of Boston and its Vichiitij. S07 



r 



of the Blue liills, the highest land in this vicinity ; and it predom- 

 inates in part of Lynn, Lynniield, Danvers, IVIalden, and Chel- 

 sea ; at these last places, it is associated with the second Porphy- 

 ry formation. 



VII. In Bienite, many varieties arc produced hy the proportion 

 and aggregation of the constituent parts, and these varieties have 

 been raised to the rank of species, by some nomenclators. Two 

 only are now admitted by modern geologists. 



1. Sienitick Porphjnj. 



^ 



The structure of this is very fiue granular^ and the Feld.^pai 

 and Hornhlende are with difficulty distinguished; it contaius 

 large grains of Feldspar and Quartz, 



S. PoT^lujriticlc Sienite. 



This has a fine granular porpliyritick struclure, or it is a finc^ 

 granular Sienite, containing imbedded large grains of Feldspar. 



■m 



Both varieties occur in the range of the Blue hills ; the first is in the 

 greatest abundance; the second is also found in Maiden, Chel- 

 sea, Lynn, &c. Sienite has not been observed stratified in this 



vicinity. 



VIII. The foreign minerals, which are imbedded in Sienite, are 

 Quartz, Mica, Epidote, rarely Garnets, and Magnetick Iron Ore. 

 From the presence of the latter, Sienite frequently affects the mag- 

 net ; the Mica sometimes forms a large proportion of the mass : 

 and hills of Sienite, of a fine structure, containing Mica in quan- 

 tity nearly equal to the other ingredients, prevail for a great ex- 

 tent, particularly atDanvers; Epidote and Quartz traverse it in 

 small veins, though not in such quantifies as in Greenstone. Si- 

 enite is liable to decomposition, but is less susceptible of the 



t 



