152 The Monoclinic Pyroxenes of New York State. 



matic zone are a (100), h (010) and m (110) equall}^ developed, but 

 the latter may become so narrow as to give the crystal a square 

 appearance. The terminal faces are ?< ( 111) and s (11 1), the latter 

 often so extended as intersect the front prism faces. A basal 

 parting is sometimes present as well as a lamellar structure. The 

 crystals have smooth but dull faces and are emerald green in the 

 thinnest sections. They show a strong,. pleochroism. C:cis52° 

 and the specific gravity is 3.60. 



A section thin enough to measure the angle of the optic axes 

 could not be obtained, as the material is so opaque, nor could any 

 determinations be made of the indices of refraction hy the Kohl- 

 rausch method, even with using phenjdsulphide. 



The composition of this black augite is: 



Percentage. Ratio. Prop, parts. 



SiOa 49.12 .8186 37 



FeO 15.98 .2220 10 



CaO 17.30 .309 14 



MgO 6.06 .151 7 



FesOa 3.53 .0220 1 



AI2O3 7.49 .0731 4 



99.48 

 Sp. Gr. 3.60. 



From this analysis we obtain the formula Ca^^Mg^Fei 0^63 Al^- 



Sig^Oiiy or 



4 (CaMgSijOg) 

 10 (CaFeSijOg) 

 2 (MgAl,SiOe) 

 MgFeaWiOg 



leaving SigOig, which is approximately SiOj 



Bogers Rock, Essex Co. — A greenish gray A^ariety of augite of 

 slender prismatic form with the planes a, b and sometimes m is 

 found at this locality (Ref. 2). One specimen was 1-8 inches 

 long and of proportionate diameter. The crystals which are 

 rarely well formed are associated with feldspar and titanite. They 

 are sometimes intergrown with the latter. 



An analysis of this augite by Seybert (Ref. 45) gave 



Percentage. Ratio. Prop, parts. 



SiOa 52.66 .8676 14 



FeO 12.30 .1708 3 



CaO 23.33 .4166 7 



