of Boston and its Vicinity, 



153 



lex ; but the infusibility of Jasp 





when the blowpipe flame 



IS urged by oxygen gas, and the compound structure of Porphy 

 readily distinguish these from Petrosilex. 



SPECIES llL—mCJ. 



Mkiif Cleaveland, p. 252. Idem, JCirwau, vol i. p. 210. Idem, ^Sikiiif p. 

 S.13. Mica or Glimmer, Jamesoiiy vol. i. p. 341. 



? 



when in thin lamina 



} 



blood red 



External Characters. 



Its colour is grey ; of which it occurs yellowisli grey, and 

 blackish grey, passing into brownish black. 

 Its lustre is splendent. 



It is translucent at the edges, and in thin lamina transparent ; 

 the blackish varietie 

 or yellowish light. 



It is amorphous. 



Its streak is greyish w^hite. 



It is flexible, elastic, and produces a snapping sound when 

 bent. 



V 



It is very soft. 



Its structure is perfectly straight foliated, with a single clea- 

 vage, and easily separable. 

 It is tough. 

 It is sectile, 

 its specifick gravity is about S.9S3. 



Chymical Characters. 

 Before the blowpipe, when in thin slender pieces, it curls 



like horn, and then fuses into a greyish white enamel. 



