Popular Science Monthly/Volume 15/July 1879/The Geological Survey of the Fortieth Parallel
|THE GEOLOGICAL SURVEY OF THE FORTIETH PARALLEL.
By Professor J. S. NEWBERRY.
THE geological survey of the country bordering the fortieth parallel of latitude was made under the direction of the War Department by a party under Mr. Clarence King, who took the field in 1867. The area covered by the survey was a belt one hundred miles wide, traversed by the Pacific Railroad between the Great Plains on the east and the Sierra Nevada on the west, approximately between the 104th and 120th meridians of longitude. The general object of the survey was to connect the region of which the geological structure has been made known through the California survey on the west with the explored portion of the Mississippi Valley, and thus to supply the material for completing a section across the continent. In addition to this it was proposed to determine by careful investigation the structure and resources of the country lying adjacent to the Pacific Railroad, which by the construction of this great work was opened to occupation, and was already invaded by a population eager to seize and develop its hidden stores of mineral wealth.
Mr. King chose as his geological assistants the brothers J. D. and Arnold Hague, and S. F. Emmons, all of whom had received good training in the geological survey of California, and proved competent and congenial co-laborers with him. Specialists were also engaged to study the materials collected in other departments of natural science; the fossils being placed in the hands of Mr. F. B. Meek, and, after his death, in those of Professor James Hall and Mr. R. P. Whitfield. The botanist of the survey was Mr. Sereno Watson, and the ornithologist Mr. Robert Ridgway, while a special investigation of the volcanic rocks collected was made by Professor Ferdinand Zirkel, the distinguished lithologist of Leipsic. The topographical work of the survey was, during the greater part of its continuance, in charge of Mr. J. T. Gardner, now director of the Topographical Survey of the State of New York.
The report of the "Survey of the Fortieth Parallel," as now published, consists of six volumes quarto. Of these the first issued (1870) was Vol. III. of the series, entitled "Mining Industry," and devoted to a description and discussion of the important mines and mining districts embraced in the territory studied. About half of the volume is taken up with a detailed description of the Comstock lode—the most productive deposit of gold and silver yet discovered—and of the methods of exploitation followed in the different mines located upon it, by Mr. King and J. D. Hague. It also contains chapters by Mr. Arnold Hague on the "Chemistry of the Washoe Silver Process," and on the "Geology of the White Pine Mining District"; the "Geology of the Toyabe Range," by S. F. Emmons; and on the "Geological Distribution of Mining Districts," and the "Green River Coal Basin," by Mr. King. This volume is accompanied by an atlas of maps, plans, and sections, which at the time of its appearance was by far the most beautiful work of its kind published in this country.
It is greatly to be regretted that the review of the mining industry of the country bordering the Pacific Railroad, so admirably begun in this volume, could not have been continued through the life of the survey, and have been presented to the public at its close. Since the publication of this discussion of the Comstock lode, the most important chapters in its history have been enacted, and it is a pity they should not also have been written. When Mr. King closed his examination the workings had reached a depth of a little more than 1,000 feet, and the maximum temperature, (of water at bottom) was about 108° Fahr.; and now the depth of 2,500 feet has been reached, the temperature of the water at the bottom is 160°, and facts have been obtained which indicate that the limits of the successful working of the lode will be determined by temperature and these limits soon be reached.
The Sutro Tunnel has also been constructed and all its geological revelations made since the appearance of Vol. III. The great bubbles of the White Pine and the Emma mines have since swollen and burst, and the important litigation between the Eureka and Richmond has been decided on geological grounds; and yet the public has nowhere received the information which it craves as to how it has happened that so many millions have been made from the Comstock mines (by their managers), and so many lost in the White Pine and Emma. The great bonanzas of the California and Consolidated Virginia have nowhere been fully described. A few geologists know that they are simply disconnected patches of rich ore, such as lie in most fissure veins; but the public at large have either no ideas at all about them, or those that are wide of the truth. So we may search in vain through all mining literature for the simple explanation of the problem involved in the Eureka litigation, and in the ephemeral productiveness of the White Pine and Emma, viz., that these mineral deposits are chambers or galleries formed in limestone beds by atmospheric water carrying carbonic acid, and subsequently occupied with ore deposited from ascending solutions which filled these cavities, just as elsewhere the simple crevices of fissure veins.
If any one can imagine the lead-bearing limestones of Missouri or the honeycombed plateau of central Kentucky broken up by volcanic action, the strata set at high angles, and their irregular cavities filled with mineral solutions issuing through fissures from below, he will get a just view of the nature and origin of these mysterious ore deposits, and a ready explanation of their irregular and superficial character.
If Mr. King could have continued his observations on the Comstock, and had investigated all the mineral deposits discovered along this rich belt, so freely opened by the active exploitation of the last ten years, in the same thorough way that he did the Comstock, he would have made a contribution to American geological literature which would have been of great scientific interest, and of a pecuniary value to mine-owners and mine-buyers to be reckoned in millions.
Vol. V. made its appearance in 1871; it was devoted to Botany, and was prepared by Mr. Sereno Watson, with the assistance of a number of our best botanists, who have made special studies of particular families of plants; as Engelmann of the Cactaceæ, Eaton of the Ferns, Tuckerman of the Lichens, etc. Vol. VI., on Microscopic Petrography, by Professor Ferdinand Zirkel, was published in 1876; Vol. IV., on Paleontology and Ornithology, in 1877; Vol. II., which embraces detailed reports by Messrs. Arnold Hague and S. F. Emmons, on the local geology of the belt of territory surveyed, also appeared in the same year; and, finally, Vol. I., written by Mr. King himself, forming a comprehensive review of the systematic geology of the country covered by his explorations, has only just now left the binder's hands.
The magnificent geological atlas intended to accompany and illustrate the reports of the "Survey of the Fortieth Parallel" was issued in 1876. This will compare favorably with any work of its kind done in the Old World, and at the time of its publication it far excelled anything which had before been attempted in this country. To the good taste and the technical skill of Mr. Julius Bien, of New York, Mr. King is largely indebted for the beauty and accuracy of this atlas; and it may be a matter of general congratulation among American geologists that it was then demonstrated that the cartographic art had been carried to as great perfection here as anywhere else in the world, and that all known refinements of graphic illustration are within their reach.
All the volumes of the "Report of the Fortieth Parallel Survey," except Vol. I., have been more or less thoroughly reviewed in the scientific journals, and it is therefore unnecessary that they should receive further notice here. It is but just, however, to say of the general character of the report of Mr. King, now for the first time collectively exhibited, that it takes high rank in the literature of the subjects which it considers, and is most creditable to the chief under whose direction the work here recorded was executed, and to his assistants, both for the great amount and excellent quality of that work, and from the good taste and care which the volumes and maps display.
Perhaps no other geologist has enjoyed the opportunity of directing the exploration of so wide and interesting a field, has been so independent and untrammeled in his action, and has had such resources at his command as Mr. King; and something of his success should be ascribed to his good fortune. Yet it is true that he has made excellent use of his exceptional opportunities, and the result can not in justice but be regarded as alike honorable to him, to the War Department under whose auspices the survey was made, and to the country.
In the volume just issued Mr. King has discussed the exposures of all the different systems of rocks which form the geological column, beginning with the Archæan and ending with the Quaternary. He then takes up the volcanic rocks, of which there is such an immense display in the western part of his field, and discusses their relations, succession, and classification. His last chapter is devoted to orography, and is a study of the different and very numerous mountain chains and axes of elevation which occupy so much of the region he has studied. In the progress of this review he has not only made great and important additions to what was before known of the distribution and development of the different geological formations throughout the West, but has subjected each group of rocks and each important topographical feature to close and careful study, with the view of evolving from its ascertained structure the details of its history. In these investigations he has touched upon some of the most profound problems that have engaged the attention of geologists and physicists; and while we can not assert, and he will hardly claim, that all the conclusions he has reached will be confirmed by further observation and mathematical analysis—for all human work is imperfect—yet it can not be denied that the facts he has reported and the inductions he has proposed will have an important influence on the progress of geology, especially in this country.
The publication of Mr. King's volume certainly throws a flood of light on the complicated and hitherto somewhat chaotic geology of our Western Territories, and it can not fail to afford important aid in the proper orientation of both observers and observations in all the great region west of the Mississippi.
It is evident that nothing like a thorough discussion of the facts and conclusions contained in Mr. King's great volume of eight hundred quarto pages can be given here; but some of the most important of his facts, and the more interesting of his generalizations, will be briefly noticed in the succeeding pages.
Archæan.—By this term, which he accepts from Dana, Mr. King designates all the great mass of crystalline schists and granitoid rocks which underlie the Cambrian system, and form the base of his grand section. These are most fully exposed in the Park and Medicine Bow Ranges of Colorado and Wyoming, and in the Humboldt and Truckee Mountains of Nevada; but there are also numerous minor ranges and summits composed of granitoid rocks, especially west of Salt Lake; and Mr. King shows that these latter exposures are portions of a broad pre-Cambrian land-surface which formed the western border of a great topographical basin that reached to the Rocky Mountains on the east. This basin was occupied by the seas from which were deposited the Palæozoic rocks. These latter were largely derived from the erosion of the neighboring land on the west, and formed a conformable series, of which the estimated thickness is over 30,000 feet. The old land which supplied the mechanical material of the Palæozoic strata extended to an unknown distance northward, and reached southward at least to the present head of the Gulf of California, in a region where it was recognized by the writer, and its relations to the Palæozoic series of the Colorado plateau pointed out in the "Report of the Colorado Exploration," 1861.
Mr. King divides the Archæan rocks into two great groups, of which the first consists at base of gray or flesh-colored bedded granite, overlain by red, massive granite, on which lie red, micaceous, bedded granites, the whole attaining a thickness of perhaps 25,000 feet. This group is characterized by the presence of quartz, orthoclase, and oligoclase feldspars, with a little hornblende and mica, the latter consisting of biotite, muscovite, and lepidomelane. It also contains more or less labradorite, titaniferous iron, magnetite, and graphite, the whole corresponding closely with the Laurentian of Canada.
The upper subdivision of Archæan rocks—found in the Medicine Bow and Park Ranges, the Uintah, Wahsatch, Humboldt Mountains, etc.—consists of true gneisses, interstratified with mica schists, often garnetiferous, hornblende schist, sometimes with zircon, etc., all very distinctly, often minutely stratified. The thickness of this group is in the Wahsatch and Humboldt Mountains, from 12,000 to 14,000 feet, in the Park and Medicine Bow Ranges somewhat less, and in the Clear Creek region of Colorado at least 25,000 feet. This upper group will be recognized by geologists as closely resembling the Huronian rocks of the East. The Archæan nucleus of the Black Hills was reported by the late Mr. Henry Newton to be composed of two groups of crystalline rocks closely resembling those described by Mr. King, and Mr. George M. Dawson found a similar double series in Manitoba and British Columbia. Without absolute proof—which it would be difficult if not impossible to obtain—the inference is at least allowable that the rocks underlying the Palæozoic series in the far West correspond to the Laurentian and Huronian Groups of the Canadian geologists, and therefore that the foundation of the western half of the continent is essentially the same with that of the eastern; and also that there, as here, a broad continental surface of these older rocks supplied by erosion the mechanical material that entered into the composition of the Palæozoic sediments, which, by successive oscillations of sea-level, were spread to varying altitudes upon its flanks.
At the close of his chapter on the Archæan, Mr. King proposes a theory of the genesis of granite and crystalline schists, which is in some respects new. In common with most of the geologists of the present day, he supposes that the granites and schists are sedimentary rocks which, having locally accumulated to great thickness, have sunk by their own weight into the yielding crust of the earth to a point where they have suffered more or less aqueo-igneous softening, and then, in his view, under varying intensities of radial and tangential pressure, they have been converted into corrugated schists or massive granite, according to the less or greater energy of the forces acting upon them. The evidence adduced by Mr. King to support this mechanical theory of the origin of granite is chiefly derived from the facts which indicate internal and bodily movement in granite, such as the dislocation of inclosed minerals, and the inclusion of masses of foreign rocks.
That there has sometimes been more movement in granites than in the schists with which they are associated—and of which they can frequently be shown to be the exact equivalents in a more metamorphosed condition—is quite certain; but it is very difficult to separate here the effects of force from those of heat. Either produces the practical plasticity which we see recorded in the obliteration of bedding, and the inclusion of foreign rocks. Granites, which exhibit the extreme phase of metamorphism, have evidently been in a plastic state, for they have been forced into fissures of other rocks to form veins and extruded mountain-crests—proofs of softening and movement which schists never afford—but whether this plasticity was the effect of greater heat or greater force than the associated schists suffered, is a question not answered by any facts yet cited. The dislocation of included minerals indicates force; the inclosure of blocks of foreign material, when these blocks are masses of metamorphic schists, indicates plasticity, but affords no test of temperature. Where the included blocks are limestone, as in the granite of the Pyrenees described by Green ("Physical Geology," p. 322), and these blocks are externally metamorphosed, internally unchanged, we have a record of softening and heat, but not the heat of fusion. The coarser crystallization of granite means that it has been more or longer softened, so that its component minerals were free to crystallize out; but no distortion or dislocation of these minerals affords stronger proof of internal movement than is furnished by the associated schists, in which the fossils often not wholly obliterated are quite as much distorted as any minerals in the granite. The inclusion in granite of blocks of slate which have been transported some distance from their place of origin, supplies, however, conclusive evidence of movement which may even be called a flow. Cases of this kind, which are of great interest and significance, were observed by Mr. Newton in the Black Hills. The granite core of this mountain-chain incloses large angular blocks of metamorphic slate which have been torn from their connections and carried bodily upward. The granite also shows a kind of slickenside-jointing, which proves that when in a plastic but not fused condition it was squeezed out of a fissure or opening in the harder overlying schists. We have here proof that the granite in its lower position has been more softened than the schists, and that it has been more moved; but we have no proof that it has been subjected to greater pressure.
In contrast with the theory of Mr. King, that granites have been produced by great pressure, is that promulgated by Mr. H. F. Walling (Proceedings of the American Association at the St. Louis meeting), in which lateral pressure is practically ignored as a cause of metamorphism. Mr. Walling, supposing, with others, that sediments accumulating along shores have sunk by their gravity, recognizing the fact that the static equilibrium must be maintained by the rising of the areas lightened by erosion in the removal of these sediments, attributes mountain elevation to this ascent, and the corrugation of metamorphosed rocks to the lateral flow of material from the sinking to the rising areas. There is certainly great force in the reasoning used by Mr. Walling to show that there must be rising as well as sinking areas, and a subterranean flow of matter from one to the other to compensate for the transfer of eroded material on the surface; but it seems doubtful whether the traction produced by the adhesion of the solid strata above to the moving mass below could produce slaty cleavage and other phenomena, which we have been accustomed to attribute to the lateral thrust produced by the crushing down of the rigid crust on a shrinking nucleus. It is hardly necessary to say that the metamorphism of granite is, according to Mr. Walling, due to the subterranean heat to which it has been exposed in its descent far below the surface. Probably the final conclusion reached in the discussion of the origin of granites, to which Mr. King has certainly contributed many new and interesting facts, will be that each of the causes, heat and pressure, should be credited with a share of the effects produced.
Palæozoic.—One of the most interesting and surprising results of Mr. King's exploration is the discovery of a section of stratified and unmetamorphosed strata said to be conformable throughout, reaching from the base of the Cambrian to the top of the Carboniferous, and attaining a maximum thickness of 32,000 feet. No section of the Palæozoic rocks of equal magnitude has yet been discovered elsewhere in the world, and the announcement will doubtless be received with some incredulity by geologists, but the accuracy with which the measurements were taken by Mr. King and his assistants, and the vindication of his classification afforded by the fossils, which were carefully reviewed by Mr. Meek, Professor Hall, and Mr. Whitfield, seem to leave no room for doubt. This great group of rocks is said, as before stated, to have been laid down in a basin bounded on the east by the ranges of the Rocky Mountains in Colorado, and on the west by a broad archæan area in Nevada. In the middle rose the lofty islands of the Wahsatch, which toward the west presented an abrupt slope of 30,000 feet, against which the Palæozoic rocks abutted. From the inclosed character of this sea most of the sediments formed in it are mechanical, and represent the wash from the adjacent land, but in the middle of the section occurs what Mr. King calls the Wahsatch limestone (Lower Carboniferous and Upper Devonian), seven thousand feet in thickness. Although no certain measure of time is afforded by the mechanical sediments—since the rapidity with which they were deposited may have varied indefinitely with the activity of eroding agents—this great limestone mass, formed as it must have been through organic agencies, represents a lapse of time which is almost beyond the reach of the imagination; and if, as Mr. King states, the Palæozoic series is essentially conformable throughout the area it occupies, we have here evidence of a stability in the physical conditions of this portion of the earth's surface, which, so far as known, is without parallel.
Mr. King gives two sections of the Palæozoic series taken, one in the Wahsatch Mountains and the other in Middle Nevada, which differ only in minor details. The Wahsatch section is, however, the most complete, as it shows the base of the Cambrian system, which is not visible farther west; it is as follows:
|Upper Coal Measure limestone
|Weber quartzite, Carboniferous, and Devonian
|Wahsatch limestone, Carboniferous, and Devonian
|Ogden quartzite, Devonian
|Ute limestone, Silurian
|Cambrian silecious schists
Mesozoic.—At the close of the Palæozoic ages great changes took place in the topography of the central and western portions of the continent, all of which are for the first time made clearly known by Mr. King's graphic and lucid descriptions in his chapters on the "Mesozoic Areas of the Fortieth Parallel." After the deposition of the great and conformable series of Palæozoic rocks in the central basin, the Archæan continent, which formed the western limit of these older deposits, and which had continued dry land to the close of the Carboniferous age, was sunk under the waves of the Pacific, and thus remained during the Triassic and Jurassic ages—long enough for the deposition of about 20,000 feet of sediments, of which considerably more than half belong to the Trias. Then the great fold of the Sierra Nevada was raised high above the ocean-level, carrying with it all the table-land of the Great Basin, which has not since been covered with salt-water. The rocks composing the Sierra Nevada are chiefly the Jurassic and Triassic beds, here often completely metamorphosed and converted into crystalline slates and massive granites, in which lie the auriferous veins that have supplied the $1,000,000,000 of gold already taken from the California placers and quartz mines. This paroxysm, or rather period of elevation, occurred before the Cretaceous age; for, in all the interval between the Wahsatch and the Sierra Nevada, no Cretaceous rocks are found. On the Pacific side of the great Sierra, however, Cretaceous strata lie nearly horizontal, abutting against the upturned Jurassic and Triassic slates, and reaching to a height of some 1,200 feet above the present ocean-level.
East of the Wahsatch a very different history is recorded, for here the Triassic, Jurassic, and Cretaceous strata were deposited not only in a series conformable among themselves, but apparently with the Palæozoic rocks below. This conformability is, however, more apparent than real, for the region between the Wahsatch and the Mississippi gives abundant evidence of elevation and subsidence during the Mesozoic ages; but these changes of level were continental rather than local, and over an area of hundreds of thousands of square miles the surface on which the strata were deposited was so nearly level that no want of parallelism in their planes of deposition is visible to the eye. That great changes of level did take place in this region is evident from the facts, first, that on the area over which the Triassic beds were deposited, extending from the Colorado to the Mississippi, mechanical and shallow water deposits alone prevail. No limestones occur here in the Trias, but it is made up of great sheets of cross-stratified and tide-swept sand of brilliant colors, chiefly bright red—due to the complete oxidation of their iron from aeration, and the absence of organic matter—with here and there heavy beds of gypsum, and formerly of salt; all the records of the intermittent action of a shallow sea. This sea-bottom over the interval between the Rocky Mountains and the Mississippi became dry land at the close of the Trias; for the Jurassic strata which occupy much of Colorado, Utah, and Nevada, scarcely reach eastward beyond the mountains; and the Cretaceous age was marked by a great submergence which carried the shore-line progressively from the Gulf of Mexico to the Wahsatch, and northward, perhaps, to the Arctic Sea, converting all the area between the Wahsatch Mountains and the Canadian highlands into a sea, in which were deposited in some places 2,000 feet of limestone, the slow accumulation of calcareous matter from the growth and decay of marine organisms.
From Mr. King's careful study of the Mesozoic rocks of Nevada, we learn that the Trias consists of alternations of limestone and quartzite, which, in Star Peak, form a continuous section of over 10,000 feet. The fossils which the limestones contain show that much the larger part of this mass belongs to the Alpen Trias of the Old World, the Halstadt and Saint Cassian beds, and those which form the passage to the Jura.
The Jurassic rocks of Nevada are mostly shales—the deposits from water too shallow for limestones—and contain few fossils. Along the eastern margin of the Jurassic area in the Black Hills, the Jurassic beds are more purely marine, and are far richer in fossils. The upper portion of these beds, which are of an estuarine or littoral character, has lately been discovered to be a vast cemetery of vertebrate animals, some of which are of unequaled size, and in their structure of special interest. Among these are the huge dinosaurs described by Marsh and Cope, some of which far exceed in dimensions any terrestrial animals before known, the largest, according to Marsh, having a length of at least one hundred feet, and a height of twenty-five or thirty.
The uppermost member of Mr. King's Mesozoic section is the somewhat famous Laramie group—the Lignitic formation of Dana, so named because it contains the most important coal-beds of Colorado. The age of this group of rocks has been much discussed by Dr. Hayden and Lesquereux, the distinguished fossil botanist, and it has been represented by them to be Tertiary, on the evidence of its numerous fossil plants; Cope, however, found the remains of Cretaceous vertebrates, and Meek, Cretaceous mollusks, in it; and hence it was said to have a Cretaceous fauna and a Tertiary flora. The writer has, however, for a long time contended that its flora was distinct from that of the Tertiary rocks, and the proof was stronger that it was Cretaceous. Mr. King adduces new and apparently conclusive evidence that it is older than the Tertiary, since, like Cope, Meek, and Stevenson, he has obtained numerous Cretaceous animal remains from it, and finds it to underlie unconformably the Coryphodon beds, the oldest portion of the Eocene.
Tertiary.—The pictures which geology presents to us of the far West during the Tertiary age are totally different from those which preceded them, and, on the whole, more varied and interesting. As we learn from various sources, at the close of the Cretaceous the widespread sea of that age was withdrawn from the interior of the continent, and all the interval between the Sierra Nevada and the Canadian highlands became a land-surface; while in the lower valley of the Mississippi, and on the Gulf and Atlantic coasts of the eastern half of the continent, the sea stood higher than before or now, for marine Tertiary strata form a broad marginal belt reaching around the old land from New York to and up into the Mississippi Valley. In the region of the Plains, the Rocky Mountains, and the Great Basin, however, we find no marine Tertiaries, but abundant evidence that, instead of the former sea-surface, a broad continental area stretched from the Arctic Ocean southward through and beyond the Territories of the United States. This continent was marked by few bold topographical features, since the Rocky Mountain system was then slowly growing, and had attained nothing like its present magnitude. The surface was, however, varied with low mountain-chains, broad savannas, strongly flowing rivers, and a series of fresh-water lakes, which in magnitude far exceeded any now on the earth's surface. The climate was mild and genial even to the North Sea, and the land was clothed with a vegetation more luxuriant and varied than that which it now bears. Of the magnitude of its forest growth we have evidence not only in the abundant remains of trunk and leaf and fruit imbedded in the old lake sediments, but in the scattered remnants of its former grandeur seen in the gigantic conifers of California, and in the cypress, magnolias, sweet gums and sycamores, which are the pride of our Eastern forests. This fertile land also sustained a fauna corresponding in richness and interest to its flora; for in the Tertiary the gigantic reptiles of the Mesozoic were succeeded by herds of mammals which far surpassed in numbers, size, and variety of species, any mammalian fauna now living. Their remains have been exhumed by thousands from the old lake-beds, where, in the long lapse of ages, they had been borne by river-floods and entombed. Thus were formed the vast charnel-houses from which Leidy, Marsh, and Cope, have drawn the treasures they have exhibited to the admiring scientific world. One after another of those great Tertiary lakes were created by topographical changes which established hydrographic basins, and, in turn, by the cutting down of their outlets, their beds were first made dry land, and afterward deeply cut by the many-branched draining streams, until they have formed the Mauvaises Terres or "Bad Lands" of the West.
The Tertiary deposits, then, cf the region west of the Mississippi are fresh-water sediments, chiefly the immediate wash of the land, containing fossils which represent not only the fishes and turtles which were their aquatic inhabitants, but the flora and fauna that lived upon their banks.
On the west, however, this lake country was bordered by a chain of volcanoes, which had from time to time their paroxysms of activity, deluging the lowlands with lava-floods, or discharging into the atmosphere clouds of ashes which, borne far eastward by the prevailing winds, were the agents of more widespread and scarcely less complete devastation. In the end the elevation of the Rocky Mountains, and the erosion of the cañons of the Columbia, Klamath, Sacramento, and Colorado, converted the greater part of the rich Tertiary plain into the only real deserts that now exist on the continent.
Through the center of the region where these great changes were wrought, the belt of the fortieth parallel survey stretches continuously for 750 miles, and yet does not reach near to its eastern border, while it covers but an insignificant portion of its north and south extent. It is evident, therefore, that but a small portion of the records which form the history of the Tertiary ages in Western America come within the limits of Mr. King's survey, but he has made wide excursions both north and south of his special field, and has availed himself of the observations of his co-laborers in Western exploration, so that he has been able to write this history much more fully than had before been attempted.
The following brief summary is all we can give of the most important points in the chapter on the Tertiary rocks, perhaps from its facts and suggestions the most interesting of any in Mr. King's report. During the Eocene four great lakes with different boundaries, and forming different series of sediments, occupied the middle portion of the fortieth parallel belt. These are named by Mr. King: 1. Ute Lake, in which the Vermilion Creek beds, 5,000 feet in thickness, accumulated, and which filled the Green River basin to the width of 150 miles, reaching to that distance north of the fortieth parallel, and to a yet unmeasured distance southward; 2. Gosiute Lake, from which were deposited the "Green River Beds" of Hayden (the Elko Group of King), 2,000 feet in thickness, which extended westward to the longitude of 116°, and eastward, perhaps, into Middle Park; 3. Washakie Lake, in which the Bridger Beds, 2,500 feet thick, were deposited; this occupied the country about Fort Bridger, reaching some 150 miles east and west, and to an unknown distance north and south; 4. Uintah Lake, a limited body of water south of the Uintah Mountains, which received the last Eocene sediments, a thin group of clays and sands containing fossils, differing from those of the Bridger Group.
In the Miocene age, the area occupied by the Eocene lakes was mostly dry land, but other lakes not less extensive, and perhaps of equal duration, occupied contemporaneously portions of Nevada and Oregon in the west, and a wide district in the great plains on the east. To the western lake Mr. King gives the name of Pah-ute, and its deposits he calls the Truckee Group. The eastern Miocene lake he calls Sioux Lake, and its basin contains the strata named by Hayden the White River Group. In the Pliocene a wide extent of the Great Basin was occupied by what Mr. King has named the Shoshone Lake, and its sediments the Humboldt Group. In the middle province was a smaller body of water called the North Park Lake, of which the area is underlain by beds to which the same name has been given by Hague and Hayden, while in the region of the Great Plains the Niobrara Group of Marsh was laid down in what King terms the Cheyenne Lake.
The details of the description of these Tertiary lakes, and of the history of their formation and disappearance, are among the many things which Mr. King's volume contains, that for want of space must be passed without notice.
Quaternary.—One of the most interesting chapters in Mr. King's volume is that which describes the records of the Quaternary age in the region which he surveyed. The salient points in this history are briefly as follows: Along the fortieth parallel in the far West during the glacial period there was no general glaciation, no continental ice-sheet, but glaciers formed on all the more important mountain-ranges, extending down from their summits to the level of from 6,000 to 9,000 feet above the sea. Traces of these ancient glaciers are seen in excavated lake-basins, glaciated valleys, and terminal and lateral moraines. The glaciers of the Uintah Mountains were by far the most important in all this region. Snow and ice crowned the Park and Medicine Bow Ranges, and extended down all the valleys which radiated from them; but the ice-covered area was small as compared with the great breadth of the country. The Uintah Mountains, however, according to Mr. King, then formed a broad-topped table-land 17,000 or 18,000 feet above the sea, all of which was one great ice-field, with local glaciers descending the valleys both toward the north and south. The whole length of the range was thus covered, and the ice-field had a north and south width of some fifty miles. Thus it formed a glacial area considerably larger than that of the Alps at present. West of the Great Basin, as we know from the reports of King and Le Conte, the Sierra Nevada was the theatre of glacial action on a still grander scale.
The topographical changes in the far West during the Quaternary age seem to have been numerous, but not consequent upon great disturbances, although this unquiet region has shown more or less of its instability to the present day. The changes which would most strike an observer were variations in the water-surface; for Mr. King, joining his observations to those of G. K. Gilbert, has shown that even as late as the Quaternary the Great Basin was a well-watered country, and contained two lakes, which in magnitude were scarcely exceeded by any of those now existing on the continent. Of these, one called by Gilbert Lake Bonneville, occupied the Great Salt Lake Valley, with a vast extension toward the south and west. Great Salt Lake occupies the deepest portion of its basin, and, with Utah and Sevier Lakes, represents the residue of its water after a long period of dryness, during which the evaporation exceeded the precipitation. The other great Quaternary lake of this region has been named by Mr. King La Hontan; it occupied western Nevada with an area nearly equal to that of Lake Bonneville, but more broken with islands and promontories. It is now represented by Pyramid, Carson, and Walker's Lakes.
These two Quaternary lakes were, of course, the products of ages during which the precipitation of moisture in the Great Basin was much larger than now; but Mr. King states that the complete history of the climatic changes in this region during the Quaternary included two moist periods with a dry interval between them, and that these have been succeeded by another interval of aridity, that of the present. Gilbert had previously shown that a period of dryness had preceded the moist period in which Lake Bonneville was filled; and Mr. King's study of Lake La Hontan indicates another period of humidity which preceded that. The reasoning by which he reaches this conclusion is extremely ingenious, and is based upon the varying chemical precipitates from the waters of Lake La Hontan. In past times the waters which drained into this lake were highly charged with carbonate of soda, and during periods when the lake-waters were concentrated by evaporation, Gaylussite, the hydrated double carbonate of lime and soda, was deposited in sheets on its sides. At other times, when the volume of water was greater, the soda was dissolved out, and carbonate of lime alone precipitated in pseudomorphs after Gaylussite. From facts of this character, which we have not space to present in full, he feels warranted in stating—1. That the lake was formed in a period of abundant precipitation, and had free drainage to the ocean; 2. In a period of desiccation, the level of the lake was reduced by evaporation below its outlet, and the saline contents concentrated to the point of formation of Gaylussite; 3. The coming on of a second flood period which filled the basin to its point of overflow, when the soluble salts were all washed out and the pseudomorph thinolite was formed; 4. A modern rapid desiccation which nearly emptied the lake-basin, leaving only a few small, weakly saline lakes as its representatives. Mr. King connects these periods of greater precipitation with two corresponding periods of glacial extension, and from these facts hints rather than asserts that elsewhere, as well as there, superabundant moisture was the cause of glacial extension, and therefore of those records which are generally regarded as proofs of a cold period. A discussion of the phenomena and causes of the "Ice period" would be incompatible with the limitations of this paper, but we may say in passing that the generalization which has been suggested by Mr. King seems hardly warranted by the facts he reports. It is certainly true that there could be no formation of ice or glaciers, however low the temperature, without precipitated moisture, and in many places the extension of glaciers is limited not by temperature, but by lack of moisture; but, to find standards of comparison with the widespread glaciers of the Ice period, we must go to the Arctic and Antarctic Continents. Here we are far removed from the theatre of most active evaporation, and where the climate is very cold, and yet the precipitation is sufficient, with the average temperature, to form continental glaciers of equal dimensions with any indicated by the records of the past, and, so far as we can judge, no other condition is necessary for the extension of the glaciers of Greenland southward to Labrador, the Canadian highlands, and the hills of New England, than a depression of temperature sufficient to congeal and retain the moisture which now flows away nearly as fast as it falls. With the arrest of the flow of the St. Lawrence, for example, and its accumulation year after year as ice and snow, it would not require many centuries to pile the ice as high on the Canadian highlands as it was in the Quaternary age.
In Chapter VI., which forms a résumé of the stratigraphical geology, Mr. King refers in his graphic and felicitous way to the conditions of deposition of the 120,000 feet of sedimentary accumulations which form the different groups we have reviewed. The tabular presentation of the stratigraphy (page 544), giving at one view the relations of the 50,000 feet of Archæan, 32,000 feet of Palæozoic, 30,000 feet of Mesozoic, and 15,000 feet of Cenozoic rocks, is the most comprehensive and impressive section which has ever been published, and one that shows at a glance the magnitude of the task which Mr. King has performed in the correlation and coordination of such a vast amount of material.
In the last two chapters of his volume Mr. King discusses at great length the genesis and relations of the Tertiary volcanic rocks, and more briefly the classification of the mountain-ranges and lines of upheaval which traverse his field of exploration. These chapters, though of great interest to the geologist, will perhaps not attract the general reader. We would, however, specially commend them to students of lithology and physical geology, as they contain a vast amount of valuable information on what have been made subjects of special study by Mr. King. There was no part of his duty for which he was better prepared than that he has done here; and perhaps none in which he has acquitted himself more creditably. The most striking generalization which he makes in this part of the book, we are, however, compelled to question. This is a new theory of the origin of vulcanism. Most geologists of the present day believe that the crust of the earth is thicker than was once supposed, and that its thickness is increased by the effect of pressure which holds in coerced rigidity a zone of greater or less depth, which is heated above the point at which it would fuse and flow under the pressure of the atmosphere only, and that local relief of this pressure would permit a greater or less mass of highly heated matter below to burst into fluidity, and perhaps find its way to the surface. Mr. King proposes erosion as a sufficient cause for the relief of pressure and the production of volcanic phenomena; but some facts suggest themselves which seem to be incompatible with this theory, viz.: 1. Erosion is so slow—on an average 3,000 years being required to remove a foot of surface—that solidification by cooling must keep pace with it; 2. Volcanoes are generally not situated in areas of erosion, but along coast-lines and on islands; and, 3. They are conspicuously associated with lines of fracture and elevation.
A simple explanation of the phenomena of vulcanism is suggested by the writer, and that is—the relief of pressure by slight arching of the crust of the earth along lines of elevation, while the pressure is maximum under the unbroken areas on either side. This unequal pressure would cause a flow of liquid or viscous matter toward and upward under the mountains that mark the lines of arch and fracture, and would permit heated matter held in solidity by pressure to assume the fluid state.
Mallett's theory, that the arching of plates of the earth's crust, and the arrest of their motion in falling, would generate heat sufficient to liquefy masses of rock, and produce volcanic eruptions, is rejected, for the reasons that the strain on arches of sufficient magnitude would be too great for the resistance of the materials composing them, and as Mr. Fisher argues, the heat generated by this method, even if great enough, would not be localized.
Mallett's theory wag framed to account for vulcanism in the crust of the earth, on the supposition that the crust was very thick, as claimed by Hopkins, Thompson, and Darwin, but Henessey and Delaunay have clearly shown that the investigations supposed to demonstrate the great thickness of the crust are valueless and irrelevant, as the premises assumed are not those of nature, and that we have as yet no evidence of such thickness of crust as would make it impossible for volcanoes to be fed from a general molten mass below the earth's solid crust.
In conclusion, we take pleasure in commending the numerous maps and plates which embellish these elegant volumes. No other scientific work known to us has so many, nor any more artistic or better adapted to supplement and illustrate the text.