Online Encyclopedia

PART VIII

Online Encyclopedia
Originally appearing in Volume V11, Page 674 of the 1911 Encyclopedia Britannica.
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PART VIII.-PHYSIOGRAPHICAL 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 ocean-floor 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 are; and that although the continental areas have often been narrowed by submergence and by denudation, there has probably seldom or never been a complete The Geological Record or Order of Succession of the Stratified ,Formations of the Earth's Crust. Europe. North America. . c-... Historic, up to the present time. Similar to the European 'de- r P. Prehistoric, comprising deposits of the velopment, but with scantier t -.a E Iron, Bronze, and later Stone Ages. traces of the presence of man. s— a, Neolithic—alluvium, peat, lake-dwell- h' 00x ings, loess, &c: Palaeolithic— river-gravels, cave-de- posits, &c. o Ta Older Loess and valley-gravels; cave- As in Europe, it is hardly pos- 0. o deposits. sible to assign a definite `c ' q Strand-lines or raised beaches; youngest chronological place to each r o moraines. of the various deposits of finis Upper Boulder-clays; eskers; marine period, terrestrial and marine. sands and clays. They generally resemble the Interglacial deposits. European series. The charac- Lower boulder-clay or Till, with striated teristtc marine, fluviatile and rock-surfaces below. lacustrine terraces, which overlie the older drifts, have been classed as the Champ- lain Group. c Newer:—English Forest-Bed Group; On the Atlantic border repre- o Red and Norwich Crag; Amstelian sented by the marine Floridian c, and Scaldesian groups of Belgium series; in the interior by a _—~ and Holland; Sicilian and Astian of subaerial and lacustrine series; France and Italy. and on the Pacific border by Older :—English Coralline Crag; Dies- the thick marine series of San tian of Belgium; Plaisancian of south- Francisco. ern France and Italy. o Wanting in Britain; well developed in Represented in the Eastern France, S. E. Europe and Italy; divis- States by a marine series ible into the following groups in (Yorktown or Chesapeake, descending order: (s) Pontian; (2) Chipola and Chattahoochee Sarmatian; (3) Tortonian; (4) Hel- groups), and in the interior vetian; (5) Langhian (Burdigalian). by the lacustrine Loup Fork (Nebraska), Deep River, and John Day groups. _ o In Britain the "fluvio-marine series" of On the Atlantic border no r" o the Isle of Wight; also the volcanic equivalents have been satis- c en plateaux of Antrim and Inner Hebrides factorily recognised, but on r'o O and those of the Faeroe Isles and Ice- the Pacific side there are i '7; land. In continental Europe the marine deposits in N. W. U following subdivisions have been Oregon, which may represent established in descending order:. (s) this division. In the interior Aquitanian, (2) Stampian (Rupelan), the equivalent is believed to (3) Tnngrain (Sannoisian). be the fresh-water White River series, including (t) Prelo- ceras beds, (2) Oreodon beds, and (3) Titanothervum beds. o ii Barton sands and clays; Ludian series Woodstock and Aquia Creek N `o of France. groups of Potomac River; ' Bracklesharti Beds; Lutetian (Calcaire Vicksburg, Jackson, Clai- o grossier and Caillasses) of Paris basin. borne, Buhrstone, and Lig- u 's London clay, Woolwich and Reading nitic groups of Mississippi. Ja Beds; Thanet sands; Ypresian or In the Interior a thick series of e. Londinian of N. France and Belgium; fresh-water formations, corn- Spardacian and Thanetian groups. prising, in descending order, the Uinta, Bridger, Wind River, Wasatch, Torrejon, and Puerco groups. On the Pacific side the marine Tejon series of Oregon and California. Daman—wanting in Britain; uppermost On the Atlantic border both limestone of Denmark. marine strata and others con- Senonian —Upper Chalk with Flints of taming a terrestrial flora re- England; Atari= and Emscherian present the Cretaceous series stages on the European continent. of formations. Turonian—Middle Chalk with few In the interior there is also a flints, and comprising the Angoumian commingling of marine with and Ligerian stages. lacustrine deposits. At the Cenomanian—Lower Chalk and Chalk top lies the Laramie or Lig- sfarl. nitic series with an abundant Albian=Upper Greensand and Gault. terrestrial flora, passing down Aptian — Lower Greensand; Marls and into the lacustrine and limestones of Provence, &c. brackish-water Montana Urgonian (Barremian)—Atherfield clay; series. Of older date, the massive Hippurite limestones of Colorado series contains, an southern France. abundant marine fauna, yet Neocomian—Weald clay and Hastings includes also some coal-seams. sand; Hauterivian and Valanginian The Niobrara marls and lime- sub-stages of Switzerland and France. stones are likewise of marine origin, but the lower members of the series (Benton and Dakota) show another great representation of fresh-water sedimentation with lignites and coals. In California a vast succession of marine deposits (Shasta- Chico) represents the Cre- taceous system; and in western British N. America coal-seams also occur. Purbeckian—Purbeck beds; Mfinder Representatives of the Middle ,. Mergel; largely present in West- and lower Jurassic forma- o phalia. tions have been found in "-' Portlandian—Portland group of Eng- California and Oregon, and land, represented in S. France by the farther north among the Arctic . thick Tithonian limestones. islands. Kimmeridgian—Kimmeridge Clay of Strata containing Lower Turns- England; Virgulian and Plerocerian sic marine fossils appear in groups of N. Prance; represented by Wyoming and Dakota; and thick limestones in the Mediterranean above them come the Atlanto- ba,sin. sauces and Baptanodon beds, Europe. North America. o t; Corallian-Coral Rag, Coralline Oolite; which have yielded so large a ti Sequanian stages of the Continent, variety of deinosaurs and other comprising the sub-stages of Astartian vertebrates, and especially the and Rauracian. remains of a number of genera Oxfordian—Oxford Clay; Argovian and of small mammals. Neuvizyan stages. Calloviain—Kellaways Rock, Divesian sub-stage of N. France. Batlioaian—series of English strata from Cornbrash down to Fuller's Earth. Bajocian—Inferior Oolite of England. Liassic—divisible into (s) Upper Lias or Toarcian, (2) Middle Lias, Marl- stone or Charmouthian, (3) Lower Lias of Sineinurian and Hettangian. ° J In Germany and western Europe this In New York, Connecticut, New •g l division represents the deposits of Brunswick, and Nova Scotia inland seas or lagoons, and is divisible a series of red sandstone into the following stages in descending (Newark series) contains land- order; plants and labyrinthodonts order; (s) Rhaetic, (2) Keuper, (3) like the lagoon type of central Muschelkalk, (4) Bunter. In the and western Europe. On the eastern Alps and the Mediterranean Pacific slope, however, marine basin the contemporaneous sediment- equivalents occur, represent- ary formations are those of open clear ing the pelagic type of south- sea, in which a thickness of many eastern Europe. thousand feet of strata was accumu- la ted. v T huringian—Zechstein, Magnesian To this division of the geologi- a Limestone; named from its develop- cal record the Upper Barren ment in Thuringia; well represented Measures of the coal-fields of also in Saxony, Bavaria and Bohemia. Pennsylvania, Prince Edward Saxonian—Rothliegendes Group; Red Island, Nova Scotia and Sandstones, &c. New Brunswick have been Autuni an—where the strata present the assigned. lagoon facies, well displayed at Autun Farther south in Kansas, Texas, in France; where the marine type is and Nebraska the representa- predominant, as in Russia, the group lives of the division have an has been termed Artinskian. abundant marine fauna. v Stephanian or Uralian—represented in Upper productive Coal- p Russia by marine formations, and in measures. U central and western Europe by numer- Lower Barren measures. ous small basins containing a peculiar Lower productive Coal- flora and in some places a great variety measures. of insects. Pottsville conglomerate. .. W e s t p h a l i a n or Moscovian—Coal- Mauch Chunk shales; lime- measures, Millstone Grit. stones of Chester, St Louis, &c. C ulm or Dinantian—Carboniferous Lime- Pocono series; Kinder hook stone and Calciferous Sandstone series. limestone. ro o Devonian type. OldRed Sand- Catskill red sandstone; Old stone type. Red Sandstone type: the Yellow E 0 JFamennian. and red strata below show the Upper Frasnfan. sandstone with ' Devonian type. Halo p tychlus, Chemung Group. Bolhriolepis,&c. ,Genesee Caithness Flag- stones with M b Givetan. Osteolepus Dip- { Hamilton Group_ a Middle Eifelian. terns, Homo- Marcellus a, {Gedientzian. stews, &c. ' Cormferous Lime- Upper Lower Gedinnian. Red and purple stone. Helder= sandstones and Onondaga Lime- berg conglomerates stone. Group. ehtes Oriskany Sandstone. w i t h c p aspis, Pier- aspis, &c. Ludlow Group. Lower Helderberg Group. Upper 'I Wenlock Water-Lime. Llandovery" Niagara Shale and Limestone. Clinton Group. Medina a (Caradoc or Bala Group. Cincinnati Group. mica Lower Trenton (Ordovician) Sl Arenig Chazy Arenig Calciferous Aren U Upper or Olenus series—Tremadoc slates Upper or Potsdam series with andLingulaFlags. Olenus and Dicelocephalus Middle or Paradoxides series—Mene- fauna. vian Group. Middle or Acadian series with Lower or Olenellus series—Llanberis Paradoxides fauna. and Harlech Group, and Olenellus- Lower or Georgian.series with zone. Olenellus fauna o In Scotland, underneath the Cambrian In Canada and the Lake Olenellus group, lies unconformably Superior region of the United a mass of red sandstone and con- States a vast succession of glomerate (Torridonian) 8000 or so,000 rocks of Pre-Cambrian age ft . thick, which rests with a strong has been grouped into the unconformability on a series of coarse following subdivisions in de- B gneisses and schists (Lewisian). A scending order: (r) Keweena- o thick series of slates and phyllites lies wan, lying unconformably on E below the oldest Palaeozoic rocks in . (2) Animikie, separated by a central. Europe, with coarse gneisses strong unconformability from below. (3) Upper Huronian, (t) Lower V Huronian with an unconform- able base, (5) Goutchiching, (6) Laurentian. In the eastern part of Canada, Newfound- land, &c., and also in Mon- tana, estat sedimentary formations low Cambr an s one have been found to contain some obscure organisms. 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. 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. r. 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 for 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 the direction and intensity of the subterranean force and the varying 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 of the United States form a great plateau or tableland, 5000 or 6000 ft. above the sea, and many thousands of square miles in extent, on which the Rocky Mountains have been ridged up. (b) It is in a great mountain-chain that the complicated structures developed during disturbances of the earth's crush can best be studied (see Parts IV. and V. of this article), and where the influence of these structures on the topography of the surface is 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 or less intervening denudation. Formed along lines of continental displacement in the crust, they have again and again givenrelief 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 Lime-stone 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 be compared. 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 deter-mined. The Alps furnish an instructive example of the long series 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 series 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. I) 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 are 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 are 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, are 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 the ground, and the influence it may have had in guiding the operations of the 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 time 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 geotectonic 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 syncline. 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 are 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 clearly 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 ofdenudation have been ceaselessly engaged since it emerged from the sea. They have excavated valleys, sometimes along depressions provided for them by the subterranean disturbances, sometimes down the slopes of the disrupted blocks of ground. So powerful has been this erosion that valleys cut out along lines of anticline, which were natural ridges, have sometimes become more important than those in lines of syncline, which were structurally depressions. The same subaerial forces have eroded lake-basins, dug out corries or cirques, notched the ridges, splintered the crests and furrowed the slopes, leaving no part of the original surface of the uplifted chain unmodified. It has often been noted with surprise that features of underground structure which, it might have been confidently anticipated, should have exercised a marked influence on the topography of the surface have not been able to resist the levelling action of the denuding agents, and do not now affect the surface at all. This result is conspicuously seen in coal-fields where the strata are abundantly traversed by faults. These dislocations, having sometimes a displacement of several hundred feet, might have been expected to break up the surface into a network of cliffs and plains; yet in general they do not modify the level character of the ground above. One of the most remarkable faults in Europe is the great thrust which bounds the southern edge of the Belgian coal-field and brings the Devonian rocks above the Coal-measures. It can be traced across Belgium into the Boulonnais, and may not improbably run beneath the Secondary and Tertiary rocks of the south 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 from any peculiarity in the general form of the ground, and even an experienced geologist, until he had learned the structure of the district, would scarcely detect any fault at all. Where faults have influenced the superficial topography, it is usually by giving rise to a hollow along which the subaerial agents and especially running water can act effectively. Such a hollow may be eventually widened and deepened into a valley. On bare crags and crests, lines of fault are apt to be marked by notches or clefts, and they thus help to produce the pinnacles and serrated outlines of these exposed uplands. It was cogently enforced by Hutton and Playf air, and independently by Lamarck, that no co-operation of underground agency is needed to produce such topography as may be seen in a great part of the world, but that if a tract of sea-floor were upraised into a wide plain, the fall of rain and the circulation 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. No such plain would be a dead-level. It would have inequalities on its surface which would serve as channels to guide the drainage from the first showers of rain. And these channels would be slowly widened and deepened until they would become ravines and valleys, while the ground between them would be left projecting as ridges and hills. Nor would the erosion of such a system of water-courses require a long series of geological periods for its accomplishment. From measurements and estimates of the amount of erosion now taking place in the basin of the Mississippi river it has been computed that valleys 800 ft. deep might be carved out in less than a million years. In the vast tablelands of Colorado and other western regions of the United States an impressive picture is presented of the results of mere subaerial erosion on undisturbed and nearly level strata. Systems of stream-courses and valleys, river gorges unexampled elsewhere in the world for depth and length, vast winding lines of escarpment, like ranges of sea-cliffs, terraced slopes rising from plateau to plateau, huge buttresses and solitary stacks standing like islands out of the plains, great mountain-masses towering into picturesque peaks and pinnacles cleft by innumerable gullies, yet everywhere marked by the parallel bars of the horizontal strata out of which they have been carved—these are the orderly symmetrical characteristics of a country where the scenery is due entirely to the action of subaerial agents on the one hand and the varying resistance of perfectly regular stratified rocks on the other. The details of the sculpture of the land have mainly depended on the nature of the materials on which nature's erosive tools have been employed. The joints by which all rocks are traversed have been especially serviceable as dominant lines down which the rain has filtered, up which the springs have risen and into which the frost wedges have been driven. On the high bare scarps of a lofty mountain the inner structure of the mass is laid open, and there the system of joints even more than faults is seen to have determined the lines of crest, the vertical walls of cliff and precipice, the forms of buttress and recess, the position of cleft and chasm, the outline of spire and pinnacle. On the lower slopes, even under the tapestry of verdure which nature delights to hang where she can over her naked rocks, we may detect the same pervading influence of the joints upon the forms assumed by ravines and crags. Each kind of stone, too, gives rise to its own characteristic form of scenery. Massive crystalline rocks, such as granite, break up along their joints and often decay into sand or earth along their exposed surfaces, giving rise to rugged crags with long talus slopes at their base. The stratified rocks besides splitting at their joints are especially distinguished by parallel ledges, cornices and recesses, produced by the irregular decay of their component strata, so that they often assume curiously architectural types of scenery. But besides this family feature they display many minor varieties of aspect according to their lithological composition. A range of sandstone hills, for example, presents a marked contrast to one of limestone, and a line of chalk downs to the escarpments formed by alternating bands of harder and softer clays and shales. It may suffice here merely to allude to a few of the more important parts of the topography of the land in their relation to physiographical geology. A true mountain-chain, viewed from the geological side, is a mass of high ground which owes its prominence to a ridging-up of the earth's crust, and the intense plication and rupture of the rocks of which it is composed. But ranges of hills almost mountainous in their bulk may be formed by the gradual erosion of valleys out of a mass of original high ground, such as a high plateau or tableland. Eminences which have been isolated by denudation from the main mass of the formations of which they originally formed part are known as " outliers " or " hills of circumdenudation." Tablelands, as already pointed out, may be produced either by the upheaval of tracts of horizontal strata from the sea-floor into land; or by the uprise of plains of denudation, where rocks of various composition, structure and age have been levelled down to near or below the level of the sea by the co-operation of the various erosive agents. Most of the great tablelands of the globe are platforms of little-disturbed strata which have been upraised bodily to a considerable elevation. No sooner, however, are they placed in that position than they are attacked by running water, and begin to he hollowed out into systems of valleys. As the valleys sink, the platforms between them grow into narrower and more definite ridges, until eventually the level tableland is converted into a complicated network of hills and valleys, wherein, nevertheless, the key to the whole arrangement is furnished by a knowledge of the disposition and effects of the flow of water. The examples of this process brought to light in Colorado, Wyoming, Nevada and the other western regions by Newberry, King, Hayden, Powell and other explorers, are among the most striking monuments of geological operations in the world. Examples of ancient and much decayed tablelands formed by the denudation of much disturbed rocks are furnished by the' Highlands of Scotland and of Norway. Each of these tracts of high ground consists of some of the oldest and most dislocated formations of Europe, which at a remote period were worn down into a plain, and in that condition may have lain long submerged under the sea and may possibly have been overspread there with younger formations. Having at a much later time been raised several thousand feet above sea-level the ancient platforms XI. 22of Britain and Scandinavia have been since exposed to denudation, whereby each of them has been so deeply channeled into glens and fjords that it presents to-day a surface of rugged hills, either isolated or connected along the flanks, while only fragments of the general surface of the tableland can here and there be recognized amidst the general destruction. Valleys have in general been hollowed out by the greater erosive action of running water along the channels of drainage. Their direction has been probably determined in the great majority of cases by irregularities of the surface along which the drainage flowed on the first emergence of the land. Some-times these irregularities have been produced by folds of the terrestrial crust, sometimes by faults, sometimes by the irregularities on the surface of an uplifted platform of deposition or of denudation. Two dominant trends may be observed among them. Some are longitudinal and run along the line of flexures in the upraised tract of land, others are transverse where the drainage has flowed down the slopes of the ridges into the longitudinal valleys or into the sea. The forms of valleys have been governed partly by the structure and composition of the rocks, and partly by the relative potency of the different denuding agents. Where the influence of rain and frost has been slight, and the streams, supplied from distant sources, have had sufficient declivity, deep, narrow, precipitous ravines or gorges have been excavated. The canyons of the arid region of the Colorado are a magnificent example of this result. Where, on the other hand, ordinary atmospheric action has been more rapid, the sides of the river channels have been attacked, and open sloping glens and valleys have been hollowed out. A gorge or defile is usually due to the action of a waterfall, which, beginning with some abrupt declivity or precipice in the course of the river when it first commenced to flow, or caused by some hard rock crossing the channel, has eaten its way backward. Lakes have been already referred to, and their modes of origin have been mentioned. As they are continually being filled up with the detritus washed into them from the surrounding regions they cannot be of any great geological antiquity, unless where by some unknown process their basins are from time to time widened and deepened. In the general subaerial denudation of a country, innumerable minor features are worked out as the structure of the rocks controls the operations of the eroding agents. Thus, among comparatively undisturbed strata, a hard bed resting upon others of a softer kind is apt to form along its outcrop a line of cliff or escarpment. Though a long range of such cliffs resembles a coast that has been worn by the sea, it may be entirely due to mere atmospheric waste. Again, the more resisting portions of a rock may be seen projecting as crags or knolls. An igneous mass will stand out as a bold hill from amidst the more decomposable strata through which it has risen. These features. often so marked on the lower grounds, attain their most conspicuous development among the higher and barer parts of the mountains, where subaerial disintegration is most rapid. The torrents tear out deep gullies from the sides of the declivities. Corries or cirques are scooped out on the one hand and naked precipices are left on the other. The harder bands of rock project as massive ribs down the slopes, shoot up into prominent aiguilles, or help to give to the summits the notched saw-like outlines they so often present. The materials worn from the surface of the higher are spread out over the lower grounds. The streams as they descend begin to drop their freight of sediment when, by the lessening of their declivity, their carrying power is diminished. The great plains of the earth's surface are due to this deposit of gravel, sand and loam. They are thus monuments at once of the destructive and reproductive processes which have been in progress unceasingly since the first land rose above the sea and the first shower of rain fell. Every pebble and particle of their soil, once part of the distant mountains, has travelled slowly and fitfully to lower levels. Again and again have these materials been shifted, ever moving downward and sea-ward. For centuries, perhaps, they have taken their share in the fertility of the plains and II have ministered to the nurture of flower and tree, of the bird of the a r, the beast of . the field and of man himself. But their destiny :s still the great ocean. In that bourne alone can they find undisturbed repose, and there, slowly accumulating in massive beds, they will remain until, in the course of ages, renewed upheaval shall raise them into future land, there once more to pass through the same cycle of change. (A. GE.)
End of Article: PART VIII
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PERTABGARH PARTABGARH

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