150 Elements of Mineralogy, 



portion and fize are extremely variable* as 

 well as their colour. The three firft confti^ 

 tute the hardeft fort of the granite, and moft 

 anciently known ; that into which the fhoerl 

 enters, is more fubjecT: to decompofition ; it 

 never has any particular texture, but confifts 

 of enormous fhapelefs mafles, of great hard- 

 nefs. 



In the finer granites, the quartz is tranfpa- 

 rent, in others generally white or grey, violet 

 or brown ; the feltfpar, white, yellow, red, 

 green, or black, it is generally the moft copi- 

 ous ingredient. The mica is alfo grey, brown, 

 yellow, green, red, violet or black, and is 

 commonly leaft copious. The fhoerl is gene- 

 rally black, and abounds in the granites that 

 contain it. Hence the colour of granites 

 chiefly depends of the feltfparor fhoerl. The 

 red granites confift commonly of white 

 quartz, red feltfpar, and grey mica. The 

 grey of white quartz, grey or violet feltfpar, 

 and black mica. The black commonly con- 

 tain fhoerl, inftead of feltfpar. The green 

 commonly contain greenifh quartz. 



If granite be expofed to a blow-pipe, its 

 different concretions fepirate from each other. 

 In a crucible, Mr. Gerhard found the feltfpar 

 melted into a tranfparent glafs ; under it the 

 jnica lay in the form of a black flagg, and 



the 



