236 ON THE DESTRUCTION OF ROCKS. 



pure. When they contain calcareous particles, it is 

 more easy to suppose that their failure has resulted 

 from the solution of that ingredient ; and, in either 

 case, it is easy to see that a considerable disintegration 

 of rock may take place, even when very small portions 

 of it have been actually dissolved. 



In Granite, this effect is often very sensible, and it 

 is no where more easily observed than in Cornwall. 

 Cavities, containing water in wet seasons, are very 

 common in the granite of that country, and they are 

 often of considerable dimensions, whiie they are ex- 

 cavated in forms so accurately curved as to resemble 

 works of art. They are well known to those who 

 have interested themselves in the antiquities of that 

 county, by the name of rock basins : having been idly 

 attributed to the favourite Druids of antiquaries. 



It is easy to trace their progress from that which 

 can contain but a single drop of water ; and, as this 

 enlarges, the work of destruction goes on in an acce- 

 lerating ratio ; while the fragments of quartz and 

 felspar, remaining, serve to prove the nature of the 

 cause. Whether, in this case, the quartz is acted on, 

 or the felspar, or both, is uncertain; but that the 

 effect does not arise from any action on ferruginous 

 matter, is unquestionable, as it takes place equally 

 where the felspar contains no iron. 



It is probable that a more indirect action of water 

 has a similar effect in producing the disintegration of 

 rocks, independently of its chemical power upon the 

 iron which they may contain. It was elsewhere shown, 

 that all rocks contained water when deep in the earth, 

 and that, so far, they are porous, however solid in a 

 general sense. By being alternately wetted and dried 

 at the surface, they may thus undergo alternate con- 

 traction and expansion, in such a degree as, in time? 



