t 



of Boston and its Vicinity, 177 



SPECIES XVni.~ARGILL.iCEOUS SLATE. 



1 



Variety I, — Argillite, 



Argillitey or Common Argillaceous Slate, Cleavelandy p. 359» Clay slate, 



r 



Jameson, vol. i. p. 334. Idem, Aikiiij p. 257. Argillite, Kirwan,yo\, i. p. 234. 



External Characters, 



Its colours are grey and red. Of grey, it occurs bluish grey 

 and blackish grey ; these are disposed sometimes in stripes or 

 spots ; and of red, it occurs brownish purple red. The surface ie 



r 



sometimes rendered beautifully pavoninej irised and chatoyant 

 from the presence of Oxide of Iron. 

 Its lustre is glimmering. 



It is opaque. 



y 



It is amorphous. 



The streak is pearl grey. 



F 



It is moderately hard, 



L 



The fracture is fine grained uneven, passing in some speci- 



i 



mens, into imperfectly straight foliated, and in others into earthy 

 and it is always slaty. 



It is moderately tough. 



It is easily frangible. 



The fragments are tabular and splintery, and often perfectly 

 rhomboidal. 



Its specifick gravity is about 2.888. 



Chymical Character, 

 Before the blowpipe it melts into a blackish brown enamel 



