ap hefesatec thi hi saan Cab bayhh iis Hiniethiratt | as % i! Meare aeatyat Motels Hf idk; digi: il ivi GRA edt BE UPR {yay alts Salttes abet i iD ating trae tieas set eiat i: Hencel ss SHitet : i i \ ' . i , ; i E 4 : ' . ; ‘ ce ; Ruri | . : 1 5 . ¥ # . : . < j i i 6 , nt ah : 4 1 ae ’ t ; ‘ 2 ; { . 3 ‘a rm i: é j = outa ye = ) B ‘ Tia ‘ f ‘ i { i ! ae \ ‘ = = Sete f 5 i\ - i ’ 4 E HELIOTYPE I oston. B 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. © 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 “ALVYANOTONOD AUNV VSO MUOY LSVA “NONVO “09 sunitg afAjorapy uopsog “JS aetyshonag, Oz SOT IS 9-424, C = < ‘TA 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 ow _—: ; \\ Oh EGE PCa Vara Uo! i oo Hettotryrr XT. Heliotype Printing Co. 220 Devonshire St,, Boston. PHONOLITE East ForRK CANON. 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 | | | ! rr ! | f | 1 | 1 1 oc | | | 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 | | ! 1 ! | \ \ ' ' 1 | ' ' 1 ‘ | 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. 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.