144 



den cooling from contact with the rock, assumed the more vitreous form 

 of obsidian." Report on the Geol. 2d Dist., p. 184. 



An analysis of a specimen of this substance gave, silica, 55.50 ; 

 oxide of iron, 32.00 ; lime, with a minute portion of alumina, 6.90 ; 

 magnesia, 4.62. If this is a correct view of its composition, it differs 

 considerably from the varieties of obsidian heretofore examined. But 

 I apprehend that its chemical character will be found liable to great 

 variations. 



PYROPHYLLITE. 



(Mineralogy of New- York, page 368.) 



Well characterized specimens of this beautiful mineral have been 

 found in Phillipstown, Putnam county. It is supposed to be a mechan- 

 ical mixture, consisting of greyish green foliated scales, looking like 

 talc or mica in a greyish white, mealy base. It is, however, very dis- 

 tinct in composition. Mr. J. E. Teschemacher thinks it identical with 

 vermiculite. Proceed. Bost. Nat. Hist. Soc-'y, 1843. 



MICA. 

 (Mineralogy of New- York, page 369.) 



This mineral is exceedingly abundant, and fine specimens have been 

 found in various parts of New-York, many of them of great beauty. 

 Attempts have been made to arrange the multiplied varieties under 

 classes or groups, but these have not been entirely successful. They 

 differ principally in their optical characters; but the difficulty which at- 

 tends their division founded on these is, that the chemical composition 

 does not always accord with them. 



Since the publication of my Mineralogy, I have examined several of 

 the New- York specimens and introduce the following notices, which, 

 although incomplete, may be of some use to those who shall hereafter 

 be induced to take up the investigation. 



MARGARITE. 



Some of the specimens from Orange county, often ticketed talc, be- 

 long to this species. The mineral has a vitreous lustre, a pale, pearly 

 gray color, rather brittle, translucent to subtranslucent. It is associated 

 with chondrodite and pseudomorphs of hornblende. It exhibits only 

 one axis of double refraction and fuses per se into a white enamel. 

 Similar specimens occur in the town of Rossie, St. Lawrence county. 

 I have not analyzed these specimens. 



