DOMAIN XI. DECOMPOSED. 



it enters, yield easily to the decomposing 

 principle. Some granites, I may say most, 

 are in appropriate circumstances not diffi- 

 cultly decomposed, the mica and felspar 

 are chiefly affected : the same may be also 

 said of most sand-stones, particularly those 

 whose cement Is argillaceous or ferruginous, 

 and many porphyries and gneisses/'* 



From these interesting observations it 

 will appear, that the decomposition of 

 rocks is not only a curious subject in itself, 

 but of the greatest importance to the arts, 

 particularly architecture and sculpture. 

 Many noble edifices have soon become 

 disfigured, because the architect did not 

 know the easy decomposition of the mate- 

 rials. Thus at Trianon the pillars are al- 

 ready decayed, because the argillaceous 

 nature of the marble of Campan will not 

 bear exposure in the open air, where it soon 

 exfoliates. At Oxford it has been observed 

 that some of the public buildings are in- 

 jured, because the builders had not studied 

 the nature of the stone, which requires to 



* Kirwan's Geological Essays, p. 143153. 



