"^ 



gQ2 



Outlines of the Minevalogij and Geology 



VII J. The external surface of Greenstone frequently presents 

 a rusty brown colour, wliicli arises fromihe decomposition of the 

 imbedded Sulphnret of Iron. The Oxide of Iron is fount! in va- 

 rious states and of various colours in the same specimen ; near 



ycllo 



pulverulent ; interiorly 



more 



compact, and its colour is reddish brown, and often of a brig 

 Vermillion. When, by exposure to the atmosphere, the Sulphur 



Iron is not only decomposed, but removed 



face of 



rock becomes cellular, and then much resembles some varieties of 

 Lava ; sucb specimens are characterized by their difficult frangi- 

 bility, toughness, and by the dark green colour and crystalline 

 structure of the Feldspar, which are very evident in the compact 

 centre of these masses. 



IX. The lava-like varieties are found principally at Charles- 

 town in rolled masses, and appear to be the result only of long 

 exposure to the atmospliere, for in no place does Greenstone 

 with such peculiar Feldspar, occur in situ, in this vicinity. 

 . X. The aspect of Greenstone is much modiDed by the differ- 

 ent proportions and aggregation of its component parts ; among 

 the varietie*s which are thus produced, two, w hich have been not- 

 ed by Geologists, are found, though not abundantly, in this neigh- 

 bourhood, viz. 



I. 



./ 



Greenstone Porphy7*i/. 



Greenstone Porphyry, Jameson, vol. in. p. 



1 



Cleaveland, p. 6lO. 



In this rock, the 

 intimate, and the mi 

 basis, which includ 



I. Porphyritick Greensfone, 



of Hornblende with 



Feldsp 



IS 



often appears homogeneous ; they form 

 large crystalline grains of Feldspar, of 



blapk or greyish black colour, with a perfectly 



