688 DYNAMICAL GEOLOGY. 



morphism in the Earth's history, this has been a prominent source of 

 the changes. 



Water in a superheated state is present in the conduits of all vol- 

 canoes ; and it is supposed that the apparent liquidity of the lava is in 

 part only a mobility among the grains, produced by this means. 



2. Through the Elements of Water directly. 



Water consists of oxygen and hydrogen, in the proportion, by 

 weight, of 8 O to 1 H. The oxygen is the element of chief import- 

 ance. But water has acted conjointly with atmospheric air, in these 

 changes ; and the oxygen produced through their united action has 

 often come from the air instead of the water. Water alone is usually 

 a protector of the rocks it covers. 



A. OXYDATION AT THE ORDINARY TEMPERATURE. The Cases 



of oxydation of widest geological influence are those of the sulphids 

 of iron, pyrite (FeS 2 ) and pyrfhotite (Fe 7 S 8 ), and those of carbo- 

 nates containing iron. In each, iron is the principal oxydizing element. 

 The oxyds of iron concerned are the protoxyd, FeO ; the sesquioxyd, 

 Fe 2 3 , or hematite, which has a red powder ; and the hydrous sesqui- 

 oxyd, Fe 2 3 -\- 11H 2 0, which has a brownish-yellow powder, and is 

 called limonite, or sometimes brown hematite. 



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- 



