690 DYNAMICAL GEOLOGY. 



sequently present in all streams, lakes, and oceans. Other prominent 

 sources of this gas in the earth's waters, and in the soil, are : (1) the 

 respiration of aquatic and underground animals, carbonic acid con- 

 stituting a large part of the air exhaled ; (2) vegetable and animal 

 decomposition, carbonic acid being an ultimate product, as it is of the 

 combustion of coal ; (3) chemical agents (mentioned beyond), sepa- 

 rating carbonic acid from carbonate of lime. 



1. Eroding Action. — Carbonic acid has a strong affinity for potash, 

 soda, lime, magnesia, and iron. If waters containing carbonic acid are 

 made to pass through powdered feldspar, mica, hornblende, pyroxene, 

 limestone, and other mineral materials containing these substances, 

 portions of them will be taken up and carried off; and the disorgani- 

 zation thus begun is attended by a loss also' of silica and alumina, and 

 ends in the destruction of the rock made of these minerals, so far as it 

 is subjected to the process. Professors W. B. and R. E. Rogers found, 

 in their experiments on the action of carbonated waters, 0*4 to one per 

 cent, of the whole mass under digestion dissolved away in only forty- 

 eight hours. 1 



Some granites and gneisses are decomposed to a depth of fifty or 

 sixty feet ; and in tropical countries, like Brazil, where a warm climate 

 favors activity in nature's chemistry, and no glacial agent has worn 

 off the earthy surface of the country, the depth of altered rock, accord- 

 ing to Liais, is sometimes a hundred yards. The decomposition has 

 been attributed mainly to atmospheric carbonic acid and moisture, and 

 to a great extent by the process just pointed out. The decomposi- 

 tion of the sulphids of iron, when present, would also aid in the 

 destruction. 



Limestones are worn, through the same atmospheric agents. Waters 

 containing carbonic acid will dissolve readily carbonate of lime, making 

 of it the soluble bicarbonate of lime ; 1,000 parts of such water taking 

 up one of carbonate of lime. Carbonic acid from other sources aids 

 in this work, and especially in the case of limestones ; that produced 

 within the soil is an important contribution to underground waters, 

 and a means thereby of making caverns in limestone formations. 



2. By preparing the way for Oxydation. — Carbonic acid helps on 

 destruction, also, by giving iron a chance to oxydize. On dissolving 

 out the iron from an iron-bearing mineral, in the manner above ex- 

 plained, it forms with this iron carbonate of iron ; and then imme- 

 diately the oxydation of this carbonate of iron goes forward, as 

 already stated, and with the same result. This process, on the part 

 of carbonic acid, of robbing minerals of their iron, and then the next 

 instant losing the iron by its becoming an oxyd, is usually going on 



1 American Journal of Science, II., v., 401. 



