Page:Popular Science Monthly Volume 4.djvu/722

702 from vapors. Oxygen and silicon, which doubtless composed more than four-fifths of the entire bulk of the gases, were separated from each other only by the elements that were needed to make up the silicates. Their compound, silica, is involatile, and even infusible by itself, under any degree of heat that we can command. The same is true of lime and the earlier-formed silicates. Therefore it is impossible to decide from their volatility which of these substances would have first condensed and reached the surface. But, as the vapor of silica, when formed, would still be of nearly the same specific gravity with silicon (2.07), and would still separate by its immense volume the oxygen from the calcium below, we may suppose that in any case silica would have to be condensed and deposited, in greater part at least, before lime, the oxide of calcium, could be formed.

Along with silica were formed and deposited the silicates of alumina—mica and feldspar; then the partially fusible silicates of magnesia, lime, and iron—hornblende, augite, and talc. There followed a numerous order of complex silicates, in which the above-named ingredients are varied by small proportions of manganese, soda, strontia, zirconia, and many other mineral bases. With, and after these, was produced the lime-deposit, the last of the minerals. The metallic vapors, which were all heavier than the mineral, were condensed and deposited chiefly during the later silicate period, and somewhat in the inverse order of their volatility, but locally and irregularly as results of great perturbations, or storms in the air.

It will further be seen, from the last column of the table, that in no respect are the materials of the earth deposited according to their specific gravities as solids or liquids. There is, in the superincumbent rock and ore masses, no order of position that would indicate in the least the floating or buoyancy of the lighter substances. Therefore, their arrangement cannot be referred to any origin from liquid conditions; and the only other theory is that of their gaseous origin.

There are many apparent anomalies in the deposition of the metallic and mineral compounds, which may require much study, and perhaps further knowledge and experiment for their explanation. Thus there is in one place a carbonate of lime—marble—and in another a sulphate of lime—gypsum. There are in certain localities sulphuret-ores of iron or copper, and in others oxide-ores; while the metals of greatest vapor-density, as mercury, lead, bismuth, and antimony, are found almost exclusively in sulphuret-ores. It will perhaps eventually be established that sulphur was combined wholly into sulphuric-acid gas, as carbon was formed entirely into carbonic-acid gas; that both were brought to the surface of the earth in solution with rain-water; and that sulphur in this form united with the metals which had failed to be oxidized upon their condensation in the air, and sulphated the quick-lime in the earth, which had not been carbonated by the carbonic solution. Then there is the exceptional production in Nature