ON THE DESTRUCTION OFROCKS. 243 



the same fact is presented in a manner perfectly un- 

 questionable. In consequence of the protection af- 

 forded to the subjacent rock by the solid mass of peat 

 with which it is covered, it remains so undisturbed as 

 to present all its divisions and concretionary forms as 

 if still in a state of integrity ; the zeolites which it 

 contains remaining also unaltered. But it is no longer 

 a rock : the spade and pickaxe cut through it as 

 through earth ; and, where natural forces have acted, 

 the whole moulders into yellow clay, leaving the zeo- 

 lites in heaps resembling banks of gravel. 



In Bute, there are found beds of a tenacious com- 

 pact clay, lying deep under a mass of solid trap, and 

 presenting what, on a superficial view, would be 

 deemed a natural and original state. On an accurate 

 examination, however, it is discovered that they are 

 interspersed with crystals of felspar, sometimes entire, 

 but more frequently reduced also to clay, yet of a 

 different colour from that which forms the base. By 

 this and some other appearances, it is proved, that 

 these have once been masses of porphyry, which have 

 thus undergone decomposition, deep in the earth, 

 while the rocks above them retain their original in- 

 tegrity. 



In the islands of Luing and Torsa, there are some 

 large veins, consisting, apparently, of that yellow 

 arenaceous claystone so well known to the geologists 

 who have visited Arrari ; and, in some places, they 

 present a porphyritic structure. No suspicion re- 

 specting these could have arisen, had it not been for 

 some deep and fresh fractures. 



Thus it is discovered that the original rock is a 

 dark compact claystone, often called basalt, in some 

 parts porphyritic; and that the yellow claystone is 

 not a natural rock, but the consequence of an inci- 



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