of Boston and its TlcinUif. 197 



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0111 



I. GRANITE. 



• L This is a compound granular aggregated rock, consisting 

 of Quartz, Feldspar, and Mica. Its predominant colour is s 

 shade of grey or red ; the colour of the Quartz is white, or black 

 isb grey; that of the Feldspar U reddish, or whitish ; and tha 

 of the Mica, hlack and grey. 



II. The size of the component parts of Granite varies fi 

 very small to large, and they are united in various proportions. 

 The Feldspar is the most abundant 'and consequently gives colour 

 to the mass ; next in proportion is the Quartz, which in some 

 specimens predominates. The Mica is sometimes in large plates ; 

 that of a black colour is in plates, three or four inches long, pen- 

 etrating the Feldspar ; and it is sometimes entirely excluded. 



III. From the various proportions of these ingredients, tlie as- 

 pect of Granite is much modiHed, and a variety of compound pro- 



duced, which are termed Gr 



Feldspar 



and Quartz form bases for other minerals. Quartz and Feldspar 

 are united to eacb other in various ways and proportions, but Mica 

 is not united to either of these constituents singly, 

 IV. Among Granitick aggregates are found, 



i. Graphick Granite. 



Tliis rock is composed essentially of Feldspar and Quartz, 

 with sometimes a very small proportion of Mica, The colours of 

 the Feldspar are flesh and vase red, and it forms a basis, ia 

 which are imbedded lamina of Quartz, of a smoke or yellowish 

 grey colour, resembling the silver grain in vegetables. When the 

 mass is broken, in a direction transverse to the plates of Quartz, 

 they presjBnt lines nearly parallel and regular, and which ranch 



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