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low : yet in speaking of the granite through which 

 runs the Garrone, he says, " This species of rock 

 braves the injuries of time, and the continual action of 

 the waters."* 



So prevalent is the opinion of the degradation of 

 mountains, that some attempts have been made to as- 

 certain the period of time requisite to accomplish their 

 total destruction, or to reduce them to a level with 

 their adjacent vallies. 



M. Gensanne has found by observations, which he 

 pretends are unequivocal, that the height of the Py- 

 rennees is lessened, by a gradual disintegration, or 

 decomposition, at the rate of ten inches in a century. 

 Upon this, supposing their height to be fifteen hundred 

 fathoms above the level of the sea, he calculates the 

 time necessary for their destruction, admitting the 

 progress to be perpetual, to be that of a million of 

 years.f 



Believing it unnecessary to notice the opinions of 

 any other authors, who are in favour of the decompo- 

 sition of rocks, I will observe that it appears some- 

 what singular that Mr. Kirwan should indulge the 

 belief, when, from his own assertion, the Runic rocks 

 "have withstood decomposition these two thousand 

 years as their characters evince."} 



But on this point, I can see but little necessity for 

 his having selected the Runic rocks as an instance of 



* Mineral des Pjrennees, page 248. f Do. 87. 

 \ Kirwan's Essays, page 436. 



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