life could come to any perfection. The solidifying of the carbonic oxide was the latest and the slowest of the atmospheric changes.
It appears that during the epoch of the hydration of the lime-rocks there occurred periods when the waters were gathered into seas, and were sufficiently cooled for the existence of marine infusoria, mollusks, and corals. Life, in some form, has ever been ready to spring into being the moment that conditions and surroundings were suitable for it. After the deposition, in those temporary oceans, of considerable thicknesses of Cambrian or Silurian strata, mixed with organic remains, some rent or upheaval has let the waters down to new beds of unslaked material, which have heated, and, as it is termed, metamorphosed those first fossiliferous deposits.
The subsequent changes which the earth's crust has undergone—aqueous, volcanic, and organic—the working up of the conglomerates and sandstones, the depositing of the deep-sea beds, the overflowing of the traps and lavas, the storing away of the carboniferous treasures, are all the story of every hand-book of geology, and pertain no more to one theory than another of the origin of the rocks. When the quarries were once made and opened, the after-work was merely mechanics and masonry.
We have heretofore assumed that the gases which originally composed the aerial envelope of the earth took up separate positions therein, according to their specific gravities. This might seem to be controverted by experiments on the diffusion of gases, in which those of very different weights, as chlorine and hydrogen, will intimately commingle, even against gravity, when brought into contact. This may be true in the narrow compass of a laboratory experiment, and yet not apply to any considerable thicknesses of the gases. Such a diffusion, of one mile in depth of chlorine, would be equal to lifting up to the hydrogen a shell of solid iron two feet thick. Whether we explain the distinguishing principle of the constitution of gases as a mutual repulsion of their molecules, or, according to a late theory, as an incessant motion and clashing of atoms, there is nothing in either to warrant the supposition of the lifting or overcoming any considerable weight in the diffusion of gases. Under the first theory, diffusion, to a limited extent, would be accounted for by the small residuum of chemical or cohesive attraction that would remain between the atoms when separated as they are in gases; and, under the last theory, by the mechanical impulsion of the molecules, through their hitting against each other. Evidently, it is a principle which operates only within narrow limits, and in the lower temperatures of the gases. The sun gives no indications of such a commingling of its gaseous elements. Spectrum analysis, when applied to its outer edges, shows first hydrogen, then the vapors of sodium and magnesium, and, lastly, those of calcium and iron. The same fact and order of position are found to exist in the more condensed layers of the sun-spots.
