of Boston and its Vicinity. 1'1'7 



External Characters. 



+ 



Its colours are white and vellowish white. The external sur- 

 face is generally tarnished of a yellowish colour, between honey 

 jellow and buff orange. 



Its lustre alternates from dull to glistening^ and is peavly, 



, L 



I 



It is translucent at the edges. 



It is amorphous and crystallized in straight planed rhcmbs 

 and in lenses. The crystals vary in size from a pin's head to the 

 size of an almond ; tliey occur variously aggregated, are gener- 

 ally hollow and the cavity is filled with minute rhomhick crystals. 

 Rhombick crystals appear to compose both the lenses and rhombs, 

 and their aggregation gives the crystals a striated surface, which 

 is frequently chatoyant, always pearly, and sometimes rough 

 with mamillary incrustations. 



The streak is white. 



It is semi-hard. 



The fracture is straight foliated, with a threefold cleavage, 

 parallel to the sides of a rhomboidal prism. Some specimens 

 present an imperfect granular structure, which arises from the ag- 

 gregation of very minute rhombick crystals. 



It is brittle. 



It is easily frangible. 



The fragments arc rhomhoidal. 



The specifick gravity is about S.90. 



Chymical Characters, 

 It is infusible before the blowpipe. The colour is changed by 

 heat to reddish brown and the mass becomes friable. 



Geological Situation and Locality. 

 It occurs in veins and in fissures traversing Argillite, at 



Charlestown. 



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