Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/388

Rh 374 instructive example of the kind of scenery that arises where a mass of high ground has resulted from the intense cor rugation and upheaval of a complicated series of stratiﬁed and crystalline rocks, subsequently for a vast period carved by rain, frost, springs, and glaciers. We see how, on the outer ﬂanks of those mountains among the ridges of the J ura, the strata begin to undulate in long wave-like ridges, and how, as we enter the main chain, the undulations assume a more gigantic tumultuous character, until, 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. Where the strata appear along the cliffs or slopes they may often be seen twisted and crumpled on the most gigantic scale. Out of this complicated mass of material the sub- aerial forces have been ceaselessly at work since its first elevation. They have cut out valleys, sometimes along the original depressions, sometimes down the slopes. They have eroded lake-basins, dug out corries or cirques, notched and furrowed the ridges, splintered the crests, and have left no part of the original surface unmodified. But they have not eﬁaced all traces of the convulsions by which the Alps were upheaved. 3. 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 served as dominant lines along which the rain has ﬁltered, and the springs have risen, and the frost wedges have been driven. On the high bare scarps of a high mountain the inner structure of the mass is laid open, and there the system of joints 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 cm 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 rock, too, gives rise to its own characteristic form of scenery. The massive crystalline rocks, such as granite, yield each in its own fashion to the resistless attacks of the denuding forces. They are broadly marked off from the stratified rocks in which the parallel bands of the bedding form a leading feature in every cliff and bare mountain slope. Among the latter rocks also very distinctive types of surface may be observed. A range of sandstone hills, for example, presents a marked contrast to one of limestone. In the physiography of any region, the mountains are the dominant features. A true mountain chain consists of rocks which have been crumpled and pushed up in the manner already described. 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. In this way some ancient tablclands, those of Norway and of the Highlands of Scotland, for example, have been so channeled by deep fjords and glcns that they now consist of massive rugged hills, either isolated or connected along the ﬂanks. The forms of the valleys thus eroded 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 deelivity, deep, narrow, precipi- tous ravines or gorges have been excavated. The canons 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 glcns and valleys have been hol- lowed out. A gorge or deﬁle is usually due to the action of a waterfall, which, beginning with some abrupt declivity or GEOLOGY [v11. rrn's1o(:n.irn1cAL. 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, as already explained (p. 276). Lakes may have been formed in several ways. 1. By subterranean movements as, for example, during those which gave rise to mountain chains. But these hollows, unless continually deepened by subsequent movements of a similar nature would. be ﬁlled up by the sediment continually washed into them from the adjoining slopes. The numer- ous lakes in such a mountain system as the Alps cannot be due merely to this cause, unless we suppose the upheaval of the mountains to have been geologically quite recent, or that subsidence must take place continuously or periodically below each independent basin. But there is evidence that the upheaval of the lakes is not of recent date, hile the idea of perpetuating lakes by continual subsidence would demand, not in the Alps merely, but all over the northern hemisphere where lakes are so abundant, an amount of sub- terranean movement of which, if it really existed, there would assuredly be plenty of other evidence. 2. By irregu- larities in the deposition of superﬁcial accumulations prior to the elevation of the land or during the disappearance of the ice-sheet. The numerous tarns and lakes enclosed within mounds and ridges of drift-clay and gravel are ex- amples. 3. By the acccumulation of a barrier across the channel of a stream and the consequent ponding back of the water. This may be done, for instance, by a lnlnlslip, by the advance of a glacier across a valley, or by the throwing up of a bank by the sea across the mouth of a river. 4. By erosion. The only agent capable of excavating hollows out of the solid rock such as might form lake—basins is glacier-ice (ante, p. 282). It is a remarkable fact, of which the significance may now be seen, that the innumerable lake- basins of the northern hemisphere lie on surfaces of intensely ice-worn rock. The stria: can be seen on the smoother rock-surfaces slipping into the water on all sides. These striae were produced by ice moving over the rock. If the ice could, as the strive prove, descend into the rock—basms and mount up the farther side, smoothing and striating the rock as it went, it could erode the basins. It is hardly possible to convey in words an adequate conception of the enormous extent to which the north of Europe and North America has had its surface ground down by ice. The ordinary rough surfaces produced by atmospheric disintegra- tion have been replaced by a peculiar flowing contour which is traceable even to below the sea—level. In the general subaerial denudation of a country, innu- merable minor features ars 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 Colispicllnws 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 are scooped out on the one hand, aml naked precipices are left on the other. The harder bands of rock project as massive ribs down the slopes, shoot up into prominent (Iif/uilles, or give to the summits the notched saw-like outlines they so often present. Tablelands may sometimes arise from the abrasion of hard rocks and the production of a level plain by the action of the sea, or rather of that action combined with the previous