AGENCY OF PYRITES AND WATER. 189 



of rocks, in this way becomes pulverized to a greater or less 

 depth.* 



The principal mineral substances with which the oxygen 

 of the air and of water unite, are iron, manganese and pyrites. 



When a rock contains iron or manganese, in a low state 

 of oxidation, these oxides attract more oxygen from the air 

 and water,! increase in bulk and split or cleave into their 

 layers ; thus affording an opportunity for the mechanical 

 agency of water, either by friction or by freezing. 



2. Pj/r/^es, or the bi-sulphuret of iron, exerts the most pow- 

 erful agency in the decomposition of rocks, and perhaps the 

 most extensive ; as this mineral is widely disseminated through 

 nearly all classes of rocks. It is composed of sulphur and 

 iron. The sulphur attracts oxygen from the air and from 

 water, and forms the well known substance sulphuric acid 

 (oil of vitriol). The iron also combines with oxygen from the 

 same source, and forms an oxide of iron. The acid and the 

 oxide now unite and produce a new compound, the sulphate 

 of iron or copperas, a substance capable of being dissolved in 

 water. Thus the rock, through which the pyrites is dissemi- 

 nated, is crumbled, thrown or changed in its properties. But 

 the action does not stop here. The sulphate of iron, being 

 dissolved in water, which is constantly penetrating the mass, 

 is brought into contact with feldspar, and both are decom- 

 posed ; the sulphuric acid in the copperas abandons the iron, 

 and unites with the potash and lime in the feldspar, forming 

 sulphate of potash and of lime, while the oxide of iron is de- 

 posited in the form of iron rust. 



* This process is called disintegration, and some examples are 

 found in Massachusetts, where the gneiss rocks have been penetrated 

 fifteen feet. The rock is said to rot. Almost every variety of rock is 

 constantly undergoing this change. 



t This action of the oxygen of the air and of water to produce dis- 

 integration, explains the effect of allowing lands to remain fallow, by 

 which their fertility in a measure is restored 



16* 



