Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/385

ORIGIN OF .IOUNT_INS.] to have been some original distribution of materials in the globe that initiated the depressions on the areas which they have retained. It has been already pointed out (ante, p. 223) that the matter underlying the oceans is more dense than that beneath the continents, and that, partly at least, to this cause must the present position of the oceans be attributed. The early and persistent subsidence of these areas, with the consequent increase of density, seems to have determined the main contours of the earth's surface. l:‘rom what has been stated in part iv., the reader will understand that rocks which were originally horizontal, or nearly so, have been crumpled over tracts thousands of square miles in extent, so as to occupy now a superficial area greatly less than that which they originally covered. It is evident that they have been horizontally compressed, and that this result can only have been achieved as a con- sequence of the subsidence of such a curved surface as that of o11r globe. The difficulty of explaining these corruga- tions on the hypothesis of the contraction of a solid globe is undoubtedly great. that the present inequalities of contour on the earth’s sur- face are from sixty—six to eleven and a half times as great as they would have been had they resulted from the con- traction of a solid globe; and he has suggested that the earth need not have become solid throughout simultaneously, and consequently may have been considerably larger than it is now at the time when a solid crust was first formed.‘ The geological phenomena long ago led to a belief in the liquidity of the earth's interior. Since this belief has been so weightily opposed by the physical arguments already adduced (ante, p. 225), geologists have endeivoured to modify it in such a way as, if possible, to satisfy the re- quirements of physics, while at the same time providing an adequate explanation of the corrugation of the earth’s crust. Mr Hopkins, Professor Dana, Professor Shaler, and Mr Fislier have, on different grounds, advocated the ex- istence of a fluid or viscous substratum beneath the crust, the contraction and consolidation of which produce the corrugations of the rocks and of the surface. “The increase of temperature,” says Mr Fisher, “though rapid near the surface, becomes less and less as we descend, so that, if the earth were once wholly melted, the temperature near the centre is not very greatly above what it is at a depth which, compared to the earth's radius, is small. Consequently, if it requires great pressure to solidify the materials at such a temperature, it probable that the melting temperature may be reached before the pressure is sufficient to solidify.” The crust, of course, must be able to sustain itself on the cor- rugated surface of the supposed viscous layer without break- ing up and sinking. The same writer has even suggested that the observed amount of corrugation is more than can be accounted for even on this hypothesis, and that the shrink- age may have been due not merely to cooling, but to the escape of water from the interior in the form of the super- heated steam of volcanic vents? More recently Herr Siemens has been led, from observations made in May 1878 at Vesuvius, to conclude that vast quantities of hydrogen gas, or combustible compounds of hydrogen, exist in the earth’s interior, and that these, rising and exploding in the funnels of volcanoes, give rise to the detonations and clouds of steam}; _ Leaving the vexed question of the condition of the earth’s interior, the hypothesis of secular cooling and contraction furnishes a natural explanation of the origin of the domin- ant elevations and depressions of the surface, and of the intense crumpling which the rocks in many regions have undergone. Taking 0'09 as the coefficient of contraction 1 C.'um7n-1'r7_r/e I’/Lil. Tr(ons., vol. xii. pt. ii., 1875. 2 1‘/Lil. 1|Ia_r/., October 1875. 3 J[:mat.s-brricht der K. ]Ii'C'21SS. Alcad. ll'z'ssensc/tafl, 1878, p. 558. GEOLOGY Mr 0. Fisher, indeed, believes" 37 l for a supposed stratum 500 miles thick, l_ying beneath 25 miles of crust, and passing from a fused into a solid state, Mr Fisher found that every 100 miles measured along a. great circle on the surface would have been one mile larger before the contraction, and that this might produce a trian- gular elevation of “ 25 square miles on a base of 100 miles, which would give a range of mountains half a mile high. If only 50 miles out of the hundred were disturbed, the range would be a mile high, and so on.”* The effects of this lateral pressure may show themselves either in broad dome—like elevations, or in narrower and loftier ridges of mountain. The structure of the crust is so complex, and the resistance offered by it to the pressure is consequently so varied, that abundant cause is furnished for almost any diversity in the forms and distribution of the wrinkles into which it is thrown. It is evident, however, that the folds have tended to follow a linear direction. In North America, from early geological times, they have kept on the whole on the lines of meridians. In the Old World, on the contrary, they have chosen diverse trends, but the last great crumplings—those of the Alps, Caucasus, and the great mountain ranges of central Asia—have risen along parallels of latitude. Mountain chains must therefore be regarded as evidence of the shrinkage of the earth’s mass. They may be the result of one movement, or of a long succession of such movements. Formed on lines of weakness in the crust, they have again and again given relief from the strain of compression by undergoing fresh crumpling and upheaval. The successive stages of uplift are usually not diflicult to trace. The chief guide is supplied by uncouformability, as explained on p. 318. Let us suppose, for example, that a mountain range consists of upraised Lower Silurian rocks, upon the upturned and denuded edges of which the Carboniferous Limestone lies transgressively. The original upheaval of that range must have taken place at the period of geological time represented by the interval between the Lower Silurian and the Carboniferous Lime- stone formations. If, in following the range along its course, we found at last the Carboniferous Limestone also highly inclined and covered unconformably by the Upper Coal—measures, we should know that a second uplift of that portion of the ground had 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 com- pared. In most great mountain—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 offer an instructive example of a great mountain chain formed by repeated movements during a long succession of geologi- cal periods. As has been already stated, the central portions of the chain consist of gnciss, schists, granite, and other crystalline rocks, partly referable to the .-lrchiean series, but many of which appear to be metamorphosed formations of Palazozoic, Secondary, and even of older Tertiary age. It would appear therefore that the first outlines of the Alps were traced out even in Arch-man times, and that after submergence, and the deposit of Palrcozoic formations along their flanks, if not over most of their site, they were re-elevated into land. From the ic- lations of the Mesozoic rocks to each other, we may infer that several renewed uplifts after successive denudations took place before the beginning of the Tertiary formations. A large part of tl.erange was, as we have seen, submerged during the Eocene period under the waters of that wide sea which spread across the centre of the Old Vorld,and i11 which the Nummulitic Limestone and Flysch were deposited. But about the close of that period the grand llplicavnl took place to which the present magnitude of the mountains is chiefly due. The older 'l‘u'tiary rocks, previously horizontal undc1' the sea, were raised up into land, crumpled, dislocated, inverted, together with all the older formations of the chain. S0 intense was the compression to which the Eocene clays and sands were subjected

4 Ccmzl/ridge P/ail. T rans., vol. xi. pt. iii.