MODE V. GRANITE. 177 



A rock of green and violet layers, being a kind 

 offelsite. Sauss. 1448. 



What he calls a trap, with a paste of a greenish 

 grey felsite, and grains of quartz and felspar. 

 2043. 



1558. The rotten-stone of England may be Rotten-stone* 

 regarded as a kind of tripoli, but is neither porous 

 nor of a slaty structure. He concludes that tri- 

 poli consists of a fine sand of felsite. 



MODE V. GRANITE. 



This important substance, which composes 

 the highest chains of mountains, and was used 

 by the Egyptians in the earliest monuments of 

 art, is chiefly composed of felspar and quartz, 

 which have been already described : a third sub- 

 stance is also indispensable in granite, namely, 

 either mica or siderite. And even when both 

 these latter are joined, the most exact mineral- 

 ogist or geologist could not refuse the strict ap- 

 pellation of granite, as different mixtures may 

 be found in no very remote parts of the same 

 rock. As some granites, instead of felspar, pre- 

 sent felsite, yet are universally admitted into this 

 class ; so the mica may pass into talc or steatite, 

 or siderite, as on the summit of Mont Blanc, and 

 VOL. T, N 



