Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/272

Rh 25.9 surface of tlie planet. The crust varies much in structure, in thermal resistance, and in the position of its isogeo- thermal lines. As the hotter nucleus contracts more rapidly by cooling than the cooled and hardened crust, the latter must sink down by its own weight, and in so doing requires to accommodate itself to a continually diminishing diameter. The descent of the crust gives rise to enormous tangential pressures. The rocks are crushed, crumpled, and broken In many places. Subsidence must have been the general rule, but every subsidence would doubtless be accompanied with upheavals of a more limited kind. The direction of these upheaved tracts would largely depend upon the original structure of the crust. It would be apt to occur in lines which, once taken as lines of weakness or relief from the intense strain, would probably be made use of again a11d again at successive paroxysms or more tranquil periods of contraction. Mr Mallet, in the paper already cited, has ingeniously connected these movements with the linear direction of mountain chains, volcanic vents, and earthquake shocks. Though the origin of the present surface configuration of the land will be more conveniently discussed in a subse- quent part of this article, after the structure of the earth’s crust has been described, a few words may be inserted here, where some allusion to the subject might be looked for. Mountains may arise from three eauses—(l) from the cor- rugation of the earth’s crust due to the eﬂ'ects of secular contraction ; from the accumulation of materials poured out of volcanic oriﬁces; and (3) from the isolation of elevated masses of ground, owing to the removal, by denudation, of the materials originally connecting them, and to the consequent formation of valleys. Mountains formed in the volcanic way are almost always conical, and are either solitary, as Etna, or occur in linear groups, like the volcanoes of Java. Those formed by denudation are of minor dimensions, and rather deserve the name of hills. Mountain-chains, on the other hand, which are the dominant features of the earth’s surface, though they may have lines of volca11ic vents along their crests, are not formed essentially of volcanic materials, b11t of the sedi- mentary and crystalline rocks of the crust which have been ridged up into vast folds. If the continental lands may be compared to great undulations of the solid surface of the globe, the mountain-chains may be likened to the breaking crests of such wave—like undulations. In their internal structure mountain-chains bear witness to the intense crumpling of the rocks during the process of upheaval. As a. consequence of the uprise of two or more parallel ranges of mountain, lines of longitudinal valley must be produced. But no sooner is a mass of land raised above the sea than it is exposed to the attacks of air, rain, frost, springs, glaciers, or other meteoric agents of disinteg1‘ation. Its surface is then worn down, the ﬂow of water dow11 its slopes cuts out gulleys, ravines, and valleys, so that eventually a very rugged surface is produced, on which, probably, no portion of the original surface of upheaval may remain, but where new lines of minor ridge and valley may appear as the combined result of internal geological structure and atmospheric denudation. The reader, however, is referred to part vii. of this article for a fuller treatment of this interesting subject. During the movements by which mountain masses have been npheaved, the stratiﬁed rocks have been so compressed as to occupy, in many cases, but a small proportion of the horizontal extent over which they originally extended. They have adjusted themselves to this diminished area by undergoing intense plication, and thus acquiring a much greater vertical depth. On the other hand, they have been abundantly fractured, some portions of their mass being pushed up, others being let down, so that the crust is GEOLOGY [111. l)YN..!lL'.L. traversed with a kind of complicated network of fissures. The discussion of these features of geological structure, however, must likewise be postponed to part iv., where the facts regarding their occurrence will be described. Seetion IV.—Hypogene Causes of Changes in the Tex- ture, Structure, and Composition of Rocks. The phenomena of hypogene action hitherto 1111der con- sideration have related almost wholly to the etfects produced at the surface. It is evident, however, that these phenomena must be accompanied by very considerable changes in the rocks which form the earth’s outer crust. These I'OL'l{S, as just stated, have been subjected to enormous pressure; they have been contorted, crumpled, and folded back upon tl1enisclve.<, as if thousands of feet of solid limestones, sandstones, and shales had been merely a few layers of carpet; they haw been shattered and fractured everywhere ; they have in one place been pushed far abovetheir original position, in anotlur depressed far beneath it : so great has been the compression which they have undergone that their component particles have in many places been rearranged, and even crystallized. They have here and there actually been reduced to fusion, and have been abundantly invaded by masses of molten rock from below. In the present section of this article we shall consider chieﬂy the nature of the agencies by which such changes have been effected ; the reslllts achieved, in so far as they constitute part of the architecture or structure of the earth‘-: crust, will be discussed in part iv. At the outset, i‘: is evident that we can hardly hope to detect many of these processes of subterranean change actually in progress and watch their effects. The very vastness of some of them places them beyond our direct reach, and we can only reason regarding them from the changes which we see them to have produced. But a good number are of a kind which we can in son1e measure imitate in our laboratories and furnaces. It is not requisite, therefore, to speculate wholly in the dark on this subject. Since the original and classic researches of Sir James Hall, the founder of experiment-.11 geology, great progress has been made in the investigation of hypogene processes by experiment. The conditions of nature have been imitated as closely as possible, and varied in different ways, with the result of giving us an increas- ingly clear insight into the physics and chemistry of sub- terranean geological changes. The succeeding portions of this section of the article will therefore be chieﬂy devoted to an illustration of the nature of hypogene action, in so fan- as that can be inferred from the results of actual experinunt. The subject maybe conveniently treated under three heads-— (1) the effects of heat; the effects of pressure and con- traction; the inﬂuence of water in effecting changes in minerals and rocks. § 1. .E'[}'cc-ts Ileal. The importance of heat among the transformations of the rocks has been fully admitted by geologists, since it used to be the watchword of the Iluttonian or Vulcanist school at the end of last century. Two source: of subterranean heat may have at different times and in different degrees co-operated in the production of hypo- gene changes—(l) the internal heat of the globe, of which some account has already been given, and (2) the heat due to the transformation of mechanical energy in the crumpling, fracturing, and crushing of the rocks of the crust, as these have been from time to time compelled to adjust themselves to the diminishing diameter of the more rapidly cooling and contracting interior. From which of these two sources, or from what combination of them, the heat was derived by which any given change in the rocks was produced, is a problem which admits