Page:Popular Science Monthly Volume 4.djvu/716

696 of the earth, then all other materials of what is now the crust must have been gases above it; and, as nine-tenths of the elements in vapor are heavier than oxygen—many of them more than ten times as heavy—this gas could never have even touched this imaginary sea of silicon. The oxidation, then, was only possible in the regions of the atmosphere where oxygen existed and abounded. There only among the free-moving gases could the incalculable amount of heat evolved in the combination be carried off.

We confidently assume, therefore, that the whole of this most abundant mineral element once existed in the atmosphere in the form of a high-heated gas; and that some time and somewhere, on the confines of the enormously-extended sphere of vapors, there was found a current sufficiently cool to condense a portion of it. If the vapor of silicon follows the general rule—that the density of gases is in proportion to their atomic weights—then it was but a fraction heavier than oxygen, and therefore not far below it in the atmospheric strata. The unceasing commotion of the elements would soon have brought this first cloud-mist of silicon into contact with oxygen, to which it has a strong affinity under high heat. Oxidized, and in molten drops of silica, or crystals of quartz, this new-formed material commenced its descent toward the centre of gravity—the first creation from the primordial elements. As it fell into the more heated regions below, it was probably soon evaporated, and, the vapor rising, carried up with it the heat taken up in the evaporation. It was again condensed, its heat given up, and it descended for another charge of the internal fires. This, in all probability, is the epitome of the process of world-cooling.

At last, the showers of melted silex reached the liquid surface of the nucleus, which the force of gravity and compression must have formed, at an early period of the nebulous globe, of less or greater extent about its centre. From this period the increasing torrents of silica, intermingled with the silicates which were forming at the same time, poured down through the heavy vapors, and filled up the furlongs-deep of granite ocean. On this vast deposit, and at about this stage of the gradual cooling of the earth, began, we must suppose, the first hardening and crusting over of the surface, since at this point, near the close of the granite age, first commences the division of the earth's crust into varieties and layers more or less distinct, as also the upbearing of the heavy metals which, without this surface-hardening, could never have floated on any molten sea of minerals. The slow cooling of the granite masses beneath this crust and under the enormous atmospheric or other superincumbent pressure, conformed them to all the acknowledged conditions of the formation of the igneous rocks.

There is found in the different beds of the granitic rocks every proportion of the admixture of silica with the silicates of alumina.