472 ON THE CONNECTION OF GEOLOGY 



latter. Mechanical powers only changing the aggrega- 

 tion of rocks, may break down their parts to a certain 

 size, according to their different nature; chemical powers, 

 again, which change the nature of substances, destroy 

 the connection of the minute parts of rocks. When che- 

 mical is preceded by mechanical action, it is much assist- 

 ed by it. The latter has a much more general effect, 

 as all rocks are subjected to its influence ; chemical de- 

 composition, on the other hand, acts only upon some 

 rocks, and in these only upon certain parts. The che- 

 mical decomposition of rocks is chiefly effected by the 

 oxygen of atmospheric air and of water ; but we are al- 

 so persuaded, that certain cryptogamic plants, intimately 

 attached to the surface of stones, Lichens namely, assist 

 in their destruction. 



The oxygen of air and water can only affect the con- 

 stituent parts of rocks, which have a great affinity to 

 it, such as the iron and sulphur forming pyrites, oxydu- 

 lous iron, oxydulous manganese, or the same substances 

 mixed with earth or carbonic acid, charcoal and bitumen. 

 Very solid and compact masses of rock, such as green- 

 stone, which are not easily affected by other means, are 

 sometimes corroded by the chemical change of the pyrites 

 contained in them, by which it is converted into a hy- 

 drate of iron *. In certain other rocks, which are also 

 readily broken down by mechanical agents, clay-slate for 

 instance, the disintegration is much accelerated by the 

 decomposition of the pyrites. The oxydulous iron of 



* The ochre yellow colour of the decayed greenstone around Edin- 

 burgh, and in general in many trap districts in this country, is caused 

 by the decomposition of the imbedded iron pyrites. 



