Delesse, Études sur le Métamorphisme des Roches, Paris, 1861: Daubrée, Études sur le Métamorphisme et sur la Formation des MINERAL VEINS AND VEINS OF ORE. These almost form a special group of rocks, and would be entitled to an equal place by the side of the three other groups, if the extent of space which they occupy in nature were not so small. They but fill up narrow fissures in other rocks. Their origin appears, almost without exception, to have been hydroplutonic. They are, for the most part, chemical precipitates from aqueous solutions formed in the interior of the earth under very different circumstances of pressure and heat than those which prevail upon the surface. Having treated these formations, which occupy so subordinate a space in the composition of the earth's crust, at length in our book on Erzlagerstätten,' we shall not devote further space to them here. 6 CONCLUSION. BEARING in mind the facts and considerations above stated, if we take a general review of the various formations and transformations of rocks, we shall discover in them a perpetual process of circulation or rotation of substances, and of their different states. The substances remain, but the forms in which they appear and the mode of their combinations vary. Disregarding for the moment the first solid products of cooling on the earth's surface, as not being capable of identification at the present day, we may most conveniently enter the circle of transmutations with the eruptive igneous rocks, as approaching most nearly to original formations. These then are constantly attacked and decomposed by chemical and mechanical forces acting from their surface inwards, and from their cracks and fissures outwards. The products of this decay are deposited either in the form of chemical precipitates or mechanical aggregates. By chemical process of precipitation cavities and fissures in rocks become filled up (amygdaloids and veins), deposits are made at the mouths of springs of limestone-tuff, siliceous tuff, bog-ore, &c.; or else, other crystalline rocks are formed, such as gypsum or rock-salt. By mechanical agency, on the other hand (partly aided by organic processes), there arise the much more important and extensive deposits of clay, sand, pebbles, marl, limestone, and dolomite; and during the process of deposit, carbon (in the form of carbonic acid from the atmosphere), water, chlorine, and some other substances are added to the previously existing materials. But, like the eruptive masses, all these deposited masses in their turn are partly decomposed and washed away by external forces, and in other part they become greatly changed internally by pressure and the action of heat. By means of heat and pressure acting during long periods, parts which thus in the first instance were only mechanically bound together, enter into new chemical combinations with each other, and assume a crystalline state more or less analogous to that of the crystalline mineral aggregates of the eruptive rocks. It is even probable in many cases that the substance of these derivative rocks has been fused and become eruptive a second time. Thus the process of destruction and new formation of rocks, be it ever so slow, and therefore difficult of observation, has never, at any time of the earth's history, been interrupted, but continues at the present day; and not only is this true of the original formations, but the new products of consolidation, of deposit, and of transmutation have always been equally subjected, and are still subject, to the same processes. This is the perpetual circulation of matter in the world of rocks. In the course of such various and renewed working up and transformation of the same substances, with the addition of those others furnished by the air and water, it cannot be matter of wonder that the variety of their groups has been always somewhat on the increase; for, if certain processes in this rotation are altogether universal in their character, recurring in the same way, everywhere and in every age, yet in consequence of the general multiplication of conditions and circumstances, and the increasing aggregate of their results, special combinations of the same processes have constantly arisen in later times and brought about special formations of rocks which were not previously in existence, or which do not belong to the normal phenomena of nature. This increase in variety of the products of later times is not confined to geological and mineral substances; a greater and more rapid increase has taken place in the organic world, where the forms of life have multiplied in an ever ascending ratio (partly in consequence of the change and increase of the conditions of existence from geological causes). The processes of change, to which the outward conformation of the globe's surface is subject, likewise mul tiply more rapidly than mere strictly geological pheno mena. Reasoning, therefore, from the past and from analogy with other kingdoms, we must expect the species of rocks and kinds of rock-formation to go on increasing indefinitely for the future, as they have been increasing continually ever since the first solidification of our earth's crust. |