IV.] THE CHEMISTRY OF THE PRIMEVAL EARTH. 41 



atmosphere upon the earth's crust. This, unlike the first, 

 which was subaqueous, or operative only on the portion cov- 

 ered with the precipitated water, is subaerial, and consists in the 

 decomposition of the exposed parts of the primitive crust under 

 the influence of the carbonic acid and moisture of the air, which 

 convert the complex silicates of the crust into a silicate of 

 alumina, or clay; while the separated lime, magnesia, and 

 alkalies, being converted into carbonates, are carried down 

 into the sea in a state of solution. 



The first effect of these dissolved carbonates would be to 

 precipitate the dissolved alumina and the heavy metals, after 

 which would result a decomposition of the chloride of calcium 

 of the sea-water, resulting in the production of carbonate of 

 lime or limestone, and chloride of sodium or common salt. This 

 process is one still going on at the earth's surface, slowly 

 breaking down and destroying the hardest rocks, and, aided by 

 mechanical processes, transforming them into clays ; although 

 the action, from the comparative rarity of carbonic acid in the 

 atmosphere, is far less energetic than in earlier times, when the 

 abundance of this gas, and a higher temperature, favored the 

 chemical decomposition of the rocks. But now, as then, every 

 clod of clay formed from the decay of a crystalline rock cor- 

 responded to an equivalent of carbonic acid abstracted from the 

 atmosphere, and to equivalents of carbonate of lime and com- 

 mon salt formed from the chloride of calcium of the sea-water. 



It is very instructive, in this connection, to compare the 

 composition of the waters of the modern ocean with that of 

 the sea in ancient times, whose composition we learn from the 

 fossil sea-waters which are still to be found in certain regions, 

 imprisoned in the pores of the older stratified rocks. These 

 are vastly richer in salts of lime and magnesia than those of 

 the present sea, from which have been separated, by chemical 

 processes, all the carbonate of lime of our limestones, with 

 the exception of that derived from the subaerial decay of cal- 

 careous and magnesian silicates belonging to the primitive crust. 



The gradual removal, in the form of carbonate of lime, of the 

 carbonic acid from the primeval atmosphere, has been connected 



