704 



DYNAMICAL GEOLOGY. 



1. The Sulphids of Iron. — The oxydation of these sulphids is one 

 of the most universal means of rock destruction ; for there are few- 

 rocks that do not contain pyrite, in disseminated grains or crystals ; 

 and only the firmer and smaller crystals of pyrite withstand the ten- 

 dency to change. Under the combined influence of moisture and the 

 atmosphere, both the iron and sulphur undergo oxydation, and often 

 produce sulphate of iron ; or, if bases are at hand, like lime, or alka- 

 lies and alumina, the acid takes the lime to make sulphate of lime, or 

 the alkalies and alumina to make alum ; and the iron, thus left free, 

 becomes a sesquioxyd, and usually the hydrous sesquioxyd, or limonite. 

 Thus the decomposition is doubly destructive. Whenever taking 

 place in a granular rock, the oxyd, becoming distributed among the 

 grains, tends to pry them apart, and so disaggregate the rock ; while 

 the acid aids in decomposing the other ingredients present. 



2. Carbonates containing Iron. — Carbonate of iron is the ore of 

 iron called siderite or spathic iron. Under exposure to air and mois- 

 ture, the iron, which the mineral contains in the protoxyd state, under- 

 goes oxydation, becoming brown, and changing to limonite. The 

 alteration goes on rapidly, to the depth that water and air succeed in 

 penetrating. Any rock, through which this carbonate is distributed, 

 will undergo rapid alteration and destruction at surface. A ferrif- 

 erous carbonate of lime, or carbonate of lime and magnesia (in which 

 iron replaces part of the calcium or magnesium), undergoes the same 

 kind of destruction, though less rapidly. The rock often becomes re- 

 duced to a bed of more or less pure limonite. Crystalline limestones 

 usually undergo this change more readily than common massive lime- 

 stone, because more permeable to moisture. 



3. Other Cases of Oxydation. — The oxydation of carbon, hydrogen, and other in- 

 gredients of vegetable and animal matter, is another important means of geological 

 change, through the oxygen of water and air. The fallen unburied leaves and stems 

 of the forest have their carbon changed by this means to carbonic acid, and so, in a 

 true sense, consumed; and if buried, the air being to a great extent excluded, part of 

 the carbon will be preserved to make coal, while other portions will be lost by this sort 

 of combustion (p. 363). Animal matters are subject to an analogous change. 



Again : Oxydation takes place through the agency of microscopic plants. The decay 

 of animal and vegetable matter is a process of oxydation. But it never begins except 

 through the presence and agency of certain kinds of vegetable organisms {Bacteria, 

 and the like) whose germs exist everywhere in the air; for it has been found by exper- 

 iment that the complete exclusion of these organisms prevents putrefaction from taking 

 place. 



Besides producing the oxydation of decay (and also that of ordinary fermentation) 

 microscopic plants, but of a different kind, oxydize the nitrogen of the atmosphere and 

 make nitrates. These organisms live in dark places, and hence it is that nitrates, the 

 source of saltpetre, (as calcium nitrate,) form in the earthy floors of caverns or covered 

 places. The process (called nitrification) often slowly erodes or disintegrates the rocks 

 of the cavern, since the nitric acid may take from these rocks the lime or other ingre- 

 dient with which it enters into combination. 



