# Popular Science Monthly/Volume 39/September 1891/A Classification of Mountain Ranges According to their Structure, Origin, and Age

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* A CLASSIFICATION OF MOUNTAIN RANGES ACCORDING TO THEIR STRUCTURE, ORIGIN, AND AGE.
By WARREN UPHAM,
OF THE UNITED STATES GEOLOGICAL SURVEY.
THE sea, in its vastness, reaching far beyond the encircling flat horizon, is a better symbol of infinitude and of eternity than is the most majestic mountain range, lifting its serrated forehead miles above the ocean-level and seeming almost to pierce the sky. The sea itself, but no part of the land forming its shores, has continued unchanged through the series of geologic eras.
"Time writes no wrinkle on thine azure brow— Such as creation's dawn beheld, thou rollest now."
But on the land, no sooner have the subterranean forces upheaved a mountain, a plateau, a continent, or an island, than the processes of subaërial erosion begin to contend against it. Rains, frost and heat, chemical change, subdue the most enduring and solid rock formations, dividing them with fractures, and pulverizing their masses and fragments to sand and clay, which gravitation by the vehicle of running water carries down and away to the sea, there to find rest until another uplift shall renew the cycle of changes. "The mountain falling cometh to naught, and the rock is removed out of his place."
The form of mountains and of their ranges and systems is due to the combination, in varying ratios, of constructive and destructive agencies. The first only seem necessary; but the second have generally been far more efficient to give the shape and outlines of all our mountains, excepting volcanoes. Constructive forces have done work that may be compared to the quarrying of the block of marble and bringing it to the artist's studio; destructive forces, producing the present mountain forms, as they stand before our vision, have done work like chipping away the greater part of the marble block and chiseling it to the finished statue. It will be convenient to speak of the constructive processes as mountain-building, and of the destructive as mountain-sculpture.
It is from observation and study of the geologic structure of mountain ranges, the diverse rock formations of which they are composed, and their attitude and relationship to each other, that we discover and understand their origin; how, by what agencies, the mountains have been built and sculptured. Structure and origin are thus very intimately connected and demand the same division under classes and types. In this classification, when citing examples of each type, we can commonly note also their ​geologic age, or the epochs of deposition of the rocks forming the mountain range, the epoch of their upheaval or successive upheavals, and the subsequent time during which they have been exposed to erosion.
Six classes of mountain ranges may be discriminated which, from predominant features of their structure, may be named as (1) folded, (2) arched, (3) domed, (4) tilted, (5) erupted, and (6) eroded. In each class the present contour of peaks, ridges, buttresses, and slopes has commonly been produced by the destroying frost, storms, and streams; but wherever some special phase of mountain-building is discoverable within the disguise which the superficial transformation has imposed, the mountain range or separate mountain is thereby referred to its constructive type. The last-named class, therefore, is intended to include only those mountains and ranges which owe their prominence to the denudation of an equal or greater thickness of the same rock formations from the surrounding country or from the valleys that divide them from other mountains, while the structure of the masses spared by this erosion does not place them in either of the five preceding classes as areas that have experienced mountain-building or orogenic movements. The sixth class belongs to areas where continent-building or epirogenic movements of widely extended elevation have been followed by so deep erosion that mountains have been made wholly by sculpture. Often the processes of mountain-building have combined in the same range the features which give names to two or more of these classes, but usually there is some chief element in such complex structure, predominantly allying the range with one of the five orogenic types. Faults, as the geologist calls dislocations of the rock formations, where the portion of the earth's crust on one side of a plane of shearing has been borne upward or forward, while the portion on the other side has fallen downward or backward, often complicate each of the six classes of mountain structure; and they are the sole or principal means of formation of the fourth, that is, of tilted ranges.
The plan of this essay is to examine the structure of each class, and to inquire what was the manner of action of the mountain-building or orogenic forces producing each of the five constructive types, and of the continent-building or epirogenic forces producing the broad, elevated expanses from which erosion has formed the sixth type, mountain remnants of destroyed high lands. Examples of each class are described, and the geologic age of their rock formations, of their upheaval, and of the ensuing erosion, is stated so far as it has been studied out, giving thus, in a brief way, both a description and a history of the mountain range or system.
​1. Folded Mountain Ranges.—Foremost in their geographic importance, and in the intricacy and significance of their geologic structure and origin, are the mountain belts which consist of folded rock formations. The strata forming the upper part of the earth's crust are bent up and down in long, nearly straight or curving, wave-like ridges and troughs, and where their disturbance was greatest the successive ridged folds are closely pressed together. The waves of the rock structure are then pushed to such steepness that their sides become parallel with each other, and the entire fold is driven forward into an inclined position. The order of the strata on the lower side of the appressed fold is thus inverted; the originally highest and last formed deposits there lie beneath older beds, in an overturned series. Subaërial erosion then wears down the undulations and the crests of the closely folded strata, often planing them off until a long section, crossing mountain ranges, passes from older to newer beds, and onward from newer to older, in several alternations, having throughout the whole a nearly constant steep dip. Owing to the interbedding of hard and enduring sandstone, quartzite, gneiss, and other rock formations, with more easily eroded limestone, shales, incoherent sandstones, or schists, the erosion commonly produces a new topography, making hollows and long valleys where the more erosible beds have been removed, and leaving ridges and mountain ranges of the harder rocks. More than this, when erosion has been continued through very long periods, it tends toward the ultimate result of removing the upward curved or anticlinal portions of the great folds and sparing the originally lower downward curved or synclinal portions, until valleys take the places which were originally occupied by the highest upheavals, while the original troughs, where the rocks were most compacted by pressure, remain now as the principal mountain ridges. Under denudation, the folded mountainous belt fulfills the prophecy, "Every valley shall be exalted, and every mountain and hill shall be made low."
The most perfect type which the world affords of this structure, or at least the example which has been most fully studied as to the age of its strata, the dates of their foldings and upheavals, and the effects of erosion, is the Appalachian mountain system. As made known by the brothers W. B. and H. D. Rogers and by later geologists, a vast series of Palæozoic strata, representing continuous deposition from the early Cambrian to the close of the Carboniferous period, is thrown into many long, steep folds in the Appalachian ranges of Pennsylvania and the Virginias, making the southeast part of this mountain system, and into plateaus and gentle undulations in the Catskill, Alleghany, and Cumberland Mountains, which are its northern and western portions. After ​the formation of the coal measures, the thick sediments that had been laid down in the subsiding eastern margin of the Palaeozoic ocean, which extended westward over the present basins of the Laurentian lakes and the Mississippi, were compressed into folds and raised to constitute a mountain mass one thousand miles long and seventy-five to one hundred miles wide, with probably much greater altitude than now. During Permian and Triassic time, according to Prof. W. M. Davis, this elevated area was channeled by rivers and finally was mostly worn down to a broad base-level or a moderately undulating expanse. Renewal of elevation, occurring in the Jurassic period, was probably attended with the remarkable overthrust faults, having apparently a maximum extent of about eleven miles of horizontal displacement, which have recently been studied out by C. W. Hayes, similar to the thrust-planes discovered by Peach and Home in northwestern Scotland. Another cycle of base-level erosion is shown by Davis to have extended from the Jurassic upheaval to the end of the Cretaceous period, reducing the Appalachian Mountains to a lowland tract, in part nearly flat and in part hilly, which he names the Schooley peneplain. This tract, almost a plain at the close of the Mesozoic era, was then a third time upheaved; and the present valleys of the Appalachian belt, divided by very long mountain ridges of uniform height, have been cut by river erosion during the Tertiary and Quaternary eras.
Closely associated with the foregoing are other folded groups and ranges of mountains, which Prof. C. H. Hitchcock has named the Atlantic mountain system, first raised as mountain masses in the Cambrian and Silurian periods, long before the great Appalachian revolution terminating the Coal period. In order from northeast to southwest, this system comprises low mountains in Newfoundland and in the eastern provinces of Canada, south of the St. Lawrence; the mountains of Maine; the White Mountains; the Green Mountains; the Hoosac and Taconic ranges; the Hudson highlands; Schooley's Mountain and other ranges in New Jersey; the South Mountain in Pennsylvania; the Blue Ridge in Virginia; and the Blue Ridge, the Stone Mountains, and the Iron, Bald, Smoky, and Unaka ranges in North Carolina. This mountainous belt, extending nearly two thousand miles, is everywhere characterized by overturned folds, and by intense metamorphism, the sedimentary strata, originally shales, sandstones, and conglomerates, being changed to crystalline schists, gneiss, and granite. Denudation of the Atlantic mountain system, and of lands stretching eastward over part of the present Atlantic Ocean area, supplied the deposits which were upheaved in the building of the Appalachian ranges.
A still older Laurentian mountain system, first upfolded in ​the Archæan, era, is to-day represented by the Adirondacks, the Laurentide highlands, and the mountains of Labrador, Baffin Land, and Greenland. Asking, then, how the mountain-building forces of eastern North America have been manifested, we see that the upper part of the earth's crust here has been folded by pressure from the Atlantic toward the central area of the continent, exerted during certain epochs of mountain formation, which have alternated with long intervals of repose and of base-leveling by stream erosion. Three chief epochs of orogenic upheaval have produced the intimately blended Laurentian, Atlantic, and Appalachian mountain systems, which geologists distinguish because of their diversity in age and in many of their physical features, but which geographers unite as the eastern mountainous belt of our continent. As a whole, it may perhaps properly be called the Appalachian, or, better, the Appalachian-Laurentide belt.
Other examples of this structure are developed on the grandest scale in the Old World, comprising the Atlas Mountains, the Pyrenees, the Alps, the Apennines, the Carpathians, the Balkans, the Caucasus, the Elburz, the Hindoo Koosh, and the Himalayas, together reaching from the Pillars of Hercules to the China Sea. These complex mountain systems may collectively be called the Alp-Himalayan belt. During the Miocene, Pliocene, and Glacial periods to the present time, compression has been exerted on each side, upbuilding its mountain chains, which cover a length of about eight thousand miles, occupying a third part of a great circle. In North America the Laurentian mountain system belongs to the remote beginning of the geologic record, and the Atlantic and Appalachian systems are very old, having repeatedly been almost base-leveled; but these principal mountains of northern Africa and of Europe and Asia are geologically very new, the highest being still in the growth of infancy and youth. When upward growth ceases, erosion triumphs and by slow degrees sweeps the mountain mass into the sea. The perpetuation of ancient mountain systems has depended on repeated upheavals, and in their present condition they are remnants spared from the erosion of areas lately elevated. Portions of this great Eurasian mountain belt began to be plicated and thrust up long before the Tertiary era, and doubtless some of its mountain systems were comparatively undisturbed during the Tertiary and Quaternary folding and upbuilding of the Alps and Himalayas; but mainly the prominence of the belt is due to the lateness of the plication, and in part to its being now in progress.
Nearly all the principal mountain systems of the world have a folded structure, but in many instances they retain no semblance of their primal undulations and earliest contour. In the mountainous plateau of Scandinavia and its outlier, the Scottish ​Highlands, only the bases of the ancient mountains remain. Eroded, perhaps repeatedly, almost to the sea-level, these plicated areas have been again elevated and are now deeply incised with valleys, fiords, and lochs. In the western mountain belt of North and South America a long and eventful history of extended plication and upheavals in many separated geologic epochs is more or less clearly revealed; but the latest accidents befalling this belt during the Quaternary era will call for special description under the fourth structural type, with which the earlier revolutions of this most prolonged mountain chain will be reviewed.
2. Arched Mountain Ranges.—Far less frequent than the foregoing, and indeed known only in parts of the Cordilleran belt of the western United States, is the arched structure, which may be best described in its most typical example, the Uinta range of northeastern Utah. According to Powell's report on the geology of these mountains, a great thickness of many rock formations has been here raised in an arch about one hundred and fifty miles long from east to west and thirty to forty miles wide. The strata range in age from the Archæan and Cambrian to the Cretaceous and Tertiary, and they appear to have reposed horizontally, as laid down in the sea, until the end of the Cretaceous period. The upheaval took place during the Tertiary era, mostly in its earlier portion, and the whole extent of the upward arching was about five and a half miles. Erosion, however, has gone forward during the growth of the arch, so that the highest peaks of the range have an altitude of only about two and a half miles, or thirteen thousand feet. Upon each side of the Uinta arch and about its ends the stratification is steeply inclined and occasionally cut by faults; but higher up the inclination diminishes and the strata extend across the top as a flattened dome, without folding or dislocation.
How were the mountain-building forces applied to form this arch? Its short extent in proportion to its width and the absence of plication make it difficult or impossible to refer it to lateral pressure, which has been regarded as the manner of application of the energy forming the great folded ranges. All the features of the Uinta range, instead, point to upward pressure as the form of mountain-building energy to which its elevation was due. It is very important, however, to note that the process of the Uinta elevation was so gradual and slow that the rivers which flowed across the area before its upheaval were not turned aside, being able to cut down their channels, which in the heart of the mountains are precipitous, narrow canons, as fast as the elevation progressed. After the consideration of the remaining types of mountain structure, we shall further examine this question of the method and the origin of the diverse manifestations of mountain-building.
​Adjacent to the east end of the Uinta arch, two similar but small upthrust mountains were formed at the same time, and repeat the same structural type in its essential features, but they have sharply arched crests, and their longer axes run from north to south, at right angles with the major axis of the Uinta range. These are Junction Mountain, about twelve miles long and four miles wide, and Yampa Mountain, seven miles long and about three miles wide. Both are cut by the Yampa River, flowing directly through them in deep canons, instead of passing around, thus showing that these very short upthrusts, like that of the larger range, were gradual, not sudden, in their development. The vertical extent of the upward arching of the strata tp form each of these mountains, counteracted meanwhile in large part by denudation, is believed to have been somewhat more than two miles; and this great elevation of so small areas was yet not too rapid to permit the river to keep pace with it in the downward cutting of its cañons.
3. Domed Mountains.—The structural type here designated is exemplified by the Henry Mountains in southern Utah, which have been elaborately studied by Gilbert. These mountains were formed as dome-shaped or bubble-like but gigantic uplifts of previously horizontal Carboniferous, Jura-Trias, Cretaceous, and Tertiary formations, by the volcanic injection of immense lenticular masses of porphyritic trachyte between the strata of the series. The injected lava mass is named by Gilbert a laccolite (cisternstone). Whereas in the first type of mountain structure the formerly horizontal strata were thrown into folds, and in the second were curved upward in great arches, they here were simply lifted quaquaversally, as a geologist would say, in vast domes. Mount Ellsworth, the most southern of the Henry Mountains, was lifted by only one laccolite; Mount Holmes, the next northward, by two; Mounts Hillers and Pennell, next in order to the north, each by one large and several smaller laccolite intrusions; and Mount Ellen, the most northern mountain of this group or range, was puffed up by many, perhaps thirty, of these cistern-like masses of lava. The Henry Mountains extend about thirty-five miles from southsoutheast to north-northwest, with a width of five to ten miles; and their highest summits rise about five thousand feet above the plateau of their base, or eleven thousand feet above the sea.
From these summits the view embraces within distances of fifty to one hundred and twenty miles northward, eastward, and southward, no less than five other mountain groups of this type, namely, the Sierra La Sal, the Abajo, La Lata, Carriso, and Navajo Mountains; and two hundred miles to the east the Elk Mountains of Colorado belong to the same class. The Henry Mountains and these other groups were all probably uplifted near the middle of ​the Eocene period, the first of the three divisions of the Tertiary era. They were contemporaneous with the growth of the Uinta range and the Junction and Yampa Mountains; "but the Henry structure represents sudden lifting by the energy of volcanic inflows of molten rock, while the Uinta structure, as we have seen, represents a very gradual upheaval. The two can not be referred to the same means of elevation, though their more remote causes were doubtless nearly related or identical. No laccolite mountains are known in other countries, and here they are found only in the region of plateaus which is intersected by the canon of the Colorado.
4. Tilted Mountain Ranges.—Next to the west of the Colorado drainage area is the Great Basin of interior drainage, which returns all its rainfall again to the clouds by evaporation. Were the lakes of this arid region to grow by increased rainfall until they should flow across the lowest points of their water-sheds and send streams to the ocean, two of them would be similar in area to the Great Lakes of the St. Lawrence. Twice during the climatic changes of the Glacial and Post-glacial epochs, these two lakes, named Bonneville and Lahontan, have so risen nearly or quite to overflowing, whereas now the former is represented by Great Salt Lake in Utah, and the latter by Pyramid and Winnemucca Lakes, with others in Nevada. Close east of Lake Bonneville rises the Wahsatch range, and west and southwest of Lake Lahontan is the Sierra Nevada, both of which are examples of tilted mountain ranges. The Wahsatch has been elevated along fault lines which form its western boundary, adjacent to the area of Lake Bonneville and the present Great Salt Lake. It is an immense mountain mass which has been tilted by upheaval of its western border and sinking of its eastern portion. The Sierra Nevada, on the other hand, has been upheaved along fault lines bounding it on the east, and is concisely described as principally a single great block of the earth's crust, about three hundred miles long from north-northwest to south-southeast, and fifty to seventy miles wide, tilted by elevation of its east side and depression of its west side. Between these grand mountain ranges which look toward each other on the east and west limits of the Great Basin, many minor ranges occur, trending from north to south, all of which have the same structure and origin through faulting and tilting, so that this is called by Powell the Basin type of mountain structure.
The great disturbances producing the Basin ranges were of late geologic date, in the early part of the Quaternary era. The resulting mountain ranges are still very young, geologically speaking, and therefore some of them rank among the most prominent on this continent. During more remote periods doubtless ​many ranges in various parts of the world have been formed in this way, their sites being now marked by profound faults which are clearly traceable, though the tilted mountains of the upheaved side of-the faults have long since passed through youth, maturity, and old age, leaving no topographic evidence of their former existence.
Many stages of mountain-building have left their impress on the great Cordilleran belt of the western part of the United States and the Dominion of Canada. The Gold and Selkirk ranges of British Columbia, according to Dr. George M. Dawson, consist of Archa3an, Cambrian, and Silurian formations, which were pushed up into mountain folds before the close of these very ancient divisions of geologic time. The auriferous slates of the Sierra Nevada, as Becker has shown, were similarly built up in a folded mountain range at the close of the Gault epoch in the Cretaceous period. During the ensuing long lapse of time to the end of the Tertiary era, this precursor of the Sierra Nevada range had been worn down to only a moderate elevation by the gnawing frosts, heat, rains, and running streams; but the beginning of the Quaternary era, according to Le Conte and Diller, brought revolutionary changes. The previously base-leveled region which now forms the Great Basin was then upheaved as a high plateau; intense volcanic activity was manifested in many parts of this area, and especially from the vicinity of Lassen Peak and Mount Shasta northward to the Columbia River and eastward along the Snake River to the Yellowstone National Park; and long faults, running mostly from north to south, divided the distended region into a multitude of orographic blocks, which, being soon allowed to sink, became tilted in their subsidence and form the present Basin ranges.
If we attempt to correlate these events with the Quaternary glaciation of the northern part of our continent, they seem to have been contemporaneous with the maximum extension of the ice-sheet of the first Glacial epoch. The ice accumulation I have attributed, on evidence derived from fiords and from river-channels now deeply submerged by the sea, to former great elevation of the glaciated areas, probably three thousand to four thousand feet higher than now. But the glacial and modified drift show that toward the end of each of our two principal Glacial epochs the land on which the ice lay was depressed nearly to its present level or in part lower. This depression of the earth's crust I believe to have been caused by the vast weight of the ice-sheets; and, in the first Glacial epoch, we have the correlative somewhat sudden elevation of a contiguous area, with outpouring of lava and formation of tilted mountain ranges, in the Great Basin and north to the Columbia. During the long interglacial epoch very ​thick subaërial deposits, called by Russell adobe, were supplied by denudation of the mountains and spread on the lower parts of the Great Basin and in the San Joaquin Valley; and the subsequent two flooded stages of Lakes Bonneville and Lahontan belong apparently to the second Glacial epoch and to a later or third epoch of glaciation in the northern part of the Cordilleran region.
Extending our view to embrace the entire belt of which the Rocky Mountains, Sierra Nevada, and Coast Range are parts within the United States, we see that it forms the western side of both South and North America. Its length from Cape Horn to Alaska is about ten thousand miles of a great circle, from which the irregular course of the chain is nowhere widely distant. These complex mountain systems, including the., Andes, the mountains of Central America and Mexico, the Rocky Mountains and parallel ranges west to the Pacific, and the Alaskan mountains, may be together named the Andes-Cordilleran belt. In Bolivia and Peru the highest portions of the Andes are found by David Forbes to be folded Silurian strata, which are so associated with Devonian, Carboniferous, and Permian formations as to imply that the principal epoch of mountain plication there, as of the Appalachian system, was at or near the close of the Palæozoic era. But later epochs of plication are also recognized in portions of the Andes, as likewise in the rocks of the Sierra Nevada, the Wahsatch, and the Coast Ranges, in the western United States. Indeed, the last-named range, and the range which culminates in Mount St. Elias, the former stated by Whitney to contain infolded Pliocene beds, and the latter found by Russell to consist of Pliocene or early Quaternary rocks, were formed by very late mountain-building, perhaps correlative, like the faulting and tilting of the Basin ranges, with great movements of the earth's crust producing and accompanying glaciation. The present height of the Andes, as of the Appalachian, Atlantic, and Laurentian mountain systems, and the Cordilleran ranges of the west part of this country and Canada, must be ascribed to Tertiary and Quaternary upheavals of this belt, portions of which had long before and at different times been folded and raised to mountain heights, but afterward had suffered erosion almost or quite to a base-level.
5. Erupted Mountain Ranges.—Volcanic action has often been developed on a grand scale along the deep fissures and fault planes which border and intersect tilted mountains and plateaus, as notably in the Andes and in Mexico, where it has built up very conspicuous volcanic cones of outpoured lavas and ejected blocks, bombs, lapilli, and ashes. Often, too, prolonged fissures, which may intersect each other (as in the Hawaiian Islands), reach down through the earth's crust to lavas that well up and build ​mountain masses and plateaus, while the crust segments have been only slightly tilted and sometimes lie wholly beneath the sea-level. Such eruptions form the Cascade Range, the mountainous plateaus of Iceland and the much-eroded Färoë Islands, the Deccan plateau in India, the volcanic chains of the Sunda, Kurile, and Aleutian Islands, and the Hawaiian Island belt.
The Cascade Range is a typical example of this class, having an extent of more than 500 miles from south to north across Oregon and Washington, showing a thickness of nearly 4,000 feet of lava where it is cut through by the Columbia, and bearing here and there volcanic peaks which rise to altitudes 10,000 to 14,000 feet above the sea. The eruptions producing this range took place during late Tertiary and early Quaternary time, being contemporaneous with the faulting and tilting of the Basin ranges, the Wahsatch, and the Sierra Nevada, and with the folding and upbuilding of the Coast Range. As Jamieson and Alexander Winchell have well suggested, the outpouring of the vast lava floods of the Cordilleran belt in the United States, a portion of which forms the Cascade Range, was probably in large part or wholly dependent on movements of elevation and subsidence of the adjacent glaciated area. Another erupted range, on a smaller scale, but very interesting in its details as described by Russell, belonging to the Quaternary era, and partly to the recent epoch, lies close south of Lake Mono, in California.
6. Eroded Mountain Ranges.—The form and contour of nearly all mountains, excepting volcanic cones, have been given to them by the sculpturing agencies of subaërial denudation. This is true of each of the foregoing classes, where mountain building energy has supplied the mass, but erosion has shaped the slopes, ridges, and peaks, the ravines and valleys. These five classes of mountain ranges have been sculptured by erosion, where previous mountain-building has raised limited areas to an exceptional altitude. But besides these orogenic upheavals, there have been broader uplifts of the whole or large parts of continents, which Gilbert and White have called epirogenic movements. The sixth class of mountain ranges, here to be noticed, is distinguished from all the preceding by its comprising mountains which owe their origin to no definite mountain-building process, being simply remnants of extensive areas which have been uplifted by epirogenic energy as great plains and since have been deeply eroded.
The plains which slowly rise from the Mississippi Valley and Manitoba westward to the foot of the Rocky Mountains afford examples of this type of mountain structure. Perhaps the most striking is the range of the Crazy Mountains in Montana, which lies immediately north of the Yellowstone River near Livingston, and ​is conspicuously seen from the Northern Pacific Railroad. These mountains trend slightly west of north, and extend about forty miles with a width of fifteen miles, attaining an elevation of 11,178 feet above the sea and 5,000 to 6,000 feet above the prairies at their base. Their structure has been thoroughly studied by Wolff, who finds that they consist of late Cretaceous strata, soft sandstones, nearly horizontal in stratification, intersected by a network of eruptive dikes. The more enduring igneous rocks have preserved this range, while an average denudation of not less than one mile in vertical amount reduced all the surrounding country to a base level of erosion. The Highwood Mountains, about 25 miles east of Great Falls, Montana, having a height of 7,G00 feet above the sea, or about 3,500 feet above their base, are described by Davis as displaying the same structure, and therefore similarly testifying of great denudation. This erosion of the Great Plains was probably in progress during the whole Tertiary era. Around Turtle Mountain, on the boundary between North Dakota and Manitoba, its amount was not less than 500 to 1,000 feet.
Original epirogenic uplifting of these plains took place at the end of the Cretaceous period, or during the early part of the Eocene. Thence onward through the Tertiary era, rains, creeks, and rivers were reducing this region nearly to the sea-level, excepting remnants like the Crazy, Highwood, and Turtle Mountains, which were being sculptured approximately to their present form. But the Tertiary era seems to have been terminated and the Quaternary ushered in by a new epirogenic differential uplifting of this continent, causing the accumulation of the ice-sheet of the first Glacial epoch. The time of great elevation initiating the Ice age, and the ensuing long interglacial epoch before the second glaciation, appear to have permitted rivers in North Dakota and Manitoba to wear away a considerable part of the Tertiary base-leveled plain, from its former eastern margin to the remarkable escarpment, in part a small eroded mountain range, of the Pembina, Riding, and Duck Mountains and the Porcupine and Pasquia Hills, which form the west border of the Red River Valley plain and of the lowland with large lakes in central Manitoba.
Reviewing this classification of mountain ranges for the purpose of discovering what elements of diversity and of unity characterize the manifestations of mountain-building energy, we see this to be of two kinds, the second being presented under four phases. The first kind of mountain-building energy, producing folds, is' evidently lateral pressure, and is ascribed by geologists and physicists to the contraction of the earth's mass by its secular cooling, with resulting adaptation of the rigid outer part of the crust to the shrinking interior. The second is energy acting ​vertically upward, which, has produced the four other types of constructive mountain ranges and masses by diverse phases of its manifestation—namely, the slow arching of limited areas, as the Uinta, Junction, and Yampa Mountains; the sudden volcanic lifting of the laccolite mountains; the upheaval and subsidence, with faulting and tilting, of the Basin ranges; and the outpouring of lava, as in the Cascade Range. Each of these four phases of vertically acting energy depends upon a viscous and plastic (neither solid nor perfectly liquid) condition of the earth's interior. Greater pressure of some portions of the crust than of others upon the plastic interior would induce each phase of upward energy in mountain-building. Where isolated blocks of the crust yielded slowly to the resulting quasi-hydrostatic pressure of the interior, mountains of the Uinta type were formed; but large areas, as the Great Basin, being swelled upward and anon subsiding, as the interior pressure increased and diminished, have become marked by tilted mountain ranges. Where the relations of intense heat, immense pressure, and chemical influences, with presence of water or its further ingress, have allowed portions of the interior, often of great extent, to become liquid lava, its extravasation by the same pressure has formed laccolite mountains and erupted mountain masses, while many volcanic cones have been mainly built up of fragments of solidified lava, much of it so fine as to be called ashes, explosively ejected.
In an appendix of Wright's Ice Age in North America, I have pointed out the source of the relationship by which these two kinds of mountain-building energy are united, both being caused by the earth's contraction in cooling, and the second or upwardly acting kind of energy being dependent on the first in the intermittent and occasional relief of stress of the earth's crust by its folding along the great orographic belts. Between the epochs of mountain-building by plication, the diminution of the earth's mass produces epirogenic distortion of the crust, by the elevation of certain large areas and the depression of others, with resulting inequalities of pressure upon different portions of the interior; and these effects have been greatest immediately before relief has been given by the formation of folded mountain ranges. There have been two epochs pre-eminently distinguished by extensive mountain-plication, one occurring at the close of the Palæozoic era and another progressing through the Tertiary and culminating at the beginning of the Quaternary era, introducing the Ice age. During the last, besides plication of the Coast Range, of the Alps, and the Himalayas, a very extraordinary development of tilted mountain ranges, and outpouring of lavas on an almost unprecedented scale, have taken place.in the Great Basin and the region crossed by the Snake and Columbia Rivers. With the ​culminations of both, of these great epochs of mountain-building, so widely separated by the Mesozoic and Tertiary eras, glaciation has been remarkably associated, and indeed the ice accumulation appears to have been caused by the epirogenic and orogenic uplifts of continental plateaus and mountain ranges. Since the disturbances, with glaciation, closing Palæozoic time, the same combination of events has not recurred until the Quaternary era, which is not only exceptional in its accumulation of ice-sheets, but also in its numerous and widely extended movements of elevation and subsidence, and in its mountain-building and renewed upheavals of formerly base-leveled mountain belts. The earth's surface is probably now made more varied, beautiful, and grand by the existence of many lofty mountain ranges than has been its average condition during the previous long eras of geologic history.