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Fast VIL—Straticraphical Geology
This branch of the science arranges the rocks of th* earth's crust in the order of their appearance, and interprets the sequence of events of which they form the records. Its province is to cull from the other departments of geology the facts which may be needed to show what has been the progress of our planet, and of each continent and country, from the earliest times of which the rocks have preserved any memorial. Thus from mineralogy and petrography It contains information regarding the origin and subsequent mutations of minerals and rocks. From dynamical geology it learns by what agencies the materials of the earth's crust have been formed, altered, broken, upheaved and melted. From geotectonic geology it understands the various processes whereby these materials were put together so as to build up the complicated crust of the earth. From palaeontological geology it receives in well-determined fossil remains a clue by which to discriminate the different stratified formations, and to trace the grand onward march of organized existence upon this planet. Stratigraphical geology thus gathers up the sum of all that is made known by the other departments of the science, and makes it subservient to the interpretation of tbe geological history of the earth.
The leading principles of stratigraphy may be summed up as follows:
1. In every stratigraphical research the fundamental requisite is to establish the order of superposition of the strata. Until this ts accomplished it is impossible to arrange the dates, and make out the sequence of geological history.
2. The stratified portion of the earth's crust, or what has been called the " geological record," can be subdivided into natural groups, or series of strata, characterized by distinctive organic remains and recognizable by these remains, in spite of great changes in lithological character from place to place. A bed, or a number of beds, linked together by containing one or more distinctive species or genera of fossils is termed a sone or horizon, and usually bears the name of one of its more characteristic fossils, as the Planorbis-zone of the Lower Lias, which is so called from the prevalence in it of the ammonite PsUoceras planorbis. Two or more such zones related to each other by the possession of a number of the same characteristic species or genera have been designated beds or an assise. Two or more sets of beds or assises similarly related form a group or stage; a number of groups or stages make a series, formation or section, and a succession of formations may be united into a system.
3. Some living species of plants and animals can be traced downwards through the more recent geological formations; but the number which can be so followed grows smaller as the examination is pursued into more ancient deposits. With their disappearance other species or genera present themselves which are no longer living. These in turn may be traced backward into earlier formations, till they too cease and their places are taken by yet older forms. It is thus shown that the stratified rocks contain the records of a gradual progression of organic forms. A species which has once died out does not seem ever to have reappeared.
4. When the order of succession of organic remains among the stratified rocks has been determined, they become an invaluable guide in the investigation of the relative age of rocks and the structure of the land. Each zone and formation,being characterized by its own species or genera, may be recognized by their means, and the true succession of strata may thus be confidently established even in a country wherein the rocks have been shattered by dislocation, folded, inverted or metamorphosed.'
5. Though local differences exist in regard to the precise zone in which a given species of organism may make its first appearance, the general order of succession of the organic forms found in the rocks is never inverted. The record is nowhere complete in any region, but the portions represented, even though extremely imperfect, always follow each other in their proper chronological order, unless where disturbance of the crust has intervened to destroy the original sequence.
6. The relative chronological value of the divisions of the
geological record is not to be measured by mere depth of strata. While U may be reasonably assumed that, in general, a great thickness of stratified rock must mark tbe passage of a long period of time, it cannot safely be affirmed that a much less thickness elsewhere must represent a correspondingly diminished period. The need for this caution may sometimes be code evident by an unconformability between two sets of rocks, as has already been explained. The total depth of both groups together may be, say xooo ft. Elsewhere we may find a single unbroken formation reaching a depth of 10,000 ft.; but it wedd be unwarrantable to assume that the latter represents ten times the length of time indicated by the former two. So far trots this being the case, it might not be difficult to show that the minor thickness of rock really denotes by far the longer geological interval. If, for instance, it could be proved that the upper part of both the sections lies on one and the same geological platform, but that the lower unconformable series in the one locality belongs to a far lower and older system of rocks than the base of the thick conformable series in the other, then it would be clear that the gap marked by the unconformability really indicates a longer period than the massive succession of deposzu.
7. Fossil evidence furnishes the chief means of comparing the relative value of formations and groups of rock. A "break la the succession of organic remains," as already explained, marks an interval of time often unrepresented by strata at the place where the break is found. The relative importance of these breaks, and therefore, probably, the comparative intervals of time which they mark, may be estimated by the difference of the fades or general character of the fossils on each side. If, for example, in one case we find every species to be Ibwmjp above and below a certain horizon, while in another locality oelj half of the species on each side are peculiar, we naturally infer, if the total number of species seems large enough to warrant the inference, that the interval marked by the former break was much longer than that marked by the second. But we may go further and compare by means of fossil evidence the relation between breaks in the succession of organic remains and the depth of strata between them.
Three formations of fossiferous strata, A. C. and H, may ocear conformably above each other. By a comparison of the fowl contents of all parts of A, it may be ascertained that, while sane species are peculiar to its tower, others to its higher portions, yet the majority extend throughout the formation. If now it is found tbst of the total nu mber of species in the upper portion of A only ooe-tkird passes up into C, it may be inferred with some plausibility that thr time represented by the break between A and C was really fce$er than that required tor the accumulation of the whole of the forma ma A. It might even be possible to discover elsewhere a thick intermediate formation B filling up the gap between A and O la has manner were it to be discovered that, whilethe whole of thefoncstioa C is characterized by a common suite of fossils, not one of the species and only one half of the genera pass up into H, the inference coud hardly be resisted that the gap between the two formatioos icarks the passage of a far longer interval than was needed for the drpc«i:x« of the whole of C. And thus we reach the remarkable coartuskm that, thick though the stratified formations of a country may be, in some cases t hey may not represent so long a total period of time as do the gaps in their succession,—in other words, that noo-oVpontion was more frequent and prolonged than deposit km. or that tbe intervals of time which have been recorded by strata have not beet so long as those which have not been so recorded.
In all speculations of this nature, however, it is necessary to reason from as wide a basis of observation as possible, seexnf that so much of the evidence is negative. Especially needful is it to bear in mind that the cessation of one or more species at a certain line among the rocks of a particular district may mean nothing more than that, onward from the time marked by that line, these species, owing to some change in the conditions of life, were compelled to migrate or became locally extinct or. from some alteration in the conditions of fossilixation, were fto longer imbedded and preserved as fossils. They may have continued to flourish abundantly in neighbouring districts for a long period afterward. Many examples of this obvious truth might be cited. Thus in a great succession of misprd marine, brackish-water and terrestrial strata, like that of the Carboniferous Limestone scries of Scotland, corals, crinoids and bracbiopods abound in the limestones and accompanying shales, but disappear as the sandstones, ironstones, clays, coals end bituminous shales supervene. An observer meeting for the first time with an instance of this disappearance, and remembering what he had read about breaks in succession, might be tempted to speculate about the extinction of these organisms, and their replacement by other and later forms of life, such as (he ferns, Iycopods, cstuarine or fresh-water shells, ganoid fishes and other fossils so abundant in the overlying strata. But further research would show him that high above the plantbearing sandstones and coals other limestones and shales might be observed, once more charged with the same marine fossils as before, and still farther overlying groups of sandstones, coals and carbonaceous beds followed by yet higher marine limestones. He would thus learn that the same organisms, after being locally exterminated, returned again and again to the same area. After such a lesson he would probably pause before too confidently asserting that the highest bed in which we can detect certain fossils marks their final appearance in the history of life. Some breaks in the succession may thus be extremely local, one set of organisms having been driven to a different part of the same region, while another set occupied their place until the first was enabled to return.
8. The geological record is at the best but an imperfect chronicle of the geological history of the earth. It abounds in gaps, some of which have been caused by the destruction of strata ov. ing to metamorphism, denudation or otherwise, others by original non-deposition, as above explained. Nevertheless from this record alone can the progress of the earth be traced. It contains the registers of the appearance and disappearance of tribes of plants and animals which have from time to time flourished on the earth. Only a small proportion of the total number of species which have lived in past time have been thus chronicled, yet by collecting the broken fragments of the record an outline at least of the history of life upon the earth can be deciphered.
It cannot be too frequently stated, nor too prominently kept in view, that, although gaps occur in tbe succession of organic remains as recorded in the rocks, they do not warrant the conclusion that any such blank intervals ever interrupted the progress of plant and animal life upon the globe. There is every reason to believe that the march of life has been unbroken, onward and upward. Geological history, therefore, if its records in the =iratified formations were perfect, ought to show a blending and gradation of epoch with epoch. But the progress has been constantly interrupted, now by upheaval, now by volcanic outbursts, now by depression. These interruptions serve as natural divisions in the chronicle, and enable the geologist to arrange his' history into periods. As the order of succession among stratified rocks was first made out in Europe, and as many of the gaps in that succession were found to be widespread over the European area, the divisions which experience established for that portion of the globe came to be regarded as typical, and the names adopted for them were applied to the rocks of other and far distant regions. This application has brought out the fact that some of tbe most marked breaks in the European series do not exist elsewhere, and, on the other hand, that some portions of that series are much more complete than the corresponding sections in other regions. Hence, while the general similarity of succession may remain, different subdivisions and nomenclature are required as we pass from continent to continent.
Tbe nomenclature adopted for the subdivisions of the geological record bears witness to the rapid growth of geology. It is a r^tch-work in which no system nor language has been adhered to, but where the influences by which the progress of the science has been moulded may be distinctly traced. Some of the earliest names are titbological, and remind us of the fact that mineralogy ind petrography preceded geology in the order of birth—Chalk, Oolite, Green sand, Millstone Grit Others are topographical, sod often recall the labours of the early geologists of England— London Clay, Oxford Gay, Purbeck, Portland, Kimmcridgc beds. Others are taken from local English provincial names, and
remind us of the debt we owe to William Smith, by whom so many of them were first used—Lias, Gault, Crag, Cornbrash. Others of later date recognize an order of superposition as already established among formations—Old Red Sandstone, New Red Sandstone. By common consent it is admitted that names taken from the region where a formation or group of rocks is typically developed arc best adapted for general use. Cambrian, Silurian, Devonian, Permian, Jurassic are of this class, and have been adopted all over the globe.
But whatever be the name chosen to designate a particular group of strata, it soon comes to be used as a chronological or homotaxial term, apart altogether from the stratigraphical character of the strata to which it is applied. Thus we speak of the Chalk or Cretaceous system, and embrace under that term formations which may contain no chalk; and we may describe as Silurian a series of strata utterly unlike in lithological characters to the formations in the typical Silurian country. In using these terms we unconsciously allow the idea of relative date to arise prominently before us. Hence such a word as "chalk " or " cretaceous " does not suggest so much to us the group of strata so called as the interval of geological history which these strata represent. Wc speak of the Cretaceous, Jurassic, and Cambrian periods, and of the Cretaceous fauna, the Jurassic flora, the Cambrian trilobites, as if these adjectives denoted simply epochs of geological time.
Tbe stratified formations of the earth's crust, or geological record, are classified into five main divisions, which in their order of antiquity are as follows: (1) Archcan or Pre-Cambrian, called also sometimes Azoic (lifeless) or Eozoic (dawn of life); (2) Palaeozoic (ancient life) or Primary; (3) Mesozoic (middle life) or Secondary; (4) Cainozoic (recent life) pr Tertiary; (5) Quaternary or Post-Tertiary. These divisions are further ranged into systems, formations, groups or stages, assises and zones. Accounts of the various subdivisions named are given in separate articles under their own headings. In order, however, that the sequence of tbe formations and their parallelism, in Europe and North America may be presented together*a stratigraphical table is given on next page.
Part VIII.—Physiocraphical Geology
This department of geological inquiry investigates the origin and history of the present topographical features of the land. As these features must obviously be related to those of earlier time which are recorded in the rocks of the earth's crust, they cannot be satisfactorily studied until at least the main outlines of the history of these rocks have been traced. Hence physiographical research comes appropriately after the other branches of the science have been considered.
From the stratigraphy of the terrestrial crust we learn that by far the largest part of the area of dry land is built up of marine formations; and therefore that the present land is not an aboriginal portion of the earth's surface, but has been overspread by the sea in which its rocks were mainly accumulated. We further discover that this submergence of the land did not happen once only, but again and again in past ages and in all parts of the world. Yet although the terrestrial areas varied much from age to age in their extent and in their distribution, being at one time more continental, at another more insular, there is reason to believe that these successive diminutions and expansions have on the whole been effected within, or not far outside, the limits of the existing continents. There is no evidence that any portion of the present land ever lay under the deeper parts of the ocean. The abysmal deposits of the oceanfloor have no true representatives among the sedimentary formations anywhere visible on the land. Nor, on the other hand, can it be shown that any part, of the existing ocean abysses ever rose above sea-level into dry land. Hence geologists have drawn the inference that the ocean basins have probably been always where they now arc; and that although the continental areas have often been narrowed by submergence and by denudation, there has probably seldom or never been a complete disappearance of land. The fact that the sedimentary formations of each 'successive geological period consist to so large an extent of mechanically formed terrigenous detritus, affords good evidence of the coexistence of tracts of land as well as of extensive denudation.
Senonian—Upper Chalk with Flints of England; Aturian and Km Siberian Stages 00 the F.uropean continent.
Turoeian—Middle Chalk with few
Albitn— Upper Greeroand and Gautt. AptiaB — Lover Greenland; Marls and
limestones of Prmence, ffcc Urfonian (Barrerr^n)—AtherfVHd day;
massive H.; puriie limestones of
souihern Frarxe. Neocomian—Wistrl day and Hastings
sand: Kauienvian and Valanginian
sub-stages of S'tuerund and France.
PurbecLua—Purbcck beds; Munder Meigd; largely preseat in West
P • •: —Portland group oi England, represented in 5. France by the thick Tithooian limrstonea.
ftmmcrtdgvan—Kimmeridge Clay of England; Wrgulian and Plerocrrian groups of N. France; represented by thick limestones in the Mediterranean
Similar to Ibe
As in Europe, it is hardly possible to assign a definite chronological place to each of >he various deposits of tnis period, terrestrial and marine They gcnt-nlly resemble the 'European series- The characteristic marine, fluriatiic and lao^trine terraces, whkh overlie the older drifts, havs been classed as the Champlam Group
On ibe Atlantic border represented by the marine Flondoa series; in the interior by a sunacrial and lacustrine series; and on the Pacific border by the thick marine ■cries of San Francisco.
a marine series
On the Atlantic border no equivalents have been satisfactorily recognised, but on the Patifk side there are marine deposits in X W. Oregon, *hkh may represent this division. In the interior the equivalent is believed to be the (rrrh-waier White River series, indiding (1) PrUoctrei beds, (1) Or«dr« beds, and O) TitttMctiunvm beds.
Woodstock and Aquia Creek groups of Potomac River; Vicl^borg, Jackson, Claiboror. Buhmone, and Lignitic groups of Mississippi
In the interior a ink* scries of fresh water forma 11 uos, comprising, in descroding order the Uinta, Bndger^ Wind River, Wasatch, Torrejon,
. oi Oregon and
Oa the .Atlantic border bo?h
taining a Icrmtrial flora represent the Cretaceous series
la the interior there is also a commingling of marine with lacustrine drposara. At the top Iks the Laramie or lagmtic series with an abundant terrestrial flora, passing down into the lacustrine and brackish-water Montana series. Of older date, the Colorado series contains an abund>nt marine fauna, yet includes also some coal-seams The Niobrara marls and limestones are likewise of marine origin, but the lower members of the series (Benton and Dakota) show another great representation of freshwater se-J 1 mentation with lignites
In California a vast succession of marine deposits (ShastaChico) represents the CrtLicccms and in western
British S America coal-seams alr-o occur.
Representatives of the Middle and lo»er -Jurassic lorroatiocH have been found in California and Oregon, and far.her north among the Arctic
Strata coctaming Lower Jurassic marine fossil* appear in Wyoming and Dalofa; and above them come the ACaiivreunu and Bapiin#4a* beds.
From these general considerations we proceed to inquire how the existing topographical features of the land arose. Obviously the co-operation of the two great -geological agencies of hypogene and epigene energy, which have been at work from the beginning of our globe's decipherable history, must have been the cause to which these features are to be assigned; and the task of the geologist is to ascertain, if possible, the part that has been taken by each. There is a natural tendency to see in a stupendous piece of scenery, such as a deep ravine, a range of hills, a line of precipice or a chain of mountains, evidence only of subterranean convulsion; and before the subject was taken up as a matter of strict scientific induction, an appeal to former cataclysms was considered a sufficient solution of the problems presented by such features of landscape. The rise of the modern Huttonian school, however, led to a more careful examination of these problems. The important share taken by erosion in the determination of the present features of landscape was then recognized, while a fuller appreciation of the relative parts played by the hypogene and epigene causes has gradually been reached.
1. The study of the progress of denudation at the present time has led to the conclusion that even if the rate of waste were not more rapid than it is to-day, it would yet suffice in a comparatively brief geological period to reduce the dry land to below the sea-level. But not only would the area of the land be diminished by denudation, it could hardly fail to be more or less involved in those widespread movements of subsidence, during which the thick sedimentary formations of the crust appear to have been accumulated. It is thus manifest that there must have been from time to time during the history of our globe upward movements of the. crust, whereby the balance between land and sea was redressed. Proofs of such movements have been abundantly preserved among the stratified formations. We there learn that the uplifts have usually followed each other at long intervals between which subsidence prevailed, and thus that there has been a prolonged oscillation of the crust over the great continental areas of the earth's surface.
An examination of that surface leads to the recognition of two great types of upheaval. In the one, the sea-floor, with all its thick accumulations of sediment, has been carried upwards, sometimes for several thousand feet, so equably that the strata retain their Original flatness with hardly any sensible disturbance lor hundreds of square miles. In the other type the solid crust has been plicated, corrugated and dislocated, especially along particular lines, and has attained its most stupendous disruption in lofty chains of mountains. Between these two phases of uplift many intermediate stages have been developed, according to Ihe direction and intensity of the subterranean force and the buying nature and disposition of the rocks of the crust.
(a) Where the uplift has extended over wide spaces, without appreciable deformation of the crust, the flat strata have given rise to low plains, or if the amount of uprise has been great enough, to high plains, plateaux or tablelands. The plains of Russia, for example, lie for the most part on such tracts of equably uplifted strata. The great plains of the western interior o£ the United States form a great plateau or tableland, 5000 or fooo ft. above the sea. and many thousands of square miles in "tent, on which the Rocky Mountains have been ridged up.
(&) It is in a great mountain-chain that the complicated Mructures developed during disturbances of the earth's crust can best be studied (sec Parts IV. and V. of this article), and "here the influence of these structures on the topography of the sarftceis most effectively displayed. Such a chain may be the result of one colossal disturbance; but those of high geological antiquity usually furnish proofs of successive uplifts with more 01 less intervening denudation. Formed along lines of continental displacement Id the crust, they have again and again given
relief from'the strain of compression by fresh crumpling, fracture and uprise. . The chief guide in tracing these successive stages of growth is supplied by unconformability. If, for example, a mountain-range consists of upraised Silurian rocks, upon the upturned and denuded edges of which the Carboniferous Limestone lies transgressively, it is clear that its original upheaval must have taken place in the period of geological time represented by the interval between the Silurian and the Carboniferous Limestone formations. If, as the range is followed along its course, the Carboniferous Limestone is found to be also highly inclined and covered unconformably by the Upper Coal-measures, a second uplift of that portion of the ground can be proved to have taken place between the time of the Limestone and that of the Upper Coal-measures. By this simple and obvious kind of evidence the relative ages of different mountain-chains may becompared. In most great chains, however, the rocks have been so intensely crumpled, and even inverted, that much labour may be required before their true relations can be determined.
The Alps furnish an instructive example of the long scries of revolutions through which a great mountain-system may have passed before reaching its present development. The first beginnings of the chain may have been upraised before the oldest Palaeozoic formations were laid down. There are at least traces of land and shore-lines in the Carboniferous period. Subsequent submergences and uplifts appear to have occurred during the Mesozoic periods. There is evidence that thereafter the whole region sank deep under the sea, in which the older Tertiary sediments were accumulated, and which seems to have spread right across the heart of the Old World. But after the deposition of the Eocene formations came the gigantic disruptions whereby all the rocks of the Alpine region were folded over each other, crushed, corrugated, fractured and displaced, some of their older portions, including the fundamental gneisses and schists, being squeezed up, torn off, and pushed horizontally for many miles over the younger rocks. But this upheaval, though the most momentous, was not the last which the chain has undergone, for at a later epoch in Tertiary time renewed disturbance gave rise to a further scries of ruptures and plications. The chain thus successively upheaved has been continuously exposed to denudation and has consequently lost much of its original height. That it has been left in a state of instability is indicated by the frequent earthquakes of the Alpine region, which doubtless arise from the sudden snapping of rocks under intense strain.
A distinct type of mountain due to direct hypogene action is to be seen in a volcano. It has been already pointed out (Part IV. sect. 1) that at the vents which maintain a communication between the molten magma of the earth's interior and the surface, eruptions take place whereby quantities of lava and fragmentary materials arc heaped round each orifice of discharge. A typical volcanic mountain takes the form of a perfect cone, but as it grows in size and its main vent is choked, while the sides of the cone arc unable to withstand the force of the explosions or the pressure of the ascending column of lava, eruptions take place laterally, and numerous parasitic cones arise on the flanks of the parent mountain. Where lava flows out from long fissures, it may pile up vast sheets of rock, and bury the surrounding country under several thousand feet of solid stone, covering many hundreds of square miles. In this way volcanic tablelands have been formed which, attacked by the denuding forces, arc gradually trenched by valleys and ravines, until the original level surface of the lava-field may be almost or wholly lost. As striking examples of this physiographical type reference may be made to the plateau of Abyssinia, the Ghats of India, the plateaux of Antrim, the Inner Hebrides and Iceland, and the great lava-plains of the western territories of the United States.
2. But while the subterranean movements have upraised portions of the surface of the lithosphere above the level of the ocean, and have thus been instrumental in producing the existing tracts of land, the detailed topographical features of a landscape are not solely, nor in general even chiefly, attributable to these movements. From the time that any portion of the sea-floor appears above sea-level, it undergoes erosion by the various epigene agents. Each climate and geological region has its own development of these agents, which include air, aridity, rapid and frequent alternations of wetness and dryness or of heat and cold, rain, springs, frosts, rivers, glaciers, the sea, plant and animal life. In a dry climate subject to great extremes of temperature the character and rate of decay will differ from those of a moist or an arctic climate. But it must be remembered that, however much they may vary in activity and in the results which they effect, the epigene forces work without intermission, while the hypogene forces bring about the upheaval of land only after long intervals. Hence, trifling as the results during a human life may appear, if we realize the multiplying influence of time we are led to perceive that the apparently feeble superficial agents can, in the course of ages, achieve stupendous transformations in the aspect of the land. If this efficacy may be deduced from what can be seen to be in progress now, it may not less convincingly be shown, from the nature of the sedimentary rocks of the earth's crust, to have been in progress from the early beginnings of geological history. Side by side with the various upheavals and subsidences, there has been a continuous removal of materials from the land, and an equally persistent deposit of these materials under water, with the consequent growth of new rocks. Denudation has been aptly compared to a process of sculpturing wherein, while each of the implements employed by nature, like a special kind of graving tool, produces its own characteristic impress on the land, they all combine harmoniously towards the achievement of their one common task. Hence the present contours of the land depend partly on the original configuration of th- ground, and the influence it may have had in guiding the operations of trie erosive agents, partly on the vigour with which these agents perform their work, and partly on the varying structure and powers of resistance possessed by the rocks on which the erosion is carried on.
Where a new tract of land has been raised out of the sea by such an energetic movement as broke up the crust and produced the complicated structure and tumultuous external forms of a great mountain chain, the influence of the hypogene forces on the topography attains its highest development. But even the youngest existing chain has suffered so greatly from denudation that the aspect which it presented at the lime of its uplift can only be dimly perceived. No more striking illustration of this feature can be found than that supplied by the Alps, nor one where the gcotectonic structures have been so fully studied in detail. On the outer flanks of these mountains the longitudinal ridges and valleys of the Jura correspond with lines of anticline and synclinc. Yet though the dominant topographical elements of the region have obviously been produced by the -plication of the stratified formations, each ridge has suffered so large an amount of erosion that the younger rocks have been removed from its crest where the older members of the series arc now exposed to view, while on every slope proofs may be seen of extensive denudation. If from these long wave-like undulations of the ground, where the relations between the disposition of the rocks below and the forms of the surface are so dearly traceable, the observer proceeds inwards to the main chain, he finds that the plications and displacements of the various formations assume an increasingly complicated character; and that although proofs of great denudation continue to abound, it becomes increasingly difficult to form any satisfactory conjecture as to the shape of the ground when the upheaval ended or any reliable estimate of the amount of material which has since then been removed. Along the central heights the mountains lift themselves towards the sky like the storm-swept crests of vast earth-billows. The whole aspect of the ground suggests intense commotion, and the impression thus given is often much intensified by the twisted and crumpled strata, visible from a long distance, on the crags and crests. On this broken-up surface the various agents of
denudation have been ceaselessly engaged since it erjerr^i from the sea. They have excavated valleys, sometimes alc~g depressions provided for them by the subterranean disturbances, sometimes down the slopes of the disrupted blocks of grorad So powerful has been this erosion that valleys cut out alcef lines of anticline, which were natural ridges, have sometaas become more important than those in lines of synchae, wfces were structurally depressions. The same subaexiai forces fait eroded lake-basins, dug out corrics or cirques, notched the ridges, splintered the crests and furrowed the slopes, leaving no part of the original surface of the uplifted dais unmodified.
It has often been noted with surprise that features cf underground structure which, it might have been cccnde^Ir anticipated, should have exercised a marked influence oa tie topography of the surface have not been able to resat the levelling action of the .denuding agents, and do not now afeet the surface at all. This result is conspicuously seen in coal-&e& where the strata are abundantly traversed by faults. Taese dislocations, having sometimes a displacement of several hundred feet, might have been expected to break up the surface c;i> a network of cliffs and plains; yet in general they do not raoery the level character of the ground above. One of the ast remarkable faults in Europe is the great thrust which bocacs the southern edge of the Belgian coal-field and briars tie Devonian rocks above the Coal-measures. It can be tr*crd across Belgium into the Boulonnais, and may not improbabif run beneath the Secondary and Tertiary rocks of the sootfe of England. It is crossed by the valleys of the Meuse and other northerly-flowing streams. Yet so indistinctly is it marked in the Meuse valley that no one would suspect its existence tea any peculiarity in the general form of the ground, and eves ta experienced geologist, until he had learned the structure of the district, would scarcely detect any fault at all.
Where faults have influenced the superficial topograph?, it is usually by giving rise to a hollow along which the suhaecal agents and especially running water can act jeffectrrery. Sack a hollow may be eventually widened and deepened intoavaHry On bare crags and crests, lines of fault arc apt to be marked by notches or clefts, and they thus help to produce the pinnaces and serrated outlines of these exposed uplands.
It was cogently enforced by Hutton and Playfair, and independently by Lamarck, that no co-operation of nadeg-Jwsi agency is needed to produce such topography as may be sers in a great part of the world, but that if a tract of sca-nooc were upraised into a wide plain, the fall of rain and the drculxL&a of water over its surface would in the end carve out such a system of hills and valleys as may be seen on the dry land now. So such plain would be a dead-level. It would have bec^saiktes on its surface which would serve as channels to guide the drairare from the first showers of rain- And these channels wodc be slowly widened and deepened until they would becon* ravisa and valleys, while the ground between them would be lei: projecting as ridges and hills. Nor would the erosion of such a systca of water-courses require a long series of geological periods :« its accomplishment. From measurements and estimates ci ihe amount of erosion now taking place in the basin of the Mississippi river it has been computed that valleys 800 ft_ deep Eight te carved out in less than a million years. In the vast tarJeiinds of Colorado and other western regions of the United States tz impressive picture is presented of the results of mere subaeri*J erosion on undisturbed and nearly level strata. Systems a st ream-courses and valleys, river gorges unexampled dsevisrrr in the world for depth and length, vast winding lines o» escarpment, like ranges of sea-cliffs, terraced slopes rising from pizieato plateau, huge buttresses and solitary stacks standing !Tir islands out of the plains, great mountain-masses toweriog tzi. picturesque peaks and pinnacles deft by innumerable gu3*s yet everywhere marked by the parallel bars of tbe borj^r-'J strata out of which they have been carved—these are the order!? symmetrical characteristics of a country where the scmer> ss due entirely to the action of subaerial agents on the one haad and