324 



ILLUSTRATIONS OF THE 



particulary of the feltfpar and fchorl, and appears 

 to be fo paffive a body, that it has been doubted 

 by fome mineralogifts, whether in this Hone 



I 



it ever affumes its own figure, except where 

 cavities afford room for its cryflallization. But 

 it is certain that, beiide the Daourian granite 

 juft mentioned, there are others, in which the 

 quartz is completely cryflallized. Of this fort 

 are fome fpecimens, found in a granite vein on 

 the weft fide of the hill of St Agnes, in Corn- 



r 



wall. The vein traverfes the primitive fchif- 

 tus, of which that hill conlifts, from fouth to 

 north nearly : the ftone is much decompofed, 

 and the feltfpar in general is almoft reduced 

 to the ftate of clay. In this decompofed mafs, 

 quartz cryftals are found, having the ftiape of 

 double hexagonal pyramids, perfedly regular 

 and complete. The fide of the hexagon, 

 which is the bafe of the two oppofite pyra- 

 mids, varies from half a tenth to a tenth of an 



inch in length, and is the fame with the altitude 



of each of the pyramids. In fome few fpeci- 

 mens, the two pyramids do not reft on the fame 

 bafe, but are feparated by a very fiiort, though 



I ^ - 



regular, hexagonal prifm. The furfaces of thefe 



f 



cryftals are rough, and fomewhat opaque, with 



flender fpiculse of ftiorl frequently traverfing 



them. This roughnefs is occafioned by flight 



furrows 



