V, 



of Boston and its Viclnit)/, 



r 



Geological Situation and Locality. 

 Occurs in a small vehi^ iii Sienite at Cohassct. Rare. 



151 



SPECIES Ih-^PETItOSILEX. 



Petrosilex, Cleavelcmd^ p. £4?. Compnct Feldspar, Jainesun, vo]* i, p. 276. 



Continuous Feldspar, Kirwan^ voL i. p/S23. Compact Feldspar, dikin. \\ 213.. 

 Chert of some niineralodsts. 



y 



External Characters. 



Its colours are white, red, grey, green and black. Of while, it 



r" 



occurs greenish white ; of red, brownish red^ cochineal red, and 

 brownish purple red; of grey, yellowish grey; of green, leek 

 green and olive green^ and of black, greyish black. All the col- 

 ours are of various intensity, and frequently two or more of them 

 occur in dots, or in stripes and zones^ either parallel or in undu- 

 lating curves. 



Its lustre is dull, and faintly glimmering. 



It is translucent at the edges. 



It is amorphous. 



It is extremely Lard passing into very hard. 



4 



Tfie fracture is from coarse to very fine splintery ; even pass- 

 ing into coarse grained uneven, and small passing into imperfectly 

 large conchoid al ; the structure of some specimens is slaty. 



It is tough. 



It is very difficultly frangible. 



m 



The fragments are indeterminately angular and sharp edged. 

 Its specifick gravity is from about S.69 to 2,79- 



% 



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