Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/277

Rh INFLUENCE or w.-.rEI:.] its weight of water to a temperature of about 400° C. At the end of a week he found the tube so entirely changed into a white, opaque, powdery mass as to present not the least resemblance to glass. The remaining water was highly charged with an alkaline silicate containing 63 per cent. of soda aml 37 per cent. of silica, with traces of potash and lime. The white solid substance was ascertained to be composed almost entirely of crystalline materials. These consisted partly of minute perfectly linipid bipyramidal crystals of quartz, but chieﬂy of very small acicular prisms of wollas- tonite. It was found, moreover, that the portion of the tube which had not been directly in contact with the water was as much altered as the rest, whence it was inferred that at these high temperatures and pressures the vapour of water acts chemically like the water itself. In the second place, tiie effect of pressure must be recognized as most important in enabling water, especially when heated, to dissolve and retain in solution a larger quantity of mineral matter than it could otherwise do. In M. Daubrée’s experi- ments just cited, the tubes were hermetically sealed and secured against fracture, so that the pressure of the greatly siiper-lieated vapour had full effect. By this means, with alkaline water, he not only produced the two minerals above mentioned, but also felspar and diopside. It is important to observe that the three conditions required for these changes—the presence of alkaline water, a high temperature, and considerable pressure——ai‘e precisely those which it can be affirmed must exist abundantly within the crust of the earth. ‘Ye must admit the possibility of rocks originally at the surface being depressed so as to come within the influence of internal heat, and to contain within their pores abundant interstitial water more or less charged with alkaline carbonates. Books under these conditions, so far as we can judge, can hardly escape internal decom- position and recomposition. Mere descent to a great depth beneath. the surface will not necessarily result in metamorpliism, as has been shown in the case of the Nova Seotian and of the South Welsh coal-ﬁeld, where sand- stones, sliales, clays, and coal—seams can be proved to have been once depressed 14,000 to 17,000 feet below the sea- lsvcl, under an overlying mass of rock, and yet to have sustained no serious alteration. Perhaps the failure of change may be explicable on the supposition that these Carboniferous strata were comparatively dry. But where rocks possess suflicient interstitial water, and are depressed within the crust so as to be exposed to a considerable temperature and to great pressure, they iinist be metamor- phosed,—the extent of the metamorphism depending partly upon the vigour of the attack made upon them by the water, partly on their own composition and proneiiess to chemical change, and partly upon the length of time during which the process is continued. A metamorphosed rock must thus be one which has siilfc-re:.l a mineralogical rearrangement of its substance. It may or may not have been a crystalline rock originally. Any rock capable of alteration (and all rocks must be so in some degree) will, when subjected to the required conditions, become a metamorphic rock. The resulting structure, however, will, in most cases, bear witness to the original character of the mass. A sedimentary rock, for example, eonsisting of alternate layers of different texture and com- position, will doubtless retain, even in its metamorphosed condition, traces of that fundamental structure. The water will travel more easily along certain layers than along others; some laminae will be more readily affected, or will give rise to a set of reactions different from those of con- tiguous layers. Hence the rearrangement and recrystalliza- tion due to nietaniorphism will take place along the prede- termined lines of stratiﬁcation, so long as these lines have not been effaced or rendered inoperative by any other geo- GEOLOGY 263 logical structure. It is doubtless to this cause that the foliated character of gneiss, mica-schist, and so many other metamorphic rocks is to be ascribed. In the process of metamorphism, tlierefere, as well as in that of fusion, to which reference has already been made, the influence of water would seem to have been always conspicuous. Indeed, as will be shown in part iv., it is extremely difficult in many cases to draw a line between the results of metamorphism and igneous fusion, or to decide whether a rock should be called igneous or metamorphic. It has been pointed out above, for example, that in many rocks which have undoubtedly been in a ﬂuid condition, as proved by their injected veins and dykes, the constituent minerals have not appeared in the order of their respective fusibilities. Sclleerel‘, Elie de Beaumont, and Daubrée have shown how the presence of a comparatively small quantity of water in such rocks has contributed to suspend their solidiﬁcation, and to promote the crystallization of their silicates at temperatures considerably below the point of fusion. In this way the solidiﬁcation of quartz in granite after the crystallization of the silicates, which would be unintelligible on the supposition of mere dry fusion, becomes explicable. The phenomena of metamorphism in the architecture of the earth’s crust are discussed in part iv. DIVISION II.—EPIG'ENE OR SURFACE ACTION It is on the surface of the globe and by the operation of agents working there that at present the chief amount of visible geological change is effected. In considering this branch of inquiry, we are not involved in the same prelim- inary difficulty regarding the very nature of the agencies as we found to be the casein the investigation of plutonic action. On the contrary, the surface agents are carrying on their work under our very eyes. Ve can watch it in all its stages, measure its progress, and mark in many ways how accurately it represents similar changes which for long ages previously must have been effected by the same means. But in the systematic treatment of this subject we encounter a difficulty of another kind. Ve discover that while the operations to be discussed are numerous and often complex, they are so interwoven into one great network that any separation of them under different subdivisions is sure to be more or less artiﬁcial, and to convey an erroneous impression. While, therefore, under the una- voidable necessity of making use of such a classiﬁcation of subjects, we must bear always in mind that it is employed merely for convenience, and that in nature superﬁcial geological action must be continually viewed as a whole, since the work of each agent has constant reference to that of the others, and is not properly intelligible unless that connexion be kept in view. The movements of the air ; the evaporation from land and sea; the fall of rain, hail, and snow ; the ﬂow of rivers and glaciers ; the tides, currents, and waves of the ocean ; the growth and decay of organized existence, alike on land and in the depths of the sea ;——in short, the whole circle of movement, which is continually in progress upon the surface of our planet, are the subjects now to be examined. It would be desirable to adopt some general term to embrace the whole of this range of inquiry. For this end the word epigene may he suggested as a convenient term, and aiiti- thetical to hypogene or subterranean action. The simplest arrangement of this part of Geological Dynamics will be into three sections :— I. AIn.—The inﬂuence of the atmosphere in destroy- ing and forming rocks. _ II. VATEP..—Tl1e geological functions of the circula- tion of water through the air and between sea and land, and the action of the sea.