198 



Outlines of the Mineralogy and Geology 



resemble Arabick characters ; hence the name, Graphick Graniiie, 

 It is one of the most superb Granitick aggregates, and receives a^ 

 beautiful polish. 



S. Purj)hjritick Granite, 



The basis of this variety is composed of Quartz aud Mica, in 

 which are imbedded large crystalline grains of Feldspar. The 

 colour of the Quartz is w bite and bluish white ; that of the Mica 

 is black. They are united in nearly equal proportions, and their 

 size is small, and consequently the basis has a fine grained struc- 

 ture. The porphyritick structure is rendered very conspicuous, 

 by the large white grains of Feldspar, which are of a beautiful 

 lamillary structure, and of a rectangular form. Small grains of 

 black Mica and crystals of Sulphuret of Iron are sparsely dis- 

 geminated in them. 



V. The minerals, whicb occur in Granite 



Garnets and 



w 



Schorl. The great and useful peculiarity of all our Granites is 

 the almost total absence of Sulphuret of Iron. In consequence of 

 the decomposition of the Feldspar, some specimens are partially 

 cellular, and their surface covered with a whitish crust. 



VI. No formation of Granite is found in this vicinity. It occurs 

 throughout the compass to which these observations are confined, 

 in rolled masses, aud it is especially abundant at Wilmingtonj 

 Carlisle, and Bedford. The Graphick Granite is found at Oar- 

 lisle, associated with coarse grained reddish Granite, and the 

 transition of these into each other is abrupt ; the Porphyritick 

 occurs at Cambridge, Newton and Needham, Neither of these 

 varieties is coramon. 



