GEOLOGY OF THE HAYSTACK STOCK 



215 



clase and quartz are poikilitic with respect to the other constituents. 

 The minerals present in order of abundance are: andesine, augite, 

 orthoclase, hypers thene, magnetite, biotite, quartz, apatite, and olivine. 

 Andesine occurs in prisms from i to 2™™ long and about 0.3"^"^ 

 wide, and commonly as carlsbad twins. Orthoclase occurs as a 

 matrix for plagioclase and the ferromagnesian minerals, filling the 

 angular interstices between them. Quartz is present only as a 

 micrographic intergrowth with orthoclase. Augite, pale violet-buff, 

 occurs as anhedra from o . 5 to i . 5°^™ long. Hypersthene is of the 

 same size and is pleochroic, green parallel to t, and reddish yellow 

 parallel to a and b. Both pyroxenes contain minute bodies of mag- 

 netite, biotite, and apatite, and a little serpentinized olivine, which 

 taken together equal about 2 per cent, of the mineral. An analysis 

 of a mixture of the two pyroxenes is given below. In the preparation 

 of the sample the crushed and sized minerals were separated by 

 Thoulet's heavy solution. The portion of the rock which came down 

 at a specific gravity of 3.1, the maximum specific gravity of the 

 solution, included, besides the pyroxenes, magnetite and bodies 

 composed of magnetite and feldspar or pyroxene and feldspar. 

 Those particles in which there was a considerable proportion of 

 magnetite were removed with the magnet, and the remainder was 

 treated with acid to remove small adhering particles of magnetite 

 from the pyroxene. The resulting nearly pure pyroxene was recrushed, 

 resized, and passed through the solution again, after which the treat- 

 ment with the acid was repeated. 



ANALYSIS OF PYROXENES FROM VARIETY E. GEO. STEIGER, ANALYST 



