jO 



of Boston and its Vicinity 



S05 



IV. Epidote is disseminated in this rock, though it occurs much 

 less frequently than in any other in this vicinity, Argillite except- 

 ed. The only metal which we have observed in it is Suiphuret 

 of Iron ; this is in small quantity only, and is principally ci)nfin- 

 ed to the varieties which have a black coloured basis. 



V. Porphyry is unstratified in this vicinity, and is intimately 

 connected with Sienitc and Petrosilex, into hot!) of which it passes. 



r 



VI. The reddish variety is the most abundant, and is found 

 principally in Maiden, Lynn and Chelsea, where it forms hills. 

 The blackish varieties are found in rolled masses at Chelsea, Na- 



\ m 



liant and Hingliam ; it approaches the Clinkstone Porphyry, and 



may probably be one of its varieties^ for it gives a ir.etallick sound 



when struck with a hammer^ and separates into tables, Tts struc- 



ture is sometimes slightly slaty^ and its fracture conchoidal and 

 small splintery, 



VII. Porphyry is sufficiently hard to give fire with steel, and is 

 susceptible of a beautiful polish ; when exposed to the action of 

 the atmosphere the Feldspar generally first decomposes, and af- 

 terwards the basis ; hence we frequently find the surface covered 

 with an earthy crust. 



V. SIENITE. 



I. This rock is composed of Feldspar, Hornblende and 

 (Quartz. It is usually defined, an aggregate of Feldspar and 

 Hornblende, sometimes admitting into its composition Quartz and 

 Mica, but as the greatest proportion of Sienite in this vicinity 

 contains Quartz, we have given it a place in our definition. 



II. The ingredients are granular aggregated ; the Feldspar 

 predoiuioates ; and the general colour of the mass is some shade 



.S7 



