706 



DYNAMICAL GEOLOGY. 



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 

 more or less slowly, whenever rocks containing these iron-bearing 

 minerals are accessible to air and moisture. The action of the car- 

 bonic acid cannot be perceived ; but the oxydation of the iron, the 

 secondary result, is very manifest in the brownish or reddish color 

 which the exposed rock acquires, and also in its disaggregation. In 

 the case of a close-textured rock, like much doleryte (trap), the 

 change gradually extends from the surface inward, making a dis- 

 colored crust. This crust loses at surface at the same rate that it pro- 

 gresses inward ; and hence its thickness, for a given variety of rock, is 

 nearly uniform. 



B. Organic Acids. — The work here attributed to carbonic acid 

 is also performed, though to a less extent, by organic acids, made 

 from vegetable or animal decomposition. They contribute to the 

 solution and erosion of limestones, and also to the process of oxyda- 

 tion. 



C. Silica. — Silica is present, in minute traces, in most natural 

 waters. 7,500 parts of water will dissolve one part of silica in the 

 gelatinous or soluble state ; and the shells of Diatoms, which are pres- 

 ent over the bottoms of most waters, are silica in tkis soluble state. 



