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to have been some original distribution of materials in the for a supposed stratum 500 miles thick, lying beneath 25 globe that initiated the depressions on the areas which miles of crust, and passing from a fused into a solid state, they have retained. It has been already pointed out (ante, p. | Mr Fisher found that every 100 miles measured along a 223) that the matter underlying the oceans is more dense great circle on the surface would have been one mile larger than that beneath the continents, and that, partly at least, before the contraction, and that this might produce a trianto this cause must the present position of the oceans be gular elevation of " 25 square miles on a base of 100 miles, attributed. The early and persistent subsidence of these which would give a range of mountains half a mile high. areas, with the consequent increase of density, seems to have If only 50 miles out of the hundred were disturbed, the determined the main contours of the earth's surface. range would be a mile bigh, and so on.”4

From what has been stated in part iv., the reader The effects of this lateral pressure may show themselves will onderstand tliat rocks which were originally horizontal, either in broad dome-like elevations, or in narrower and or nearly so, have been crumpled over tracts thousands of loftier ridges of mountain. The structure of the crust is so square miles in extent, so as to occupy now a superficial complex, and the resistance offered by it to the pressure is area greatly less than that which they originally covered. consequently so varied, that abundant cause is furnished for It is evident that they have been horizontally compressed, almost any diversity in the forms and distribution of the and that this result can only have been achieved as a con- wrinkles into which it is thrown. It is evident, however, sequence of the subsidence of such a curved surface as that that the folds have tended to follow a linear direction. In of our globe. The difficulty of explaining these corruga- North America, from early geological times, they have kept tions on the hypothesis of the contraction of a solid globe on the whole on the lines of meridians. In the Old World, is undoubtedly great. Mr 0. Fisher, indeed, believes on the contrary, they have chosen diverse trends, but the that the present inequalities of contour on the earth's sur- last great crumplings—those of the Alps, Caucasus, and face are from sixty-six to eleven and a half times as great the great mountain ranges of central Asia—have risen along as they would have been had they resulted from the con- parallels of latitude. traction of a solid globe; and he has suggested that the earth Mountain chains must therefore be regarded as evidence need not have become solid throughout simultaneously, and of the shrinkage of the earth's mass. They may be the consequently may have been considerably larger than it is result of one movement, or of a long succession of such now at the time when a solid crust was first formed. movements. Formed on lines of weakness in the crust,

The geological phenomena long ago led to a belief in the they have again and again given relief from the strain of liquidity of the earth's interior. Since this belief has been compression by undergoing fresh crumpling and upheaval. so weightily opposed by the physical arguments already The successive stages of uplift are usually not difficult to adduced (ante, p. 225), geologists have endeavoured to trace. The chief guide is supplied by unconformability, as modify it in such a way as, if possible, to satisfy the re- cxplained on p. 318. Let us suppose, for example, that quirements of physics, while at the same time providing a mountain range consists of upraised Lower Silurinn an adequate explanation of the corrugation of the earth's rocks, upon the upturned and denuded edges of which the crust. Mr Hopkins, Professor Dana, Professor Shaler, and Carboniferous Limestone lies transgressively. The original Mr Fisher have, on different grounds, advocated the ex- upheaval of that range must have taken place at the istence of a fluid or viscous substratum beneath the crust, period of geological time represented by the interval the contraction and consolidation of which produce the between the Lower Silarian and the Carboniferons Limecorrugations of the rocks and of the surface. "The increase stone formations. If, in following the range along its of temperature," says Mr Fisher, “though rapid near the course, we found at last the Carboniferous Limestone surface, becomes less and less as we descend, so that, if the also highly inclined and covered unconformably by the earth were once wholly melted, the temperature near the Upper Coal-measures, we should know that a second uplift centre is not very greatly above what it is at a depth which, of that portion of the ground had taken place between compared to the earth's radius, is small. Consequently, if the time of the Limestone and that of the Upper Coalit requires great pressure to solidify the materials at such

By this simple and obvious kind of evidence a temperature, it is probable that the melting temperature the relative ages of different mountain chains may be commay be reached before the pressure is sufficient to solidify.” pared. In most great mountain-chains, however, the rocks The crust, of course, must be able to sustain itself on the cor- have been so intensely crumpled, and even inverted, that rugated surface of the supposed viscous layer without break- much labour may be required before their true relations can ing up and sinking. The same writer has even suggested be determined. that the observed amount of corrugation is more than can be The Alps offer an instructive example of a great mountain chain accounted for even on this hypothesis, and that the shrink- formed by repeated movements during a long succession of geologi. age may have been duo not merely to cooling, but to the cal periode. As has been already stated, the central portions of the escape of water from the interior in the form of the super- partly referable to the Archæan series, but many of which appear to

chain consist of gneiss, schists, granite, and other crystalline rocks, heated steam of volcanic vents. More recently Herr be metamorphosed formations of Palæozoic, Secondary, and even of Siemens has been led, from observations made in May 1878 older Tertiary age. at Vesuvius, to conclude that vast quantities of hydrogen

It would appear therefore that the first outlines of the Alps were 829, or combustible compounds of hydrogen, exist in the traced out even

in Archæan
times, and that after submergence

, and

the deposit of Palæozoic formations along their flanks, if not over earth's interior, and that these, rising and exploding in the inost of their site, they were re-elevated into land. From the refunnels of volcanoes, give rise to the detonations and clouds lations of the Mesozoic rocks to each other, we may infer that of steam.3

several renewed uplifts after successive denndations took place before Leaving the vexed question of the condition of the earth's

the beginning of the Tertiary formations. A large part of the range interior, the hypothesis of secular cooling aud contraction

was, as we have seen, submerged during the Eocene period under

the waters of that wide sea which spread across the centre of the furnishes a natural explanation of the origin of the domin- Old World, and in which the Nummulitic Limestone and Flysch were ant elevations and depressions of the surface, and of the deposited. But about the close of that period the grand upheaval intense crumpling which the rocks in many regions lave chiefly due. The older Tertiary rocks, previously horizontal under

took place to which the present magnitude of the mountains is undergone. Taking 0.09 as the coefficient of contraction

the sen, were raised up into land, crumpled, dislocated, inverted, Cambridge Phi. Trans., vol. xii. pt. ii., 1875.

together with all the older formations of the chain. So intense was Phil. Mag., October 1875.

the compression to which the Eocene clays and fands were subjected Monatsbericht der K. preuss. Akad. Wissenschaph, 1878, p. 568.

Cambridge Phil. Trans., yol. xl. st. li.

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that they were converted into rocks as hard and crystalline as many steeper folds as they are traced away from the plains, until of the Paleozoic masses. It is strange to reflect that the enduring they are found at last folded back upon themselves, and thaterials out of which so many of the mountains, cliffs, and pin the older are made to overlie the younger. Instead of overnacles of the Alps have been formed are of no higher geological tiquity than the London Clay and other soft Eocene deposits of the lying the central and more ancient masses of the range, they south of England. After the paroxysm of elevation had ended, one seem really to dip into and under them, so that a section or more large lakes were formed along the northern base of tho

across the region might convey the impression of a great mountains. In these hollows the Swiss molasse accumulated to a depth of more than 6000 feet-a great pile of slowly formed gravels, syncline instead of a great and complicated anticline. This sands, and clays. That the sea gained occasional access to the region fan-shaped arrangement of the rocks may be observed even is shown by the interpolation of bands containing marine organisms, in the single mountains of a great chain Mount Blanc is

a as already stated (ante, p. 363). Not improbably a gradual sub- a familiar example. sidence of the region was going on during the formation of the molasse. But towards the close of the Miocene period another great

Another piece of geological structure is sometimes brought epoch of mountain-making was ushered in. The lakes disappeared, vividly before us by the examination of these regions of and their thick sediments were thrust up into large, broken, moun. disturbance. Not only have the rocks been crumpled and tain masses. The Righi, Rossberg, and other prominent heights inverted; they have likewise been traversed by great disalong the northern flank of the Alps are formed of these upturned locations. Those on one side of a fissure have been pushed lacustrine deposits. Since that great movement no paroxysm seems to have affected the Alpine region. Ceaseless changes, bodily over those on the other side, or they have experienced indeed, have been in progress, but they have been due not so much a vertical displacement of hundreds or even thousands of to subterranean causes as to those su baerial forces which are still so

feet. As a rule, however, dislocations are more easily traced, active.

if they are not also larger and more numerous, among the The gradual evolution of a continent during a long succession of geological periods has been admirably worked out for North low grounds than among the mountains. One of the most America by Dana, King; Hayden, Newberry, Powell, Dawson, and remarkable and important faults in Europe is that which others. The general character of the structure is extreme sim. bounds the southern edge of the Belgian coal-field. It can plicity, as compared with that of the Old

World. In the Rocky be traced across Belgium, has recently been detected in the Mountain region, for example, while the Palæozoic formations lie unconformably upon the Archwan gneiss, there is, according to King, Boulonnais (ante, p. 350, note), and may not improbably run 8 regular conformable sequence from the Lower Silurian to the beneath the Secondary and Tertiary rocks of the south of Jurassic rocks. During the enormous interval of time represented | England. It is a remarkable fact that faults which have a by these massive formations what is now the axis of the cüntinent remained undisturbed save by a gentle and protracted subsidence. vertical displacement of many thousands of feet produce In the great depression thus produced all the Palæozoic and a great little or no effect upon the surface. The great Belgian faolt

, part of the Mesozoic rocks were accumulated. At the close of the for example, is crossed by the valleys of the Meuse, and other Jurassic period the first great upheavals took place. Two lofty, northerly-llowing streams. Yet so indistinctly is it marked ranges of mountains,-the Sierra Nevada (now with summits more

in the Meuse valley that no one would suspect its existence than 14,000 feet high) and the Wahsatch,-400 miles apart, were pushed up from the great subsiding area. These movements were

from any peculiarity in the general form of the ground, and followed by a prolonged subsidence, during which Cretaceous sedi. even an experienced geologist, until he had learned the ments accumulated over the Rocky Mountain region to a depth of structure of the district, would scarcely detect any fault at 9000 feet or more. Then came another vast uplift, whereby the

all. Cretaceous sediments were olevated into the crest of the mountains, and a parallel coast-range was formed fronting the Pacific. Intense

With the fractures along mountain chains we may connect metamorphism of the Cretaceous rocks is stated to have taken the hot springs so frequently to be met with in these regions. place. During the Tertiary ages the Rucky Mountains were perma- But the most important connexion with the heated interior nently raised above the sea, and gradually' elevated to their present is that established by volcanic vents. The theory of secular height. Vast lakes existed among them, in which, as in the Miocene basins of the Alps, enormous masses of sediment accumu.

contraction, while affording a rational explanation of the lated. The slopes of the land were clothed with an abundant vege origin of the great terrestrial ridges, serves at the same time tation, in which, as already stated (ante, p. 365), we may trace the to show why volcanoes should so frequently rise along these ancestors of many of the living trees of North America. One of the most striking features in the later phases of this history man ang diminishing the pressure on the parts beneath the upraised

ridges (ante, p. 254). The elevation of the crust, by of lavas from points and fissures over a vast space of the Rocky Mountains. in tracts, permits them to pssume a liquid condition, and to the Snake River region these lavas have a depth of 700 to 1000 rise within reach of the surface when, driven opwards by feat, over an area 300 miles in breadth.

the expansion of superheated vapours, they are ejected in These examples show that the elevation of mountains the form of lava or ashes. has been occasional and, so to speak, paroxysmal. Long It appears therefore that the present contours of the intervals elapsod when a slow subsidence took place, but at earth's surface must be due in large measure to the effects last a point was reached when the descending crust, unable of the contraction of a ccoling globe. The crust has been any longer to withstand the accumulated lateral pressure, repeatedly corrugated, sometimes suffering sudden and was forced to find relief by rising into mountain ridges

. paroxysmal shocks, at other times undergoing slow and longWith this effort the elevatory movements ceased. They continued upheaval and depression, were followed either by a stationary period, or more usually But these subterranean movements form only one phase by a renewal of the gradual depression, until eventually of the operations by which the outlines of the land have relief was again obtained by upheaval, sometimes along new been produced. They have ridged up the solid crust abova lines, but often on those which had previously been used. the sea-level, and have thus given rise to land, but the land

We seo also how, by such enormous compression, the as we now see it has acquired its features from the prolonged rocks should have acquired a cleavage structure (ante, and varied action of the epigene agents upon rocks of very P. 306). Soft clays have been squeezed and folded till varied heights and powers of resistance. they have become hard fissile slates. So intense have been It is evident that, as a whole, the land suffers ceaseless the corrugation and compression that the strata have under- erosion from the time that it appears above water. It is gone a chemical rearrangement of their particles ; they likewise clear, from the nature of the materials composing have been “metamorphosed” or changed into echists and most of the rocks of the land, that they have been derived gneisses, if indeed some portions of them have not been from old denudations of the same kind. And thus, sids actually fused and intruded into the surrounding masses as by side with the various upheavals and subsidences there igneous rocks.

has been a continuous removal of materials from the land, The consideration of these changes enables us to realize and an equally persistent deposit of these materials under why the strata of a great mountain chain should rise into | water, and consequent growth of new rocks.

The pro

This degradation of the surface may be aptly compared distributed over the whole surface of the land. If zoooth to a process of sculpturing, which begins as soon as the land of a foot is the mean rate from the whole surface, then some emerges from the sea, and never ceases so long as any por- parts, including the more level grounds, must lose very tion of the land remains above water. The implements much less than that amount, while other parts, such as the employed by nature in this great work are those epigene slopes and valleys, must lose very much more. forces whose operations have already been described. Each portions between these extremes must continually vary of them, like a special kind of graving tool, produces its throughout every country, according to angle of declivity, own characteristic impress on the land. The work of rain, nature of surface, amount and distribution of rainfall, aod of frost, of rivers, of glaciers, can be readily discriminated, whether the rain is spread over the year or concentrated though they all combine harmoniously towards the achieve-1 into a short period. ment of their one common task. Hence the present con- The proportion between the area covered by the more tours of the land must depend partly (1) on the vigour with level ground of a country, where the rate of denudation is which the several epigene agents perform their work of least, and that of the declivitios, valleys, and stream chanerosion, (2) on the original configuration of the ground, nels, where that rate is greatest, may be assumed as pine to and the influence it may have had in guiding the opera one. The extent of the annual waste may be further taken tions of these agents, and (3) on the varying structure and to be nine times greater over the latter than over the former, powers of resistance possessed by the rocks.

so that, while the more level parts of the surface have been 1. Taking a broad view of denudation, we may conveni- lowered 1 foot, the valleys have lost 9 feet. Taking the entlŷ group together the action of air, frost, springs, rivers, mean rates of waste over the whole area to be coogth of glaciers, and the other agents which wear down the surface a foot per annum we find that on these data the annual loss of the land, under the one common designation of subaerial, amounts to fths of a foot from the flatter grounds and 5 and that of the sea as marine. The general results of sub- feet from the valleys in 6000 years. This is equal to a loss aerial action are—to furrow and channel the land, to erode of 1 foot from the former in 10,800 years and from the valleys, to sharpen and splinter the ridges of mountains, later in 1200 years, or to Ith of an inch from the one in and thus, while roughening, to lower the general surface 77 and from the other in 83 years. At this rate of erosion, and carry out the detritus to the sea. The action of the å valley 1000 feet-deep nray be excavated in 1,200,000 ses, on the other hand, is to plane down the land to the years. These estimates are only approximations to the level at which the influence of breakers and ground-swell truth, but they are valuable in directing attention to the ceases to have any erosive effect; the flat platform, so often real efficacy of the apparently insignificant subaerial denudavisible between tide-marks on a rocky exposed coast-line, tion now in progress. Any other estimates of the relative is an impressive illustration of the tendency of marine amount of material worn away from the different parts of denudation. The combined result of subaerial and marine the surface may be taken, but the mean annual loss from action, if unimpeded by any subterranean movement, would the whole area, as ascertained by the river discharge, reevidently be to reduce the land to one general level under mains unaffected. If we represent too large an amount as the sea. For, except in that upper marginal zone where removed from the valleys we diminish the loss from the the waves and tidal currents play, the waters of the ocean. open country, or if we make the contingent derived from protect the solid rocks which they cover. And the rocks the latter too great we lessen that from the former. indeed can find no permanent protection anywhere else. 2. From this reasoning it follows that, apart altogether But to reduce a large area of land such as a continent to from irregularities of surface due to inequalities of upheaval, the condition of a submarine plain, would require a longer every area of land exposed to ordinary subaerial action period of time than seems to have elapsed between two must, in the end, be channeled into a system of valleys. epochs of upheaval Traces of ancient plains of marine de- Even a smooth featureless tract elevated uniformly above nudation are to be met with in Scandinavia and in Scotland, the sea would eventually be widely and deeply eroded. on but a comparatively small scale, as if there had been time Nor would this require a long geological periud, for, at for only a narrow platform to be formed before the next the present rate of waste in the Mississippi basin, valleys paroxysm of contraction and uplift completely renovated | 800 feet might be carved out in a million years. Un. the geography of the region.

doubtedly the original features superinduced by subterInstead of trying to estimate how much work is done by ranean action would guide and modify the operations of each of the subaerial agents in eroding the land, we gain a running water, though their influence would certainly wane much more impressive idea of the reality and magnitude as the features themselves slowly disappeared. " In no case of their work as a whole by treating their operations as one probably would the aboriginal contour remain through a great process, the effects of which can be actually measured. succession of geological periods Traces of it might still be The true gauge of the present yearly waste of the surface discernible, but they would be well-nigh effaced by the new of the land is furnished by the amount of mineral matter outlines produced by the superficial agents. In the vast carried every year into the sea by rivers. This mineral tablelands of Colorado and the other western regions of mattor is partly in mechanical suspension, partly in chemical the United States an impressive picture is visible of the solution, and is to no small extent pushed in the form of results of mere subaerial erasion on undisturbed and nearly shingle and sand along the bottoms of the streams. Some level strata Systems of stream-courses and valleys, river data respecting its amount have been already given (ante, gorges anexampled elsewhere in the world for depth and pp. 274, 278). If we take the ratios furnished by the length, vast winding lines of escarpment, like ranges of seaMississippi as a fair average, which, from the vast area and cliffs, terraced slopes rising from plateau to platean, huge varied climatal and geographical characters of the region buttresses and solitary stacks standing like islands out of drained by that river, they probably are, then we learn that the plains, great mountain masses towering into picturesque oboth of a foot is wom away from the general surface of pcaks and pinnacles cleft by innumerable gullies, yet everythe land every year. At this rate, if the present erosion where marked by the parallel bars of the horizontal strata could be sustained, the whole American continent, of which, out of which they have been carved—these are the orderly according to Humboldt, the mean height is 748 feet, would symmetrical characteristics of a country where the scenery be worn down to the sea-level in about 41 millions of years is due entirely to the action of subaerial agents on the one - comparatively short period in geological chronology. hand and the varying resistance of perfectly regular stratified It is obvious, however, that the denudation is not equally | rocks on the other. The Alps, on the contrary, present an

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instructive ezarple of the kind of scenery that arises vlore precipice in the course of the river when it first commenced a mass of high ground has resulted from the intense cor to flow, or caused by some hard rock crossing the channel, rugation and upheaval of a complicated series of stratified has eaten its way backward, as already explained (p. 276). and crystalline rocks, subsequently for a vast period carved Lakes may have been formed in several ways. l. By by rain, frost, springs, and glaciers. We see how, on the subterranean movements as, for example, during those which outer flanks of those mountains among the ridges of the gave rise to mountain chains. But these hollows, unless Jura, the strata begin to undulate in long wave-like ridges, continually deepened by subsequent movements of a similar and how, as we enter the main chain, the undulations nature would be filled up by the sediment continually assume a more gigantic tumultuous character, until, along washed into them from the adjoining slopes. The pomerthe central heights, the mountains lift themselves towards ous lakes in such a mountain system as the Alps cannot be the sky like the storm-swept crests of vast earth billows. due merely to this cause, unless we suppose the upheaval The whole aspect of the ground suggests intense commotion of the mountains to have been geologically quite recent, or Where the strata appear along the cliffs or slopes they may that subsidence must take place continuously or periodically often be seen twisted and crumpled on the most gigantic below each independent basin. But there is evidence that scule. Out of this complicated mass of material the sub- the upheaval of the lakes is not of recent date, while the aerial forces have been ceaselessly at work since its first idea of perpetuating lakes by continual subsidence would elevation. · They have cut out valleys, sometimes along the demand, not in the Alps merely, but all over the northern original depressions, sometimes down the slopes. They liemisphere where lakes are so abundant, an amount of sth. have eroded lake-basins, dug out corries or cirques, notched terranean movement of which, if it really existed, there and furrowed the ridges, splintered the crests, and have left would assuredly be plenty of other evidence. 2. By irreguno part of the original surface unmodified. But they have larities in the deposition of superficial accumulations prior not effaced all traces of the convulsions by which the Alps to the elevation of the land or during the disappearance of were upheaved.

the ice-sheet. The pumerous tarns and lakes enclosed 3. The details of the sculpture of the land have mainly within mounds and ridges of drift-clay and gravel are exdepended on the nature of the materials on which nature's amples. 3. By the acccumulation of a barrier across the erosive tools have been employed. The joints by which all channel of a stream and the conséquent ponding back of the rocks are traversed have served as dominant lines along water. This may be done, for instance, by a landslip, by which the rain has filtered, and the springs have risen, and the advance of a glacier across a valley, or by the throwing the frost wedges have been driven. On the high bare up of a bank by the sea across the mouth of a river. 4. scarps of a high mountain the inner structure of the mass By erosion. The only agent capable of excavating hollows is laid open, and there the system of joints is sewn to have out of the solid rock such as might form lake-basins is determined the lines of crest, the vertical walls of cliff and glacier-ice (ante, p. 282). It is a remarkable fact, of which precipice, the forms of buttress and recess, the position of the significance may now be seen, that the innumerable lakecleft and chasm, the outline of spire and pinnacle. On the basins of the northern hemisphere lie on surfaces of intensely lower slopes, even under the tapestry of verdure which ice-worn rock. The striæ can be seen on the smoother nature delights to hang where she can over her naked rocks, rock-surfaces slipping into the water on all sides. These we may detect the same pervading influence of the joints striæ were produced by ice moving over the rock. If the upon the forms assumed by ravines and crags. Each kind ice could, as the striæ prove, descend into the rock-basins of rock, too, gives rise to its own characteristic form of and mount up the farther side, smoothing and striating tho scenery. The massive crystalline rocks, such as granite, rock as it went, it could erode the basins. It is harilly yield each in its own fashion to the resistless attacks of the possible to convey in words an adequate conception of the denuding forces. They are broadly marked off from the enormous extent to which the north of Europe and North stratified rocks in which the parallel bands of the bedding America has had its surface ground down by ice. The form a leading feature in every cliff and bare mountain slope. ordinary rough surfaces produced by atmospheric disintegraAmong the latter rocks also very distinctive types of surface tion have been replaced by a peculiar flowing contour which may be observed. A range of sandstone hills, for example, is traceable even to below the sea level. presents a marked contrast to one of limestone.

In the general subaerial denudation of a country, innuIn the physiography of any region, the mountains are merable minor fentures are worked out as the structure of the dominant features. A true mountain chain consists of the rocks controls the operations of the eroding agents. rocks which have been crumpled and pushed up in the. Thus, among comparatively undisturbed strata, a hard bed manner already described. But ranges of hills almost resting upon others of a softer kind is apt to form along its mountainous in their bulk may be formed by the gradual outcrop a line of cliff or escarpment. Though a long mang! erosion of valleys out of a mass of original high ground. of such cliffs resembles a coast that lias been worn by the In this way some ancient tablelands, those of Norway and sea, it may be entirely due to mere atmospheric wasta. of the Highlands of Scotland, for example, have been so Again, the more resisting portions of a rock may be seen channeled by deep fjords and glens that they now consist projecting as crags or knolls. An igneous mass will stand of massive rugged hills, either isolated or connected along out as a bold hill from amidst the more decomposable strata the Aanks. The forms of the valleys thus eroded have been through which it has risen. These features, often su governed pertly by the structure and composition of the marked on the lower grounds, attain their most conspicuous rocks, and partly by the relative potency of the different development among the higher and barer parts of the denuding agents

. Where the influence of rain and frost mountains, where sabaerial disintegrtion is most rapid. has been slight, and the streams, supplied from distant The torrents tear out deop gullies from the sides of the sources, have had sufficient declivity, deep, parrow, precipi- declivities. Corries are scooped out on the one hand, and tous ravines or gorges have been excavated. The canons naked precipices are left on the other. The harder bands of the Colorado are a mngnificent example of this result of rock project as massive ribs down the slopes, shoot up Where, on the other band, ordinary atmospheric action has into prominent aiguilles

, or give to the summits the notched been more mpid, the sides of the river channels have been saw like outlines they so often present. attacked, and open sloping glens and valleys have been hol- Tablelands may sometimes arise from the abrasion of hard lowed out A gorge or defile is usually due to the action of rocks and the production of a level plain by the action of a waterfall, which, beginniug with some abrupt declivity or the sen, or rather of that action combined with the previous

degradation of the land by subaerial waste. But most of when, by the lessening of their declivity, their carrying
the great tablelan ls of the globe seem to be platforms of power is diminished (p. 276-7). The great plains of the
little-disturbed strata which have been upraised bodily to a earth's surface are due to this deposit of gravel, sand, and
considerable elevation. No sooner, however, are they placed loain. They are thus monuments at once of the destructive
in that position than they are attacked by running water, and reproductive processes which have been in progress un-
and begin to be hollowed out into systems of valleys. As ceasingly since the first land rose above the sea and the first
the valleys sink, the platforms between them grow into shower of rain fell. Every pebble and particle of their soil,
narrower and more definite ridges, until eventually the once part of the distant mountains, has travelled slowly and
level tableland is converted into a complicated network of fitfully downward. Again and again have these materials
hills and valleys, wherein, nevertheless, the key to the whole been shifted, ever moving downward and sea-ward. For
arrangement is furnished by a knowledge of the disposition centuries, perhaps, they have taken their share in the fer-
and effects of the flow of water. The examples of this tility of the plains and have ministered to the necessities of
process brought to light in Colorado, Wyoming, Nevada, lower and tree, of the bird of the air, the beast of the field,
and the other western territories, by Newberry, King, and of man himself. But their destiny is still the great
Hayden, Powell, and other explorers, are among the most

In that bourne alone can they find undisturbed
striking monuments of geological operations in the world. repose, and there, slowly accumulating in massive beds, they
, The materials woru from the surface of the higher are will remain until, in the course of ages, renewed upheaval
spread out over the lower grounds. We have already traced shall raise them into future land, there once more to pass
how streams at once begin to drop their freight of sediment | through the same cycle of change.

(A. GE)

ocean,

INDEX.

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333

Acadien formation, 331. Contemporaneity, geolo- France, geology of, 350, Leucite, 228.

Plication, 261, 298.

Strike, 298.
Air, moter eats of, 364; gical, 322.

356, 358, 362, 363. Lias, 354.

Pllocene deposits, 364. Structural geology, 213,
geological operations Contraction, effects of, Frost, 265, 280.

Life, geological effects of Porphyry, 234.

291.
of, 264.

261, 264, 371.
Gabbro, 235.

plant and animal, 289. Post - Tertiary deposits, Submerged forests, 256,
Alps structurcof, 371, 373. Coral islands, 257.

Garnet, 228.
Lignite, 238.

86.5.

321
America, North, geology Coralline Oolite, 354, 356. Gas-spurte, 294.

Limestones, 232-238. Prehistoric forinations, Subsidence, movements
of, 328, 339, 345, 350, Cornbrash, 854, 355. Gault, 357, 359.

Lingula Flags, 323.

368, 369, 370.

of, 256, 321, 367.
352, 354, 359, 364, 369. Cosmical aspects of
Geognosy, 213, 220. Liparite, 234.

Pressure, effects of, 261. Sulphates, 228, 232.
Andesite, 235

geology, 213.

Geological record, 213, Llandeilo Flag group, 332. Primordial zone, 829, 330. Sulphides, 228.
Animals, geological oper- Crag deposits, 364.

323, 326.

Llandovery group, lower, Prismatic structure, 249. Sulphur, 228
stions of, 289.
Cretaceous System, 357. Geology, definition of, 212; 332.

Propylite, 233.

Sun-cracks, 294.
Apatite, 228.
Currents, marine, 283. cosmical Aspects of, Llandovery group, upper, Pumice, 234.

Superposition, order of,
Archean rocks, 327. Delessite, 228.

213; dynamical, 213, 334.

Purbeck group, 334, 856. 295, 321, 325.
Arenig group, 331.
Deltas, 278, 31
239; structural, 213, London Clay, 861. Quartz, 227.

Switzerland, ge of,
Atmosphere, 220, 265. Denudation, 872.

291; paleontological, Longmynd group, 329. Quartz-porphyry, 238. 353, 359, 362, 363, 87.
Austria, geology of, 327, Deronian System, 340. 213, 319; stratigra- Ludlow group, 335. Quartz-rock, 237.

Syenite, 23.
361
Diabase, 235.

phical, 13, 325; physio- | Man, antiquity of, 368. Quaternary deposits, 865. Table-lands, 374
Augite, 228.
Diorite, 235, 309.
graphical, 213, 370. dan as geological Rain, 267.

Talc, 228.
Bagshot group, 361. Dip, 298.

Germany, geology of, 328,

agent, 291.
Rain-prints, 294.

Tarannon Shale, 334.
Bala group, 332
Dolomite, 228, 232.

338, 339, 841, 350, 351, Mangrove swamps, 289. Raised beaches, 266. Tertiary systems, 360.
Basalt, 235.
Dust-showers, 266.
358, 357, 358, 363, 366. Marlstone, 854.

Recent period, 368, 370. Thanet Sand, 361.
Bath Oolites, 355.
Dykes, 311.
Glacial drift, 365.

May Hill Sandstones, 834. Rhætic beds, '352, 368. Tides, 283.
Belgium, geology of, 31, Dynamical geology, 213,
periods, 219. Menevian group, 329. Ripple-mark, 293.

Tiil, 367.
350, 858.

239.

Glaciation of Europe, 866; Metamorphic rocks, 314, Rivers, 272, 873 ; geolo- Tourmaline, 228
Dlown sand, 237.

Earth, planetary relations of America, 369. Metamorphism, 263, 315, gical action of, 273, Trachyte, 234.
Bohemia gcology of, 330, of, 214; forin of, 215; Glaciers, 281.

372.

frozen, 280.

Trees, influence of, in
movements of, 216; Gneiss, 236.

Rocks, characters of, 229; ! geology, 289.
Boulder-clay, 367.
stability of axis of, Granite, 230, 233, 308. Mica-schist, 231-236.

Mica, 228.

crystalline, 229, 230, Tremadoc Slates, 330.
Bracklesham beds, 861. 216; changes in centre Graphite, 228.

Millstone Grit, 319.

231, 807; fragıpental, Thassic Systero, 852.
Bradford Clay, 304, 355. of gravity of, 217; crust Greenland, 359, 363, 364. Mineral veins, 817.

229, 231, 235, 292; Tuffs, 239.
Breccia, 237.
of, 222, 227, 971; in- Greensand, 357.

Miocene rocks, 802.

microscopic structure | Unconformability, 318.
Bricain, geology of, 827, terior of, 223, 371 ; Greywacke, 231, 287. Molasse, 363.

of, 220; schistose, 235, United States, geolony
328, 331, 340, 342, 846, Internal heat of, 223; Gulf-stream, influence of, Mountains, origin of, 258, 314; volcanic fraginen. of, 328, 331, 839, 845,
351, 352, 351, 357, 360,
age of, 226.
in climate, 219.
370, 871, 374.

tal, 239, 242; permea- $50, 352, 354, 337, 350,
362, 361.
Earthquakes, 254.
Hail, 280.

Neocomian series, 857, bility of, 267; stratified, 864, 369, 372, 373.
Brooks, 272.
Ecliptic, change in ob- Harlech group, 329.

359.

292, 370; joints of, Upheaval, movements of,
Bunter group, 352, 353. liquity of, 216.

Haüyne, 228.
Nepheline, 228.

297; inclination of 298; 255, 321.
Cainozoic formations, 360. Eocene formations, 360. Heat, effects of, on rocks, Nosean, 228.

curvature of, 298. Veins of intrusion, 811.
Calciferous Sandstones, Eozoon, occurrence of,828. 1

258, 262.
Obsidian, 230, 234.
Rock-salt, 228.

Volcanic action, causes
346.
Epidote, 228.

Hippurite limestone, 358. Oceanic circulation, 284. Rocky Mountains, struc-
Calcite, 228
Epigene or surface action, Homotaxis, 322, 327. Oceans, 221, 282.

ture of, 372, 373.. Volcanoes, 223, 240; pro-
Cambrian System, 329.

263.
Hornblende, 228.
Oll-shale, 238.

Russia, geology of, 338, ducts of, 241; action
Canada, geology of, 328 Egninoxes, precession of, Human period, 368. Old Red Sundstone, 340, 342, 367.

of, 243; fissures at,
339, 345, 350, 354, 369.

216.
Huron, lake, 828, 370.

342.
Sandstone, 237.

244; explosions of,
Caradoc group, 332. Europe, gcology of, 827, Hypogene action, 240. vine, 228.

Scandinavia, geolo ef, 24; showers of stones
Carboniferous Limestone, 330, 338, 841, 850, 851, Ice, geological action of, Oolites, Lower or Bath, 331, 338, 858 366, 367. from, 245; geological
317.
353, 356, 358, 362, 363, 281, 366.

355 ; Middle or Oxford, Sen, geologica: erations effects of, 249; mud
System, 346.
364, 865.

Ice-cap, effects of polar, 256; Upper or Port- of, 284; deposits in, from, 250 ; structure
Carrara marble, 231, 232. Excavation by rivers, 275.

217, 219.
land, 856.
287, 290.

of, 251; submarina,
Caverns, 271, 320, 369. Expansion, effects of, 264. Igneous rocks, structures Overlap, 295.

Sea-water, composition of, 252; distribution of
Chalk, 258, 359, 360. False bedding, 292.

of, 307.
Oxford Clay, 354, 356.

in space and time,
Chlorite, 228
Faults, 261, 301, 372. Interbedded rocks, 313. Palæontological geology, Secondary or Mesozolc

252 ;

vapours from
Clays, 237.
Fjords, 257.
Interglacial periods, 220, 213, 319:

systems, 852

241, 251.
Clay-slate, 238
Flint, 238.

367.
Palæozoic formations, 328. Serpentine, 228, 232

Watei as a geological
Cleavage, 261, 306, 372. Fluor-spar, 228
Intrusive rocks, 307, 310. Pearlstone, 231.

Siderita, 228.

agent, 267; subter-
Clif-debris, 237.
Foliation, 315.
Iron, oxides of, 228, 232. Peat, 238, 290, 819. Silurian System, 331.

Tunean, 269; Influence
affected by Foraminiferal ooze, 290. Joints, 297.

Permian System, 351. Snow, 280.

of, in hypogeno
movements, Forest marble, 355. Jurassic System, 854 Phonolite, 234.

Soil, 265

change, 262.
218; by ocean, 285; Formations, table of geo- Kellaways rock, 854, 856. Physiographical geology, Speeton Clay, 357. Wares, 285.
by man, 291; indicated logical, 327.
Keuper group, 352, 853. 213, 370.

Springs, 270; hot, 223 Wealden series, 357, 359.
by fossils, 321.

Fossils, use of in geology, Kimeridge Clas, 354, 356. Pitchstone, 231, 234. Stonesfield slate, 354, 353. Weathering, 265, 268.

296, 321, 324 325; Lakes, 279, 319, 374; Planets, relative densities Strate, 293, 370; alterna- Wenlock group. 338.
Coal-measures, 349.
nature of, 319; how frozen, 280.

of, 214.

tions of, 295; persis- Woolwich and Rrading
Colonles, doctrino of, 823. preserved, 319; relative Laminæ of rocks, 293. Plante, geological opere- tence of, 205; groups

beds, 161.
Coneretions in rock, 29 1. valne of, in geology, Laurentian rocks, 827. tions of, 237, 319.

of, 296, 327.

Zeolites, 228.
Conglomcratc, 287.
320; relative age of 321. Lavas, 242, 246

Pleistoceac deposits, 363, ! Stratification, 292. Zircon, 228.

of, 253

221.

Climate,

earth's

Coal, 238.

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