The phenomena of Hypogenie Action must be accompanied with very considerable changes in the rocks which form the earth's outer crust. The importance of heat in tho transformations of rocks is fully admitted. Two sources of subterranean heat have had their agency in the production of hypogenic changes: 1, the internal heat of the globe; 2, the heat due to the transformation of mechanical energy in the crumbling, 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. In pursuing the investigation we have to consider the tempera ture, from the lowest at which any change is possible up to that of complete fusion; the nature of the rock operated upon, some materials being much more susceptible to change from heat than others; the pressure under which the heat acts, the potency of this agency being much increased with increase of pressure; the presence of water, whereby chemical changes take place which would not be possible in dry heat. It may be concluded that the manner in which rocks have been melted within the crust is not that more simple fusion which we can accomplish artificially, but that it has involved conditions which have not been successfully imitated in any laboratory or furnace. It may be considered that while some rocks, like obsidian or pitclistone, which so closely resemble artificial glasses, may have been derived from a simple igneous fusion, such as can be imitated in a furnace, the great majority of rocks have had a more complex origin, and in a great number of cases can be proved to have been mingled with more or less water, while they were still fluid. In the second place, there can be no question that, in the great hypogenic laboratory of nature, rocks have been softened and fused under enormous pressure. In one instance such pressure has been calculated to equal that of an overhanging mass of rock 50,000 ft. high.
The- process called sublimation, by which mineral substances can be obtained in a crystallized form from the condensation of vapors, maybe the result of the mere cooling and reappearance of bodies which have been vaporized by heat and afterward solidified by cooling, or, from the ,solution of these bodies in other vapors or gases, or from the reaction of different vapors upon each other. These operations frequently occur at volcanic vents and in the crevices of recently erupted and still hot lava streams. They have been successfully imitated by experiments. Superheated steam is endowed with a remarkable power of dissolving that intractable substance, silica; artificially heated to the temperature of the melting point of cast-iron, it rapidly attacks silica, and deposits the mineral in snow-white crystals as it cools. Besides the influence of pres sure in raising the melting point of subterranean rocks. and in permitting water to remain fluid among them at temperatures far above the boiling point, even at a red, or perhaps, a white heat, we have to consider the effect produced by the same agent upon rocks already solidified. The simplest and most obvious result of pressure upon such rocks is their consolidation, as where a mass of loose sand is gradually compacted into a more or less coherent stone, or where a layer of vegetation is compressed into peat, lignite, or coal. If pressure becomes extremely unequal, or if thq rock can escape from the influence in one or more directions, there will be a disturbance or rearrangement of the particles which are by this means made to move upon each other. These disturb ances are: 1, cleavage, from strong lateral pressure; 2, pebbles and organic remains squeezed into each other; 3, the formation of jets ,of metal or rock material by some great pressure; 4, compression, or plieation, produced by the cooling and shrinking of the earth, as shown in contracted rocks; 5, faults or dislocations resulting from elev ation or upheaval.
While subterranean heat has had a large part in the construction of the materials of the earth's crust, water, ou the other band, has performed a hardly less important share of the task. Fire and water have often co-operated in such a way that the .result must be taken as their joint achievement; but we are now to consider the changes produced by water, pure or otherwise, and at ordinary or other temperatures. All rocks at or near the earth's surface contain water, not chemically in combination, but in their pores. Most of it evaporates when the stone is freely exposed to the air. Rocks differ in water absorbing capacity. Gypsum will take from one-half to one and one-half per et. by weight; granite a third of one per et.. ; quartz scarcely anything; chalk 20 per et. All surface rocks contain water, and no mineral substance is strictly impervious to the passage of liquid. It is now well understood that there is probably no terrestrial sub stance which, under proper Conditions, is not to some extent soluble in water. The mere presence of pure water within the pores of subterranean rocks must change their composition. Some of the more soluble materials must be dissolved, and as the water evaporates, must be deposited in a new form. But water in a natural state is never chemically pure. In its descent through the air it absorbs oxygen autl carbonic acid, besides other impurities, and as it filters through the soil it abstracts more carbonic acid, as well as other results of decomposing organic matter; thence it effects numerous decompositions of the rocks underneath. The nature of these changes may be inferred from the composition of spring water. Two important kinds of chemical decomposition must evidently arise from the action of such infiltrating water. 1. The presence of the .organic matter must exercise a reducing power on oxides. This will be more especially the case with those of iron, the nearly insoluble haematite being reduced to the protoxide, which, converted into carbonate, is readily removable in solution. There can be lrttle doubt that by this means a vast amount of ferruginous matter is extracted from subter ranean rocks and carried to the surface. • 2. The presence of carbonic acid enables the water to attack vigorously the mineral coristituents 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 farther reactions among the rocks through which they are carried. "In the decomposition of rocks," says Bischof, "carbonic acid, bicarbonate of lime, and 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 most solid-looking, unweathered rocks, are found to have been affected by such metamorphism. Their hydrous magnesian silicates, for
example, are partially or wholly converted into such hydrous forms as serpentine, chlorite or delessite. 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-products are dispersed in filaments or irregular patches through the base of the rock, or gathered together and even recrystallized 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 pseuclornorphs, 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 replaced it. Serpentine representing olivine, clay taking the place of rock-salt, silica that of wood, and marcasite that of molluscan shells, arc familiar examples. There is no reason to doubt that these changes may, in the course of ages, have been effected at ordinary temperature 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. 1. 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° Fahr. may be reached. Experiments have shown that the chemical action of water is vastly increased by.lfeat. Thus M. Daubree exposed a glass tube containing about half its weight of water to a temperature of about 400° centigrade. At the end of a week he found the tube so entirely changed into a white, opaque, powdsry mass as to present not the least resemblance to. glass. The remaining water was highly charged with all alkaline silicate containing 63 per et. of soda and 37 per et. 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 min ute, perfect, limpid hipyramidal crystals of quartz, but chiefly of very small aelaular prisms of wollastonite. 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,vapor of water acts chemically like the water itself. 2. The 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 otherwise could do. In M. Daubree's experi ments just cited, the tubes were hermetically sealed and secured against fracture, so that the pressure of the greatly superheated vapor had full effect. By this means, with alkaline water, he not only produced the two minerals above mentioned, but also feldspar 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 —are precisely those which can be affirmed to exist abundantly within the crust of the earth, We must admit that rocks originally at the surface may have been so depressed as to come within the influence of internal heat, and may contain within their pores abundant interstitial water more or less charged with alkaline carbonates. Rocks under these conditions, so far as we can judge, can hardly escape internal decomposition and recomposition. Mere descent to a great depth beneath tile surface will not necessarily result in metamorphism, as has been shown in the Nova Scotian and South Welsh coal fields, where sandstones, shales, clays, and coal-seams can be proved to have once been depressed 14,000 to 17,000 ft. below the sea-level, 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 sufficient interstitial water, and are depressed within the crust so as to be exposed to a considerable temperature and to great pressure, they must be metamorphosed—the extent of the metamorphism depending partly upon the vigor of the attack madeupon them by the water, partly on their own composition and proneness to chemical change, and partly upon the length of time during which the process was continued. A metamorphosed rock must thus be one which has suffered a mineralogical rearrangement of its substance. It may or may not have been a crystalline rock origi nally. 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 some cases, bear witness to the original character of the mass. A sedimentary rock, for example, consisting of alternate layers of different tex tnrJ and composition 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 contiguous layers. Renee the rearrangement and recrystallization due to metamorphism will take place along the predetermined lines of stratification, so long as these lines have not been effaced or rendered inoperative by any other geologicalstructure, 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, therefore, 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, 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 igne ous or metamorphic. It has been pointed out, for example, that in many rocks which have undoubtedly been in a fluid condition, as proved by their injected veins and dikes, the constituent minerals have not appeared in the form of their respective fusibilities. Scheerer, Elie de Beaumont, and Daubree have shown how the presence of a compara tively small quantity of water in such rocks has contributed to suspend their solidifica tion, an.d 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, unintelligible on. the supposition of mere dry fusion, becomes explicable.