230 THE CANADIAN NATURALIST. [May 



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 pre- 

 cipitate the dissolved allumina 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 lime- 

 stone, 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 com- 

 parative rarity of carbonic acid in the atmosphere, is less energetic 

 than in earlier times, when the abundance of this gas, and a higher 

 temperature, favoured the chemical decomposition of the rocks. 

 But now, as then, every clod of clay formed from the decay of a 

 crystalline rock corresponded to an equivalent of carbonic acid 

 abstracted from the atmosphere, and equivalents of carbonate of 

 lime and common salt formed from the chloride of calcium of the 

 sea-water. 



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

 position 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 sub-aerial decay of calcareous 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 with 

 areat changes in the organic life of the globe. The air was doubtless 

 at first unfit for the respiration of warm-blooded animals, and we 

 find the higher forms of life coming gradually into existence as 

 we approach the present period of a purer air. Calculations lead 

 us to conclude that the amount of carbon thus removed in the form 

 of carbonic acid has been so enormous, that we must suppose the 

 earlier forms of air-breathing animals to have been peculiarly 

 adapted to live in an atmosphere which would probably be too 

 impure to support modern reptilian life. The agency of plants in 



