240 ON THE DESTRUCTION OF ROCKS. 



change of colour, and, in consequence, preserve their 

 tenacity when rocks of other kinds, of the same 

 colours, fall to powder, or, at least, become softened. 

 It has not hitherto been suspected that the car- 

 bonat of iron was a common ingredient in rocks ; and 

 though its effects in accelerating their decomposition 

 is very trifling, it is proper that it should be enu- 

 merated among the rest % It is frequent in the white 

 veins of carbonat of lime that traverse the schists, 

 and is detected by the brown colour which it acquires 

 from the action of water. Its existence would as 

 little be suspected in the white quartz rocks. Yet, 

 in these also, it is discovered by their becoming rusty 

 on exposure, sometimes to the depth of even a quar- 

 ter of an inch ; as, by the same means, it is ascer- 

 tained to exist in some of the whitest compact fel- 

 spars. It ought to be added, that the rust thus 

 formed, appears to be soluble in the water of the 

 atmosphere ; as such rocks become bleached at the 

 surface, while the brown stain occupies an inferior 

 lamina. It is tjms that we must account for the 

 whiteness of the powdery surface so often found on 

 the decomposing argillaceous schists and felspars, 

 even where we are sure that these contain iron. If 

 there are any other chemical causes for the decom- 

 position of rocks, they are still unknown to us, and 

 must remain for the investigation of future chemists. 



Of the Decomposition of deep-seated Rocks. 



Although the phenomena in question occur most 

 conspicuously where rocks are exposed to the atmo- 

 sphere, or to the action of air and water together, 

 they also take place deep in the earth, as already re- 



