Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/276

Rh 262 G E O L 0 G Y § 3. Inﬂzmm’ of lI'.r(er. In the great geological contest fought at the beginning of the century between the Neptunists and the Plutonists, the two gre it battle-cries were, on the one side, Water, on the other, Fire. The progress of the science since that time has shown that each of the parties had truth on its side, and had seized one aspect of the problems touching the origin of rocks. If subterranean heat l1as played a large part in the construction of the materials of the earth’s crust, water, on the other hand, has performed a hardly less im- portant share of the task. They have often co-operated together, and in such a way that the result must be re- garded as their joint achievement, wherein the respective share of each can hardly be exactly apportioned. In the following brief résumé of this subject we shall consider the changes produced by pure water, by water charged with substances in solution, and by water raised above ordinary temperatures. By numerous observations it has been proved that all rocks within the accessible portion of the earth's crust contain interstitial water, or, as it is so1ue_times called, quarry—water (eau-dc-carrzérc). This is not chemically combined with their mineral constituents, but merely re- tained in their pores. Most of it evaporates when the stone is taken out of the parent rock and freely exposed to the atmosphere. The absorbent powers of rocks vary greatly, and chieﬂy in proportion to their degree of porosity. Gypsum absorbs fron1 about 0'50 to 1'50 per cent. of water by weight ; granite, about 0'37 per cent.; quartz from a vein in granite, 0'08; chalk, about :?0'0; plastic clay, from 196 to '24':3. These amounts may be increased by exhausting the air from the specimens and then immer.-:- ing them in water. The water enclosed within the interstices and crystals of igneous rocks may be either an original constituent, deriving its origin, like any of the component minerals, from molten reservoirs within the earth’s crust, or it may have descended fron1 the surface to the incandescent rocks. Many facts may be adduced in support of the greater probability of the second view. Besides the general proximity of volcanic orifices to large sheets of water, we have abundant evidence of the actual descent of water from the surface, both through ﬁssures, and also by permeation through the solid substance of rocks. All surface rocks contain water, and no mineral substance is strictly impervious to the passage of liquid. The well- known artiﬁcial colouring of agates proves that even the mineral substances apparently most homogeneous and im- pervious ca11 be traversed by liquids. M. D-aubrée has instituted a series of experiments to illustrate the power possessed by water of penetrating rocks, in virtue of their porosity and capillarity, even against a considerable counter- pressure of vapour; and, without denying the presence of original water, he concludes that the interstitial water of igneous rocks may all have been derived by descent from the surface. The presence of interstitial water must affect the chemical constitution of rocks. It is now well understood that there is probably no terrestrial substance which, under proper conditions, is not to some extent soluble in water. By an interesting series of experiments, made many years ago by Messrs Itogers, it was ascertained that many of the ordinary mineral constituents of rocks could be dis- solved to an appreciable extent even by pure water, and that the change was accelerated and augmented by the presence of carbonic acid.‘ Silica, alkaliferous silicates, and iron 0XlClO3 can be taken up and held in solution by pure water, even at ordinary temperatures, in considerable quantities. 1 -tmr-,-.'_-5'72 Juurn. Sciazce "3, V. 401. [III. DYN..fI(‘. L. The mere presence of pure water therefore within the pores of subterranean rocks cannot but give rise to changes in the composition of these rocks. Home of the more soluble materials must be dissolved, and, as the water eva- porates, n1ust be redeposited in a new form. }ut water in a natural state is never chemically pure. In its descent through tl1e air it absorbs oxygen and carbonic acid, besides other impurities (seep. 267), and as it ﬁlters through the soil it abstracts more carbonic acid, as well as other results of decomposing organic matter. It is thus enabled to effect numerous decompositions of the rocks underneath. The nature of these changes may be inferred from the composition of spring water, to which reference will subsequently be made (p. 270). For the present it will be sufficient to remark that two important kinds of chemical decomposition must evidently arise from the action of such inﬁltrating water. The presence of the organic matter n1ust exercise a reducing power on oxides. This will be n1ore especially the case with those of iron, the nearly insoluble lizrmatite being reduced to the protoxide, which, converted into carbonate, is readily removable in solution. There can be little doubt that by this means a vast amount of ferruginous 1n-atter is extracted from subterranean rocks and carried to the surface. The presence of carbonic acid enables the water to attack vigorously the mineral constitu— cuts of rocks. Alkaline carbonates, with carbonates of lime and magnesia, and protoxides of iron and manganese, are produced, and these substances borne onward in solution give rise to further reactions among the rocks through which they are carried. “In the decomposition of rocks,” says llischof, “carbonic acid, bicarbonate of lime, and the alkaline carbonates bring about most of the decompositions and changes in the mineral kingdom." The microscopic study of rocks has thrown much light upon the mineralogical alterations in rocks due to the influence of percolating water. Even the Inost solid-looking, unweathered rocks, are found to have been affected by such metamorphism. Their hydrous mag- nesian silicates, for example, are partially or wholly con- verted into such hydrous forms as serpentine, ehlorite, or (lelessite. The process of conversion may often be watched. It can be seen to have advanced along the fissures or cleavage—planes of the minerals leaving the intervening sections still fresh; or it may be observed to have proceeded in such a way that diffused alteration-pro- ducts are dispersed in ﬁlaments or irregular patches through the base of the rock, or gathered together and even re- crystallized in cavities; or the whole rock, as in many serpentines, has undergone an entire transformation. Much information regarding such internal alterations of rocks may be obtained from the study of psemlomorp/ts, that is, crystals having the external form of the mineral of which they originally consisted, with the internal structure and composition of the mineral which has replacedit. Serpen- tine representing olivine, clay taking the place of rock-salt, silica that of wood, and marcasite that of molluscan shell, are familiar examples. There is no reason to doubt that. these changes may, in the course of ages, have been effected at ordinary temperatures by water descending from the surface of the ground. But two other considerations require to be taken into account in the discussion of the internal transformations of rocks by subterranean water. ( l.) In the first place, the water has often been at a high temperature. Mere descent into the crust of the earth will raise the temperature of the water until, if this descent be prolonged, a point far above 212° F-alir. may be reached. Experiinents have shown that the chemical action of water is vastly increased by heat. Thus M. Daubrée exposed a glass tube containing about half