64 CHEMISTRY OF THE ROCKS. 



have first condensed and reached the surface. But as the vapor 

 of silica when formed would still be of nearly the same specific 

 gravity (2.07) with silicon (1.97), and would still separate by its 

 immense volume the oxygen from the calcium below, we may 

 suppose that in any case the 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 alu- 

 mina mica and feldspar; then the partially fusible silicates of 

 magnesia, lime, and iron hornblende, augite, and talc. Then 

 followed a numerous order of complex silicates, in which the ' 

 above-named ingredients are varied by small proportions of man- 

 ganese, 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 perturba- 

 tions 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 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 expla- 

 nation. Thus there is in one place a carbonate of lime mar- 

 ble 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 mer- 

 cury, 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 



