ON THE DESTRUCTION OF ROCKS. 237 



to lose their integrity, as far as that operation can 

 reach ; independently of the frequent solution and 

 precipitation of their soluble parts which must thus 

 take place. 



The destruction of the calcareous rock of Malta, 

 described by Dolomieu, may be noticed in this place, 

 though it is a peculiar instance, not arising from water 

 alone, and apparently limited to that rock. When 

 wetted with sea water, a crust is soon formed, which 

 exfoliates and is followed by others, till the whole 

 stone falls to pieces. It is still more remarkable that 

 a single drop is capable of producing this effect ; and 

 that it even extends through a whole wall, where the 

 salt has not reached. 



It must be remarked that those rocks which are 

 subject to decompose in considerable masses, as if by 

 an universal caries, even when situated deep in the 

 earth, contain alkali as an ingredient in their compo- 

 sition. It is in granite, in gneiss, and in the "trap 

 rocks, that this circumstance is peculiarly frequent and 

 remarkable ; and, in all these, either potash or soda 

 is found, in some, both. This effect, unless when it 

 arises partly from changes occurring in the ferruginous 

 ingredient, must probably be attributed to the solvent 

 power of water ; and, however difficult it might be 

 imagined for water to disengage alkali from such a 

 combination, the possibility is confirmed by the effect 

 which it is known to produce on glass. 



The changes which are experienced by iron, are 

 however the most conspicuous, and apparently the most 

 important of the chemical agencies by which the 

 decomposition of rocks is effected. This metal exists 

 in stones in different conditions, all of which, it is 

 probable, have not yet been ascertained by chemical 



