not perpendicularly inwards towards
the axis of the plateau, but very obliquely, cutting off the gable-like end
of Blue Mountain. This name is given to that high knob which stands
upon the eastern verge of the plateau, at the end of the transverse ridge
which now marks the locus of one of the centers or axes of eruption. The
excavation of the valley has cut off the eastern face of this ridge and laid
open the structure and arrangement of the various beds. This arrange-
ment is quite similar to what would be expected and to what has often been
observed in great volcanic piles. From the central axis the sheets are seen
dipping away in both directions at variable angles never very great. On
the northern side they descend towards the northeast and on the southern
side to the southeast, the lower beds dipping more than the upper ones.
All of these lavas seem to have welled up in mighty floods without any of
that explosive violence which often characterizes volcanic action, and so
great was the volume of extravasated matter, that it at once spread out in
wide fields, and deluged the surrounding country like a tide in a bay flow-

ing over all inequalities. How far these floods extended it is difficult to

NORTHERN FOCUS OF ERUPTIONS. 233

say. To the westward they are cut off in the great wall which faces
Sevier Valley with an altitude of nearly 6,000 feet above the river. To
the east they are likewise cut off by the oblique valley, though they
reappear at lower altitudes on the other side, and are instantly lost again
under soil and waste, but evidently descend into Grass Valley, and may
commingle with the equally grand floods emanating from the Fish Lake
Plateau to the eastward. But south and north they-are displayed in im-
mense volume. Those which flowed north and northeast are spread out
in the vicinity of Salina Canon and one great coulée stretched beyond the
canon, which now cuts off a portion of it, leaving it as an outlier. Large
portions of these old lavas have been swept away. The mauvaises terres
south of Salina village were once covered with it. Standing prominent
among these bad lands is a conical butte-like mountain of singularly per-
fect form. It is a remnant left by circumdenudation, and upon its summit
is a “tip” or cap about 250 feet thick, consisting of this same lava reposing
upon the sedimentary strata, out of which the peak has been carved in
cameo. This mountain is called the Black Cap. The augitic trachyte,* of
which its summit apparently forms a remnant, is the same as that which
extends across the Salina Canon. This flow reached a distance of 30 miles
from its source. South of the cafon and nearer the source sheets of argil-
loid trachyte rest upon the augitic and hornblendic, and heavy beds of con-
glomerate derived from the ruins of both kinds of rock are interspersed.
To the northeastward, extending as far as 25 miles, similar aggregates of
massive superposed -coulées are displayed, having a thickness of nearly a
thousand feet and increasing in bulk as we approach the Sevier Plateau.
The hornblendic trachytes are in the larger proportion, but the lighter gray
trachytes, and especially the ‘argilloid’ varieties, are almost as voluminous.
They are much degraded by erosion, and several fine canons have been cut,
ramifying into broader ravines, with big rough swelling hills between them.

*This rock isa conspicuous one. It has many crystals of sanidin, but the less conspicuous plagio-
clase is very abundant. The line is difficult to draw—perhaps impossible—between some andesites and
augitic trachytes. The texture is sometimes the only basis of a distinction, and this should be used
with great caution, and never without reservations. Still the textures of the two groups are usually
distinct and characteristic, and the rock assumes in most cases the one aspect or the other even when
the mineralogical constitution is doubtful. In the very few cases where there is no means of forming
a decided distinction it would seem asif the old term ‘‘trachydolerite” might be useful. It has the
advantage at least of being non-committal.

234. GEOLOGY OF THE HIGH PLATEAUS.

Southward from the northern center of eruption of the Sevier Plateau
the floods are piled up in grand succession sheet upon sheet. No narrow
streams or rivers of lava were here, but great deluges, which welled up and
rolled majestically over vast Phlegrzean fields, and, spreading out in broad
lakes, left after their congelation an even stratification, which may be read
miles away from distant summits. Standing upon the verge of the Awapa
Plateau and looking across Grass Valley, these old floods are seen lying
calmly and evenly with an outward resemblance to dark stratified rocks cut
by ravines and terraced off into trappean ledges. Ten or fifteen miles
southward they have commingled by intercalation with the coulées from
the middle eruptive focus of the plateau.

The eruptions from this middle locality were inferior in magnitude to
those from the northern vents, though absolutely they were by no means
small. Its lavas differ somewhat in character from those derived from the
northern vent. Trachytes are present in considerable volume, and here as
elsewhere alternate with dark doleritic lavas. They succeeded the ande-
sites in the order of eruption. Here we find also the same inclination of
the pseudo-strata which is observed in the Blue Mountain, the layers dip-
ping away from the central mass in opposite directions.

Around this eruptive locus we find also those great beds of conglom-
erate which are so conspicuous throughout the entire district and especially
in its southern portions. A mighty wall of .this material is presented
towards Sevier Valley, just north of the middle vent, and extends for about
8 miles in that direction, where it thins out; but before being quite lost by
attenuation is cut off by erosion. It is well stratified and weathers into an
abrupt cliff’ Here, as elsewhere, it was formed in an ancient valley, lying
between the two vents, and has the alluvial-cone structure. The great
Sevier fault has cut the formation, and its continuation is seen upon the
eastern slopes of Sevier Valley, 3,000 feet below. Upon the southern side
of the vent the conglomerate is seen in still greater mass. In truth, its
magnitude here becomes astonishing.

Upon the Grass Valley side of this central eruptive locality is seen

what is undoubtedly a remnant of a very ancient volcanic cone, afterwards

CENTRAL VENTS OF THE SEVIER PLATEAU. 235)

completely buried in the seas of lavas which were poured out around it.
At a later date it has been excavated by the erosion of Grass Valley and
one side of it exposed. ‘This is a large tufa-cone, which must once have
been nearly 1,800 feet high, and was formed by showers of small frag-
ments blown from the orifice. They are seen dipping to the southeastward
in a large ravine recently excavated in the side of the plateau, and the
angle of dip is from 28 to 30 degrees near the summit, but decreases towards
the base. The fragments are mostly augitic andesite and are closely com-
pacted with very little cementing material. They are very sharp and
angular, showing no evidence at all of attrition The stratification is quite
perfect and the entire mass is thoroughly consolidated into a coherent body
of stratiform layers. It is noticeable that the fragments are seldom of large
size, rarely exceeding in weight ten or fifteen pounds. Only a small seg-
ment of this cone is now exposed, and such portions as have been excavated
have been ruthlessly attacked by the waters, which have incised deep
ravines, which are destroying the cone almost as fast as they are unearthing
it. Far above it rise the massy sheets of trachyte and the pediments
formed in the projecting sheets lap around it on both sides. Probably it is
a very common thing in the history of a volcanic pile for its earlier cones
and monticules to be overwhelmed and buried by later outpours. But it
may give some notion of the magnitude and grandeur of the eruptions of
the Sevier Plateau to see a cone of this magnitude inclosed in rock, as if it
were a mere trifle.

The conglomerate forms the principal mass of the plateau south of the
central vents for a distance of nearly 20 miles, where it becomes confluent
with similar beds derived from the volcanic masses disgorged from the
southern vents. It is frequently intercalated with enormous sheets of horn-
blendic trachyte, erupted during the long period occupied by the accumula-
tion. The conglomerate forms the intervening summit of the plateau
between the eruptive localities, and has a thickness never less than a
thousand feet and several exposures show more than 1,600 feet of it. Into
its composition enter all the varieties of the andesitic and trachytic rocks
forming the series of eruptive masses to the northward, which are cemented
together by volcanic sand and decomposed fine detrital matter. The

236 GEOLOGY OF THE HIGH PLATEAUS.

degree of consolidation is always considerable and is quite sufficient to
enable the edges to stand in great mural fronts many hundreds of feet in
height. In this respect it is as consistent as any of the calcareous sand-
stones of the region. It is, however, more easily attacked by the rains
and frost than the volcanics or even than the more massive kinds of sand-
stone. ‘The included fragments exhibit all degrees of roundness by attrition;
are often quite sharp and angular; most frequently a little worn by current-
action; sometimes greatly so. Where the fragments are least worn they
are most abundant In many places the amount of cement is much less
than others, while in some places the fragments are relatively few. In size,
the fragments vary from a mere granule to two or three tons. The con-
glomerates are seen upon the slopes of Sevier Valley at the foot of the
western front of the plateau usually flexed upward a little and then cut off
by the great fault. On the east side of the plateau they slope down
towards Grass Valley (which is in great part a valley of erosion), and are
cut off in some places and dip beneath its floor in others, but reappear in
the western front of the Awapa Plateau. Whether these beds which are
seen in the Awapa are continuations of those in the Sevier Plateau is not
absolutely certain, but I think they are.

About midway between the middle and southern eruptive centers the
Sevier Plateau is cut completely in twain by a mighty gorge called the
East Fork Canon. It is the old story—erosion. The plateau rose athwart
the course of the stream and was sawed in two. It is not a narrow chasm,
but a valley walled by ledge upon ledge. The dissevered beds above stand
a couple of miles or more apart facing each other across the depths; below,
the walls are from 1,000 to 2,000 feet assunder. The total depth varies in
different parts from 1,400 to 3,700 feet. The structure of the plateau is
thus clearly revealed. The upper rocks are volcanic conglomerate of
immense thickness, with intercalary sheets of coarse trachyte, the former
well stratified. The lower rocks are of a highly exceptional character, and
will be treated of at length in the latter part of this chapter.

The third eruptive focus of the Sevier Plateau stands east of the head
of Panquitch Canon. It bears a strong resemblance in its features and the

character of its emanations to the northern vent (Blue Mountain). It is

SOUTHERN ERUPTIVE CENTER. 237

not, however, so well exposed, and much less can be said about it. A
grand ravine has eaten its way into it from the western side and disclosed
at the base propylite and hornblendic andesite in great masses, and exhibit-
ing evidence of an early period of great erosion followed by the eruption
of augitic andesites and many forms of trachyte, which buried the ancient
piles beneath their floods. A few fragmentary exposures of old conglom-
erate, consisting of the ruins of the most ancient lavas, are also revealed
near the base. Some of these have been so thoroughly metamorphosed
that they form almost a homogeneous mass, in which the cement has an
aspect closely resembling the fragments it envelops, and is shot through
with minute crystals of feldspar and secondary hornblende. When broken,
the surface of fracture cuts the pebbles and cement indifferently. The
propylites and hornblendic andesites are more profusely charged with horn-
blende than those of the northern vent, and the propylites are rather finer
in texture. The great mass of rocks now visible in this part of the plateau
are of the trachytic series and later in age. They are mostly of the
‘argilloid’ varieties, but contain fewer porphyritic crystals of orthoclase
than are usually found in such lavas, and are heavily charged with ferritic
matter, giving them a dirty brown appearance. Those eruptions which
flowed westward commingled with those which emanated from Dog Valley,
about 12 to 15 miles westward. Of those which flowed eastward I know
but little. I have no doubt that they are well exposed in many of the
ravines which descend from the crest of the plateau towards the foot of the
Aquarius. I have hastily crossed them once, but have no conception of
them sufficiently clear to justify me in attempting to describe them. My
field-notes indicate a broad expanse of trachytic and andesitic rocks inter-
bedded with volcanic conglomerate sloping gently towards the east and
appearing to emanate from the above-mentioned source.

The eruptions from this source did not extend more than 6 or 7
miles southward. On the west side of the Sevier Plateau the last that was
seen of them was ina deep canon-like ravine, called Sanford Canon, open-
ing into Panquitch Valley about 6 miles south of the head of Panquitch
Canon. Here the strictly eruptive part of the plateau ends, and the con-
tinuation of it southward is composed of Tertiary beds of the Bitter Creek

238 GEOLOGY OF THE HIGH PLATEAUS.

group, overlaid by an enormous mass of volcanic conglomerate. Between
the two are thin layers of those fine-grained marls and sandstones which
have been derived from the decay of ancient lavas, and which were evi-
dently deposited in water. Of the age of these intermediate beds it is pos-
sible to say but little. They are apparently conformable to the Bitter Creek
below, but the conformity is no proof of continuity of deposition. They
contain no fossils. The finer marly and arenaceous deposits are often of
an exquisite apple-green color, and in some of the exposures the color is
most charmingly delicate. The larger masses are from strong gray to
white, when the grain is fine, and brown when it is coarse. Small decayed
granules of volcanic sand, hornblendes, mica, and a green mineral, which may
be epidote or “‘ viridite,” are intimately commingled. Veins of chalcedony
and agate often cut the beds, and the fragments strew the soils and bad-
land at the foot of the cliffs.

The fault which uplifts the plateau has not been affected in any notice-
able manner by its passage from the volcanic to the sedimentary region.
_ It cut through a country which had apparently been long in repose; where
time had been gradually smoothing down the inequalities which had been
produced by volcanic activity. When this new disturbance set in it seems
to have laid out its line of operations regardless of existing inequalities,
splitting whatever it found in its way. In the southern part of the Sevier
Plateau it has sheared the old volcanic pile, and passing southward among
the sedimentaries and conglomerates it treated them in the same fashion.

The termination of the Sevier Plateau southward is effected by cliffs
of conglomerate fringed with buttes. The conglomerate attenuates in that
direction, and when its thickness has diminished to about 600 feet itis cut off
by the undermining of the sedimentaries upon which it rests. At the end of
the plateau the Sevier fault has diminished its throws to less than a thousand
feet, and farther southward the throw reaches a minimum of about 600 feet,
and thenceforward it increases again. This has produced a very slight sag,
in which les the Panquitch Hayfield, a broad valley-plain having an abso-
lute altitude of a little less than 7,000 feet.

SUCCESSION OF ERUPTIONS—SECTIONS. 239

SUCCESSION OF ERUPTIONS IN THE SEVIER PLATEAU.

The following successions of volcanic beds were observed in the Sevier
Plateau. An effort was made to obtain some good sections in the Monroe
amphitheater, but proved unsuccessful, partly owing to the difficulty of scal-
ing the rock faces and penetrating the clefts, and partly to the fact that the
chaotie condition of the rocks in many places makes the section of doubt-
ful value. Thus lavas of later age, filling ravines scoured in older floods,
occupy lower positions than the latter, and the contacts are lateral instead
of by superposition. Some present thick lenticular outcrops, some recur
(probably) at different altitudes. There is much local shattering and fault-
ing which cannot be restored, and many masses vary so much in thickness
that it would be misleading to state it without qualification. Most of the
heavy masses are presumed to consist of several distinct coulées, but the
separation is rarely visible or accessible. These difficulties and many
others increase towards the base of the series and are troublesome near the
summit. The chief value of a collection of sections is the illustration it
furnishes of the secular order of eruptions of the various groups of rocks
and their intercalary character.

Section I.

Commencing at the summit of Mount Thurber and descending south-
west; altitude, about 11,160 feet.

Feet
I. Granitoid trachyte, composed of layers, ranging from 30 to 80 feet in thickness,
the number of which is unknown, and varying but little in lithological
QUIK St oR ae nao Ram ke aoe toccon Sach Soe T en Een maS Ramee RAB ereL 280
2, Ocenia GOK, Sanael IPRS 54405 consan Sadao pcosaagudassddeaoacsdsudeus 60
3. Somewhat finer dolerite, but with well-marked porphyritic plagioclase... .. -. 30
4yeAroiloidstrachy te; reddish browaerceirse cia) ae cise ise \o eels sees tetecnes LAO
5. Gray granitoid trachyte..--- PN NPA Ne aral aatts aici Stavskersigi aisvarei ad Ristaae ate aremene 40
GRDoleritemveryeine-orainedsandecompackeeeereaes ss ene eee eee eee 3: 2
(eearcillordstrachybemseyeralulayersee errr sericea eae. eee er seenee eee ae ser 110
8. Very coarse and porphyritic dolerite, dark gray, many layers. - - - eee eae Sa 85
9. Granitoid trachytes, several layers, thickness unknown; only 60 feet meas-

ELEC Capen emma NT Vr rer ie eect eter (eae, ue ch ll fee ileal. ee erate a Tene yd 60

240 GEOLOGY OF THE HIGH PLATEAUS.

ly

Section I].—Monror AMPHITHEATER.

Beginning at the verge of the upper amphitheater and descending west-
southwest; altitude, about 10,100 feet.

Argilloid trachyte, reddish brown, with large orthoclase erystals..........

. Granitoid trachyte, very coarse and somewhat hornblendic, three layers

and probably more: i=)2..4 a= eens ewe sierinie ss eiocis <sonid Sisto eitake ea eee

7 Mine-orained ‘dolerite. 8--..jeeciene set tercie ey Sine Setar ee rere ee
. Fine-grained dolerite, perhaps two layers........-.--... Dich ha cee reheat
. Hornblendic trachyte, rather fine grain..... ...........-...- ode Sie Soret

Granitoidtrach yte@s. sos cca e teieoe eta saree ioe eis aie eee ree

wlightineditrachytesbrick-likemexturesaeeee aerate eee eee eee eet eae
. Argilloid trachyte, light gray, with small crystals and grains of magnetite,

and! probably,/six orisevenilayerss- 1-15-24 seis e eo ei ee eee ea

. Augitic andesite, very massive and in many sheets .......-...---.-..-.-.-

Hornblendic#trachytesascas ce ence ene ee ne eee Oe eee ene

~wOoranitoidstrachyite. coarse rain ease e a - Peer aeeee ren cece reste eee eee

DOleTIbO ys M5. iy ite co sinns cin De eee ee SER EERE OEE eee Ronee eee
Granitoid trachyte, unknown thickness.

Section II] —Monroz AMPHITHEATER.

Beginning near the base of the great upper cliff on the northern side

of the amphitheatre and descending south-southwest; altitude, about 9,800

feet.
1. Granitoid trachyte, light reddish-brown, with crystals of magnetite... -....-
2. Granitoid trachyte, light gray, coarser than the foregoing, containing mag-

Netitere sae eee ee we Ek io es Re ES Se tes ee

. Argilloid trachyte, very heavy masses, probably several layers but divis-

ional lines not readily made out, dark-colored porphyritic crystals, much

; Augiticitrachyte\(?); several layerss---2>-+-> ose eens eee tee eects
+ ELornblendic: trachy,te(?)ssacccec. see circu ieee tetera ner sre eae eer
» Areilloiditrachyte.siehtireddish color: sss seen eee eee eee ener

SECTIONS IN THE SEVIER PLATEAU. 241

Section 1V.—Monror AMPHITHEATER,

Beginning near the central part of the upper verge of inner amphi-

theater; altitude, about 9,400 feet and descending west.

Feet
1. Granitoid trachyte ..... Slee tie nas Shae eth Pact cay ast ectatuar anny ciate cyte ape ella foi ated sistas 80
2. Dolerite, brownish gray, much weathered on the surface, much shattered
and splintered and falling apart in slabs and tiles, probably numerous
layers noudistinctlyseparables secre er eit ise ie eto lateral Celene = 160
3. Granitoid trachyte, rather dark gray, slightly hornblendic, and somewhat
NA PENNA! -S oe cabo concoasete saanes casem neces udasos GuoUps E55 ettcrssers 35
4, Granitoid trachyte, finer than above and rather lighter in color.....-..---- 30
DS mGranitoidatrachytes likesNos od merecieeeceee sear ese eania see SOORASE A 50
6. Granitoid trachyte, a little darker and coarser than the preceding ..... --. 65
RD) OLELIGE Ns Beis ci tere ccseioee is ome se ee me tel Qtcive Minne Salers Cathe uierepe el aereerereregeyaee 15
8. Argilloid trachyte, numerous areas very massive and not distinctly sepa-
TUE eye iat a= Sheela te one ein aatoveea ates otters tc ele Lin steno eee Sheets Se oe spare ee 420
QHD OCU a aise rsenere tes sce de la\chore ce es a syeyelevn wis anste ws ate alge os nies else isyerereelsters saiseree 50
105 -Aroilloidistrachyteys ss. cs sis oslo e ssrste s soe selec sere ders eels acim eines 50
dieAtroalloidigtrachiyitoremcn recrseiasals (eee stare Piste rae -etereenee tater ere 60
12. Argilloid trachyte. .... Nac F eects oare eave eta als cuchenwssiater ad ef neya sto eucratare deteyaretoe eres 55
1S}, UME Tyne COMmAlnVetits 6655: s5a500 S4ocDs ous pope opEeeaooeeoe anGgc0 cscC 140
14. Granitoid trachyte, dark colored, coarse grain, very hard and compact, and
in very massive layers..........-. ee eRe acon ant adc tar Ste 180
15. Dolerite or augitic andesite (?).....-.-....5.-.--.--..- a SR faa Oceano 100
16. Hornblendic trachyte, dark and rather fine grained.... -...........-----. 110
lfG IEG sal HNO TBO Ossscccss cocsoausadumnus suon0e noo uEasbo Sunase ED oSe 40
18. Hornblendic trachyte.....-...-.... eer eete aja aleve cletadeassetareustetaceucts iar eneecioere 50
19. Dark granivoid trachyte ...-....-. apes ereyare ciSisiceaceisiapectese stace od wie ksiseeieers 50
20 reEormblendichtrachytetec asserts eraser i tee) sei sieve, ayes) ofeievaye eesti 95
Pil, (Capsiernar dl Hee AK, IE ORV como ecasce Soos5000beaESousas yeoeoaGasEse 33
22. Hornblendic trachyte, dark, coarse grained. This and the preceding num-
bers below 15 probably consist of several layers each, not well separated - 75
23. Augitic andesite in many layers, hard, compact, fine grained, and all very
Tairieye sin CN KEIM NK a coakoudncdas seqquaes saogon sags boue naenss DEDbEeR 260
24. Conglomerate, containing fragments of hornblendic amelie and hornblen-
Gickandesitemereereecl sree eta Sibyoiets yar siateve wisi EN eee meses 60
25. Hornblendicitrachyte, numerous layers.2----.--:.- «2.2 .-..2----92-- == (2)350
26. Hornblendic andesite, no good estimate possible, but not less WIM cbcqcco6 200

16 HP

242 GEOLOGY OF THE HIGH PLATEAUS.

Section V.—Monroe AMPHITHEATER.

In the middle gorge of the amphitheater, beginning at 7,750 feet alti-

tude and descending to the west.

wh

Feet
FaGranitoidytrachy tee cee sence oe eee eee Eee eee eee ener 110
. Augitic andesite, in layers varying much in thickness along the exposure.. 200
. Hornblendic trachyte, very rough and coarse in texture, probably four or
1Y/O IBN sscesses cdocpesocecs coo sHsSsoscpoEooobEn aboddSbecoge cSSeés 330
/RTENINO GUNES coos conescssesoods or cosbesecoscode Saco wsosse pede cocons 80
Hornblendicitrachyitesssae- cts.) e-rineee eee ek eee enit Scere eae oe eee 40
wHornblendicitrachytessemeecereern teeter eater ee RAM re acts ta 40
Hornblendic trachyte--.....-..-.. nae Ieee Ger ne sendeone nosose 70

. Conglomerate, with fragments of hornblendic andesite and trachyte and au-

gitic andesite: 222532 oe nck Shee SES eee eee Oe eee eee 55

. Hornblendic andesite of unknown. thickness in numerous layers, with very

uneven divisional lines.

. Modern alluvial or torrential deposits of unknown thickness.
. Tufas, water laid, of unknown thickness.
. Hornblendic andesite, possibly forming a part of the same mass as No.9,

lying upon the eroded and highly-inclined surface of propylite; thickness
unknown.

. Hornblendic propylite, rising in a precipitous wall or barrier far above the

last-named mass, but also extending beneath it and unquestionably of
oreater antiquity sac coi ae sacs oc ieee ease ee ae ee Sisters ieee 775

Section VI.—Sevier PLareau.

In a large ravine about 34 miles south of Marysvale Peak, beginning

on the south side of the ravine and descending westward; altitude, 7,700 feet.

1. Hyaline trachyte, with a few porphyritice crystals, somewhat resembling a oe
liparite, but no free quartz... -. Oe mage Sin Rc MAS Et ent Sane 35
2. Granitoid trachyte, rather darks erayiess=—--) ee eee eee eee eee eee eee 22
3. Granitoid trachyte, dark gray and coarser than preceding ...--......----- 20
4, Granitoid trachyte .......- Be Pen Oa OOo ero GOSS 32
owGranitoid trachylet «2 coke nce ees pe ay eae ee et 28
6. Dolerite, with large crystals of plagioclase in 2 very fine base.......-....- 15
fiseriyolinetrachy te Similan GON Ose lege eyes oars eae ee 25
Sa Eyaline)trachyte; bricht reddish color =2--=--5--e eee eee eee eee eee 37
9. Argilloid trachyte, in several layers, varying slightly in character........- 90
10. Dolerite in several layers, similar to No. 6, with smaller crystals of plagio-
ClASO Las cect mee wat seek eee Ae see eee See eee eee 75
fi FAreiloid trachyte, in numerous layers|=>--52 >>> 42 see eee eee eee 160
12

- Tufas of unknown thickness, but u visible exposure of.--.....--......-- - 220

TUFACEOUS BEDS OF EAST FORK CANON. 243

EAST FORK CANON.

East Tork Canon is a great chasm cut through the Sevier Plateau
transversely at its narrowest part, dividing that uplift into two portions.
It is wholly the work of erosion, and is an excellent example of tlie persist-
ence of a river channel in spite of the great displacements of the country
along its course. The East Fork of the Sevier River carries the entire
drainage of Grass Valley, and has evidently done so through several long
geological periods. Grass Valley, as will be seen by the map, is the long
narrow depression lying at the eastern base of the Sevier Plateau, and is
parallel to Sevier Valley, lying west of the plateau. Between the loci of
these two valleys the plateau has been, through the later periods of geolog-
ical time, gradually hoisted several thousand feet. The uplift has been
greatest upon the west side of the table, which is bounded by the great
Sevier fault. From the western crest-line the plateau slopes eastward ; at
first very gently, then with a more pronounced descent as far as the wall
of the Awapa Plateau. There is no fault on the east side of the Sevier
table, but in some portions there is a cliff or abrupt slope caused by long
ages of erosion.

Ten or twelve miles north of the canon are the central vents of the
Sevier Plateau, already described as of very ancient date. Twelve or thir-
teen miles south are found the great andesitic and still greater trachytic
centers of eruption. Far back in Pliocene time this fork flowed between
these volcanic piles from east to west, joining the main stream of the Sevier
River at the foot of Circle Valley. The great changes of topography pro-
duced by the elevation of the Sevier Plateau have in no manner attected
the location of the fork, which has only sunk its channel as the table slowly
ascended. Very grand and imposing is the valley which it has carved
through this uplifted mass. It is not one of those deep, narrow chasms cut
into the earth, but a terraced valley of notable width, a distance of 2 to 5
miles separating the summit walls, with only a narrow bottom below. In
the natural section thus made nearly 4,000 feet of beds, composed wholly of
volcanic materials, are exposed. The river near the point of maximum cutting
just grazes the top of the yellow Tertiary lacustrine beds, exposing only a few
acres, but enough to assure us that we have here the entire volcanic series.

244 GEOLOGY OF THE HIGH PLATEAUS.

As we enter the lower gateway of the gorge ascending from Sevier
Valley we at once recognize the nature of the displacements which have
occurred. On the north side are seen immense beds of volcanic conglom-
erate dipping at angles varying from 12° to 25° to the westward. There
is much repetitive faulting here. Again and again the beds have sheared

Tic. 4.—FAvuLts AT LOWER END OF East Fork CaNon.

and slipped, the throws varying from 200 to 350 feet, all of them being
thrown to the eastward. More than 2,000 feet of conglomerate, beautifully
stratified in huge massy layers, with intercalations of dark hornblendic
trachyte of the roughest description, are exposed in this part of the gorge.
Suddenly we miss the conglomerates. They appear to end abruptly at a
lateral ravine which enters the main canon from the north, and on the
opposite side of the ravine the rocks are of a totally different character.
Through that ravine runs the main throw of the great Sevier fault, here of
about 2,500 feet of displacement. As we look beyond it and up to the tow-
ering crags of the principal plateau mass, we again recognize the continua-
tions of the conglomerates in the palisades bounding the tabular summit.
Beneath them another series of strata has been brought to light by the lift
of the fault and the erosion of the canon. These are tufaceous deposits,
presenting features of great interest.

The general aspect of these beds is shown in Heliotypes V and VI*
It is obvious at once from their very aspect that they are water-laid, yet
when closely examined all of them are seen to have been subject to altera-
tion in varying degree, which gives them the appearance of massive volcanic
rocks. There is one member about 120 feet in thickness which has the

character of a voleanie rock so pronounced that no person would doubt that

*The summit of the plateau is not visible from the points where the photographs were taken, as
the upper walls of the caiion are beyond the summits of the lower walls.

HleLioryer V

TTeliotype Printing Co., 220 Devonshive St., Boston,

TurA BEDS METAMORPHOSED. East FORK CANON.

TUFACEOUS BEDS OF EAST FORK CANON. 245

such is its real nature, if confining his examination to a hand-specimen and
unaware of its mode of occurrence. It has, however, some peculiarities not
common in lavas, though not sufficiently marked to justify their exclusion
from that category. It is an acid rock, carrying as much silica as some
rhyolites or extremely siliceous trachytes. Feldspar,- chiefly monoclinic, is
very abundant and in conspicuous, though not very large, crystals. The
most notable peculiarity is the abundance of accessory minerals, which is
not a common character in voleanic rocks so highly charged with silica.
Although they are seldom destitute of accessory minerals, my own observa-
tion has given me the impression that they are almost always scantily sup-
plied with them. These minerals are chiefly mica, hornblende, and plagio-
clase. There is also an unusually large quantity of peroxide of iron in a dif-
fused state, which has given the rock a strong reddish or pink color. It is
excessively hard and compact, and one of the most difficult to fracture of any
in the whole district. Its chemical composition allies it most nearly to rhy-
olite, but in texture and in mineral constituents it does not conform so nearly
to that group. The base, when examined microscopically, is similar to that
which is seen in rocks with a well-marked porphyritic habit. None of these
peculiarities would be alone sufficient to affect the conclusion that it is a
voleanic rock. My doubts have arisen from other considerations. Both
above and below it are thin beds composed of materials which more or less
closely resemble it, some so nearly that no appreciable distinctions can be
drawn, and these are surely sediments deposited and stratified where they
lie and altered by metamorphic action, some more, some less. A transition
can be traced, by selecting from the different layers, ranging from tufas
which have been but little altered to the extremely hard rock of pro-
nounced volcanic appearance. All of the little altered tufas show that they
are composed of water-worn volcanic sands and gravel, and in some which
are greatly altered the original pebbles are still visible.

The strata which are composed of volcanic débris seem to be extremely
susceptible to metamorphism. This is true not only of fine tufas, but of
conglomerates which have a pulverulent matrix. But what is most remark-
able is that the result of the alteration is not a wholly crystalline rock,
like gneiss or diorite or hornblendic schist, but one consisting of an amor-

246 GEOLOGY OF THE HIGH PLATEAUS.

phous base holding porphyritic crystals, which is the dominant and dis-
tinctive characteristic of a voleanic product. Not only are the various
stages of this alteration displayed here, but they may be seen in many
other localities within the district; and I infer that similar occurrences are
found in many other portions of the western mountain region.
Immediately beneath these tufas, in the heart of the cation, there is a
very small area of common sedimentary beds. Their age is not known,
since no fossils have been taken from them, but judging from their litho
logical character, they resemble the Upper Bitter Creek Tertiary; and
lithological correspondence here is of much more value than is elsewhere
attributable to it. They show no trace of alteration, which is all the more
remarkable when we find so much change in the beds which overlie them.
This relation of altered volcanic clastic beds to underlying unaltered Ter-
tiaries is also presented in the southern part of the Sevier Plateau. These
facts appear to emphasize still more strongly the assertion that tufaceous
deposits are extremely susceptible to metamorphism. Perhaps this ought
not be regarded as surprising. Ordinary sediments consist of materials
which have not only been comminuted, but also chemically decomposed
and separated into aggregations much simpler than those constituting
eruptive rocks, and their chemical correlatives among the metamorphics.
Among the common sedimentaries we find chiefly siliceous, argillaceous, or
calcareous deposits, with these ingredients commingled; but only now
and then presenting such components as would yield by metamorphism
yocks corresponding chemically to the volcanics. They are very poor in
alkali. The tufas, on the other hand, consist of materials which, though
thoroughly comminuted, are not so thoroughly decomposed as those con-
stituting the common sediments, and contain the constituents which by
mutual reaction are capable of yielding feldspars, hornblende, and mica.
The geologist in the field is often called upon to note instances of local
metamorphism for which he can discover no adequate local cause. On the
other hand, he often finds occurrences where metamorphism has not operated,
though the conditions seem to be identical with those which are elsewhere
believed to have produced it. The phenomena of contact metamorphism
have been sufficiently studied to enable us to say confidently that the

METAMORPHISM OF TUFACEOUS DEPOSITS. 247

proximity of heated magmas or the prevalence of high temperature within
a mass of strata are not the only conditions requisite for the activity of that
process. Strata traversed by eruptive dikes are sometimes altered for
many hundred feet from the contact and sometimes are wholly unaffected.
This fact alone indicates that something besides high temperature is re-
quired to produce such an alteration. Nor do all the conditions appear to
be fulfilled when strata containing suitable constituents are subjected to a
high temperature, for cases are common where rocks so constituted and con-
ditioned are not altered. Although we do not know all the requirements of
metamorphic action, we may feel confident that they are somewhat complex
and numerous. One inferential condition is that of a high degree of molecu-
lar mobility in the constituents, whereby a free interchange of molecules
among the clastic particles or fragments is made possible But precisely
how this is effected is a matter of conjecture. It may be by the permea-
tion of heated waters or other liquid or vaporous solvents which may not
require a very high temperature, and which may even be effectual at quite
moderate temperatures. How far we are required to postulate the absorp-
tion of foreign constituents (alkalis and earths) by the entire metamor-
phosed masses or the elimination of constituents which the masses origi-
nally contained are problems too conjectural in their nature for present
discussion. That the tufas of Hast Fork Canon should have been meta-
morphosed while the Tertiary (?) strata upon which they rest are wholly
unchanged is not a matter so wholly surprising. In the former beds all the
conditions precedent have been satisfied, in the latter they have not.

An examination of the heliotypes (V and VI) will show one member
more massive than the others which is about 120 feet in thickness. Under
ordinary circumstances this would have been pronounced an eruptive sheet
without much hesitation. But such a decision would raise some difficult
questions. Other layers much thinner, and in some cases not exceeding ore
or two feet in thickness, are composed of rock very similar to it. Others
show a transition from material apparently identical into unaltered or very
little altered tufa. In most of the beds rolled pebbles are found, and as the
varieties become more and more metamorphosed these pebbles become less
and less distinct; and in the massive sheet itself some of these pebbles may

248 GEOLOGY OF THE HIGH PLATEAUS.

still be discovered upon weathered surfaces, though in fresh fractures they
appear to have gained an aspect very nearly homogeneous with the general
mass. This phenomenon of the gradual vanishment of pebbles is not con-
fined to the tufas, but is frequently seen in the conglomerates, some of
which have been greatly altered and converted into a hard semi-crystalline
rock strongly resembling andesite and hornblendic trachyte. Moreover,
the inferior boundary of the larger sheet is indefinite in many places, and
near the fault it appears to have passed lower down and involved beds
which are not so much affected farther up the canon. The lines of bed-
ding near the fault are nearly obliterated, and the thickness of the lava-like
mass has greatly increased. I entertain very little doubt that the sheet is
not a lava, either contemporaneous or intrusive, but is a metamorphosed
tufaceous deposit.

Farther up the East Fork Canon, upon the north side, stands an iso-
lated mass, consisting of phonolite, represented in Heliotype No. XI. It is
a hill about 1,400 feet high, with steep flanks, covered with talus. Near
the summit the cleavage of the rock in vertical planes is exhibited with
clearness. Upon closer inspection a secondary cleavage, perpendicular to
the foregoing, is also disclosed, and the viscous vitreous character of the
lava is very conspicuous. Under the microscope it discloses very few
crystals, and these are very small, consisting of nephelin. No feldspar was
detected. The specimens brought home, though fair in appearance, proved
to be much weathered and hardly suitable for microscopic or chemical
investigation. The plateau mass around this hill was much eroded, and
the eruption of the phonolite appears to have occurred after the erosion
had far advanced, for it is an isolated mass, and its lavas flow over rugged

ridges and ravines upon its northern side.
GRASS VALLEY.

Separating the second and third ranges of tabular uplifts is a broad
depression, named Grass Valley; a name which has done great service in
the West, for it may be found in every State and Territory. It is properly

an appendage of the Sevier Plateau, from the platform of which it has been

GRASS VALLEY. 249

in part eroded. The eastern wall of the valley is the uplifted side of the
third plateau range, comprising the Fish Lake table at the north, the Awapa
in the middle, and the Aquarius at the south. This wall is everywhere
due to displacement. The western side of the valley is a wall of erosion
formed by the river sinking its channel and the subsequent decay of the
mesas by secular waste. The origin of the valley apparently antedates the
last general uplifting of the plateaus by a very long period, and its course
and general arrangement were probably determined by the configuration
of the country which was made at the close of the trachytic epoch of erup-
tions. The valley then lay between two long lines of volcanic vents, one
in the Sevier Plateau, the other in the Awapa, with a broad lava field
between them. The vertical movements which subsequently upheaved
those tables did not displace the course of the drainage, which only estab-
lished itself the more immutably in its original position.

The lowest point of the valley is not at either end, but a little south of
its mid-length, opposite the head of East Fork Canon. To this point two
streams flow, one from the north, the other from the south, and their waters,
here uniting, pass through the cation to join the Sevier. It was evidently
so from a remote epoch. The great cation itself was at first a mere depres-
sion between the central and southern trachytic vents of the Sevier Pla-
teau, but as that mass was upraised, the fork persisted in holding its
thoroughfare and cut the rising platform in twain. At one epoch the rate
of elevation was sufficiently rapid to dam the fork and create a lake in the
valley, which may have been 15 or 20 miles in length. Remnants of
old lake beaches are still visible on the southern and eastern sides of the
valley, and these possess considerable interest. They are best displayed

“where Mesa Creek merges from its gorge in the northwestern angle of the
Aquarius. They consist of beds which are composed of a mixture of the
ordinary detritus which comes from the waste of sedimentary sandstones
and that which is derived from the decay of volcanic rocks. Where the
former greatly preponderates, the resulting strata have the usual aspect of
the lacustrine Tertiary deposits; and where the latter is in great excess the
beds have the same appearance and characters as the stratified tufas else-

250 GEOLOGY OF THE HIGH PLATEAUS.

where described as of middle or late Eocene age. The case is also pre-
sented where the same stratum, traced horizontally along its exposure,
passes gradually from one kind into the other. These beds are probably
of greater antiquity than the Bonneville beaches around the shores of Great
Salt Lake, being in a much more dilapidated condition and only occasional
remnants being preserved. Near the head of East Fork Canon, a large
‘““meadow”* or bog, formed by the accumulation of the finest river silt,
deposited by slack water, still indicates the recency of the same struggle
between the uplifting of the plateau tending to dam the stream and the
agency of the running water in carving its channel and lowering its outlet.

Perhaps the most striking phenomena which may be seen in Grass
Valley are the great alluvial cones now forming in the northern and mid-
dle portions of it. The great gorge of the Fish Lake Plateau opens into it
near the northern end, and a very flat cone, with a radius nearly 33 miles
in length, has been built of the detritus brought down from that chasm.
Viewed from the summit of that table, which rises 4,300 feet above it, the
periphery of the cone is seen to be very nearly circular through an are
of about 120°, becoming confluent with another great cone south of it.
Many others of equal magnitude and quite perfect in form are displayed
down the valley, most of them sloping from the Sevier side. They are
composed of fragments which are not much abraded or rounded by attri-
tion, and whatever waste they have suffered seems to be due as much to
slow weathering as to abrasion. They are held in a matrix of soil which is
highly fertile when watered, but too stony for the plough. They vary in
size from a few ounces to a few pounds, and near the apices of the cones
they are found weighing many hundreds of pounds. At numerous places
the shiftings of the streams have enabled them to cut into the cones locally,
and the sections always reveal a pronounced stratification. Comparing
them with the ancient conglomerates now exposed in the plateau walls on
either side of the valley, it is impossible to doubt the identity of the pro-
cesses which have accumulated both.

No sedimentary formations are found in the northern part of Grass

“In the West, a marshy locality formed by the accumulation of vegetable mold and river silt,
yielding a peculiar wild grass, is called a ‘‘meadow.” Ina moister country it would be simply a bog.

PAUNSAGUNT PLATEAU. Di

Valley. In the southern portion, the lift of the Awapa fault has brought
up the Tertiary strata and exposed them in the wall of that plateau. The
rocks exhibited in the valley proper are all volcanic. They are chiefly
trachytic, and only here and there project above the masses of alluvial
matter which is. gradually burying them. Just south of East Fork Canon
a few large coulées of basalt are seen, and they appear to have emanated
from the vicinity of the Awapa fault. They form broad terraces, rising one
behind another, and ending in cliff and talus 60 to 80 feet in height. They
have been much battered by erosion and are no doubt of considerable
antiquity. Basalts of similar character are found overspreading consider-
able tracts upon the summit of the Awapa Plateau near its western verge
and upon the northwestern edge of the Aquarius.

PAUNSAGUNT PLATEAU AND PARIA VALLEY.

Crossing the Panquitch Hayfield we reach the foot of a very gentle
slope, which rises almost insensibly to the southward, forming a plateau of
the ordinary type called the Paunsdgunt.* Its length is about 25 miles
and its width from 8 to 12 miles. It lies in the southward prolongation of
the major axis of the Sevier Plateau, from which it is separated by the
shallow depression of the Panquitch Hayfield. Its western front is formed
by the uplifted side of the Sevier fault. Its eastern front is a cliff of ero-
sion looking down into the Upper Paria Valley; a valley of erosion drain-
ing into the Colorado. There is a fault a little distance from the eastern
wall running north-northeast, but the Paunsigunt is upon the thrown side
of it. So great has been the erosion in Paria Valley that, notwithstanding
the greater altitude of the strata within it than the altitude of their con-
tinuations in the plateau, the valley is from 3,000 to 3,500 feet below the
plateau summit. If the denuded strata could be restored, they would make
the locus of the valley nearly 2,000 feet higher than the plateau.

The Paunsdtgunt is composed wholly of sedimentary beds: Eocene
resting upon Cretaceous. The stratification is sensibly horizontal, though
at several localities on the eastern flank the junction of the two series is

*Paunsigunt means the ‘‘place of the beavers.”

952 GEOLOGY OF THE HIGH PLATEAUS.

unconformable. In the cliffs of the eastern and southern margins the fol-
lowing series is presented:

Feet.
1. Gray calcareous sandstone ........-.--- BEE BHBEED POU SUF oUKHUR aoe Sho percent -aleN0)
QE \iviniligs MTN OWE So Gicss adosescpsoacd LUG acooopabeuHDoo DDO OOdCOssoCSseS 160
3. Red marly limestones and calcareous shales ......-. Se Se ee 300
47 Red) pinkish limestone seen chee ie eer ase eee eee eee eee 450
5. Conglomerate, with small pebbles and gravelly sandstone...........-...--- 190

1,280

Below these are the characteristic gray Cretaceous shales, somewhat
arenaceous, forming long spurs and foot-hills. They do not here form
cliffs, but long slopes, descending into the lower regions adjoining. From
the southern extremity of the Paunsdigunt they rise with a slight inclination
towards the south and are beveled off by erosion. At one point the sec-
tion crosses (southward) a decided monoclinal flexure with a maximum dip
of about 10° to 12° trending east and west, but quickly reflexing back to
a dip of 8° to 4°. One after another the formations end in cliffs and
ledges, and the profiles drop at each crest-line upon lower beds, until at a
distance of about 23 miles from the southern end of the plateau the carbon-
iferous forms the final platform, and rises gently but continuously to the
Grand Canon.

The western side of the plateau looks down from its northern half
upon the valley which carries the upper waters of the South Fork of the
Sevier River. Across this valley the gentle slopes of the Markdgunt rise
towards the west. Along this base of the Paunsigunt runs the Sevier fault,
but before reaching the end of the plateau its course changes from south
to the southwest. Just where this change occurs is the divide between the
valley of the Sevier and the headwaters of the Virgin, a tributary of the
Colorado. The wall of the plateau thenceforward becomes a cliff of ero-
sion gradually swinging to the southeast, then around the end of the table
(which projects southward like a great promontory), and finally trends to
the northward. The summit of the table has a central stream which
gathers all the drainage and carries it northward to the Panquitch Hay-
field, thence into the East Fork by the way of Grass Valley, and finally
through East Fork Canon into the Sevier River.

MNIG

"SHAITO

‘ANHOOW YMAMOT

‘OVEALVTG( LNOASVSNOVG

“OY Suit aparjouyay

*UOSOT “7S 2.42 SUORICGE OTT

NITAT]

‘ITA

PARIA AMPHITHEATER. Dye

From the southern cape of the plateau we look southward over an
immense expanse. The Kaibab is in full view, stretching away south-
ward until its flat summit and straight palisade is lost in illimitable distance.
To the southwest Mount Trumbull is seen nearly a hundred miles away.
To the southeast a farrago of cliffs and buttes of strange forms and vivid
colors breaks up the monotony of the scene. But the eastern and north-
eastern view is one which the beholder will not easily forget. It is the
great amphitheater of the Paria.*

An almost semicircular area, with a chord 30 miles in length, has been
excavated into a valley by numberless creeks and brooks, which unite into
one stream named the Paria. This stream is at present a mere thread of
water flowing southward to the Colorado, which it reaches at the head of
the Marble Caton. During nine months of the year so feeble is the stream
that it sinks in the sands before reaching the Colorado, but it is a raging
torrent during the months when the snows are melting. The many tribu-
taries which ramify in all directions are generally dry during the greater
part of the year, but a few of them are perennial. [very one of these
little streamlets has cut its canon, and nearly all of them are abrupt and
impassible save by very difficult and tortuous trails made by Indians and
preserved from obliteration by the few herdsmen who pasture cattle in the
vicinity. Yet it seems that at a comparatively late geological epoch the
climate may have been much moister than at present, and these many
water-ways carried perennial streams. Such a climate in all probability
prevailed during the glacial period and during the Miocene age. The
amount of erosion which has here been produced is very great. By refer-
ence to the stereogram it will be seen that the locus of the Paria Valley is
constructed as a great uplift. The strata which are found within its con-
fines occupy much higher horizons than their continuations beneath the
Kaiparowits Plateau on the east and the Paunsdgunt Plateau on the west.
In these two plateaus the erosion has been small for some reason, while in
the Paria Valley it has been very great, approaching in extent the vast
erosion which has taken place to the southward in the Kaibab district.

* In the pronunciation of this name the vowels have the German sound, and the accent is on the
middle syllable (Pah-ri-ah). It is the Ute name for elk.

254 GEOLOGY OF THE HIGH PLATEAUS.

From the center of the great Paria Valley or amphitheater the dip of the
strata is semi-quaquaversal; that is, towards the east, north, and west, and
all intermediate directions ; but towards the south the strata incline upwards.
The erosion has been greatest in the center of the amphitheater, and has
proceeded radially outwards just as in the San Rafael Swell. This process
has left the strata in terraced cliffs facing the center of the amphitheater,
and as we look across from the southern cape of the Paunsdgunt to Table
Cliff and Kaiparowits Peak, more than 30 miles distant, we behold the
edges of the strata, sculptured and carved in a fashion that kindles enthusi-
asm in the dullest mind. At the base of the series the vermilion sandstones
of the Upper Trias are seen in massive palisades and gorgeous friezes,
stretching away to the southward till lost in the distance. Above them is
the still more massive Jurassic sandstone, pale gray and nearly white,
without sculptured details, but imposing from the magnitude and solidity
of its fronts. Next rises in a succession of terraces the whole Cretaceous
system more than 4,000 feet in thickness. It consists of broad alternating
bands of bright yellow sandstone and dark iron-gray argillaceous shales,
the several homogeneous members ranging in thickness from 600 to 1,000
feet. But the glory of all this rock-work is seen in the Pink Cliffs, the
exposed edges of the Lower Eocene strata. The resemblances to strict
architectural forms are often startling. ‘The upper tier of the vast amphi-
theater is one mighty ruined colonnade. Standing obelisks, prostrate col-
umns, shattered capitals, panels, niches, buttresses, repetitions of sym-
metrical forms, all bring vividly before the mind suggestions of the work
of giant hands, a race of genii once rearing temples of rock, but now
chained up in a spell of enchantment, while their structures are falling in
ruins through centuries of decay. Along the southern and southeastern
flank of the Paunsdgunt these ruins stretch mile after mile. But the crown-
ing work is Table Cliff in the background. Standing 11,000 feet above
sea-level and projected against the deep blue of the western sky, it presents
the aspect of a vast Acropolis crowned with a Parthenon. It is hard to
dispel the fancy that this is a work of some intelligence and design akin to
that of humanity, but far grander. Such glorious tints, such keen con-

trasts of light and shade, such profusion of sculptured forms, can never be

PARIA AMPHITHEATER. 255

forgotten by him who has once beheld it. This is one of the grand pano-
ramas of the Plateau Country and typical in all respects. To the eye which
is not trained to it and to the mind which is not inured to its strangeness,
its desolation and grotesqueness may be repulsive rather than attractive,
but to the mind which has grown into sympathy with such scenes it con-
veys a sense of power and grandeur and a fullness of meaning which lay
hold of the sensibilities more forcibly than tropical verdure or snow-clad
Alps or Arcadian valleys.

The Amphitheater or Upper Valley of the Paria seems from the sum-
mit of the Pink Cliffs to be a slightly rugged basin, but like most of the
Plateau Country it is found to bea difficult field to traverse. A network of
sharp canons several hundred feet in depth ramifies through it, and the
traveler is apt to become entangled in their mazes, and find himself con-
fronted every few miles with an impassable chasm, never seen until he is
almost upon the point of driving his mule into it. A few tortuous traits
wind deftly among them, leading by break-neck paths into their depths
and out again, and finally into the broad and grotesquely picturesque bot-
tom of the Paria River.

The Paunsdgunt is the southernmost extension of the system of the
High Plateaus, and is a promontory thrust out into the terraces which step
by step drop down to the Kaibab district. In this series of terraces are
exposed the edges, almost always cliffwise, of the entire Mesozoic system
of the region. Just here the Cretaceous does not form such conspicuous
cliffs as it presents farther east, but the Jurassic and Triassic series are seen
to the southward in their most typical forms. The exposures are truly
magnificent. While the cliffs front southward, presenting in naked walls
their entire thickness and disclosing every line, they are also cut from north
to south and sometimes diagonally by cations, which reveal their dip and
structure. But as these terraces are more properly a part of the Kaibab
system, no detailed description will be given of them here. The Paunsa-
gunt itself is a simple tabular block of Lower Eocene beds, of which a
section has just been given. It is exceedingly simple inyits structure, and,
further than has been already described, presents very little matter for
special remark. It is destitute of eruptive rocks, except at its northern

256 GEOLOGY OF THE HIGH PLATEAUS.

end, where a number of basalt streams appear to have burst out of the
western wall near the summit and poured down upon the talus and
slopes below. They are of small extent and mass, and are noteworthy
only as an instance of the peculiar positions from which basalt sometimes
breaks out.

A few miles to the south of the southern cape of the plateau is another
small field of basaltic eruption. It is located in the bottom of a rather
broad valley or basin. A large cinder-cone is still standing singularly
perfect in symmetry and perfect also in its preservation. The cup at the
summit is not broken down, but still preserves a continuous rim. From
this cone streams of basalt flow southward, and entering a canon in the
Jurassic sandstone reach the front of the White Cliffs nearly 12 miles from
their source. The individual streams have spread out very thin, and are
in some places very slender, with every indication of extreme fluidity at
the time of their passage. In the canon the basalt is nearly all swept
away by erosion, only a few small patches (in situ) being left to indicate
its former existence. But beyond the canon larger remnants are seen, and
these evidently formed the terminations of the coulées. It is impossible to
affirm anything as to the age of this basalt, though I have little doubt that
all the damage it has suffered from weathering and erosion might surely
have been accomplished in the period of a thousand years and perhaps in
a shorter time. On the other hand, it may be several thousand years since
the vent became silent. Four miles to the west of this cone stand half a
dozen others, perched high upon cliffs or mesas, and sending their streams
into the upper canon of Kanab Creek. These appear to be older and more
weather-beaten, though evidently belonging to the most recent geological

history of the country.

Opist NIP AN 1D) 1k CI IE.

THE FISH LAKE PLATEAU.—THE AWAPA.—THOUSAND LAKE
MOUNTAIN.

Southern extension of the Wasatch monocline across Salina Caho.1.—Its bifurcation into the Sevier and
Grass Valley faults.—Strawberry Valley.—Ascent of the northern slopes of Fish Lake Plateau.—
Summit Valley.—Tertiary exposures.—Iish Lake Plateau.—Its summit.—The great gorge and
cliffs.—Sources of the volcanic sheets.—Origin of the gorge.—Fish Lake.—Moraines.—Reversal of
the course of the drainage.—Alcoves in the plateau wall.—Succession of beds.—Trachytes and
dolerites.—Augitic andesites.—Location of the vents and sources of the lavas.—Outlet of the lake.—
Mount Terrill.—Mount Marvine.—Origin of Summit Valley.—Isolation of Mount Marvine from
its parent mass.—Moraine Valley.—Exposures of Tertiary beds.—Mount Hilgard.—Gilson’s Crest.—
Lavas of Mount Hilgard.—The Awapa.—Its general configuration and structure.—Its desolate
character.—Great variety of rocks displayed in the Awapa.—Hornblendic and granitoid tra-
chytes.—Conglomerates.— Propylites.— Basaltic fields of ancient date.— Rabbit Valley.—Its
structural origin.—Erosion of the lava sheets around the borders of the valley.—Accumulation
of modern alluvial conglomerates.—Exposures of Tertiary beds in Rabbit Valley.—Thousand
Lake Mountain.—A remnant of the grand erosion of the Plateau Province.—Lava Cap.—Under-
lying Tertiary.—Absence of the Cretaceous and unconformity of the Tertiary with the Jurassie.—
The Water Pocket flexure and its age.—Jurassic sandstone.—Triassic beds.—The Shinérump
and its sculptured cliff—The Red Gate.—The separation of the mountain from the Aquarius
Plateau.

The third range of plateaus, including the Fish Lake, the Awapa, and
the Aquarius, are not inferior in interest to those already described. Con-
nected with them are the masses of Mounts Marvine and Hilgard with the
intervening valleys. Far to the northward, in the extension of the same
line, is the Wasatch Plateau, of which the structure has already been
described. The great monoclinal slope which forms its western flank splits
gradually into two displacements in its southward extension, one of which
forms the Sevier fault, and the other, passing gradually from a monoclinal
into a sharp dislocation, forms the Grass Valley fault on the eastern side of
Grass Valley. The uplifting along the course of the Sevier fault has pro-
duced the Sevier Plateau. The uplifting along the other branch or Grass
Valley fault has given rise to the Fish Lake table and the Awapa Plateau.

257

17 HP

258 GEOLOGY OF THE HIGH PLATEAUS.

As we go southward from the Wasatch Plateau, crossing Salina Canon near
its middle, we at once begin to ascend the northern slopes of the third
chain. We are among the sedimentaries, which dip gently to the westward ;
and descending from the south, a noble valley opens into the middle of
Salina Cation, with the edges of the lowest Tertiary beds walling it abruptly
on the west and the surface of the Upper Cretaceous rising gradually on
the east. This lateral valley is named, locally, Strawberry Valley—a name
which recurs with great frequency throughout the mountain regions of the
West. As we move upward towards the south the dip of the beds increases,
and the very long and gentle inclination of the strata at length becomes
wrinkled into a monoclinal of large proportions. We perceive this readily
when, at a distance of 4 or 5 miles south of Salina Canon, we climb the
western wall of Strawberry Valley, and see directly in front of us to the
southward the Tertiary beds covered with immense sheets of old lava,
but exposed beneath in a deep ravine. We see them rising monoclinally
from the west and smoothing out eastwardly to a sensibly horizontal posi-
tion at a high altitude. The underlying sedimentaries are well exposed,
for erosion has carved away much of the country to the northward and
given admirable sections transverse to the main structure lines and axes.
Three days’ inspection of these northern flanks will convey a full concep-
tion of the general features of the structure, for they are very easily read.

Climbing the western wall of Strawberry Valley, we reach a platform
about 2 miles wide, from which start the long slopes leading up to higher
levels. Immediately in front is the Fish Lake Plateau, full 4,000 feet above
us. To the south-southeast is an easy ramp leading up to Summit Valley,
an elevated interspace between Fish Lake Plateau and Mount Marvine.
As we ascend this grade, we have on the right a deep ravine carved into
the general plateau mass, laying bare, in an admirable manner, the sweep-
ing curyes of the Tertiary beds, overlaid by trachyte, both being bent into
typical monoclinal form. The strike of this monoclinal is visible, extending
south-southwest nearly 15 miles, giving origin to a slope varying in inclina-
tion from 18° to 20°, and with no other ravines than the one just mentioned.
It conveys to the eye an impression of singular smoothness—like a vast roof.
The slope we are ascending is much more uneven; and, at an altitude of

=

SUMMIT VALLEY. 259

about 9,300 feet, brings us upon the floor of Summit Valley. Upon the west
is a sharp crest-line, constituting the eastern verge of Fish Lake Plateau,
which overlooks the valley from an altitude of 11,000 to 11,400 feet.
Upon the east side rise two conspicuous masses—Mount Terrill and Mount
Marvine. This valley is an excellent starting-point, from which we may
make excursions radiating in many directions, and study in detail the diver-
sified objects which compose the surrounding country. And, first, let us
look at the nature of the valley itself.

Not the smallest among its attractions for the geologist is the fact that
it is a most eligible summer camping-place. In the daytime, through-
out July, August, and most of September, it is mild and genial, while
the nights are frosty and conducive to rest. The grass is long, luxuriant,
and aglow with flowers. Clumps of spruce and aspen furnish shade
from the keen rays of the sun, and fuel is in abundance for camp-fires.
Thus the great requsites for Western camp-life, fuel, water, and grass, are
richly supplied, while neither is in such excess as to be an obstacle to pro-
eress and examination. :

The valley floor is, for the most part, Lower Tertiary. For a consider-
able portion of the length the edges of these beds are exposed upon the
eastern side of the valley, forming the lower slopes of Mounts Terrill and
Marvine. They are also seen at the base of the Fish Lake slopes; but a
little higher up they are covered with ancient lavas. Northward, however,
lavas form the floor of the valley. Proceeding in that direction a few miles,
the mountain-walls which inclose the valley rapidly decline in altitude
and die away in steep slopes, while the platform on which we travel at
length becomes the summit of a plateau, having an altitude about 2,000
feet lower than the neighboring tables; and projecting 4 or 5 miles farther
northward, it ends in abrupt volcanic cliffs, from the crests of which we
overlook all the space which intervenes between them and the Wasatch
Plateau, 20 miles distant. ‘The thickness of the lava at these cliffs is about
700 feet, and is composed of hornblendic trachytes in very massive sheets,
alternating with augitic andesites, which are much thinner. Retracing our
steps and traveling to the southern end of the valley, we find its floor undu-
lating with little hills, a part of which are Eocene beds and a part are old

260 GEOLOGY OF THE HIGH PLATEAUS.

terminal moraines, of which more will be said hereafter. A fine stream
runs along the valley, and at the southern end is joined by a still larger one,
issuing from Fish Lake, a few miles to the south-west.

FISH LAKE PLATEAU.

An easy way of reaching the top of this plateau is by ascending its
northeastern angle from Summit Valley. If the route be well chosen, we
may reach the highest point without once dismounting. The summit is
about 12 miles in length and 2 miles in width; is nearly level, or very slightly
undulated; and stands about 11,600 feet above sea-level. On every side it
is bounded by precipitous cliffs, except along a part of its southwestern
flank, but here and there the walls are broken and notched. Along the side
facing west-northwest runs a cliff of vast proportions, second only to
the western front of the Sevier Plateau in magnitude and grandeur. Upon
the very brink of this wall is the highest point of the plateau, from which,
ina clear day, we may easily discern the peaks of the Wasatch around
Salt Lake City and beyond. These are more than 150 miles distant.
Mount Nebo, 70 miles northward, seems like a near neighbor, and the gray
peaks of the Tushar are seen towering beyond the heights of the Sevier
Plateau. To the southward looms up the grandest of all the plateaus—the
Aquarius—its long straight crest-line stretched across the whole southern
horizon, and seeming but a few hours’ ride away from us. Here we do not
feel that sense of being upon a plain which impresses us while traveling
upon the other plateaus, but we realize that this summit is at a great eleva-
tion; for we may look afar off in every direction to valleys and plains
which lie thousands of feet below us, and beyond which we perceive other
summits rising to altitudes nearly or quite equal to our own. But perhaps
the most impressive feature of the scenery lies almost beneath our feet. It
is a grand amphitheater, eroded deep into the plateau mass. Its dimensions
and grandeur are surpassed only in the great amphitheater in the Sevier
table near Monroe. It is less rugged and diversified than the latter, but is
more picturesque, chiefly because the eye can command the whole of it at
once. The summit upon which we stand is upon the edge of a straight
unbroken wall 4 miles long and nearly vertical for 1,200 feet, then descend-

FISH LAKE PLATEAU—THE GRAND GORGE. 261

ing in steep slopes to the central line of depression, which declines to
the westward until the gorge opens into Grass Valley, 4,300 feet below.
Across the abyss rises the other wall, somewhat less lofty and abrupt than
this, and we can look over it to the great irrigated farms of the Lower Sevier
Valley, 40 miles away. In this gorge a grand section of volcanic rocks is
exposed, of which the total thickness now visible will aggregate very
nearly 4,000 feet. The exposure, however, is not so advantageous for
study as might be desired, since the upper third is inaccessible cliff and the
lower two-thirds are heavily mantled with soil held in place by forests of
spruce and aspen, or are hidden beneath huge banks of coarse talus. The
disconnected exposures, however, are very many; and, so far as each one
individually extends, it exhibits distinctly the local attitudes of the rocks.

The first inquiry which arises is, whence came all these lavas? The
question is not easy to answer satisfactorily, for they were erupted far
back in Tertiary time, and the changes which the country has undergone
since their outpouring are very great. ‘The nearest great centers of erup-
tion which we are now able to identify with certainty are Blue Mountain,
nearly 12 miles distant across Grass Valley, and Mount Hilgard, nearly as
far in the opposite direction. As for the Fish Lake table itself, it does not
furnish very decisive indications of being an eruptive center. In the cliff
wall which faces the great amphitheater the successive sheets are seen to lie
nearly horizontal, parallel, and continuous over great distances. Although
they cannot be reached from below, yet they can be distinguished by their
colors, which are apparently identical with those in the great west wall of
the Sevier table overlooking Monroe. The beds are very massive and are
dark iron-gray (hornblendic trachyte), alternating with a number of shades
of red (augitic andesite, argilloid trachyte, and dolerite). No distortion or
confusion of the layers and no dikes were observed. None of those signs
of a voleanic core or center which are seen in Blue Mountain or in por-
tions of the Monroe amphitheater are here apparent. Nevertheless, it seemed
to me that the source of these lavas could not be far distant. Since the face
of the great cliff is parallel to the general course of the structure lines, it is
not surprising that the evidences of an eruptive center should be few and
inconspicuous, or even escape notice altogether.

262 GEOLOGY OF THE HIGH PLATEAUS.

The gorge itself is the work of erosion, and its apparent history is
worthy of passing mention. The course of this valley cuts obliquely across
the great monoclinal flexure which forms the western flank of the Fish Lake
Plateau, and was in process of excavation before that flexure was formed.
Like almost all other valleys, its position and direction are quite independ-
ent of the structural features of the country, and when the final uplifting
took place it did not divert here the course of the drainage. Its only effect
was to increase the amount of excavation to be done. The position of the
great gorge upon the shoulder of the monocline and running obliquely
across it is very striking, and might have given rise to a great deal of specu-
lation as to its origin, were we not able to apply to it the exceedingly simple
solution of the antecedence of drainage courses to the structural features
of the country and their persistence in spite of changes of great magnitude.
At first the interior of the gorge suggests a vast caldera, like those described
by Lyell in the Cape de Verde Islands or the Val del Bove at Aitna. But.
it is neither a caldera nor a Val del Bove, as a study of the surrounding
country abundantly proves.

Passing across the nearly level summit a distance of 2 miles we reach
the southeastern verge of the plateau, whence we may look down upon the
beautiful surface of ish Lake. This sheet of water, about 54 miles in length
and a mile and a half in breadth, is walled in by two noble palisades. The
one on which we imagine ourselves to stand—the plateau summit—is about
2,600 feet above the water; the other is nearly a thousand feet less lofty.
The lake itself is about 8,600 feet above the level of the sea. No resort
more beautiful than this lake can be found in Southern Utah. Its grassy
banks clad with groves of spruce and aspen; the splendid vista down
between its mountain walls, with the massive fronts of Mounts Marvine and
Hilgard in the distance; the crystal-clear expanse of the lake itself, com-
bine to form a scene of beauty rarely equaled in the West.

The subjects of geological interest to be found in the vicinity are nu-
merous. First may be mentioned the origin of the lake itself Mr.
Howell’s first impression was that glaciation had played an important part
im its excavation. Mr. Gilbert expressed the opinion that it might have
been caused by the sinking of a block between two faults. But I have

FISH LAKE. 263

been unable to discover sufficient evidence to sustain ether view. Although
the traces of ancient glaciers are conspicuous in the vicinity, nothing can
be more sharply defined than the places where they terminated; and we
are able to affirm confidently, by a comparison of places in close juxta-
position, that in one place the sculpture is due to glaciation and in another
it is not. It does not appear anywhere in this part of the plateaus that the
glaciers ever extended much below the 9,000 feet level, for at about that
level the terminal moraines cease and give place to other forms of sculpture.
As regards the possibility of a sunken block between two faults, it seems to
me that the evidence is not suflicient to establish it, and there is decided
evidence that it is an ancient valley of erosion, having its main features
marked out and partially developed before the present elevation of the
country had been reached. At the southwestern extremity is a low divide,
scarcely 30 feet above the water Jevel, which forms the local watershed
between the Colorado drainage system and that of the Great Basin. At
present the lake drains into the Colorado system; but at no distant epoch
it apparently drained into the basin system, flowing over this low divide.
Its ancient channel, leading down into Grass Valley (tributary to the Sevier
River), is as distinct and unmistakable as if it had dried up only afew years
ago. Mr. Howell, who recognized this channel and its obvious meaning,
supposed that the barrier now forming the divide had been produced by
morainal débris brought down from the Fish Lake Plateau and deposited
athwart the channel. More careful scrutiny, however, shows that the bar-
rier consists of volcanic rock in place. Hence it appears that the course of
the drainage has been reversed. Originally it flowed out of the lake to
the southwest; but as the gradual uplifting went on the whole lake basin
was tilted, so that it began to flow out of the opposite end and over a low
barrier to the east and southeast. A very slight tilting only was required to
effect the change; and a drop of 40 or 50 feet on the western side would
again reverse it to its original channel and pour it down the Awapa wall
into Grass Valley.

A journey along the bank of the lake towards its outlet is instructive
as well as entertaining. The trail (I believe there is now a wagon-road)
leads along the base of the plateau wall, rising more than 2,000 feet above

264 GEOLOGY OF THE HIGH PLATEAUS.

us and notched deeply here and there by great recesses of peculiar form
and appearance. These alcoves are half a mile or more in width, and set
back into the plateau mass a mile or two. They are filled with coarse
broken rubble or talus, over which it is extremely difficult to make progress,
but still practicable. These alcoves are the work of ancient glaciers, and
extending from the opening of each of them is a pile projecting out towards
or even into the lake basin and forming a terminal moraine. Near the
lower end of the lake is a moraine a projecting a mile and a half from the
plateau, and consisting of soil, rubble, and bowlders piled in a confused
mass to the height of nearly 200 feet and having a width of nearly a mile.
It almost divides the lake into two. The summit of the moraine holds
many pools of water embowered in aspens and bushes of many kinds, invit-
ing to lovers of the picturesque, but disappointing to him who accepts the
invitation. This is the largest moraine in the vicinity, though absolutely
it is not a very extensive one. It is instructive chiefly because it indicates
how small a part glaciation has played in the sculpture of this country.
There is never any difficulty in distinguishing the work which has been per-
formed here by ice from that which has been accomplished by the more
usual processes of degradation. The effects of glaciation are distinct and
peculiar, and cannot easily be confounded by a skilled observer with the
results of any other action. Doubtful cases do not seem to occur; at least
I cannot recall any which conveyed doubt to my own mind. The ice which
formed the ancient glaciers of course accumulated upon the summit of the
plateau. That summit is about 12 miles in length and 2 to 3 miles in width.
It is very nearly level and is not deeply scored by ravines in the central
parts, but only upon the edges of the walls which bound the table on nearly
all sides. The ice may have accumulated to a considerable thickness upon
this summit, so broad and so nearly level, before attaining sufficient mass to
flow readily. Most of the effects were exerted upon the eastern and south-
eastern walls of the plateau, for such inclination as it possesses is in those
directions. The grander wall, which overlooks the great gorge, is not per-
ceptibly affected by glacial action, and it is not probable that the ice flowed
over it to any considerable extent.

In the glacial gorges the rocks are very accessible for study. They form

VOLCANIC ROCKS OF FISH LAKE PLATEAU. 265

a great ageregate thickness of trachytes, alternating with augitic andesites
and some dolerites. The intercalary relations of the trachytes with the augitic
sheets is conspicuously markeu, as is also the transition from hornblendic
trachytes near the base of the exposures to argilloid, granitoid, and even
hyaline trachytes at the summit of the exposures. These older trachytes
are dark gray, sometimes with a greenish or olive tinge, suggestive of the
andesitic group, but retaining a predominance of the trachytic characters.
Among them are found what appear to be augitic trachytes, but they have
not yet been studied very critically, and they differ notably in their macro-
scopic facies from the more abundant and voluminous augitic trachytes
lying at lower levels around Salina Canon. About the middle, or a little
below the middle, of the mass are found very heavy beds of argilloid
trachyte. Throughout the northern part of the district there is no single
variety of rock which occurs in such massive beds or with such frequency.
Its texture and habit are strongly individualized and peculiar. It varies
somewhat in color, ranging from dull red to a dark purplish hue; in
fact, having the same range of colors as common clay-slate. It is soon
recognized in the great walls of the plateaus by its color, especially at
sunset, when the cliff faces the west, or in the morning when the cliff
faces the east. At such times the color characters come out strong and
clear, and the greater thickness of the beds also adds confidence to the
recognition. Higher up many varieties of light-gray trachyte are found,
belonging to the sanidin-trachyte group. Many of these have the char-
acters of clinkstone (not phonolite), being resonant and foliated in a peculiar
manner. Some of the sheets are broken up by a system of cleavage joints
into regular tiles an inch or two in thickness, and having from one to three
square feet of surface in the broader faces. In other sheets the cleav-
age, though conspicuous, is not so regular. Upon the extreme summit of
Fish Lake Plateau is a small remnant of an ancient cowlée, which was once
no doubt of large proportions. It is of the granitoid variety, and all that
now remains are some large blocks (as large as cottages), looking like huge
bowlders clustered together. Several of these are poised upon smaller
blocks, and during a keen blast of hail and snow I had once an occasion to
feel grateful for the shelter afforded me when I crept beneath one large

266 GEOLOGY OF THE HIGH PLATEAUS.

mass supported upon four corner-stones. Where lavas are disjointed into
large blocks of this kind it is not uncommon to find them, in the last stages
of decay, taking the aspect of a heap of gigantic bowlders. Granites and
massive sandstones sometimes exhibit the same behavior. The compan-
ions of these blocks have in this case probably been carried off by ice into
the gorges, and thus, instead of being erratics, they are the source from
which many erratics have probably emanated.

In addition to the trachytic rocks of Fish Lake Plateau, many flows
of augitic andesite and dolerite are also found. These occur as intereala-
tions between the trachytes, and are very numerous; but as they lie in much
thinner sheets their aggregate mass is much less. The augitie andesites are
older than the dolerites, and are seen in greatest frequency at the lower
horizons. They vary considerably in character, some being hardly dis-
tinguishable from the augitic varieties of trachyte, and having a grayish
color, while others merge into dolerites. Several varieties were found, which
were of a bright red color, and which might, upon hasty examination, have
been very deceptive. The iron contained in these varieties appears to be
largely in the form of peroxide, and both the magnetite and augite have been
altered, not by ordinary weathering, but by some metasomatic change which
I have not met with elsewhere. It does not appear to be identical alto-
gether with that alteration which reddens the scoria of basaltic cinder cones,
though the two changes may have much in common. In these varieties
the plagioclase crystals are well developed and retain their lively polariza-
tion, and are exquisitely striated.

No particular portion of Fish Lake Plateau could be designated as a
focus of the very many eruptions which constitute its mass. Nothing like
a cone or crater is anywhere discernible, unless in some spot there may yet
remain the ruins of such a feature so nearly obliterated as to escape ordinary
or cursory observation. The several beds appear to lie in well stratified
sheets, somewhat irregular in form, occasionally highly so, but on the whole
decidedly like a series of coarse sedimentary strata in their general group-
ing. This, however, does not necessarily involve the inference that the
lavas came from a distant source or were not erupted from numerous fis-
sures and orifices in the vicinity and within the plateau mass itself. In

MOUNT TERRILL AND MOUNT MARVINE. 267

truth, it seems little doubtful that the Fish Lake Plateau is a great center
of eruption. A general fact in support of this view is that in three direc-
tions—north, south, and east—and in all intermediate directions, the mass
of erupted material attenuates gradually. Whether this be true also of the
west side it is impossible to say, because the great monocline carries every-
thing down beneath the alluvium of Grass Valley. But in the other direc-
tions we can form a fair notion of the general arrangement of the total
extravasation, and the attenuation and radiation from a central locality is
sufficiently clear. The most probable view of the original arrangement is
that the lavas emanated from many orifices and fissures scattered over the
surface of an extensive volcanic pile, not unlike that of Mauna Loa, but on
a smaller scale.

From the outlet of Fish Lake, at its northeastern end, we may pursue
our way down the noble valley which carries the effluent stream. About
4 miles from the outlet we again enter Summit Valley, and, turning north-
ward, we may ascend it to the first camping-ground from which we started
to ascend the plateau. On the trail thither we pass two great terminal
moraines projecting from the openings of gorges cut back into the plateau
mass. Like the one projecting into the lake, they are well preserved and
quite typical in their features.

MOUNT TERRILL AND MOUNT MARVINE.

Upon the eastern side of Summit Valley rise two conspicuous masses,
which present to the eye nothing suggestive of a plateau. The northern
one is Mount Terrill, the southern is Mount Marvine, both being in the
prolongation of the same axis. Although in external form they are great
mountain piles, their origin is due to circumdenudation, just as a great
butte owes its individuality to the removal of the strata around it. They
consist of lavas, resting upon Lower Tertiary calcareous beds, and both the
lavas and the sediments are nearly horizontal so far as stratification is con-
cerned; but the lavas were obviously outpoured over a much eroded sur-
face, with hills and valleys of some magnitude. The volcanic sheets may
have been continuous with those of Fish Lake Plateau., since they have the
same lithological characters and varieties as the more striking trachytic

268 GEOLOGY OF THE HIGH PLATEAUS.

members, but are less numerous and of less thickness in the aggregate.
Whether once continuous or not, it seems evident that the separation of
these two mountains from the plateau was effected by the gradual excavation
and enlargement of Summit Valley. As we view the objects on the ground
and try to reconcile ourselves to this notion, the magnitude of the process
seems to male it incredible. Yet, as a common canon yalley is the self-
evident result of erosion, so may such a valley as this be produced by the
operation of the same general process, if sufficiently long continued. And
this valley is very ancient. It is a remnant of a topography existing before
the general uplifting of the platform on which the plateau and mountains
stand. The volcanic rocks are probably as old as the Miocene, and the
inception of Summit Valley may have occurred late in that age or in the
early Pliocene. Judging comparatively by the effects of erosion here and
in the adjoining country, the isolation of such a mountain as Mount Mar-
vine is by no means a disproportionate work, when the duration of the
process is considered. This view is abundantly confirmed when we exam-
ine the positions of the Tertiary strata beneath the lavas. There has been
no downthrow sufficient to cause the valley, and the beds are seen to curve
gradually downwards towards the west in their normal attitudes on the
shoulder of the great monoclinal. (See Section 3, Atlas sheet, No. 6.)
Mount Terrill is a long narrow ridge, consisting of trachytic lavas, rest-
ing upon calcareous beds of Lower Eocene age. The trachytes are rather
thin, their ageregate thickness being from 250 to 450 feet only. The
varieties are very similar to those of the Fish Lake Plateau. The extreme
summit is a remnant of a light-gray clinkstone (not phonolite, but a
sanidin-trachyte), which weathers into slabs about 3 inches thick by hori-
zoutal planes of cleavage and by vertical joints. Underneath is a large
mass of light-red argilloid trachyte and several bodies of light-gray
trachyte, and one dark mass which may be an augitic variety. The sedi-
mentary beds upon which they lie are not well exposed. Asis almost
always the case at such high altitudes (over 10,000 feet), they are covered
with soil and talus. No fossils were discovered, but their continuity has
been traced with strata of known age, and these are found in the ravine

MOUNT MARVINE—MORAINE VALLEY. 269

under the northeast corner of the Fish Lake, Plateau. They are Lower
Eocene, equivalent to the Bitter Creek of Powell.

The altitude of the ridge forming Mount Terrill declines towards the
south until a lofty col or “saddle” is reached, which divides it from Mount
Marvine. The latter is one of the most striking features of the region. It
is along ridge reduced to a mere knife-edge at the summit, and having
rocky fronts on either side, sloping about 60°. A transverse section of
the upper 2,000 feet of the mountain would be an equilateral triangle.
For several years it was named by our parties The Blade. When seen
from the south or north it has a most abrupt and peaked appearance, which
becomes more pronounced the nearer we approach it. Viewed laterally
from Summit Valley at its base, it presents a serrated summit, notched with
many gaps and bristling with many cusps. The altitude of the mountain
above the valley is about 2,700 feet and 11,400 feet above the sea. It con-
sists of alternating trachytes and augitic rocks, resting upon Lower Eocene
strata. The thickness of the volcanic beds is, in the aggregate, from 1,200
to 1,800 feet, being least at the northern end, and increasing towards the
south. There is a succession of beds having the same general lithological
characters as those in Fish Lake Plateau, except that the augitic members
seem to be less numerous but more massive. Here, also, the dominant rock
is the argilloid variety of trachyte.

The origin of this mountain becomes quite apparent when studied from
both sides. It has been isolated, like a gigantic butte, from the adjoining
country by the erosion of the valleys upon either flank. The inception of
this work is very ancient, since it undoubtedly antedates the uplifting of the
platform on which the mountain stands, and may therefore be referred to
any epoch more ancient than the latter part of the Pliocene and more
recent than the Eocene.

MOUNT HILGARD AND MORAINE VALLEY.

Before proceeding southward it is desirable to look briefly at Mount
Hilgard and at the intervales which separate it from Mounts Terrill and
Marvine. From Summit Valley we may easily cross the col which separates
the two latter summits, and descending the other side we find ourselves in

270 GEOLOGY OF THE HIGH PLATEAUS

a broad valley parallel to the one just left. This has been named Moraine
Valley, from a rather large and conspicuous relic of glacial times, which
could not escape observation because it is so well preserved and tells its
story so plainly. It fills a lateral valley, heading near the summit of Mount
Terrill and extending eastward into the broader expanse of Moraine Valley.
It is covered with pools and lakelets bowered with aspen and spruce, and
has the ordinary terminal character where its proper bed opens into Mo-
raine Valley; beyond which no traces of glaciation are recognizable. The
altitude of the termination is very nearly 9,000 feet, showing the same gen-
eral fact which has already been spoken of, that the glaciers did not, in
this part of the country descend to low levels, but were confined to the
highest parts of the region.

In the northern part of Moraine Valley the sedimentary beds are occa-
sionally revealed in insulated exposures surrounded by trachytic and ande-
sitic beds in an advanced stage of decay. They are of Tertiary age and
are found on the western side of the valley, where considerable spaces are
uncovered. They dip slightly towards the east, being, in fact, the eastern
branch of an anticlinal swell, while the beds of Summit Valley form the
western branch and Mount Terrill occupies the summit. (See Sec. 3,
Atlas Sheet No. 6.) Their Tertiary age is inferred from their position,
but no fossils have been obtained from them. The volcanic rocks of the
northern part of the valley seem to have been of much greater volume
formerly than at present, and to have been much wasted by erosion, though
it is also inferred that they were never so extensive and massive here as to
the southward. They are all, so far as observed, trachytic; some of them
belonging to the dark hornblendic division, others to the sanidin division,
the latter predominating upon the western side of the valley.

The principal drainage is to the southward, running parallel for a con-
siderable distance to that from Summit Valley, and at leneth the two unite
and form a noble stream as large as the Sevier, which has volume enough
to reach the Colorado. The northern part of the valley is drained by a
few rills, which find their way into Gunnison Valley to the northeast and

thence through Salina Canon to the Sevier. Thus the divide between the

MOUNT HILGARD. Dit

Colorado and Basin drainage systems crosses the upper part of Moraine
Valley transversely, and the same is true of Summit Valley.

Mount Hilgard is a lofty headland, rising upon the eastern side of Mo-
raine Valley to an altitude of about 11,000 feet. Towards the north and
east it presents inaccessible battlements of dark volcanic rock, consisting
chiefly of hornblendic trachyte and augitic andesite. ‘Towards the west it
presents an abrupt face, which, however, is easily scaled. To the south it
extends in a long ridge of diminishing altitude until it reaches the vicinity
of Thousand Lake Mountain. To the eastward are seen the sedimentary
formations stretching away indefinitely. ‘They are Cretaceous, with a thin
fringe of Lower Eocene capping them just at the base of the volcanic wall
of which Mount Hilgard forms the loftiest part. To the northward the
volcanic wall extends at an altitude 2,000 feet lower than the mountain top,
and gradually swings westward until it nearly joins the wall which forms
the northern salient at the head of Summit Valley, being divided from it
only by a narrow ravine heading near Mount Terrill. This northern
extension of the volcanic battlement has been named Gilson’s Crest. This,
together with the great ridge formed by Mount Hilgard and its southern
extension, forms the eastern boundary of the great eruptive masses which
cover almost the entire expanse of the District of the High Plateaus. There
are, however, two or three outlying patches to the eastward of small extent,
evidently independent centers of eruption, but no special significance seemed
to attach to them.

The ridge of which Mount Hilgard is the culmination is evidently a
chain of volcanic vents along a fissure, and the extravasation appears to
have taken place along its entire extent. There are no individualized
peaks or cones suddenly springing up at various points of the chain, but a
broad summit platform, slowly and pretty regularly diminishing in altitude
through a distance of nearly 20 miles, and it is difficult to point to any par-
ticular spot as possessing a more distinctly focal character than the others.
The outpours appear to have occurred all along the line, with an approxi-
mation to uniformity, or possibly with a gradual increase of magnitude and
frequency, as we approach the summit of Mount Hilgard. From that head-
land southward the top of the platform widens out, becoming 4 miles wide

Die, GEOLOGY OF THE HIGH PLATEAUS.

at a distance of 8 miles south. The mass of lavas appears to be of great
thickness, and the sedimentary beds are not seen beneath them until we
approach the vicinity of Thousand Lake Mountain. The eruptions were of
the most massive character, being in some instances more than a hundred
and twenty feet thick, and presenting ledges of rock several miles in length.

The eruptive materials are of the same general character as those ob-
served in the Fish Lake Plateau. They are mostly trachytic, with subor-
dinate though considerable masses of augitic andesite and dolerite interca-
lating. All of the trachytes have a dark, somber appearance, and belong to
both of the divisions of that group. The older varieties are hornblendic,
with considerable plagioclase, and among them are also found augitic tra-
chytes. The younger members are chiefly of the argilloid varieties, and, as
elsewhere, they occur in immense beds.

THE AWAPA PLATEAU.

The Awapa and Aquarius Plateaus have not been studied in detail, and
my knowledge of them is such only as has been derived from a few rapid
transits across the former in different places, and about three weeks spent
upon the flanks of the latter. Their area is very great, and, in order to
acquire sufficient data to give any detailed account of them, much more
labor and travel is necessary. I have been much indebted to some notes
prepared by Mr. E. E. Howell, whose observations have supplemented my
own in some very important particulars, and have prepared the way to the
determination of many points; especially those relating to the stratigraphy
and structure of the Aquarius Plateau.

The separation of the Awapa from the Fish Lake Plateau is probably
more justifiable on the ground of convenience of discussion than of reality,
for the latter passes directly into the former. A sudden descent across a
steep slope brings us from one to the other. For a short space the lake
may be regarded as a natural barrier, but east of the lake it is not practi-
cable to say where the one ends and the other begins. The Fish Lake
table has an altitude of more than 11,000 feet, and where its southern end
drops down upon the Awapa the altitude of the latter is less than 9,000
feet; and generally the altitude of the Awapa is the least of the several

THE AWAPA PLATEAU. Dike

masses constituting the High Plateaus. The slopes of the entire mass all
converge towards a central depression called Rabbit Valley. It is not an
inclined plane or roof, but the segment of a dish. Everywhere the general
inclination is very slight, though undulating with low hills. The western
boundary is a wall from 1,800 to 3,000 feet high, plunging down into Grass
Valley, sometimes by a grand precipice, sometimes by a steep, difficult
slope and always abruptly. The length of the plateau, though its bound-
aries are for the most part difficult to locate with precision, is about 35
miles and its breadth about 18 miles.

It is a dreary place. Upon its broad expanse scarcely a tree lifts its
welcome green, save a few gnarled and twisted cedars. Its herbage con-
sists only of the ubiquitous artemisia and long nodding grasses. Not a spring
or stream of water is known upon all its area, except at the lowest part, where
its slope merges into the floor of Rabbit Valley. And yet most of its sur-
face is at an altitude where verdure and moisture abound, and where the
summer is like the spring of more favored regions. But here the snows of
winter are melted early, and the summers are nearly as hot and dry as those
of the plains below. A ride across it is toilsome and monotonous in the
extreme. It takes us over an endless succession of hills and valleys, clad
with a stony soil, usually just steep enough to worry the animals, but not
enough so to require us, or to even encourage us, to dismount. Here
and there a sharp canon opens across the path in an unexpected manner,
compelling a long detour to find a crossing. These are usually shallow,
rarely exceeding 400 or 500 feet in depth, but are as typical in their forms
or sections as the cafons in the sedimentary strata.

The whole mass of the Awapa consists of volcanic materials. The
only localities where sedimentary beds are seen are in Rabbit Valley and
low down in the western flank opposite East Fork Canon in Grass Valley.
In the latter locality the great fault has brought them to daylight, and
the ravines have stillfurther opened them to view. They are Lower Ter-
tiary, corresponding to the Bitter Creek beds of the northern plateaus.
Above them are 3,000 feet of volcanic conglomerates and lavas. In Rab-
bit Valley beds presumed to be of the same age are also discovered

beneath thin cappings of trachyte and basalt. Hence it appears that the
18 HP

QTA GEOLOGY OF THE HIGH PLATEAUS.

lavas gradually attenuate from west to east, or rather from the periphery
of the plateau towards its central depression.

The variety of the rocks dispiayed is truly astonishing. It seems as
if two exposures rarely presented the same description of lava. The great
majority of them belong to the trachytic group, and it is surprising to see
what numberless changes can be rung upon materials which vary so little
in their ultimate composition. This manifold variation is displayed in the
Sevier Plateau and in the Markagunt; but for some reason I was more
profoundly impressed with it in the Awapa than elsewhere, though, pos-
sibly, it may be no greater in the latter than in the former. But assur-
edly the number of distinct coulées is extremely great, and it is hard to find
two precisely alike. Some of the trachytic beds are quite thin, being not
more than 20 to 25 feet thick, and successions of these variegated layers are
frequently met with. On the other hand, some of the grandest and most
massive sheets in the High Plateaus are found here. On the north side of
Rabbit Valley the plateau slope ends in a low wall about 100 to 120
feet in height and nearly 4 miles in length, which seems to be one indi-
vidual sheet of argilloid trachyte. Some very grand sheets of hornblendic
trachyte are also displayed hard by, having the exceedingly rough, coarse
texture which is so characteristic of that variety. In the large coulées
of hornblendic trachyte, and sometimes also in the granitoid variety,
may be seen that rough, broken aspect of the lava suggestive of flow-
ing in a very viscous and almost solid state, as if the whole mass were
continually rending itself into fragments, as it crept along like a huge
glacier, while more fluent portions from within the flood worked their way
into the rifts, and there congealed. In the smaller and thinner sheets this
phenomenon is not seen; but a more mobile character is indicated. The
grander flows generally belong to the granitoid and argilloid varieties, while
the smaller and more fluent ones are sometimes hyaline and sometimes
augitic trachyte. Vitreous products also are common, and at the parting of
the beds trachytic obsidian is found in abundance. No rhyolites have been
detected in this plateau, but a few pitch-stones, with the trachytie character
rather than the rhyolitic, were observed. This indicates a considerable
range in the chemical constitution, and is accompanied with a correspond-

THE AWAPA PLATEAU. 2D

ing range in the superficial aspects of the beds. In the northern part of the
plateau the dark argilloid and hornblendic trachytes predominate. They
agree in their characters and aspects with those which occur in the Fish Lake
Plateau and Mount Marvine. There is also decided evidence that the
main sources from which they outflowed were around the southeastern bor-
ders of the lake. This evidence is substantially the fact, that the sheets
increase in thickness and become more rugged in that quarter, and all the
phenomena of flow indicate movement from that direction. The supposed
location of the vents, however, was not visited. No augitic andesites were
noticed intercalating with the northern trachytes of the Awapa, though
large bodies of them may have escaped observation, owing to the superficial
and cursory character of the investigation.

No conglomerates or tufas were seen in the northern part of the Awapa,
and these would have been noticed if they really exist there in masses of
any importance. As such bodies are usually very bulky and conspicu-
ous, it is hardly possible to overlook them. In the western part of the pla-
teau, however, they are found in great volume. They are stratified in the
usual manner, with nearly horizontal bedding or with that peculiar cross-
bedding which may be seen in Panquitch Canon (Heliotype No. 4). In
the western wall of the plateau, a little north of East Fork Canon, these
conglomerates form a grand cliff and talus rising about 3,400 feet above
Grass Valley, and the total thickness of the fragmental beds is roughly
estimated at 1,600 feet. Large masses of hornblendic trachyte are found
beneath them, and granitoid trachyte above them. ‘These beds of conglom-
erate stretch north and south from this point, forming the most conspic-
uous part of the plateau wall, for a distance of 21 or 22 miles. Their deg-
radation gives rise to a precipitous escarpment, broken in several places by
ravines and gorges. They are also found in great bulk in the canons which
cut into the heart of the plateau. They are believed to be alluvial in their
origin. The fragments which they contain are exceedingly varied in their
composition and texture. Hornblendic andesites and trachytes are commin-
gled in the same stratum, and of each kind there are very many varieties.
In one of the deeper cafons some propylitic fragments were found, but

276 GEOLOGY OF THE HIGH PLATEAUS.

whether they were derived from conglomerates or from rocks in situ farther
up the gorge was uncertain.

A considerable number of basalt fields are found upon the surface
of the Awapa. In no instance was any considerable mass of this lava
encountered, and wherever found it formed only a rather thin local veneer
rarely so much as 100 feet thick, and generally much less. These basaltic
sheets have been greatly ravaged by erosion, and their fragments scattered
far and wide. No trace of a basaltic cone or monticule was anywhere
seen. If any such ever existed it has been totally demolished. I incline
to the opinion that none were ever built in those portions of the Awapa
which were visited, but rather that the basalt quietly outflowed in the same
manner as it did from some of the very recent vents on the Markégunt.
From the orifices it seems to have spread out at once in thin, diffuse pools
or lakes, where it has slowly weathered away. Wherever it occurs, it is the
most recent of the eruptive masses. None of it belongs to so late an epoch
as those of the Markigunt or the southern terraces overlooked by the Pink
Cliffs. In some localities the sheets of basalt are wasted to mere heaps of
disjointed blocks, thickly strewing the platform and partly buried in soil.
In others, the continuity of the sheets is tolerably well preserved. It is
impossible to fix the age of those eruptions, though I infer that none are as
old as Middle Pliocene; perhaps not so old as the close of that age. They
seem to have been erupted after the movements of displacement which
blocked out the plateau had well advanced, and these are held to be
among the most recent events of Tertiary time.

I have not attempted to delineate these basalts upon the geological
map, being uncertain as to their extent and outlines.

RABBIT VALLEY.

The slopes of the Awapa all converge towards a central depression
called Rabbit Valley. The trachytic beds descending towards it end sud-
denly, sometimes in low cliffs, sometimes in steep slopes. The eastern
side of the valley is walled by the great uplift of Thousand Lake Mount-
ain. Along the western base of that mass runs one of the great faults of
the district, with a maximum throw of more than 4,000 feet. On every

RABBIT VALLEY. PAT

side the valley is girt about by imposing masses; on the north and west
by the slopes of the Awapa, and on the south by the Aquarius. Its floor
is a broad alluvial plain, receiving the wash of all the surrounding uplifts,
and carrying a noble stream, which is fed from all directions by rivulets
which have brought down their loads of débris and, reaching the nearly
level bottom, have deposited it. Those coming from the Awapa are always
dry in summer, excepting one which heads near the foot of the slope, but
the other tributaries from the north and south are perennial. The accumu-
lation of detritus through the ages has produced a broad expanse of alluvial
plain through which the Fremont River meanders, and nothing but a moist
atmosphere is wanting to make this valley an Eden.

It is somewhat unusual to find so large an area in this elevated region
in which the accumulation is in excess of the power of the rivers to carry
it away. But this exceptional condition appears to have prevailed in
Rabbit Valley for a considerable time. It was apparently brought about
by the last stage in the uplifting to the eastward across the great fault, or,
what is the same thing, the downthrow of the valley itself; for these
vertical movements must be considered in a purely relative sense and as
meaning simply the difference of elevation between the lifted and thrown
sides, respectively, of the displacement. The Thousand Lake fault cuts
across the outlet of Rabbit Valley, which passes between Thousand Lake
Mountain and the northern salient of the Aquarius, and it has had the effect
of an increasing barrier to the outflow of the Fremont River and has
slackened its waters within the valley. Hence the loads of ditritus which
its affluents bring down from the plateaus on every side are thrown down
in the valley. Since the last paroxysm of uplifting the river has taken to
meandering, in consequence of the progressive building up of its channel
and has repeatedly shifted its bed over different parts of its flood plain. Old
canons in the borders of the lava sheets coming down from the Awapa have
been partially filled up and the river has abandoned them. In truth, a con-
siderible number of low caions of this sort are still discernible in the lower
portions of these old trachytic beds, and it is apparent at a glance that they
have nothing in common with the canons and ravines which descend the
slopes of that plateau, except to form the old trunk channel into which the

278 GEOLOGY OF THE HIGH PLATEAUS.

latter debouched. The Fremont River, however, still maintains its course
in one of those old canons for a distance of 4 or 5 miles. It leaves the low
flats of the valley to enter the rising slopes of the Awapa and flows through
a rocky gorge which becomes four or five hundred feet deep. Thence it
emerges into the valley plain again and pursues its way to the foot of the
valley, where a salt marsh, covered with saline pools, has been built up by
the accumulation of fine silt.

It is interesting to pursue this subject further and to view it in relation
to future instead of past time. The river leaves the valley through the
great gap between the mountain and the Aquarius, and the passage has
been named the Red Gate. Thence it flows off into the heart of the
Plateau Country, reaching the Colorado by a profound canon. Throughout
the greater part of this distance the river is a rapid stream and is slowly
sinking its channel. Its rapid descent begins half a mile beyond the point
where it crosses the great fault, and it is apparent that here, too, it is
lowering its bed; for old terraces of river gravel and loess are seen at
different levels within the Red Gate in an excellent state of preservation,
and the river has cut a broad and deep channel through them. It is only
a question of time how deep the channel may be cut, for where it leaves
Rabbit Valley the altitude is almost exactly 7,000 feet above sea-level, and
the junction of the river with the Colorado is less than 4,500 feet. Esti-
mating the course of the stream between the two points at 100 miles, the
average descent is not far from 25 feet to the mile, which is about the same
fall as prevails in nearly all the tributaries of the Colorado in this part of
the country. All of them are evidently corrading their beds. Here and
there local flood plains are formed, occurring along stretches of the streams
where the fall is slight; but such flood plains are merely temporary in the
secular life of the river. They are succeeded by rapids which are grad-
ually eating their way backwards, and in a brief period the stretches of still
water will become rapids in turn. In time, then, the Fremont River will cut
down its channel at the outlet of Rabbit Valley unless the fault at the Red
Gate increases its throw. In the absence of such increase in the fault the
stream will ultimately carry back the excavating process into the valley
and the extensive alluvial beds will be gradually attacked and eroded away.

ALLUVIAL FORMATIONS—THOUSAND LAKE MOUNTAIN. 279

These alluvial masses are partly conglomeritic in texture, especially
near the borders of the lava sheets and at the foot of the plateau slopes.
Towards the middle of the valley they become finer, shading into sandy or
fine gravelly deposits. They are instances of the formation of conglom-
erates upon a considerable scale by the alluvial process, but under condi-
tions somewhat different from those disclosed in Sevier and Grass Valleys.
The included fragments upon the western and southern portions are always
volcanic and exceedingly varied. The débris derived from Thousand Lake
Mountain on the eastern side consists mainly of fine quartz sand coming
from the decay of the Jurassic and Triassic sandstones of that structure.

In the northwestern part of Rabbit Valley a few exposures of Ter-
tiary beds are found beneath the terminal trachytic sheets. Upon the
eastern side of the valley are still better exposures upon the lowest slopes
of Thousand Lake Mountain. In the latter locality they are soon cut off
by the great fault, and reappear nearly 4,000 feet above, beneath the lava
cap upon the summit. Below they abut against the Lower Trias. Laterally
they run beneath extensive outpours of basalt, which, though not of very
modern origin, are still comparatively recent.

THOUSAND LAKE MOUNTAIN.

Thousand Lake Mountain is an exceedingly interesting object. The
name was given by the Mormons who pasture flocks in the valley below.
They derived it from a group of pools of glacial origin upon the sum-
mit. Structurally and morphologically it is a small plateau, in some
respects very similar to the other and larger members of the district, but
possessing, also, features peculiar to itself. The country to the west of it is
thrown down by a profound fault forming the depression of Rabbit Valley,
The country to the east of it is the inner region of the Plateau Province,
from which thousands of feet of strata have been removed by the grand
erosion of Tertiary time, while the mountain itself has been left like a
gigantic butte or cameo upon the border of the region. Upon its southern
flank the Fremont River has cut a wide passage, which has separated it
from its mighty parent, the Aquarius Plateau.

Upon the summit is a lava cap from 400 to 500 feet in thickness and

280 GEOLOGY OF THE HIGH PLATEAUS.

quite flat, giving a tabular summit to the mass about 5 miles long and
nearly 2 miles wide. An almost impassable talus surrounds the scarped
edges of this cap, and renders the ascent difficult except at a few points
upon the eastern side. ‘The lavas are hornblendic trachytes and augitic
andesites, heavily interbedded and made up of numerous flows. These
rest upon a layer of Lower Tertiary, of which the thickness is not precisely
known, but which cannot well exceed 700 feet. Whether the diminished
volume of the Tertiary here is due to an originally small amount of depo-
sition or to an erosion of the upper members prior to the volcanic overflow
is not yet determined, but I incline to the former explanation. And the
general indications seem to be that over the area occupied by the eastern
part of the Aquarius and Thousand Lake Mountain the Tertiary deposition
was locally much thinner than elsewhere. Immediately beneath is the
Jurassic white sandstone. The Cretaceous is absent from its place in the
stratigraphical series. Yet a few miles to the northeastward the whole
vast Cretaceous system is rolled up and cut off on the slopes of a grand
monoclinal.

This monoclinal is the Water Pocket Fold, which is probably the grand-
est feature of the kind in the Plateau Country, so far as known, and per-
haps the most typical. Its first appearance is beneath Thousand Lake
Mountain (see Atlas Sheet No. 4), where the trend is east-southeast and it
gradually swings around towards the south-southeast, reaching to the Colo-
rado River, in the heart of the Glen Canon. It crosses the river into
unknown regions. Upon the northwestern side of the mountain it is coy-
ered up by Tertiary beds and lava sheets and is wholly concealed, so that
neither its northern nor its southern terminations are at present known.
The great fault upon the west side of the mountain cuts across this mono-
clinal nearly at right angles, and has dropped the platform to the west~
several thousand feet. The age of the great flexure is evidently older than
Tertiary time, for the Lower Eocene beds lie nearly horizontally across
the upturned edges of the whole Cretaceous system and upon the deeply-
eroded surface of the Jurassic sandstone. Inasmuch as the entire body of
Cretaceous strata, including the Laramie beds, appear in succession as we

cross the strike of the flexure and as they are all upturned upon its flanks,

Ilenioryrre X.

THeliotype Printing Co.,

220 Devonshire St., Boston.

THE RED GATE. LOWER TRIAS. SHINARUMP.

THOUSAND LAKE MOUNTAIN. 281

the first conclusion seems to be that the movement took place after the
Laramie beds were deposited and before the Tertiary strata were laid down.
The contacts, however, between the Tertiary and Laramie beds have not
yet been studied and analyzed, nor have any good exposures of those con-
tacts in this vicinity been discovered. It is not impossible that through a
large portion of Cretaceous time this area was a part of an island under-
going a slow erosion, while just beyond the flexure to the eastward the
later Cretaceous members were accumulating upon an island coast; that at
a later epoch the island was submerged, and received a deposit of Lower
Eocene beds. This supposition has considerable support in facts which
will be brought forward in the next chapter, and leads to the conclusion that
a long interval of disturbance and erosion separated the Cretaceous from
the Tertiary throughout this part of the Plateau Province. The absence
of more than 5,000 feet of strata between the Lower Eocene and the for-
mation upon which it reposes is a very striking fact, and the simplest expla-
nation is here the best.

The Jurassic white sandstone is disclosed all around the mountain. It
has the same familiar facies which has been adverted to in the preceding
chapters upon the Markagunt and Paunsagunt Plateaus—a grayish-white
massive sandstone, wonderfully cross-bedded, and weathering into inacces-
sible domes of peculiarly solid and bold aspect. The upper Jurassic shales
appear to be absent, at least they were not detected, and the eroded con-
dition of the sandstone at the time of the deposition of the Tertiary is a
sufficient reason for presuming that if the shales once existed here, and I
doubt not that they did, they have been swept away.

Beneath the Jurassic appear in normal order and relations the Ver-
milion Cliff sandstones (Upper Trias) and the Shindrump shales. These
formations have the same aspect as in the lower terraces which front the
Kaibabs in the Grand Canon District. The Vermilion Cliff series has the
same succession of sandstones and siliceous shales, usually bright red, but
sometimes patched with bright yellowish brown. They are best exposed
upon the southern flank of the mountain at the Red Gate. The Shinarump
has the same band of conglomerate, consisting of fragments of silicified
wood imbedded in white sand, which is seen in the vicinity of the Hurri-

282 GEOLOGY OF THE HIGH PLATEAUS.

cane fault, 150 miles to the southwest. The shales also present the same
striking and constant appearance as if in all that interval not a layer or line
had lost its identity. At the base of the mountain, upon the southern side,
the Shinérump shales form a broad platform or terrace skirting the south-
eastern flank, and ending in a beautifully sculptured cliff about 600 feet
high, eminently characteristic cf the formation. The architecture is repre-
sented in Heliotype X, but the colors are such as no pigments can portray.
They are deep, rich, and variegated, and so luminous are they, that light
seems to glow or shine out of the rock rather than to be reflected from it.
The Red Gate has already been alluded to as the passage by which
the Fremont River leaves Rabbit Valley and flows off into the heart of the
Plateau Country. As we approach it from the west the flaming red of the
Trias is seen reaching out southward from Thousand Lake Mountain in a
rocky wall which has been breached by the river. These beds curve down-
wards on the south side of the gate and disappear beneath the spurs of
the Aquarius. The great fault along which Thousand Lake Mountain has
been upheaved continues southward across this passage, cutting into the
mass of the Aquarius. The downward flexure of the Trias is simply the
effect of diminished uplift on the south side of the gate. The passage itself
has been cut by the river, which has occupied its present locus for an im-
mense period, which may reach back as far as Miocene time. Some changes
may have occurred in its course through the repeated outflows of lava across
and into its valley. But there are independent considerations which lead
to the conclusion that the Fremont River is one of the more ancient tribu-
taries of the Colorado, born with the country itself far back in Eocene time,
though its upper branches may have been much modified by the violent
changes accompanying the great volcanic activity of the Middle Tertiary.
Beyond the Red Gate the relations of the river to the structural features of
the region through which it flows, and also to the imposed sculpture of the
country, are such as to compel the conviction that the river must antedate the
Tertiary deformations of the strata which are there found, and also antedate
the great erosion of the Plateau Province. Through all those vast changes
by displacement and erosion the river has ever maintained its thoroughfare.

The passage through the Red Gate is part and parcel of the same history.

THE RED GATE. 283

We may in imagination look back of an immense geological period to an
epoch when the platform of the Aquarius reached far beyond its present
boundaries and included the whole mass of Thousand Lake Mountain and
the whole country as far as the eye can reach to the eastern and southern
horizons. The cutting of the passage through the Red Gate is but an insig-
nificant factor in the total process, and falls far short of what we know has
been accomplished in other portions of the wonderful country of which it is
the portal.

CHAPTER XIII.

THE AQUARIUS PLATEAU.

Distant views and the approach to the Aquarius.—Its grandeur.—Its summit.—Scenery and vegeta-
tion.—Glacial lakes.—The lava cap.—The southern slopes.—Panorama from its southeastern
salient.—View to the northeastward.—The Water Pocket fold.—Inconsequent drainage.—View
of the Henry Mountains and La Sierra Sal.—The Circle Cliffs.—A labarynth of caiions.—Cantons
of the Escalante River.—Exposures of the Jura and Trias.—Navajo Mountain.—The great wall of
the Kaiparowits Plateau.—Distant view of Table Cliff and Kaiparowits Cliff.—The great southern
amphitheater of the Aquarius.—The grand erosion.—Former extension of the Cretaceous and
Eocene strata over the Plateau Country.—General structure of the Aquarius.—Faults in the cen-
tral portion.—The Escalante monocline and its Pre-Tertiary age.—A Cretaceous island.—Western
wall of the Aquarius.—Trachytes, andesites, and basalts. —Complicated faulting.—Table Cliff.i—
Kaiparowits Peak,

The Aquarius should be described in blank verse and illustrated upon
canvas. The explorer who sits upon the brink of its parapet looking off
into the southern and eastern haze, who skirts its lava-cap or clambers up
and down its vast ravines, who builds his camp-fire by the borders of its
snow-fed lakes or stretches himself beneath its giant pines and spruces,
forgets that he is a geologist and feels himself a poet. From numberless
lofty standpoints we have seen it afar off, its long, straight crest-line stretched
across the sky like the threshold of another world. We have drawn nearer
and nearer to it, and seen its mellow blue change day by day to dark som-
ber gray, and its dull, expressionless ramparts grow upward into walls of
majestic proportions and sublime import. The formless undulations of its
slopes have changed to gigantic spurs sweeping slowly down into the
painted desert and parted by impenetrable ravines. The mottling of light
and shadow upon its middle zones is resolved into groves of Pinus ponde-
rosa, and the dark hues at the summit into myriads of spikes, which we

know are the storm-loying spruces.
284

THE: SUMMIT OF THE AQUARIUS. 285

The ascent leads us among rugged hills, almost mountainous in size,
strewn with black bowlders, along precipitous ledges, and by the sides of
canons. Long detours must be made to escape the chasms and to ayoid
the taluses of fallen blocks ; deep ravines must be crossed, projecting crags
doubled, and lofty battlements scaled before the summit is reached. When
the broad platform is gained the story of ‘Jack and the beanstalk,” the
finding of a strange and beautiful country somewhere up in the region ot
the clouds, no longer seems incongruous. Yesterday we were toiling over
a burning soil, where nothing grows save the ashy-colored sage, the prickly
pear, and a few cedars that writhe and contort their stunted limbs under a
scorching sun. To-day we are among forests of rare beauty and luxuriance;
the air is moist and cool, the grasses are green and rank, and hosts of
flowers deck the turf like the hues of a Persian carpet. The forest opens
in wide parks and winding avenues, which the fancy can easily people with
fays and woodland nymphs. On either side the sylvan walls look impene-
trable, and for the most part so thickly is the ground strewn with fallen
trees, that any attempt to enter is as serious a matter as forcing an abattis.
The tall spruces (Abies subalpina) stand so close together, that even if the
dead-wood were not there a passage would be almost impossible. Their
slender trunks, as straight as lances, reach upward a hundred feet, ending |
in barbed points, and the contours of the foliage are as symmetrical and
uniform as if every tree had been clipped for a lordly garden. They are
too prim and monotonous for a high type of beauty ; but not so the Engel-
mann spruces and great mountain firs (A. Lngelmanni, A. grandis), which are
delightfully varied, graceful in form, and rich in foliage. Rarely are these
species found in such luxuriance and so variable in habit. In places where
they are much exposed to the keen blasts of this altitude they do not grow
into tall, majestic spires, but cower into the form of large bushes, with their
branchlets thatched tightly together like a great hay-rick.

Upon the broad summit are numerous lakes—not the little morainal
pools, but broad sheets of water a mile or twoin length. Their basins were
formed by glaciers, and since the ice-cap which once covered the whole
plateau has disappeared they continue to fill with water from the melting

286 GEOLOGY OF THE HIGH PLATEAUS

snows. Larly in autumn the snows have disappeared and the lakes cease
to outflow, but never dry up.

The length of the Aquarius from northeast to southwest is about 35
miles, and its breadth from 10 to 18 miles. Its altitude varies from
10,500 to 11,600 feet above sea-level. Over three-fourths of its periphery
is bounded by massive cliffs, while along the remaining fourth it declines
gently to its confluence with the Awapa. Its upper portion is a lava-cap
of vast dimensions, varying from 1,000 to 2,000 feet in thickness. Its lavas
are seen in greatest mass at the northwestern flank, overlooking the south-
ern part of Grass Valley and the Panquitch Hayfield. Upon the southern
and eastern sides, at the foot of the volcanic wall, the long slopes begin,
which reach far out into the mesas of the inner Plateau Country. Their
descent is slow and easy to all appearance, but they are deeply gashed with
profound canons and terrible gorges, among which it is dangerous to ven-
ture. To traverse these slopes it is necessary to keep high up near the base
of the lava-cap, where the ravines head, and where they are sufficiently
open to afford a practicable trail. Even here the journey around the base
of the cliff is laborious, involving the constant ascent and descent of vast
gorges and amphitheaters, and requiring many days to accomplish it. Yet
the traveler who has abundant strength and perseverance will be amply
rewarded, provided he has chosen his way with prudence and good judg-
ment. Upon these slopes the structure of the plateau is revealed.

In truth, there is but little “structure.” The plateau is simply arem-
nant left by the erosion of the country around its southern and eastern
flanks. A few of its minor features are due to displacements, and its west-
ern wall originated in a great fault or rather in several faults. The rest of
the mass owes its pre-eminence to circumdenudation. We may gain some
notion of the stupendous work which has accomplished this result by taking
our position upon the southeastern salient at the verge of the upper platform.

It is a sublime panorama. The heart of the inner Plateau Country is
spread out before us in a bird’s-eye view. It is a maze of cliffs and ter-
races lined off with stratification, of crumbling buttes, red and white domes,
rock platforms gashed with profound cations, burning plains barren even of
sage—all glowing with bright color and flooded with blazing sunlight.

WATER POCKET FOLD—INCONSEQUENT DRAINAGE. 287

Everything visible tells of ruin and decay. It is the extreme of desola-
tion, the blankest solitude, a superlative desert.

To the northeastward the radius of vision reaches out perhaps a hun-
dred miles, where everything gradually fades into dreamland, where the air
boils like a pot, and objects are just what our fancy chooses to make them.
Perhaps the most striking part of the picture is in the middle ground, where
the great Water Pocket fold turns up the truncated beds of the Trias and
Jura, whose edges face us from a great quadrant of which we occupy the
center. Where the strata are cut off in this way upon the slope of a
monocline they do not present to the front a common cliff and talus with
a straight crest-line, but a row of cusps like a battery of shark’s teeth on a
large scale. But even in this relation the Jurassic sandstone is peculiar,
for itis here of enormous thickness and so massive that it is virtually one
homogenous bed, and the great gashes cut across the fold or perpendicular
to the face of the outcrop have carved the stratum into colossal crags and
domes. By these tokens we can trace the Water Pocket fold from the
eastern slopes of Thousand Lake Mountain around a quadrant, whence its
course flies off in a tangent far into the south and is lost to view beyond
the Colorado. Its total length thus displayed must be about 90 miles.
Across this monocline run the drainage channels which head in the amphi-
theaters along the eastern front of the Aquarius. It is interesting to note
how completely independent are these streams of the structural slopes of
the country. They rush into a cliff or into a rising slope of the strata as
if they were only banks of fog or smoke. It matters not which way the
strata dip, the streams have ways of their own. The Fremont River and
the creeks which flow down from Thousand Lake Mountain present a very
striking relation to the strata. They at first run very obliquely into the
fold, and thence by an equally oblique course run out of it again. Nearer
to us Temple Creek plunges right into the flexure perpendicular to its strike
and in the somewhat uncommon relation of a stream running with the dip
of the strata. Still nearer, Tantalus Creek runs across the fold in the same
general relation but meanders about within it.

In the first chapter I have explained this independence of drainage
channels of the structural slopes and attitudes of the strata by the general

288 GEOLOGY OF THE HIGH PLATEAUS.

proposition that the rivers are older than these structural features, that
their courses were initially determined by the configuration of the surface
when the region emerged from its lacustrine condition in Middle Eocene
time, and have persisted in holding those initial positions in spite of all
changes. It happens, however, that in the cases before us the flexure is
much older than the rivers. The age of the Water Pocket monocline is
Pre-Tertiary, at least in the northern part, and we infer that the whole
monocline is of one age. ‘This seems at first to be in contravention of the
law. But the anomaly is apparent only and not real. For we have seen
that in Thousand Lake Mountain the Tertiary lies nearly horizontally
across the denuded edges of the Cretaceous and Upper Jurassic and rests
upon the Jurassic white sandstone. The same relation is found in the
Aquarius. In the eastern half of the plateau the Cretaceous is wanting
and the Tertiary rests upon the Jura. A little west of the middle of the
plateau upon the southern flank is seen another ancient monocline with its
throw in an opposite direction to that of the Water Pocket flexure. This,
too, is of Pre-Tertiary age, and upon its slopes the Cretaceous again comes
in with full force, and across its beveled edges lies the Lower Eocene hori-
zontally. Thus while this pair of flexures was forming the intervening
uplifted block was undergoing erosion, and at a later epoch it was submerged
to receive a blanket of Lower Eocene strata. If now we attempt to replace
the beds which have been stripped off by the later erosion of Miocene and
Pliocene time, we must extend the Tertiary beds eastward (and southward)
indefinitely, so as to cover the Water Pocket flexure unconformably, and
also to cover the Cretaceous mesas which lie beyond it. Thus, after the
Middle Eocene, the locus of the flexure was covered with a sensibly hori-
zontal stratum of Lower Eocene beds upon which the local drainage sys-
tem was laid out. As the erosion went on the streams sank their channels
and the upper strata were deauded. The Water Pocket fold was in time
exhumed and the streams cut down into it from above. And since its
exhumation it has been greatly ravaged by erosion.

Directly east of us, beyond the domes of the flexure, rise the Henry
Mountains. They are barely 35 miles distant, and they seem to be near
neighbors. Under a clear sky every detail is distinct and no finer view of

CIRCLE CLIFFS—ESCALANTE CANONS. 289

them is possible. It seems as if a few hours of lively traveling would
bring us there, but it is a two days’ journey with the best of animals. They
are by far the most striking features of the panorama, on account of the
strong contrast they present to the scenery about them. Among innumer-
able flat crest-lines, terminating in walls, they rise up grandly into peaks of
Alpine form and grace like a modern cathedral among catecombs—the
gothic order of architecture contrasting with the elephantine. Beyond the
spurs of Mount Ellen may be seen the northernmost summits of the Sierra
La Sal, 120 miles distant; but the main range is hidden by the mass of the
Henry Mountains.

The view to the south and southeast is dismal and suggestive of the
terrible. It is almost unique even in the catagory of plateau scenery. The
streams which head at the foot of the lava-cap on the southern wall of the
Aquarius flow southward down its long slopes. The amphitheaters soon
grow into canons of profound depth and inaccessible walls. These pas-
sages open into a single trunk canon, and their united waters form the
Escalante River, which flows out of Potato Valley due eastward for 12 or
15 miles, and then turns to the southeastward, reaching the Colorado about
50 miles from the turn. It enters its cation at the foot of Potato Valley
(see map, Atlas Sheet No. 1), and at no point can its walls be scaled.*
Numberless tributary cations open into it along its course from both sides, so
that the entire platform through which it runs is scored with a net-work of
narrow chasms. The rocks are swept bare of soil and show the naked
edges of the strata. Nature has here made a geological map of the coun-
try and colored it so that we may read and copy it miles away. The rocks
exposed are Trias and Jura, each preserving emphatically its characteristic
color and architecture.

The descending spurs from the southeastern salient terminate upon a
spot which is about as desolate as any to be found on earth. It is a large
plain, about 25 miles long and 10 miles wide, elliptical in shape and girt
about by a circuit of cliffs of great altitude. On the eastern side are the

* Mr. Jacob Hamblin, of Kanab, entered this chasm and traversed it nearly to the Colorado River,
but at length found it impassable on account of quicksands and fallen rocks. His journey was a terri-
ble one, and he sought in vain to reach the country above. The depth of the Escalante Caton where
its river first enters the Monocline is about 1,600 feet, and increases as the river flows on.

19 HP

290 GEOLOGY OF THE HIGH PLATEAUS.

domes and crags of the Water Pocket fold, huge promontories of red and
white massive sandstone, separated by narrow clefts, many of which are
cut down to the level of the plain and even lower, so that they carry a
portion of the drainage from within the ‘Circle Cliffs” to the Water Pocket
Cation. On the west side of the plain the mesa which looks down upon it
is slashed by many narrow and profound canons, which wind about within
it and open into the canon of the Escalante. These carry the remaining
drainage of the plain—. e., when there is any to carry, which I warrant is
seldom enough. The floor of this cliff-bound area is Lower Trias (Shind-
rump), and the walls which inclose it upon the west are Vermilion Cliff
Trias, and those upon the east are the same, with the Jurassic sandstone a
little beyond them. ‘The plain is barren, treeless, and waterless, so far as
known. It constitutes one of the centers of erosion of this part of the
Plateau Country, from which the waste of the strata edgewise has pro-
ceeded radially outwards. Probably the Cretaceous was eroded from its
surface prior to the Eocene, and the Tertiary afterwards deposited upon the
Jura in the same relation as is now seen high up on the flanks of the Aqua-
rius. The late erosion has removed the Eocene, the Jura, and the Upper
Trias.

Far to the southeastward, upon the horizon, rises a gigantic dome of
wonderfully symmetric and simple form. It is the Navajo Mountain.
Conceive a segment of a sphere cut off by a plane through the 70th parallel
of latitude, and you have its form exactly. From whatsoever quarter it is
viewed, it always presents the same profile. It is quite solitary, without
even a foot-hill for society, and its very loneliness is impressive. It stands
upon the southern brink of the Glen Canon of the Colorado, at the junction
of the San Juan River. Its structure is believed by Mr. G. K. Gilbert to
be laccolitic. Its summit has not yet been reached by any exploring
party, and the approaches to it from all sides are extremely difficult.* On
the north side runs the profound chasm of the Colorado, on the east the
canon of the San Juan, and on the west another side gorge. South of

*Professor Powell, during his descent of the Colorado River, climbed out of the caiion and ascended
about half-way to the summit. He believed that if time had permitted he could have gained the top of
the mountain.

NAVAJO MOUNTAIN—KAIPAROWITS CLIFF. 291

it, for 60 miles, the country is dissected by a net-work of deep, narrow
chasms, among which are trails of a most intricate and difficult nature,
known at present only to Indians. The mountain is inhabited by a band
of renegade Indians, chiefly Navajos, who are very jealous of all intrusion
into their fastnesses, and great caution is requsite when venturning near
their retreat.

Due northward rises the great wall of the Kaiparowits Plateau. This
giant cliff is 60 miles in length and nearly 2,000 feet high. Throughout
its course it wavers but little from a straight line. Almost all the great
cliffs of the Plateau Country are very sinuous, being in fact a series of pro-
montories, separated by deep bays, like the lobes of a ‘“digitate” leaf.
The cause is readily discerned The bays are produced by the widening
of the canons, which, in a great majority of cases, emerge from the cliffs
and seldom run down into them. Erosion thus not only saps the main
front of the cliff, but attacks it through these side-cuts. But the Kaiparo-
wits cliff has only a single canon emerging from it, and this is near the
northern end. From the very crest-line the drainage is to the southwest,
while the cliff faces northeast, and thus the eroding agents can attack it
only in front. Since the strata are homogeneous in their horizontal exten-
sions, and heterogeneous vertically, the effect of erosion has obviously been
to produce a straight wall, broken only at the point where the single canon
emerges from it. The beds of which the Kaiparowits is composed are
Middle Cretaceous. We can see, from our standpoint, their characteristic
colors, which present a very striking appearance. Broad bands of bright
yellow sandstone, alternating with the dark gray of the argillaceous shales,
produce a contrast which is not only visible, but even emphatic, at a dis-
tance of 60 miles. These belts of light and shade are 300 to 400 feet
thick, and apparently quite horizontal.

To the southwest rise Kaiparowits Peak and Table Cliff, of which
more will be said hereafter. Between those points and our own position is
a great depressed area, of which the lowest part is Potato Valley. The
altitude of its floor is about 5,600 feet above the sea. Towards it conver-
ges the drainage of all the highlands lying north, west, and southwest, and
the confluence of the streams from those directions forms the Escalante

292 GEOLOGY OF THE HIGH PLATEAUS.

River. The country which thus concentrates its waters into Potato Valley
may be regarded as a vast amphitheater, with a radius vector varying in
length from 12 to 18 miles, and of which the ramparts of the Aquarius and
Table Cliff form the upper rim. The amphitheater is the work of erosion,
being a westward extension of that vast denudation which has removed
thousands of feet of strata from the whole region spread out before our
gaze.

As we study the panorama before us, the realization of the magnitude
of this process gradually takes form and conviction in the mind. The
strata which are cut off successively upon the slopes formerly reached out
indefinitely and covered the entire country to the remotest boundary of
vision. Their fading remnants are still discernible, forming buttes and
mesas scattered over the vast expanse. The same process of reasoning by
which the mind joins the edges of strata across the abyss of a narrow
canon enables us to join their edges across wider intervals. The restora-
tion of the Trias to its Pre-Tertiary condition is made almost at a glance,
since the vacant spaces are few. The restoration of the Jurassic and Cre-
taceous is precisely the same in nature and equally simple, though the
spaces to be covered by it are much wider. The Tertiary is wholly want-
ing to the eastward. ‘There remains only a single outlier to the southward—
Kaiparowits Peak. But its former extension over the whole of the Plateau
Country admits of no serious doubt after we have once mastered the plan
of the drainage system and of the Post-Eocene displacements. The rivers
alone might not be sufficient to demonstrate the conclusion, nor would a
restoration of the displacements, but the two together admit of no other
interpretation. How far eastward and southward the lava-cap extended
cannot be determined. Remnants of alluvial conglomerates, with large frag-
ments of trachytes and augitic andesites, are found more than 20 miles
eastward, and they are indistinguishable from the rocks now forming the
summit of the plateau. But how far they have been carried is a question
which it is impossible to answer.

The altitude of the eastern front of the Aquarius above the country
which it overlooks is upon an average about 5,500 to 6,000 feet, and the
thickness of the strata removed from its vicinity is probably about 4,000 to

STRUCTURE OF THE AQUARIUS. 293

5,000 feet. In some localities the denudation has been much greater, in
others considerably less. The preservation of the Aquarius has no doubt
been due to its immense roof of hard lava.

The eastern part of the plateau is the loftiest, being about 11,600 feet
above sea-level. Its platform here is believed to be nearly horizontal, as
indicated by the projection of its summit against the sky from every point
of view around the horizon. When seen from Thousand Lake Mountain,
which is very nearly as high, no peak, nor even a hill, breaks the monotony
of the almost level crest. But the summit is so densely forest-clad that no
effort was made to penetrate its interior spaces. The upper wall of dark
volcanic rock is seen to extend completely around the eastern third of the _
plateau. A little east of the center of the plateau a fault throws down the
platform west of it from 600 to nearly 1,000 feet. This fault is a south-
ward extension of the one which runs along the western base of Thousand
Lake Mountain and across the Red Gate. South of the Gate its throw
gradually diminishes, and on the southern slopes of the Aquarius, a few
miles south of the lava-cap, it runs out. This fault is comparatively recent

for the most part, and is probably coeval with the other great displacements

of the Pliocene-Quaternary system. On the northern slopes it splits into two
branches, which reunite near the southern verge.* This movement has
produced a sag in the central part of the plateau, but the altitude of the
summit is nearly all regained towards the west by a gradual ascent.

Of the rocks upon the summit I can say but little, having traversed
only the central part of the plateau. Those which were observed were
chiefly dark hornblendic trachytes commingled with very extensive masses
of augitic andesites. In their general aspect they resemble those which
are found on Thousand Lake Mountain and northward as far as Mount
Hilgard, but with a somewhat larger proportion of augitic lavas. The
bedded lavas exposed edgewise in the upper cliffs are highly varied within
their limits of chemical and mineral constitution. No acid rocks were
observed, and only a few very basic ones. But the sub-acid and sub-basie

*Mr. Gilbert is of the opinion that the displacement is much more complicated. Ascending the
face of this fault and reaching the summit, he found a narrow valley near and parallel to the fault,
which valley he believes was caused by the sinking of a narrow wedge. He has also suggested to me
several other minor features of inequality in the surface which he regards as due to minor faulting.

294 GEOLOGY OF THE HIGH PLATEAUS.

rocks present a great deal of variation in their aspect. A body of lavas so
enormous as that which caps the Aquarius cannot be discussed with profit
until it has been studied long and patiently, and inasmuch as my own
observation has been extremely superficial, I do not feel justified in attempt-
ing to give any further account of them.

The structure of the plateau is best studied upon the southern slopes.
Here the most striking feature is a large monoclinal, already alluded to as
a companion to the Water Pocket fold. It comes up from the southeast,
crossing the lower end of Potato Valley, and trends along the slopes north-
westwardly, disappearing beneath the lava-cap. The throw of the mono-
cline is to the westward. Upon its flanks the Cretaceous system is turned
up and dips westward beneath the southwestward extension of the general
plateau mass. The edges of its strata are truncated by erosion, and over
them lies unconformably the Tertiary. (See Atlas Sheet No. 7, Section
No. 7.) The upthrow of the monocline heaves up the Jurassic white sand-
stone, which is seen rolling up in a huge wave 1,200 to 1,800 feet high
across the lower end of Potato Valley. The position of this flexure rela-
tively to the plateau mass is peculiar and very striking; indeed, at first
sight it appears altogether anomalous. We are accustomed in the western
regions to see the strata rolled up on the flanks of a mountain range like a
great wave urged onward towards a coast and breaking against its rocky
barriers. But the Escalante flexure is like a wave sweeping along parallel
to the coast, the crest-line of the wave being perpendicular to the trend of
the shore. Its line of strike runs up the slope and disappears beneath the
Tertiary near the summit of the plateau. A fine steam of water (Winslow
Creek) runs upon this monocline parallel to its strike, precisely as Water
Pocket Creek runs upon and parallel to the course of that flexure.

The age of the Escalante monocline is evidently Pre-Tertiary. It has
been exhumed by the general erosion after having been buried beneath
Eocene strata, and after these strata had been overflowed in great part
at least by many hundreds of feet of lavas. The stream had its course
laid out prior to this erosion, and held its position after it had cut through
lavas and Eocene beds into the underlying Jurassic sandstones.

The area included between the Escalante fold on the west and the

WESTERN PORTION OF THE AQUARIUS. 295

Water Pocket fold on the east appears to have been, during the latter part
of the Cretaceous age, an island. It is apparently possible to designate
roughly the positions of large portions of its east and west coast-lines. In
a word, those coast-lines may have been approximately coincident with the
axes of those two flexures. The northern part of this island cannot at
present be ascertained, because the lavas have deeply buried it, and there
is not even sufficient basis for conjecture. But of the portions now indi-
cated it is possible to infer that the length of this island must have been at
least 90 miles and its maximum width about 35 miles.

The northwestern angle of the Aquarius is laid open by an immense
gorge. A mass of lavas and conglomerate more than 2,000 feet thick is
revealed, and beneath them lies the Tertiary. Near the opening of this
gorge the Grass Valley fault cuts across it, throwing down the platform to
the west Along the western base of the Aquarius the faulting becomes
very complicated, and the displacements are great in their vertical extent.
The faults are repetitive, or ‘‘stepped,” with numerous instances of the
dropping of large blocks between faults of opposite throw. These blocks
usually sag in the middle, and there is occasionally some chaos produced
in the component masses. An effort was made to find the proper restor-
ation, but I am doubtful whether it has been very accurately done. (See
stereogram. )

The western wall of the Aquarius, which looks down upon the south-
ern portion of Grass Valley and the Panquitch Hayfield, is of great gran-
deur, rising more than 4,000 feet above the valley below. Apparently it
is composed of volcanic materials from top to bottom, but the thickness of
the volcanic masses is less than it seems at first. The wall rises by success-
ive steps, and each step represents a fault, so that the aggregate thickness
of lava and conglomerate probably will not exceed 2,000 feet on the aver-
age. The rocks are mainly trachytic, but a large proportion of augitic
andesites is associated with them. At the summit of the plateau near the
western crest and upon the thrown blocks which are successively passed
as we descend, are numerous fields of ancient basalt much eroded, and
presenting a similar appearance to the scattered basalts spoken of in the
preceding chapter as occurring upon the surface of the Awapa. Their

296 GEOLOGY OF THE HIGH PLATEAUS.

extent and distribution is not accurately known. They cover a consider-
able area, but in a disconnected way, and their eruption appears to have
occurred prior to the principal epoch of faulting. The mass of conglomer-
ates is very great. They are composed wholly of the débris derived from
the destruction of the more ancient trachytes and andesites, and are well
stratified in layers which are nearly horizontal.

The age of the principal eruptions of trachyte and andesite cannot be
ascertained, but it is very ancient, going back probably into the early Mio-
cene. The same indications of great antiquity are found here which have
been observed in the Sevier Plateau and in the Tushar—eruptive epochs in
which lavas in enormous quantities were outpoured with hundreds and per-
haps even thousands of individual eruptions, epochs of erosion during which
were accumulated heavy beds of conglomerate, periods of faulting and dis-
location which have given a new topography to the country, periods of
renewed activity of volcanic forces, and a long final period of waste and
decay. All this conveys the impression of immense duration; how long
the era may have been we do not know, even in terms of the geological
calendar. But the interval which separates us from the Kocene must in
some way be filled, and these operations are all that we have to fill it with.

The western front of the Aquarius, from the grand gorge of Mesa Creek
to its southern termination, is about 17 miles in length. The lavas and
conglomerates are heaviest at the northwestern angle, and diminish in bulk
towards the south. The northwestern part of the plateau seems to have
been one of the great centers of trachytic and andesitic eruption from which
the extravasated masses flowed outward in all directions. No cones or
mountain piles, however, are now visible. If any formerly existed they
have been leveled down nearly to a common platform, and can no longer
be distinguished from the rolling hills which have been sculptured by the
protracted erosion. There is, however, this peculiarity in the locality:
the lava-sheets are less stratiform and more chaotic than in localities where
they are collectively thinner. They are also more varied in kind and in
texture. As we recede from this locality the sheets become more uniform
and even in their bedding, as if they had spread out and become thinner.

TABLE CLIFF AND KAIPAROWITS PEAK. 297

Many dikes are also visible around the gorge of Mesa Creek, while none
were observed in the bedded lavas farther south.

TABLE CLIFF.

The southwestern cape of the Aquarius ends at a high pass separating
the Escalante drainage from that of the Panquitch Hayfield. This pass is
thus in the main divide between the drainage system of the Colorado and
of the Great Basin. At this cape the lava-cap of the Aquarius terminates,
but beneath it the Tertiary thrusts out a long peninsula to the southward.
The altitude of these beds is very nearly 11,000 feet above the sea, and the
peninsula which they form is Table Cliff. Upon its summit is an outlying
remnant of lava a few hundred feet thick, which was once, no doubt, con-
tinuous with the lava-cap of the Aquarius. The table is practically a large
butte left by the denudation of the surrounding country. I have explained
in the first chapter how the degradation of the Plateau Country has to a
great extent proceeded from a number of centers, extending radially out-
wards, wasting the edges of the strata, partly by direct attack upon the
fronts of cliffs, partly by the interlacing of canons, but each series of beds
being gradually wasted backwards, and their terminations forming ever-
expanding circles facing the center of erosion. The erosion of the Tertiary,
which spread from the center now occupied and inclosed by the Circle
Cliffs, has met the outward-spreading erosion from a center now occupied
by Paria Valley, and the cusp formed by the meeting of the two circles is
the locus of Table Cliff. The table is interesting on account of the splen-
did exposures of the Cretaceous system upon its western and southwestern
flanks. While the beds in the mass of the table are nearly horizontal, the
ledges of the Cretaceous projecting towards the west are turned upwards
at a very moderate inclination, and in passing to the floor of Paria Valley
we cross the whole Cretaceous system, of which the thickness here is
5,000 feet. The series consists of heavy members of bright yellow sand-
stone and gray argillaceous shales. Each member is from 300 to 500 feet
in thickness. The cliff sculpture is about as fine as any in the Plateau
Country. We have noted its appearance from the western side of Paria
Valley at the foot of the Paunsdgunt slopes (Chapter XI), and a nearer

298 GEOLOGY OF THE HIGH PLATEAUS.

view, though less pleasing, is no less impressive. None of the cliffs are
lofty, but the grandeur of the spectacle consists in the great number of
cliffs rising successively one above and beyond another, like a stairway for
the Titans, leading up to a mighty temple. The Eocene beds which form
the upper table are rosy red, and carved ina manner which is so suggestive
of intelligence that it is difficult to persuade ourselves that the blind forces
of nature could have achieved such a result.

KAIPAROWITS PEAK.

Kaiparowits Peak is a mountain-like butte south of Table Cliff, capped
by Tertiary beds, with the Upper Cretaceous upon its flanks. It is obvi-
ously a mere remnant of the continuous Eocene formation which formerly
stretched indefinitely southward. Its slopes descend to the platform of the
Kaiparowits Plateau, which is composed of Middle Cretaceous beds. This
plateau is properly a member of the Kaibab system, and is one of the most
interesting. It is a broad causeway, reaching to the Colorado, where it is
cut off momentarily by the Glen Canon. Beyond the river the Cretaceous
beds continue far into Arizona, and expand into the great mesas and ter-
races which cover a large part of that Territory. Along this plateau there
are still preserved the unity and virtual continuity of the formations which
constitute the District of the High Plateans and the mesas of New Mexico
and Arizona, while elsewhere throughout the heart of the Plateau Province
they have been removed by the great erosion. The little remnant of Ter-
tiary beds upon the summit of Kaiparowits Peak is one of the many indi-
cations that the Lower Eocene also once reached across the same interval.

INDEX.

Page.
Acid rocks. (See Rhyolite.)
Air estofieLrupulons qe aie cicc cess cae seiseeice mele sicinseivics's Oo bogode Gda6 GoceK000 gn0000 posea8 39, 56, 59, 61
Allluvialiconessascna cma cececends see econ ts aa eelncclcscocsccseccan soaceeeces soeoceueacere 39, 214, 249, 277
Alluvial conglomerates, general discussion of ...........--. 22-2 se-se0 222 cece eeeee -oe- ee eee 214
JN TET NED THN ROLE ANG THEE) Sasa ease Gna BOOG00 S000 GUCGO0 OG BIOORG DAG OO Sue GOD UCOSEO OESEED Osbn 89, 117, 123
Andesite, augitic:
Classification of -.....-...---- eee miana te cine wictonisoe crn e cinta eecinte area erenae cnpoeee 101, 108, 109
Occurring in the—
Aquarius)Plateau----- -s-c-.1e--sce sos -in> ACTDHS Sco cobO dan oad COdeRa BE cOOS a Sed enanae 293, 295
ARR TIOE RIGS con aa50 b808C0 GenS 020550 cOn6 cnEEed COOCbE pdadoD 6ecass ogDeG6 DESHSO 275
Mish bakepblateatyss sasscae cease acces voce wancestescceceniansee aseccosetecce sence 261. 266
WOME LEnNEAIL. Saco S000 c6a5 2600 6000 0500 A006 600.0000 6605 Bade DAG0dcEo Ba06 sass esne 272
Marka cuntpblateauemarciares ancl ceinecscsecicicce cba cinis ae eailaelensclosecis ecposes 196, 204
MonroevAmphitheabereaceessoccitisccicscclesciecciceceisoeciececeeisctes DORR ROBES ACO.cabS 229, 239
Sevierpbla teats sasen weve as Saisie sctesinaen cee stine sion ciscde acacia sewers cies 229, 232, 235, 242
US aT eeepc eis ce aac or ietencisieiie tieaeis (ore wareseceeeee ISR CEM B ESSe Oe MaBCee eoee 177, 181
Orderolysequencelesssecisassieneslenesiscisciecis ee 100605 pebsos0 d608 aeH0.0 500 ao50 8608 63, 65, 67, 131-137
Andesite, hornblendic:
Glassificationt ober met cee ece eae oceme cae ce orca eet oe ncn sete ce Suncom eons tisemermeseee 110
OcCurrenCeserte eae cee eee ace tee SES nies Seba lce een sotira oo eclootaleoneee 230, 237, 242, 260
Orderiofssequencemese=ssseree ce eceeseeneoee SESE BREN ECE B EE OSC OS Baap Sobaarics 67, 131-137
Andesite, quartz. (See Dacite.)
Aquarius Plateau, general discussion of (Chap. XII)-....:...2.-.------ s----- s--------- ------ 284
Relations to District of the High Plateaus. ..---.--.. ..---. 2200 2-2 + coc eee nnn conn ee ee ones 5
Argilloid trachyte. (See Trachyte.)
ArchmansTocksmeeassace cates cener ccs seaccmueceee FSS BOO CER ECA ECBO EOE SOCGED Gaon Osmare Ceee 143
Awapa Plateau, general discussion of. :---.....--- .---------.--2-- --- eabHoS boca mae bosSobeS 272
RelationstoDistrictiof the High) Plateausaccscesssce =a o-oo ee ceeee eo neem eeescces== a 4
Dal dyseMOUlMbger cece ects reese ere eos okie aie aoe eee caine bow a eeloc cmimeue re vamiscaciassectascee 172
Basalt:
Chemicalkconstitutiontofeesesae sca eeeccitee ese seiseecsectes cceeee cena aeeaiseccseiaceoe= 123
Classificationvore ans aaye See ine ee Sere rat ae oa eis bin os loos esa diane cateeeeeeeeinne oe aceaoees 101, 110
IDEN ESS encgndnced Secs OCC TE SOCAL HOUOUISEOC BOOS OO ESE Berne ter Se Rese BE Se RESIS 111
Miquidityofeasa-ceesseeee = BEA SOO SCCM Ona ConeS ide Sees wade ee noe See eek eee renee se sieese 135
IMG ONG Mis SSS ca koScbn Saao eDOCS CCOD SDOCcS COSSES CES BEeoEee CEB ROCREeO Bce- Hoop oAreReeHeSes lili
Orderofi sequence ast se cec ee siccein sateen c sane Son ninos atoec Houses tees seeeeciescomeee 67, 181-1387
Superfusiomofsas scene ce ese earn eee ER aia meas mate ngams = ees aia eee ne mvc pe oe 135 -
Syntheticicharacterofie sa. cce sess hoses ene cae Serco eons eee aan) oe eeee eo et eae eee 122

300 INDEX.

F Page.
Basalt—Continued.
Occurring in the—
Aquarius Plateau........-....--- 900008 6 ob0050 Oo SoESED Habe SOgGS5e6 GaSb caSeos odes 295
JARO DEM = BA SOS GOOEO HOS 8ODO50 HHOSEU GEOS00 CANHOOOCESGS 000e 66606005 0056 ep osoade 93
JAR OR REYES ecb sa5 550660 SO0D09 060650 SESDGODS00 6d0G asaocenSa cad Goa0 sRSSS Bede 276
Carboniferousieassemaacees o-oo ieee ce celseecle sins ee Serene eee eee eee ee eee ene 93
IDOE WON ooo 6 Go65 505560 CREdEO SEO. C558 HO6C BORO COOS ROBE CO GOOD ConaKe Sonase Sdoads 190
nmGE AVENE? Ss scepcobinas esocap bade! 6 SHSdoS Cand SHES Kode Ueno Uda Sas e546 Sena ces. a5ee 250
Markacuntiblateanier cteeajemaieaeiee seiceiaciacis ceieieseieanciencetsem cence ences 197, 199, 200, 202
JERI ENCE S366 o5cc5 GooceD 6S65 S05b 5050 Haas SESe Sno ode dees Coot Sods S555 256
Permian ----- Jose se Suessesaeosetssee eteses peste seesn Coes sejece eee eae eee eee 93
Silurian. 525 $2222 oon do scsese sedis ces cee nad cease aan since Sec eer ee eee eae 93
TriaSSiC), 225-382 oes esos ass ss etee cose ofsceciseee caress oe ee eateee eee ae eee eee 93
MusShaLzeecccesce sesh ce cwe ecco scece seSdeebsdscsceewecsts ocicioceiscs eoeeosecece 177, 180, 183

Basaltic::cones s-.\22 ss. ssh ses sete nse occs eh ocae cece crees cece oc iceccles se se ecm e peice Sete Oe NL IG8256
Basic rocks. (See Basalt.)

Basin| Province; its) topopraphy=—s-- csc oeisees ee eelen a monies seni se ses ee ese eseecesieestieeee ee 6
WONT, WANS) 6 Shed 2od5NS 5556 Sond SoS Sao Godeco SENS gOS S05 SUS SUNOHSS Ga6Hdecsn5 css Coos 47
Structural features. 22 scc~ 2 cc ecoe svececoe coco neaie ce «See Seee eee cites ae eee ceoeecee 51

BeamPeales 235. aoc cc eae con eacee Orewa se ehes ooo renee ee eet eer mae eee ee teres econo 192

Beart Valley: =2-: 2... 5520.02 sos tee noes tems con oee sae ciesecceeeee eee ssi one secon eases 191

IOS neyP, MIG son 6c5. sae coseOS Coos SEE ESS HATiRos MSS OD ODS OO RadS case aaciaee CHS HASHSo Oace coeSes 172

Bitter Creek beds (see HOcenoNe BERSLOSOGCH) DOSS BESS Cac Edas 65060) Gaaeo0.q05000 6008 12, 73, 158, 159, 195, 238

BluevMountiins..woacsccccw acco ace <oloe seleee ae ee Seen nS eee nee eee oreo 228, 232

Bonne villowialke=sses=s=oeeees eee ee BeOS HOO RES Rape ese seca sa coniice fee. 41,211

UbLeye) tel C2 by <n eae ee i ee ee Re er Sa Soc ooooSToaEd 161

Bullion: Canonite a oc ste es Se er ee aoe cole oe nln ne ee eee ee see eae 174

C@anonsieeeeees Usain deihacicSaveSeal cae able o eeis se See eee coast n ah Soke Ne are Sea a eee eo oO,

Carbonaceous sinion, Beate FESS Seas BAS ee eee Met ate Bek ee eee es oceans mance 10, 156

Carboniferous stratacss. osc os ose oye eo Sea Sc es Ss ere ee ee ey en A 1 A!
Conters'of'erosion*:.otrrk sat es oat Se eee ean Se a eae ie eee Se eee cee eee eae a

CircleiCliffsy. 52s cee scones teawtie can lece conte ce as nae e tee ee aoe Ee mem cle ete eet 289
CinclomVialloy oi... fon Sosa oo ee eee cic oes Stet cents sine tan isc Nelecia en ete 213
Chalcedony case aaewre co ae= se eee clone eae eee ence cea alone noise = aaa len ana aero a tesee ena atase 205, 238
Ghemicalicharactersiof volcanic Locks =se-ees aos a nena eae eases eenieeee see ase nee = ae 88, 117, 123
Classificationtofavolcanic)rocks| (Chap liv) seesemestaa seat esse eee eee eee 82
{O01 ete ee ee oe eas ee ese hooae cou csue.coseed Boos 155, 156
@ones: basaltic: ; <<. sec soacee soe ee See See Se Se ae nner OS Lo neou
@olorados River: <q s<2cs.c lees ccceae let asec ene oe clos o ae eee ee ee ee eee ee eae ONenO Reo uAeoD,
Cotta, B. von, Eee Terti PAY GUNG OU Slee omer odcborace coon Cone ESHo cee cbse ooacsoaSsser bees 96
Cc parlor nerates, general discussion of the formation of..-....------------- ------ ------ -------- 214
Conelomerates Shins Com pee ante et ee ee ee eee 147
Conglomerates, voleanic......-...---- -- Boseetae ne soe neeets cpeeeeees 39, 70, 75, 178, 214, 237, 238, 275, 295
Cretaceous, the—
IBrackishsyalen! Ded S Oh mee ee erate ne ee ee ee 10
Carbon aceous) sha) es sessed 10
Condition of Plateau Country during...... ..-...----- -- ane won e 2 nnn 7 ow ww wns woe 9
Gonrelations Of cass cae cases Sone aan ce te na mais seas sie o Senet eee a atte 9
ixtentiof, in) tho, WeStss.s-seees noses ean else an eee sane es claw crete aie atelier ates 9
TIGL ae eeee Sete Se aE ose RSS OSS Ss OSS SSe ere nbcone aeactcleaciatocssiossecs 295
Of the—
Aquarius) PR] ateat sere see. Soa ae eae ae ee eae oo ee ee ee eee 288
Kehr hate AH Up ees aos Gass aes 5 ceo s saa eos caoano peeaeo ooo ose sc cesascsiasecs 291

Memnkaoun tibia teases cea seo ne ee ee alam al ae eee 206

INDEX 301

Page,
Cretaceous, the—Continued.
Of the—
TE TRG TIM IPIB EA os c6q50e96 Se55C8S00080 S800 B00 COOH D600 BCEO D660 5050 CEES ERED wassee 951
ANDI ONG) (CHEE 5 cone Sh50 ee5000 aosanp co0060 soStetodSSoS Hbeace cans CoSCOS Cone nenS Hpesscoces 297
Wasatch Plateau. ---.-...5-...-...2 --- Gobood acu Hocoed COSCON cans becOsD Eades cecdcs 162
Simei DIY OES cog scoo ce bane cHod Suse en bO0qDD SBOKED sHOSoCEDSOboD edae Bone BeoESaceonSE SEES 154
Subsidenceroteescereeraatateaeiaciasaeieseeiercisicee d0eO60 coco dene co06eS BaSeeG Cane doce dseces 3
UmcontormitiesiabuhercloseOteneseiesmseelsacniacincsecclese re cie=saeeieae ceases sees 10, 156, 168, 280, 288
(CHS A DATES 555 osedeg Coeedo cand coSgDN Dan Sa0550 O00 CS COU aSo cOSdEd DESbeDcooceSnennbs cooSeS 153
Dacite:
Classificationioteeseieesecstacceisaaetesciesiote scars 0d SoDdoO Sans aoBo qadcaD consaDoassacocsoeces 110
Ong Gi FECWONGD OfacsSca s6c6os Soocde céon déoboncqES Leceaa 6 re oalesjcineeiiee iaes See ee ceeeee 10, 138
Dawson, J. W., Triassic basalts - ...-.. - 2... +202 2. eece coon ee ce cnes acnne peSeod DoSBoC Cane 25H6 93
IDIAMO, IMOWEND > 6605 6060S SSeS FOO GOD Goud Bde COOOSH SOOO GUdnUS 6506 Seda nENS COEboaOESSeg Geer 172
Mensiuygotel avasseesee es eee ee eteeisektcee ale QOODUT Gadd 6800 O8aS Ebon SooGe0 Saddae boone nce 88, 90, 132, 134
MesiccationvotsthembocenceWWakescaasatcccsienceee see ceceicscicceccntiesscisetniceeeec aeeeiecssicoee 15,73
IDA DARE, TOG OF CCOTERINGD scodcc sand ea0s daS050 Hone BoDuES BReotS COdeES esannO baa TcEd socecc 95
Diorite, mode of occurrence. ---~ .--.---. «---e- ween 0Odad0 one C568 HaUd GO5500 G5 nanESOGe OSCDES 95
IDOE WAM = onda caches a500n0 cR00 ShoDdS onde cbosOe poOdHD DoGeDG HEOGobe55e66 064000 cans ecacKe Asse 182, 189
Dolerite:
Classificationtofees-seeeremceerieclesesiescces CEUOHO HOCOOU GEE Eos Bobd senS BESO nonobaEcoe meas 111
Occurring in—
IDO Walley7o coscoc odeo cacdad coSoUR dooGK0 5000 bone doSSHO 6GES S50 0Q00 Gane BEES BOGeGS 190
Fish) Gake! Plateduse. sone scccccoccocecclee tices onc nicaweee lene soul cscs cletenseecse 261, 265, 266
Hilgard, Mount .............-.. d0don0 Sab600.poSsaSb NnOSSU.CEae5D HaDboD DHOORSS OO SS 272
SeviertPlateaw sasscsmceaicecetee mine ais ne echeniniseuccGe crm aclae issues enenccsroesc 230, 234, 239
Bu sh arsprse reac ae esate eels fale oe ote ie ae eee oie ee elo ain ee eae tec cies Sleceteleatesteeres 177
Orderof sequent se sone cs ace soe estes ese ce wlesiewesienc ows se sabe alae eens coeeseeces 67, 131-137
Drainage eiNnconsequen bertce ewes asta te sae lss cere cna ciee ceo cecisiesintees cins sme censeimon = eeelonate 162, 287
SystemiofePlateamMero van Celsemaericeiccele ccc cstecesen celeste ceiniee cisco seccee ceniseiseioeceee 16
Dynsmicsioferuptionsmeassae sass eee os see Soe ia oan eine wale eo cicaceaecisconeclsecetoacscsses 125
Mastibork Canon ae peseae Sancta snisceiseecoc cee csccestecses ces Heeed Bac eeesosc Seevenieces 70, 236, 243
cho Clitieflexnreys sso ae sare cee cisae Secale sis cw wiccieiae wats ols ou wioeis civics siecc ste cewiecd/seiseeciecsses 44
Sai bers aaa Sse ace nse Giaale Sic ein Soe Sc late ois isla cis ae Sa ciaus ch cis aisloe oes ienteuiewsintign de eereeisises 50
Hocenewlacusuine;conditioncdurin puLhO sea ameeieeesmeceacseelssoaacies eee citecccceeecieeeeee es 12
Of the—
AXGWETHIS) 3 65.00 con sop eben edacds 6606 66.0000 G605 0000 JacHED Go9D bond cdodeE od00es dB00be 288, 290, 292
PACED Dey asleenial ei asae mete tate orem ielemie nieve ne enya ein lele's shelialeine iis oe oe aniniccelointcisleimeieraers 273
MishbhakewPlateawace sca cfoes ec atssteulncoow sees cele cutee ceioce eee eset cas eeeecces 257
il card pMOoun tpn esse secescisceele se ese cians seston loca cles eisceawclasscieaeteeeces 271
Kaiparowits!BReakyasccics seas ssineaslae cee ctsle sae closes ne caelesee Selsociciciereceejeeeciccweees 298
IWIGTESCTING 25 conGdo S05 040500 Haba G09. 0006 06 HOTS 6500 SORS CODE O8U0 CCN OG00 OSHS COOK 189, 191, 204
IETS U IND sccoe Sasesa50 56 56. 0000 660060. 885005 050000 dass ORNU DES ecaS SoEGQsSessooSSSS 250
SVM bese ers save sere aie see Oa Saeki oa mos eleec Siesnces cine Semye co ecedicccsiceiececseessee 170
TBE VOOG Welly .ctso5 coos5e se Sob ase cad6 BodE eGo Go GODS coscoocens Saccas HaseooESouSses 273
Seyrersblapeapes teeta ao eeeic te micle isn csbae wie luis wists ee See islcicrapcuisaiemoe oe cn eenena teens 237, 246
Riemer \WeMlen7 66S 56a Stoo cooous poaScO SOCn Rone DOS OHS OG HU OsOn Hane Hocd Daca qackoo GES Agen)
Dab ley @ lashes ereran cess oe ae ate roe es Rowe eee he cea Gidee Sone ciecciea Subse oe ocecisbe ces ces sce Deen
Thousand LakepMountainesss a= scoseeceise Se oc wee escieceoss .ces seccieccsicceeceeeeaae 280
WasatchiPlatewuseae-eeoescen ceases sare cee Byoe a eeeise acta ciao se ceeea 166
SSHOre RIM Spec peee ee epee eye eae ae a eR ae ree a he ican oon ale wie ieee oan ela le sien eee seeeeee 71, 184
WnecontormityawithiCretaccous\=se-ece ese eciserccoeeeeee eee eee seer ae eee ened coUneo eee
Erosion of the Plateau Country-....-.-...-.......-.------14,17, 21, 36, 37, 161, 239, 253, 286, 290, 292, 298

302 INDEX.

Page.
Eruptions, Causes Of ~~~ -- <<< -- <n ene nnn www ew nn one on aw oenw vonnen see seenee 118, 125, 129
Centersxofsss. coe sae ee lenos ee cece at aces ct eke oeeese sean sod to ESI ee scene coe oa eee 18, 297
Cychiealkcharacter0fsasee-o cae eeisee oe cle ale meee eee eee eee ese eee eee 126
MscalamtexC si OMs yp tecte ace ste tee ce cia mw tim mest eel tem are ole ee ee rate a eee Dare Pee 9239
IMIG ONIN 3 odooculnsooce ponte soaded oaeess obaSecsaa05 ans a50665 SaSSSe bSbhes Sasens ce'bao= 288, 294
TP 0 SSeS gOS ESSE Bas CHE EES DOOEES BEDE Ee BNC BE REES Ger OR Se a Con Ses Gabe od Cobaas aokSsosEacoe 291, 292
ihanltssceneralediscussiomoh (Chapa) pase eees cece ean a ie enema ete sea ee eee 25
lhomolosyioLawithomonoclinal slexuresteceee se se see alae ieee ieee ease eae ee eee eee 26
locationyo fed qu ari ns teem ee eselaee emai elena ene ere rare eee arate Sa 293
TSAR G UGH) 6655.05 coecce cooSes Hass Gsonce H5o5 GoDESOSSSgSSScsS GS CoaSeS tanceéossese 32
Mast MUSINAl 2 ccee sees eae ces acc Cece eet # oars Sopa aCe canis Meee Eon ee eee 34
Gram @iWrashitscsciaicoe nos otis c ce nac\ sites ein sel eleteieo alee ce nana aioe eer eee os an 7,28
GrassgVallleyiemenstucerscec scot oem See ce ee eee eens ioaginsbe we sebebeteuen 82, 256, 273, 295
(EmnmTston) VEN Ey 55566069 550660 035509 SSSES5 SOSESS SaaS SoaSaS Scan oSSasS conesa cose cece 162, 164
TBP ROL Ses e565 one 554 505685 coStas Sa0Snd Cacche Sass cho55s cooase cone odes SOSdes OosE See6 33, 295
1B IRMA ERG 5 oo sco coe saos 6ae9 S255 O80 S500 pances OSSeSs os0S00 KobEdS Osccee 7, 28, 189, 194, 202, 208
Tete) WANE 35565 cose os50. 005605 5405 5059 5656 6659 555556 nacc00 caSSos Hanon Habone osooee Se 33
RGAE oes So Sas55c0- SSS cone SoSEOe Sons SaSh Sedo HOSS OSa6 SoeSso SEs sSeceséo cecEs oS MGR, Drs S518)
Aba aTeAnG | IDE Cas Se sse6 esp nese see SaSsea aS coce SaaS esse cosces sSseessocsse sessise 33, 277, 293
ANON OH Ns Sond oscoee gascosabes soon BSc sete Sac6 Saonse Good Sone nSHOoS cane Seco CoSdSs HO)
MITTS TENE 5 poadioo ca cooeaseansas Saesce sass cess SoeSs6 Sa5e0 Togs essscasosace skescseces 29, 178, 180
\WYGEI (Cant Oo 355555 Soa 550 caso cose sosSas Sone s5 SeaU sbocsesosesonceseo sesese ches 164
Western Kalba) Dienataeselaae alee ael ee eae eee eee eee aa eae ee ee 32
TTR] DPN) S66 35 caso oss6 COCR E CEO CaS 0 BSE BECO SSSR SOS ASSO CSSON SoS8 Goss ocoS SecSed naSoSaeS 262
Fish Lake Plateau, general discussion of (Chap. XII).--....-.... ......---. .--------- 2-2 = ---- 256
Relations to District of High Plateaus.-.... -.-- 5... - 2-2 22. s220 «+2225 - ~~ ese 4
ISRS TON WANK os tonne: e5o6 Sres cena aescoSsgsa0 chess Gass se saaa sas cshoso ose access send 270, 277, 287
osibilihyofny. Ol cant COGS sete se elem ee ee eaten 88, 90, 132, 134
Geikie; A’; ‘Carboniferous dolerite and’ basalta == 222 soe eae cee en eee ee eee ee ee ee eee 93
Gilberts Gee accolitic mocks sae pee ntete eat eleanor 94
INE) DUDES SS oo Shoo sess Sate ascces nOSssS cts SoS SOSESS SosS ons Sot ote onoosbb2e5 250 290
Ovig intofs Misha kOe sean te samme em le wl ell ol ee 264
Waiter OCK@ byt 0) Oem le lle 45
Glaciall Period Saas eee eee le 35, 41
Phenomend <<< 22). Sc sen ce serwe sco ceceeciceieneee soup aie sasisaeceeseceseieeeceeore 35, 41, 264, 270, 285
Grand iWashifault)< < <5 Js. cess cses ce coene soccer ems aecaesocee saee see onaee es esee mr aeieeee 7,28
Grandt@anonofsthe) Colorado sae eee ela ae el lll er 18, 20, 37, 209
Granite-porphyry- ---. .--- Space HSdSS Hohe Hascse cess soos sess seca csee cosines csesco oneseocesces 119
Grassy Valle ye CISC NSSIO 00 fies ane ee eee aoa ee eee 227, 248
INN RSA 5 S-aoas moose ceoass ostone secctsos Siesta se esse Gos cteesstesansese- 32, 257, 273, 295
Relations of, to District of High Plateaus ~ ---. .- 2. 022-2. coe. 22 enn w= oon = wo ones === 4
Gray Cliffs (see also’ Jurassic sandstone) 22 == cose oo ow alee enn care en wiew ean womans oneene 207
Greenvhiver Deds sacerceton sata cee eee atelier eee 159, 167, 205
(Gomrmncon Waly = seo ceeds ass Gace Sot oss ocean ac cose cobs choc soos seetesosesse 23 ce00 36 - 162, 267
13 Gray We 8 5 So Sess Sttsceco coscesibass Sasaco Sash. ones tose ns 18, 19, 288
High Plateaus, component members of the-.......--.---.---------- ---------- ---- ------ ------ 2
TEMS IVE sono seer eee ees acne so 2S ans 05060 SESS HO SLES nod SESEOE On00 SOTeSS CSzeeo nose S526 271
Hornblendic andesite. (See Andesite.)
Hornblendic propylite. (See Propylite.)
Hornblendic trachyte. (See Trachyte.)
Howell, Edwin E., on—
Aquarius? Platean. sus < 23. cose ccc cece nec cleounalieet ees Roce en banat eee eee ee eee 272
QOrigin.of WishiLake -- <<. -ss5-.5.ccacsccmacascosecasenes soaeeeesceees see eee Eee 262
Mertianiesiot the Markérunti=. -o2- = -eseaeeianee sacs eee eee eae eee eee ee tee 204

Wasatch Plateat; <.. 62.2 é.asc0ccc< bccuserncceccncdecaesetee cote ee eee eee eee 164

INDEX. 303

Page.
Hunt, T. S., on Archean trachytes and basalts. .---.. ecce eacene noc na06 6c0s Hobe cose AnaCOUe=e* 93
Hydrostatic theory of eruptions. .-...----. ------ 222+ 122-22 cee eee eee eee nee cee eee eee nee eee 114
TrONOxd Lowe asal bese see reek cae oe io etins sine ce acca eserosttecjasee bac ccser cc enlesecesectecce 122
\WOLGRIMC ROGER 2555 050000 590005 opROsobS0S SHEd oS DateCe Saag esse aed0 5865 Baeg oe cen6 coos 117, 123, 245
UMConse quent CLAIM AG Creare te alee aloe elare oma al alam iat l= emma) minim ale alm = [alein) alo elem ein ol aim m= ole minimle 162, 287
Junghuhn, Batavian voleanoes.-----.----- ---- ---- -- 202+ ++ 222 enn ne eee ene 22 ee eee 128
dimes, Chnlbonmirros IMACS scooe> c6e5 go5eso coccos Sede Deo Sbes os0ce0 Héeeeu concas oS bee6 oneen= 93
UTASICHSANGSbLON Opes tees Seeiae erates eaiteeeinneisnceies erie acini eee 20, 150, 176, 184, 281, 287, 289
SINAIES Scccbee ca5ea0 Ga0as0 6500 DODO. HS0e Gado COO BOUIOdO05 0H56 GoDOdU caDn soEacoon ese 6ecc cose 153, 163
\WARQISS) s conSon accods Ceonsacedoos se6o00 6060 Seeeeriencee s DODO Sted Oooo eSSEcD ccopesosesosess 163
TAM ODD IDRC seoceo Gob cnaesboSbcdo SNOSU Bond SaeSas Gotond o806 SHS Boon SOsS oScoss HoabecSsee 209, 254
Reba abet acer scacrccaccae seas sce se aleseistscciecaicecls cieeinieeeicicmemtice see chcisececsicenerece 209, 252
TGR SEDRON US), IRBAI KES soot Sake scdo'ce S566 609000 do0e 5d0006505 1ad0s BOGOR bobaOD OaSa Baca naSSac 253, 291, 298
TORO WAIS, IPIUGEM, 6 565665 G50 Go od00 sé00 od 0000 Senn bb0000 a6c0 SO0SHo Gees coadE0 atES 6n5s 23, 151, 252, 291
TGA), TO mATEMTOMS GROWING G656 4506 bag0 600050 Sado SOON BOOS baEd GNSS C558 Sods S6aSS6e sdoscs ssocdse5 151
Kino Celi queract Onlotw) avasea see eeiceseeiseeiscelanecssemiametderieereracinia BDSOSEOHELEES USO DDS 128
IPETNO-(CHYADOMNEOUS sosos cocaGo oseaSs EGHS Goode Hood cho coD cosa b0eRss dons CaaS cuDEde SHbE 146
Seorecation! of crystals in lavas) ~ -oo soccer clecccer cecees come ei --las ere core ens -== =a == 124
ILAECOITNG AUAIMERBIRG oo 5660 sac oan Sooo BOOT CODERS co5D HOST DEO cabs SaOT SoSO BHC BOGOnD GeabbeROcsHS 50
OF NarAyO MOMENT. 5 25 so. Gen6 cea6 cade GeHGee DOObED E569 Gade Sos oboHSN Sods Hades seSO=e 290
ILAIRGS, MIMNCEN So 5 coos 5a056es sosaos SSecKe cannes Beads oobo00 5005 5oS605 coBbos BoGeES CHOg bes Bees 285
aramiesbeds|(seeyalso) Cretaceous) pecmaciseeiseciseeeeeisise clocietiseeeclens eles slecieeieeselenice 10, 155, 280, 281
ILGWCID: popoae dace sep onoos baSée6 CoO RSE Sooo abo 06 SNOHbe SdOGED SoeCuS BSS SeeboD Hb Boon sosD cece 89, 102
WeucitesbasalumclassiticavlOntotemasseepee eee ees sieaeieccioe-cieesescsseiceciessriseecitacie sere lil
TERRI cog cas ese eSN CONS Dd SaaOBASQSS SESS GODS COOUSU GaOb cede GnOG00 DoRO SEao sees base boSe ees BEE 156
IE THAE) THN CHUN NE) TROON) 555 cop CbGonS BoC EOOD COC UOC a HbS50 Coon Hau HOoooU Sacens Hono conesdasceac 89, 117, 123
ILIG UGH HY OX WES S65 5c6s5q00 GoGo C000 DOSE EOE SeOd 0a: 0o0H0D EacKed Gob eaeo DneS CHoEHE Seog. GSES 135
Liparite. (See also Rhyolite.) ;
(CHECSITOMTON OE scooes GHnaad cgb50 baa0 Decco ddonSe De0d coacES ROdSo saeean BeSeeCanaa eeeESE 104
WOrderofpsequ ence peace ee ee eae ee seme o ee eee tecen iiss eevee eeminaesce memes 67, 131-137
Ofgtheiphushaes ree reine sincere patew ioc on ogee e eines tere aca mieisavesm omer mn sicie G aeronears ees 175
rit bleLC ree Leek esllepra teres eters es als fev ctaye eos eine minis Meera iar alee Cann ale ais ciate wel ee ers eslaeciemtseroses 192
Logan, Sir W., Archean basalts. ...--..---- 0.6886 Ce dOD0 GOOD ERHS DEO SOO GoGanE osob Sooeabbcocnecs 93
Lower Trias. (See Shindérump.)
Maenesinn Me ripulyemoclseameneemetsscstiscisecheeceiecieactsecleccinaite-teeeiiacimecieceieeniaeeteees 89, 117, 123
Mairnin othe Creeks eerste en ctarec toca pe caierei ste alyaete clei eisiotaiclaw ee paistereinjetceecisievecie Saleveejarctaeye ies 211
Markégunt Plateau, general discussion of (Chap. IX). ....-..-.222-- 2. 0-2-0 222+ eee ee === = 188
Relations to the District of the High Plateaus -...-........-..--..----.--------------+--- 3
Marsh, O. C., mammals of Lower Eocene....--....----- .------------- sacs asdosd sa05 cHuN CDSCC 11
Mat VAT OO Umber ljercaerercie are ctecoaie arate orsioreiay wate vinta veleteisieielnicicit eine ieiwislecle Siselectiaeaices eeeeen 259
WR GVHIG we Abe osee fosg.Gassades Sadek dOnGs pon Oo SmbObad Codeed Sececo Caos Beenaccoseen can cagcos 174
Mesar@ reeks ayers sare cla einer 5 onic Sa ate sek an Ssledsesleinyatejinlasolsie oie be score eee céjacespsewcacisees 249)295
Metamorphicrocksicomparedawlthieruptvelseacesaasset nee ase aeeiaieee nes nee aieeesieee sae 117
Mufastof hast Morks Canon sacs ce- sie aeisieee aces scsi ceca cine ei seiceiwss st esse acl oneaeass 234
Metamorphismioteoutaceoussbedssse=emeacmeatcerneeceaeacee eraser eaaeeree ee saeeeneeeeesenes 79, 243
MidoetsiCrestionenemacsieaie sansa oer Sante tee cise aoe San are oe toes Semen ewincyes Soe esosecqemeees 181
IMG CEN Ee ELOSLON saat aa pats says a ce Se ce oe eee Setinc as ecteie aac eee eee Kesce eaawee dun eowecoee ees 21, 40
HELIN Creare esr cre ree iare iia bus Cl Sia ialla Ses Sa ale alee ore Sie eine ee Sia we ecient Ro sieeehe cee cmidee Met ememae 21, 40
Mon Ochim ale Ox UT es ys secre ears ae oe teyae enters oe eats aioe ie era Selo tate ores Sere See Ace eenineio seeeeee 26
BasteMatbabey sce er sae Mean tails 1h) SS led sting caste arene oe easeea ee esiaces 32
CHOC MI TSW stoves See cies cele SE Sate eine wae we ae wk Rc eRiOe ee eee ae ee inet eee ee 44
HE Scallamber awa egen evens sen tee mee aisles wire oes Wow cid aes dacieae se wic meer aaa Bee ee Leet ese eee 288, 294
GrassnVialley- cee sececee co a see nce aoe Se eta amie See Sate a aaec Dae oae oe eee bs Seea eG caus aeeaes 32, 257
SanpRatacliectesaasecc cence wanceetessoces meee sees soececemeecseee Se CaN GaDaCCaECaS ee 44
WWVitS Bt CDi ye meyer atenofeteie eras ate Meta ae wie cise as aa eee ee cee Se roles emis no cis ine aeiee cn pane eenues 160
PWialerseock eterna ci cue aeieneee meh aoa iaecl ite cher See lee rer eee ob ee nee eee bare Sees 44, 287

304 INDEX.
Page.
Monroe Amphitheater ......--.--------- --------- weet eee enee Bonn SS COE eE Scone Bose Seo oc0 SoES 56, 227
Nr) WEMIER 7 oo Sncs Bode Soba coda cosas -e5ecs coac.oS56 260000 Ses. ce0 ocseseoge9 c000 o050 ca5SeC 267
INIT 3 os ceee case cosecu Goce Sa66 caso bodees ssoussadcoos ScIDEHO BESO CSCI ee55 Hono Ssde coecee sacs 169
Navajo Mountain....--..--22. 2-2 ------ 22-222 one ne eee == AoIbecO sas cose copa ceScau cSaees 290
Nebo; Mounti----.-- ---- ---- we ene eee ee cee ce eee ce ee ee eee eee 6 Sanco Seno oSeceEcoceCco cond 2, 260
INGA = ooo cao. c65s Gen6 BSSS ORS 660 GES5 SSS BESOSSSSS BtnS 949905 a0ds SSO SSEdsoSSd6 coNs66 9Ss050 66 89
Nephelin-basaltielassiticatl om Ofte sem ame miele ae eat a eal atelier 111
Nephelin-dolerite, classification of -.....---.----.--------------- cuacee cdo5 sons osHUS see cées 111
Nevadite. (See Rhyolite.)
OSHA = oS 586 S550 c6enee Sepp iSscd BOSOE9 FESS DNSE00 SSSSC0 SSS 505050 sees acbose das Sse SeS8c0 103, 107
QUAD So Ssce SocSS6ensoSSs 8500 GRS8GEC0 COSSHO SO00 OOSDES SSOC SCS Sono taro Shes Gosced csecso codecs 89
OIG OF GRUB .5 ccsadooosde0 Séod.ccoD Gabo de GSe0 CScosS0cc Bobac0 edooasecon Haan Sadocca8e6 62, 131-137
ORO GHA NING TONE) pooecs 2566s SaSSe0 605 SESE SOSOSS Sa05 oNBoaS Secu onde Hod oadD coup ODESEOacOSeO 51, 54
TERIESOVAONG TORREY NO) NSS 55555 Sodced SSScd0 SESSSO aSHHS HOdSoF SoScoSsScE ScoceE es coSasoccoonneane 143
TPERNG pI 1 CRIT Ns oa S856 coeces coQ050 GOSEoS GSSSES ESCEST ASS S558 SScccs oS5Sce SaadoN O5S0 NaSSOO s+ 212
TEER AEE Ls ooe Sa 6ae0 COSCO SOSBIG SSS500 SS SBS05R55 CSOOS0 DASHEE SacHSe Sash Sc56 GaQnGD BSC68e 238, 295
YEE ese CS 00501955556 BOSS DOSOS9 S608 BOSSES GASSES H5SCOd o056 SascEs boSaGo GeSDOOSadEOOSO SS 199
WANES? 3065506 606660 S500 GO55 SESSRS Hose R Sedo Ses a055 FesSOSSeoo Deco Gand SedooSSSoeccoSesES 212
1etmHes WANE fos oes hoop cosas So nbaSceco Sas boosie SaSSso pogo Cocesmoso ace Sede coco cHad agoces dase 252, 297
Paria folds (see cho) Clifts? monochine)2)s--.-. coast eae eee ee ease ee eeien as eciee sees eeeise cee 44
LER HN TEI 555 bas6 dooeao6oonos S655 650500055500 o656 Sano SasdaS aSsRes oohSSN cesoSsoase 3, 251
TENN S coches Soc6 Sasa Os6d Gacaa5 cOOSoS 83085 050059 S865 coos as SaSHSS Hens oaaS cose acoseSacesee 3, 1€9, 170
ParkeMOun tals pease ae = ae ee ata eee (eat a lee te eee ee ae 438
JER LER DUNES sac ctao SSO SSO CSIESSD Good SASH CASSII S50 Bane SS bdo ssabcos5 daSacs son Boos esac SSeS 3
Parallelismyofetaults 2m emma sae renee ae ete ieee las eae fee ee ee 27
hOvANGL ON tSHOLE WNC ae ieee em see ee ae 45
Permian or Permo-Carboniferous (see Shinarump).-.---..-..-. ------------ -----. ---------- «<5 146
Phonolite classifica ton Ofjee—=sel seme e les ae aon eae eta ee 107
Opt Ws 5 18a el CH san5 Soc665 SoS ESS SESSS5 choc osse GHacs Rabe ndonoS TasE BobCOb Ssioo DsEc.ase% 248
Temps (OPER oe a os a Ro oa se asin ose oS ORS SOS oSodaS SESS CSS So cocS IEE) ANE Orit, aR}
TES OO OE UNG) GEN 5 35 55 so Begg Son SSK e seo Soc cnns GOSS Os65 ahoS caso nods ceosossas= 20
JRE EENL TER ARN s R655 95555 654550 CHSSSD HOSSS ONS555 aodnoS soca eadisce ssosSssoeschobasaesessssc 5, 8, 49
TRIG GOR S 5-65 sao sees Sees ces conccs Cees Seno cen SaObosososcbteSooses -cosco Ene 8, 208, 252, 286
TA CR TOs sess cacao osse0e C562 050559 O020 GOSS Sas ance HoSSoSSsenns cose Cans Dosa sess oSsenSeS 45
TRIS OCOMEVCNOSI OM ae arate tele te se 21,40
LEM On oa saps coos 5ssosen6 paso RoSS So ceSO oe Sac SaaS osSsas ooeSse sess ates dedeos esse seoe 28, 34
Romphyni ther rani te see as ee eae nso ae eile er 119
TRS GRR = 65 SOSA Coen OSS ES S550 CAI SSA HOSES SSas One Sees See Sone sons esos teesosen seas 92
WINEOIR AR hme Socas seasco decane secocd Rass OSS SSSSS Sass ches Sabs cond caso cso Sebo cegeseos 94
RotashyinivOlcanic woo kseer esac ee alee see eee alee ee ee 89, 117, 123
TREY \YAIE eo Sec sea once. g5eo Coco sn Sad san ss oosRassan SEE OSH oacecen Scns atceso meso scostec¢ 289, 291
PEO GGUL dis Wieseaiss cose cas==s nose soncoo caso a5 candice tances stdocctadooo ces sasoncs: 5, 19, 27, 143, 158, 290
Pre SNentl any XU CS fate ete ee ate ee ee 43
Tew abe ae ae Roe oe boc e secon SSO ASO h6s6 OSasOD SISSEE SoSoas Hocd caesebescaessces 114, 124
Propylitevanci tic Clissiication Of oman eaneasee a= nanaiaameneise cess === eee ee 109
CASSIS COT OTOL ers ae ea oe 102, 108
Marlies ern ptlOns Ofes sees sae lene ele el 39, 57
Hormblendic® classification! Of —--- secs see ie een ane eleeacie ea ele ee eee ee 108
AWAD a VIATOR ccs oo ae Se Societe mele ce cle te Se len ene eee ee a ee an eS 275
Monroe Amphitheater 2.5.0 252- <sccn0 cosinjcc oo no ceee sams oaseaaes eneneaen Sao Eee 229, 242
Sevier Plateau -.2...s.-sceteces eos coca spec caus snjaw cous cose sacs conn eee eee 229, 237, 242
Orderofsequente sa. cae e secs es eee eiaa teen ae eae ee see eee se aaa 63, 67, 138
CT Spe peng AROS OCICS TROIS SEES Rey SOHO BOO Tae SO SECO SISO ROD ARG DEAD eSae aces BaSSosco cooae2 69, 109
TEENIE) as Seem Ons eco CASED OCS BOSD CRED RECS SOO AS OSOS CERO RSS CHECEE Goat essasss sess sess sade 103
Quartz propylite manos sence ee ane eneaiee see eee eae ee ease eee ee ee 69, 109

i eee

ie. -— =

INDEX. 305

Page.
Rabbiigvyslleyeccswesaasescesee salsa ciseesceeseciesecieeee see tecce SnOtSSo Deod mod osaenos= 273, 276
ReEcen cyzofe aul tetemencemernrecciecceiseyar eats ccies seer iae ease aa eciS ee nist ee aes ne eee 42
RScesslomohclifisesseeeaceceeseecciemaiececiceccisecereees bono eonccd caqje HOC Db0000 Cone DEAS ppasen 19
RediGaterseer eae aci nace cc cieue sissies dae csecicicicosciseisien ele cnc becsyeciee teed eece cocetleccloosece 78, 281
TI ClbING, GSEHNERITON GE = cc05 c0 coc] 6956000059 865608 0506 CODdSS Ga66 bode DEEeaS HEEEEO O50 SEEREE 101, 103
Occurrence in— ; 3 ;
INIGRESAGUENIN= = 6cose0 cong cood noes cand. ea tea9 b9 Cada obEedE boda 9000 Sb sUDe coon ocra Bobd 61, 66, 193, 197
RSenmgr Walley. cocsec 0000 o500 sodoes concse Codes OnGo Saud Go HEGRE coneSeOSOSED Gees CCEA SE 66, 213
PRushareemenemace ment ecctsemeise eer meneiae cee ctteel moee re cersencicns ere cnicens 60, 66, 175, 177, 180
ORG OH EOTMEINED .co0c5 c505d0 basesb cenosontc Seb aseoss caseee cnRHEd coceeoUESon End 63, 67, 131-137
Richthofen classification of volcanic rocks\s..-2--2s a2 - sees soe eeecleseccececceees cee seeee 103, 105
Oneler OF SEGMENES OF GAUINAONS cocoons aces ons chon donn Gone doso6S HHUA osEdooSe SoSH eHenSb ease 62
IGG) TOCUSIELEINC®) Of ooce code b050 sondeU odo GeN6 Send DUSDSsEE Kose Ss CHlbcoM oonadassoned cone sane 16
Tomi CMW .o65 cock. ogee BeseRS ocoU aEan bosabe Se be RE g4ER Cob Ge cana Gocd coGaOGoH Bens Senccsencece 161
RS SUIT TD pets ta eee ate wees aparece = Sle Paya lers ane nyo cons {ered aatafelosyala einjate miavee eave eo Oe oyna winecike ie cisiceewige 170
Shabir CHMOD. sé50 sceces econ cd0G0d spSaoU coDeaS coda 5600 GHbOHA acbOSS soDSEN Sons conececeneLase 163, 256
Shin URE ISIE coco cacnss so0esc yosmbo sdeene bob Gaenee CONSS0 GNDO 0DOONS B60GS8 seSube cone sEdood 290
Sanehataelemonoclineerass serene asec es yee rete iis oral sie Ee ce einen niet isties ee ohvess cise ses eee 44
Sain IRA Ryall Sosec ocosae SGnSe6 Sano oDeCOs dodSen Ga0g a9NDS0 caDS HGOKSS code sdebbabUSE SESOOSEO 19
Sanphetemelateauleese cers rtes eects nieaerece ere eeetnanmiee mieenicicetecm Sectcntecie ce aoeeees 165
Sem IRA Walllesyoodode oss6ss Gado ce Sodas cose cons e500 CobSSN dose SoDnOS CedteS Seda cannes coeds 161
Some; Ce 12h, WENA WING AWNGH ANG. comcop se SoLe cage eoScee coud Bao Sodece ones enon cose csc seee 56, 231
SelenitepniJjurassicishalesteeseceescemseeeeciarer ine sere ccieck aban neemaeeeee eet teeter enna 154
SEGMEMCS OE GRU NITOMBL coa5o9 sadsou sodeot os podSOD REDO EDS sont Hosooa bees Seebes coMase quecuS se 62, 131-137
SONAIEE TEI 66 cponac ssee cose0e sbobSD caecnS BSGDBO SE6b05 SnoO SoSHaS coHS coded cedeer Gens SoSHOLl), MGR} PRs 285s}
Sevierselateauyceneraleaccountiota (Chap px) sacmriserisece ease estes ee estes eces ose seee neces 225
RelationsktomDistrictioh ee hyblateauseeccer et teeaerscencseioe cis eecisesee cisceieciseceeeeees 3
Wrestermfrontisicercaxce tome <2 cps cern aseeteciiee ae ne decisis citer ceckecwece esas scseeeesceses, 111,206
SNC IMG sco socosucace costco cecesc poDEDE OdE6 SaDEOS anbas0 GSdOaER HnaD S6eboens HonseSenepcaas 212
SMe Walley; ccsscc csesconses Sous scse dbo0Ed cane Gace Baasad segca9 caccen Benes coed BESS es06 3,211
SHIA RAT), NOs oooscc sens ade cons PAdes oeoons cubs eo sobs oboe Sade sogSns Ebc0 SSeES5.9qS0e5 case 144
WIE TEREST. ooo0os sétcoe Seen becaNS HoaD50 Hose Sade SnnG Gone cane bone a8 0505 SbOSeHD coSaeN bosS 208
MhousandylsalceyMountalmerssere vere sae secrets pak sateen erase isis ecne ine seceieee sees 281
Cinclei@ itis sewese ree = nee ees se ateeeione sine etaielaists wile ees eieisiow's ciat ayeeneicnisonniceceimssacees 290
Shorevlinelotetherhocene Wakes sasas-ceceiciecisciesect ae sac Sues scicice a locsee sseceeccecoceesesacee 44,184
STEHRD ANN O50 cooase Gposdo casso ose cspbte code copaad DdoSEd ddoEeS SSDS cesoER eoaaGe cogasboacesC 50
SierrapbaySaleecoewacateecnccets scree sislscicierescie ote sco0 codSDD OEGe nda S600 6d0060 bans cHOGce ssSocees 289
Siliemtie diswioodeersseecee eects eter siete el-lsisisein sna sees Saeeelesicincls/ae ae wasn eeeen eseaice sciences 147, 207
SHNCR, PEREEUAT® Ol TIRES > oceac quan coomes Case S600 Cd05 050000 SSES56 coOdeD BESH SdCSCO acne 88, 117, 123
SOG Tim TOleaMiIG ROCK 55 scoocounaccae cope ndeb UCOOHS OSSOSd Haus cece cao neo Seb eg SEcROogcSO Cees 89, 117, 123
Sirahisraphy of thevkligh Plateaus (Chaps Wil)= coco. so-so. sees oe a ee emiceae ee eslcere=ss= 243
SIRO OGERy WEES oo5es sono ds Coad Seb abe sedag secens udobae SdsboU pocasU Sede S650 HSdoceceonsS 258
Simmer pxamloeny (Clieis) 100) conc segaco cadtob sosdeu seqcne comsan sagen. cagboN osa6 coccoy eaeee= 25
Sub-acid rocks. (See Trachyte.) ;
Sub-basic rocks. (See Andesite.)
Subpsidencerolg@retaceous-Wocene) Peds=mere acta ane eee ese ee eet enecine seein eats nee nae 13
Summitavialleygeseecact eect secre ee sey neeeeeeoe ene eae tiene so aremeicc see nl ee eeaeoeors 259
Tablet @littecseteeceses are ewer aan Soe cae asters Se mintemiealetaienicesectiucye See eeee cnc ccmmenee 253, 291, 257
Machyliter saat cee sess eee nek cece dongs isin s ee wate ae Sawieina lew nie Sees nae ees eae Seca aeowraties lil
A TRNUEN LEE): (Che tee, nemoas. catered GACH E Oo CELOCOGSo Gere SEROUS CODER OOd SEB On SoSEiod GSSocp eoSstSoesecs 287
Rempley@ reels jee se eee areas nee Coe male ee Sep ecise oe eee nae So ciee Ss Seieteaiaree i aenssets 287
SiG au ON GT occmem a cuacnenodoec Bapred son ceodd FebSde CTE obo Soap SECOSSeOu ance GoStiod aHecas 259, 267
Tertiary formations....-.-...... mails witstoraterie Slewrale Caremie lo cles ania Simimateia eran Saat ene otc u wines cee 158

20 HP

306 INDEX.

; Page.
Tertiary formations—Continued.
Of the—
I XGWES HITS = poo. nob5 pooad0 Coceoo cadeeoSedKc™ Sood Sau500 odea oe sODa oNdEDS BobaoD cacHan meals} eld), 22)
AWADQ. .-- 02. <2 28 o-oo oe eee ene we ne ee eee wee we nts een eee eee ween 273
Wihmgequinln.. So 55a0G0c0 cass aede yoades soausdusas esdada doccu0 Edacco caocos Coso SaaceC 189, 191, 204
Wioramiane Wallis coche A56556 poc6 sacsod cussen.coacec0Ns Basse dceonn Saa0.e5ee50 000060 60S5 270
TEENA co500 oso6ed c9S055 dadose. cecsuN Seria Scosadaces psobes Sass coeds aocea2 booGDa.SccO 170
Tio} Onin WeHlleyeocoee cescos csuS aoc] secede NS6s Geo dodce™ Sous sosaapcuaode amadcd Sooace Go 273
Summit Vial Gye oes Oh a Se lars annie es cis Hers se asistel sy iets erate ete ene ye ee a eee 259, 267, 269
The Chit? coos so5c cocaso asesoe anSeod bedece de adaceo Sasan0 cbdce9 SaSE00 cons boee 2060 253, 297
Thousand Lake Mountain ..-.-- OC bEBGho SnD SosDua cosa SSbo HoNo SacHbS cenosq asus cene Sade 280
{UWEHES doacne seaceacces padenh cochay Gopeda cogSes cose asoaEe aspene Seas onc ch0ecg cdsase 184
\WWESBIOIN IDIRIGHIL ooo Sce5 ssansnoso0 sagcan Budo Soda doSoco sae ObSSbe Snes SHabSs HOSS deSeSS 166
Uncontorniubyawabhy CretaceOUs ee peer elem == eis ee ates ee aaa ee eer ne nena eee 11, 158, 280, 288, 290
WUEPSITUERS OE MOND AR 3565 copobonogsoS sauces eos sanece soar sascaRonaaod BsoSce spose oss asso qses 97
Aine) WAG sseocso Heo sods sad catsce se coesaSsess ossab Sond coocgessdoee sueeReeasemee 74
Violcaniewocks esses esa saeeeeeeies see aces BRE RAGA Ree oeea pereiso turer tas eSCeesece 88,91
Woomera) Ibe yids) WINS Ok Canc oAkS neoso nonocs KeSanb odMeSb Hosadd secb/sHnccoeAEaE 33, 277, 279, 291, 297
Mhousandwiake Mountain sstsseceeeeccecae ena a= sae es sae sens se neler ae eee 280
LITRONY ECORI), ooccconesco sens ss6nHg Gone SSno¢ acon oSn508 pans Sao S DEUS HOSE CHnESbONoSeS SeSs 65

Drachyteyaceoteripblons Of-j-s----teee saat eee eee eee sennen ae see peat eee eae ee aU
Argilloid --- 22. - 5. ose 6 220 cae s oss enn = 104) 196) 229) 2395241, 242) 261, 265, 268) 269) 2705272, 274
INORG See e os Oncnee S250 68500 Gone O5-645 9c05 bososo cos sooo! DIMI} TIC h/ PRB} OF (0) ofall, ora), Otte) 72)

Classification of .2.2.<,22-2s5sece teem canecisie ose ecinnce serene cee cece eae ea ace case Seer ae 101, 104
Gramitol dtr eecsee sete te ee alee ala eaiee aes eee 104, 190, 229, 239, 241, 261, 265, 268, 274
orn blend cheememcs eee eet ee nee ete ene ieee 190, 229, 240, 242, 258, 260, 265, 272, 295
TEV AMOS sp pmeo pipe cae coo ncr Gees cond Sop sos Sos Sod pasesu bosons aescosorooce coun beeceueden 104
Ordersofese quence kasama ae aa ee ee ete ete 67, 131-137
IRorphyriti@me ese =e ae ee On BOS SHooso caddis OkSoSe oe ooo End coba. S505 odes decor sseos 104
Sanidineees see eee eee eee eee eee eee sa Spa rae jaan clea vaietoeete as eideis aetteias 101, 268
Occurring in—
FXG TERNS le Ber ese Socetodon Sonam Oo oon daSAne eos bepAbotobocsdaos Sacdeduacadnes 293, 295
AON ic oeBoopoeseas cooces SoSH NISES9 HatSos Onosdanccase ore sadosoces Sera bosdoass 274
Dop: Valleys 222 2osenn ec scene cecen ete eae cleo eae noe eee ee aanee waeeeceaeee en eane 190, 212
ish Dake! Plateaus ssisce eee ca eee ees eee eee aenree tee. ebGelscns CoS ee eee 257, 265
Le OD Fer R HM GEN Sac 8 S.505n anon San SROs aad Asc to One mas aSdeesosoocsnssossesensees 271
Markéount-elatean=:-.---.----+2----2 s----- inacne Saumen cures Coke ae Ree eae 196, 199
Marvine, Mount 2.2. sacscceceh wccces one sem eses oe eosin ae eee eee eee eee 269
Sevier Plate ates e scares a ae etre late ea oie eer ae 59, 229, 239, 241, 242
Sevier. Valley t22<o5-2 cena e sees ects eee napeemsin es ots So Seniesa pacrnmccooce 212
DWE MIG ness Gsecn6 bods BsecnosSs Bear Sena sccn con OAoSasction Sstolacta cose cesses 258

Tusharcacsseseseacts sass cea seas «eee nes em Sere ae ee ees eon CEL EL OU MLO L
HIE AMANO NS LEN Nien Sele Sq Pena ees ISU SOC so acco oo COKiae = ebEacons ceon canacuastode R274

Mrachitie centers of eruption’ -2--<!----— -sceee ce eele= se sel e=s eae ease aaa cle seneeseeerec sees 59
DRSIS (WNT) S555 ceselnsonco sosesecace cess nbocne Seno eon osepicao naos soghotasesescsance case 20, 147
G@ircle@littsiess eee Sieh ose Mae alah ae ee aCe eM ne ie cern ise See oes es ee 289
Mscalante Basinie--. senses secon cose eee esos ce nese seen ee aise eee amen see eer 290
Markount: Joccic- 2. veen socnisnesaashe see cae eset eeeee sot eae eet oaee eee rete ae ee een 207
San Rafael swells sed seese ccs oS cosa ds tcceWe duce. a veca scan Censeerccdelte seice eee eee eee 20
Thousand! Wake: Mountain <7 32 222c2 cons cncces acct acccosseceascue wtowe Se Ree 281
Water=Pocket fold: 2.225.255 ce05d o2 econ set ens ook ome eens ee ee cle ee ae ee 287
Prom Dull s Mounts ooo oss sccee case ote bas cic coin eee eee ee cee ae see en eee eee 252
MRT AS she soars, cache ee er Se Sain aa Saas Siamese is ee re ee i se ee ee eee FOO WR Oom OA TOAD)
Tushar Range, general discussion of (Chap. VIII) .----.-.-. Sodas oso batecee Eee eee oe eee 169

Relationsito District of Mich Plateans- —--5 eso eee oo ene ee eee eee eee eee ee eee 3

INDEX. 307

Page.

Uncontormibyiof Lertiary andi Cretaceous =\.--.-- e- aecnewiee = ccce oo soos cece eens oone 11, 57, 280, 281, 288
Upliftins, ati centers! of Crosion) se sce. 2 n= ence oes ome oer eer sean = =e mopsconcencsoscees — iheh Wy

Git JMleniGaw TEROVANOS) co50 0608, e506 soma enSode so9cE0 000009 Sans coed 605 bone oSaNSeesaneness case 15,18
Wermilion’ @littsh (seeralsovlniassl©) seaeeseiseee see eisneieicee steeeai-== else seine =e S00000 CODSERETSS 148
YAT Tain ISOs, COMOME Ol: saccao soaasa- dana HOSae Bobb Abe OCEHOG Gad SOR CouSser Daesco ReneS BSSsc 209
Wolcaniciconslomeratesersertsceceieeice eee see encase cess 39, 70, 75, 178, 190, 214, 237, 238, 244, 275, 295
Wolcaniczerup tions causesiot (Chap hive) perisciciachee ee mele = eines eines ae ele eee SE Stee eee 113
\H/EISEHIGIN Te nO ChNGS -Sobc6 caGaad cono-s8500n o5e8N6 HOD4SOD dosadd soueee eon Goes cebccoeeseeeneeoe IKTILOY/
Wasatch Mountains .........----------- 000000 620909 sougSacgGas 505000 C500 BOO UD BoSEHO.OORON0 =e 2
Wasatch Plateau, general discussion of (Chap. VIN).-.--. ....------.2---- 2-2-2 eo e eee nen - oes 160

IROTHOMS 10 Ieee IPRONTINGS) ssocce coa5 Goce cqaes9es95 960006 LeabngoS Sooo iDepo coco acODcese 4,19
Waiters ocle ttt ol dSeyamremmer eisai aee sme niseietetice ee siseeese cer rsice cis ciniccinieseetem seen ee 44, 280, 286, 287
\VGRE Cin son PUI doo coon quenboResHoonaenodonueds dodeed COnSa6 quae AACea a uenemEcaeCson osc 164
AWiesbrlMatbalbptaull tianytateste see meine eran erent ato ae eee eek teee «ionminae aie necicciceaesias secre 32
ZATICO REP eat CULL CaN CeSlbeS matter entree seer miele sete eit eee ence see nenee ee cee eaee 65

PLO PYIES wer wise wcaevec cise ei seiseis see see sees el-miao= eaete Seas els sels ees seinenineecins coneeceeeecee 108

Rhyolites!ess-sscee= Fag ets SEGRE Ea SRS a OUR ONS SS CIC OO REE a ee eer eee 104
Hone oidiversedisplacementresreeeertertsaeieeccsenteetsaesieecricecitcencceeicaccencceeericceees 162

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