ON THE DESTRUCTION OF ROCKS. 239 



been once lead-coloured, may be seen in many of the 

 secondary limestones, where the change can be traced; 

 but it is scarcely suspected that this process has been 

 carried on in so extensive a manner in the sandstones, 

 which I shall nevertheless hereafter show. 



Iron, in the form of pyrites, is known to destroy 

 many of the secondary sandstones in which it exists. 

 But it is not sufficiently common to produce any 

 extensive or conspicuous effects ; and, in the state in 

 which that mineral occurs in the primary rocks, it is 

 scarcely susceptible of decomposition. 



It is in the state of the diffused or combined pro- 

 toxyde, that the power of iron in decomposing rocks 

 is most remarkable ; and it is of sufficient importance 

 to be numbered among the causes that are most 

 active in the work of destruction. The chemical 

 process is simple ; as this oxyde combines easily with 

 water and carbonic acid, thus producing rust, which 

 is indicated by the yellow and brown colours which 

 the rocks that contain it acquire on exposure. It is 

 probable that, in these cases, the actual decom- 

 position is the consequence of the increase of bulk 

 which the stone thus acquires ; and therefore, in 

 many instances, where the quantity of this ingredient 

 is small, the rock does not moulder, although it 

 becomes more tender. That is an event not unfre- 

 quent in many limestones and sandstones ; as well as 

 in some of the claystones of the trap family, which 

 retain their integrity after their colour has been 

 changed. Where, as in many basalts, the iron 

 abounds, the event of this change is to resolve the 

 whole into clay. There is yet however some ob- 

 scurity in this subject, which it requires the future 

 aid of chemistry to dispel. Many shales and slates, 

 though apparently containing this oxyde, resist all 



