The Monoclinic Pyroxenes of New York State. 141 



but those which far exceeded all the others in numbers were rec- 

 tangles, whose longer side made an angle of 25°, with the edge 

 010 A 110. The upper sides of these rectangles are parallel to the 

 intersection of 010 and Oil. 



A triangular figure was produced on the negative unit pyramid 

 face, whose acute angle pointed toward the edge 111 A 111. The 

 base of the triangle made an angle of 15°, with the edge IIIAIIO. 



Yery unsatisfactory figures were obtained on the basal pina- 

 coid. The majority were squares whose sides were respectively 

 parallel and perpendicular to the plane of the orthopinacoid. 

 There were also a few pits whose outline was closely that of an 

 equilateral triangle. 



The results noted above agree very closely with those obtained 

 by Wulfing* on diopsides from Ala., and by Greimf. Greim 

 states that the triangular pits were sometimes truncated by a 

 fourth plane, but this was not noticed by the writer on any of 

 the crystals etched by him. 



Chemical Composition. 



As previousljr stated pyroxene is divided into several varieties 

 of definite theoretical composition. It rarely happens that the 

 analysis when calculated out gives a formula which corresponds 

 exactly to any of these, but instead, the pyroxene is found to 

 consist approximately of an isomorphous mixture of several 

 metasilicates. 



Rammelsberg J put forth the theory that in aluminous pj^rox- 

 enes, the ferric iron and alumina were present as sesquioxide and 

 mixed with the normal metasilicates, thus giving the general 

 formula n R SiOg+RgOg, while the p3'roxenes with no alumina 

 were simple mixtures of isomorphous silicates. 



Tschermak § subsequently showed that in many cases in the 

 alumina free augites Ca^Mg+Fe, while in the aluminous ones 

 Ca<Mg+Fe. 



The writer has attempted in each case to calculate the mixture 

 of metasilicates present. (See detailed accounts of localities). 



*Beitrage zur Kenntness der Pyroxenfamilie in Chemischer und Optischer 

 Beziebung. — Heidelberg, 1891. 



tNeues Jahrbucb fiir Mineralogie, 1889, p. 252. 

 t Zeitschr. d. d. geol. Ges., 1876, p. 496. 

 I Min. Mittbeil., 1871, p. 17-46. 



