Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/386

Rh A) "- I that they were convertcd into rocks as hard and crystalline as many of the I’al-.eozoie masses. It is strange to reflect that the enduring materials out of which so many of the mountains, cliffs, and pin- nacles of the Alps have been formed are of no higher geological an- tiquity than the London (‘lay and other soft Eocene deposits of the south of l-In-glaml. After the par-o__vsm of elevation had ended, one or more large lakes were formed along the northern base of the mountains. In these hollows the Swiss molasse accumulated to a depth of more than 6000 fect—a great pile of slowly formed gravels, sands, aml clays. That the sea gained occasional access to the region is shown by the interpolation of bands containing marine organisms, as already stated (ante, p. 363). Not irnprobably a gradual sub- sidence of the region was going on during the formation of the molasse. hit towards the close of the Miocene period another great epoch of mountain-making was ushered in. The lakes disappeared, and their thick sediments were thrust up into large, broken, ruoun- tain masses. The Righi, Rossbcrg, and other prominent heights along the northern ﬂank of the Alps are formed of these upturned lacustriue deposits. Since that great movement no paroxysru seems to have affected the Alpine region. Ceaseless changes, indeed, have been in progress, but they have been due not so much to subterranean causes as to those subaerial forces which are still so active. The gradual evolution of a continent during a long succession of geological periods has been admirably worked out for North America b_v Dana, Iiing, Ifayden, I'ewberry, Powell, Dawson, and others. The general character of the structure is extreme sim- plieity, as eonrpared with that of the Old 'o1-ld. In the Rocky Mountain region, for example, while the Palreozoic formations lie unconformably upon the Archrean gnciss, there is, according to King, a regular conformable sequence from the Lower Silurian to the Jurassic rocks. During the enormous interval of time represented by these massive formations what is now the axis of the continent remained undisturbed save by a gentle aml protracted subsidence. In the great depression thus produced all the. Palazozoic and a great part of the Mesozoic rocks were accumulated. .t the close of the Jurassic period the first great upheavals took place. Two lofty ranges of mountains,—the Sierra Nevada (now with summits more than 14,000 feet high) and the 'ahsatch,—400 miles apart, were pushed up from the great subsiding area. These movements were followed by a prolonged subsidence, during which Cretaceous sedi- ments aceurnulated over the Rocky Mountain region to a depth of 9000 feet or more. Then came another vast uplift, whereby the Cretaceorrs sediments were elevated into the crest of the mountains, and a parallel coast-range was formed fronting the Pacific. - Intense metarnorplrisrn of the Cretaceous rocks is stated to have taken place. During the Tertiary ages the Rocky Mountains were perma- nently raised above the sea, and gradually elevated to their present height. Vast lakes existed among them, in which, as in the Miocene basins of the Alps, enormous masses of sediment accumu- lated. The slopes of the land were clothed with an abundant vege- tation, in which, as already stated (ante, p. 365), we may trace the ancestors of many of the living trees of Xortlr America. One of the most striking features in the later‘ phases of this history was the outpouring of great ﬂoods of trachyte and other lavas from many points and ﬁssures over a vast space of the Rocky Mountains. In the Snake River region these lavas have a depth of 700 to 1000 feet, bver an area 300 miles in breadth. These examples show that the elevation of mountains has been occasional and, so to speak, paroxysmal. Long intervals elapsed when a slow subsidence took place, but at last a point was reached when the descending crust, unable any longer to withstand the accumulated lateral pressure, was forced to ﬁnd relief by rising into mountain ridges. With this effort the elevatory movements ceased. They were followed either by a stationary period, or more usually by a renewal of the gradual depression, until eventually relief was again obtained by upheaval, sometimes along new lines, but often on those which had previously been used. Ve see also how, by such enormous compression, the rocks should have acquired a cleavage structure (ante, p. 306). Soft clays have been squeezed and folded till they have become hard ﬁssile slates. So intense have been the corrugation and compression that the strata have under- gone a chemical rearrangement of their particles; they have been “ metamorphosed ” or changed into sclrists and gneisses, if indeed some portions of them have not been actually fused and intruded into the surrounding nrasses as igneous rocks. The consideration of these changes enables us to realize Why the strata of a great mountain chain should rise into .. GEOLOGY ['II. I‘lIYSIO(iIl-l'IIIC.l.. steeper folds as they are traced away from the plains, until they are found at last folded back upon themselves, and the older are made to overlie the younger. Instead of over- lying the central and more ancient masses of the range, they seem really to dip into and under them, so that a section across the region might convey the impression of a great syrrcline instead of a great and complicated anticline. This fan-shaped arrangement of the rocks may be observed even in the single mountains of a great chain. Mount Blane is a familiar example. Another piece of geological structure is sometimes brought vividly before us by the examination of those regions of disturbance. Not only have the rocks been crumpled and inverted ; they have likewise been traversed by great dis- locations. Those on one side of a ﬁssure have been pushed bodily over those on the other side, or they have experienced a vertical displacement of hundreds or even thousands of feet. As a rule, however, dislocations are more easily traced, if they are not also larger and more numerous, among the low grounds than among the mountains. One of the most remarkable and important faults in Europe is that which bounds the southern edge of the Belgian coal-ﬁeld. It can be traced across Belgium, has recently been detected in the Boulonuais (ante, p. 350, note), and may not irnprobabl y run beneath the Secondary and Tertiary rocks of the south of England. It is a renrarkable fact that faults which have a vertical displacement of many thousands of feet produce little or no effect upon the surface. The great Belgian fault. for example, is crossed by the valleys of the Mouse, and other northcrly—ﬂowing streams. Yet so indistiuctly is it rnarked 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. With the fractures along mountain chains we may connect the hot springs so frequently to be met with in these regions. But the most important connexion with the heated interior is that established by volcanic vents. The theory of secular contraction, while affording a rational explanation of the origin ofhthe great terrestrial ridges, serves at the same time to show why volcanoes should so frequently rise along these ridges (ante, p. 254). The elevation of the crust, by diminishing the pressure on the parts beneath the npraiscd tracts, permits them to assume a liquid condition, and to rise within reach of the surface when, driven upwards by the expansion of superheated vapours, they are ejected in the form of lava or ashes. It appears therefore that the present contours of the earth’s surface must be due in large measure to the effects of the contraction of a cooling globe. The crust has been repeatedly corrugated, sometimes suffering .srrdden and paroxysmal shocks, at other times undergoing slow and long- continued upheaval and depression. But these subterranean movements form only one phase of the operations by which the outlines of the land have been produced. T hey have ridged up the solid crust above the sea—level, and have thus given rise to land, but the land as we now see it has acquired its features from the prolonged and varied action of the epigenc agents upon rocks of very varied heights and powers of resistance. It is evident that, as a whole, the land suffers ceaseless erosion from the time that it appears above water. It is likewise clear, from the nature of the materials composing most of the rocks of the land, that they have been derived from old deuudations of the same kind. And tlrns, side by side with the various upheavals and subsideuces, there has been a continuous removal of materials from the land, and an equally persistent deposit of these materials under water, and consequent growth of new rocks.