126 



DYNAMICAL GEOLOGY. 



destructible mineral is a chief constituent become covered with a rusty crust 

 which is ever encroaching inward ; and this crust is slowly reduced to a 

 rusty earth, having parted with all soluble ingredients ; or, losing the rusting 

 mineral, it finally falls to earth or sand. A porous granite or gneiss contain- 

 ing black mica may become deeply rusted, and finally reduced to a weak mass 

 of quartz and unaltered feldspar, — good material for a granitic sandstone. 



If marcasite or pyrite is present in any rock, there is not only oxidation, 

 but corrosion from the sulphuric acid that may be formed, which attacks any 

 lime present in tlie minerals of the rock, or any magnesia, or potash, or soda, 

 or alumina, and makes sulphates with each. The aluminum sulphates are 

 alums, but strictly so only when potash, soda, or some other base is also 

 present. Some beds of shale containing iron sulphide are impregnated or 

 interlaminated with alum which has been thereby made, the shale affording 

 the alumina of the alum. 



Limestones, even the whitest of marbles, often contain a trace of iron 

 or of manganese in combination, and occasionally masses of the iron car- 

 bonate, siderite. The iron carbonate, unless in a massive state, readily 

 oxidizes ; and so also does the iron of the limestone on exposure for a few 

 months ; and this is a commencement of the change in the whole mass to 

 limonite. The work in progress is illustrated by Fig. 132, representing an 



Impure limestone decaying to limonite. 



Same, with caloiferous echist. D. 



impure ferriferous limestone as it appears where the alteration is going 

 on at the Amenia Ore-pit, N.Y., southwest of Salisbury, Conn. ; and Fig. 

 133, the same, with the calciferous schist adjoining also changing. If one 

 per cent of iron is present, a limestone will rust and decay ; if as much man- 

 ganese is present, it will become covered with black stains. The massive 

 siderite changes slowly over the surface and in rifts. 



Limonite — the yellow-brown oxide of iron, or yellow ocher — is the most 

 common result of the oxidation; but hematite, of red-ocher color, is often 

 produced in warm and rather dry climates. Nearly all red, yellow, and 

 brown rocks, sand-beds, or earth-beds, owe their color to iron in one of 

 these two forms. 



Oxidation of the iron in pyroxene gives the yellow-brown fronts to trap 

 bluffs — not their gray and black tints, which are due to lichens; and has 

 spread delicate surface shades of red and yellow over sandstones in the 

 Yellowstone Park, and other dry parts of the Rocky Mountains, through 

 the oxidation of the little iron inside. 



