153 Outlines of the Mineralogy and Geology 



Chymical Characters. 

 li is fusible without addition, before the blowpipe. Diflferent 



+ 



specimens are fused wiili greater or less ease ; generally they 

 must be subjected to the blowpipe flame for one or two minutes, 

 and even then, the edges and angles only become converted into 

 a whitish frit or enamel, filled with bubbles. Some specimens 

 yield a blackish glass. 



Geological Situation and Localities, 



Petrosilex is one of the most abundant minerals in the vicinity 

 of Boston. It enters into the composition, and forms the basis 

 of Porphyry in Maiden and Lynn, and of itself, sometimes forms 

 hills and presents high mural precipices. Elegant striped varie- 

 ties are found at Milton ; rolled masses and fragments are found 

 in alluvial soil in Cambridge, "West Cambridge, Medford, New- 

 ton, Roxbury, &c. and is disseminated in Amygdaloid at Hing- 

 ham. It is one of the most frequent pebbles on Nahant, Nan- 

 tasket, and Chelsea beaches. 



Bernards. 



L 



The external surface of this mineral often appears earthy ; 

 this arises from its great tendency to decomposition. Epidote, 

 Feldspar, Quartz and Sulphuret of iron are not unfrequently im- 

 bedded in it, but not so abundantly as to render it porphyritick ; 

 some specimens are covered with beautiful dendritick impressions 

 of black oxide of manganese, which often gives a uniform dark 

 colour to a large surface. 



No mineral in this vicinity has so often been confounded 

 with Jasper and Porphyry, by roineralogical students, as PetrosI- 



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