58 CHEMISTRY OF THE ROCKS. 



metallic precipitations, there was continually falling over all the 

 earth the white impalpable powder of lime the element calcium 

 condensed into cloud-mist and oxidized in the upper regions of 

 the air. 



These were the great chemical periods of our world ; when 

 the cooling vapors of the swollen sphere were struggling to unite 

 and hold fast the embrace against the antagonist force of heat ; 

 when the conjoined elements were pouring down their fiery tor- 

 rents, and the air was laden with the falling cinders and ashes of 

 aerial conflagrations ; when the vast workshop of Nature was 

 forming and sorting its raw materials. 



We do not however wish to be understood as insisting that all 

 these minerals and metals came down in just the form and order 

 that we have indicated, or that they were regularly deposited 

 and left the orderly traces that perhaps our hasty sketch would 

 seem to imply. There were unquestionably constant and pro- 

 found commotions in the atmosphere, and the commingling of 

 the most diverse elements. There were doubtless repeated melt- 

 ings and chemical recombinations at the surface, and the rending 

 and comminuting of the newly-formed crust by internal forces. 

 The history of the earth's irregularities and disorders forms the 

 greater part of geology. 



But what we do claim as certain is, that all the constituents of 

 the outer shell of our globe existed at one time as elemental 

 gases above a sea of matter that was held in condensation by 

 superincumbent pressure ; that as the earth gradually cooled, 

 these gases condensed somewhat in the order, inversely of their 

 fusibility, and directly of their nearness to the outer bounds of 

 the atmosphere, and fell to the surface like rain and snow from 

 water-clouds ; that they formed chemical combinations at the in- 

 stant of their condensation or subsequently, according to the 

 power of their affinities or the elements that were present ; and 

 that, excepting the more recent displacements by mechanical 

 forces, they now lie in the earth as they fell from the heavens. 



The silica and silicates which form the base and by far the 

 greater part of the earth's crust, became oxides of their several 

 selement because oxygen was the superabundant gas in the earth's 



