78 THE CHEMISTRY OF CREATION. 



the hard rocks and minerals exposed to their 

 action, we may proceed to select a few in- 

 stances which will exhibit the importance of 

 their operations on the large scale. D'Aubuisson 

 relates that the granite -country of Auvergne 

 and the Eastern Pyrenees is often so much 

 decomposed, that the traveller may imagine 

 himself upon large tracts of gravel. And to 

 show that this process, under favouring circum- 

 stances, may be of sufficient rapidity to be 

 observable in a few years, the same author 

 mentions, that in a hollow way which had been 

 only six years blasted through granite, it was 

 found on examination that its walls were so 

 much decomposed by the influence of carbonic 

 acid, that the solid rock, to the depth of three 

 inches, was in a crumbling condition. Dolo- 

 mieu calls the peculiar effect produced by this 

 gas a " disease of the granite," la maladie du 

 granite. In such districts, masses of granite 

 are found, which look quite solid, yet when 

 touched by the hand, or trodden by the foot 

 of the traveller, fall to powder. Such is the 

 influence of this decomposition in granite, that 

 it is found in the quarries at Dartmoor, to the 

 depth of fifty or sixty feet, to be more or less 

 decomposed. Consequently, this granite, which 

 is called surface granite, is less durable than 

 that obtained beyond the influence of decom- 



