360 



W. Cross — Soine secondary minerals 



The jyyroxene appears almost colorless in thin sections and 

 is not visibly pleochroic. In optical orientation it corresponds 

 with diopside or augite, and chemical analysis shows it to be- 

 long to the aluminous variety. The material for analysis was 

 obtained by dissolving the calcite cement and separating the 

 pyroxene from quartz by the Thoulet solution. This examina- 

 tion was carried out by Mr. L. G . Eakins in the laboratory of 

 the IT. S. Geological Survey and yielded the following results : 

 Si0 2 54-87, Al 2 3 6-34, Fe 2 3 2-88, FeO 4=61, MnO 0-14, CaO 

 15-87, MgO 1447, JSTa 2 0-28, fl 2 0\31,- 99*77. There was a 

 small amount of quartz attached to some of the grains, which 

 explains the high silica percentage, and some brown horn- 

 blende with its alteration products was included with the pyrox- 

 ene, but these latter impurities cannot have materially influ- 

 enced the result. 



The original hornblende associated with the augite is dark 

 brown in color, strongly pleochroic, showing : a = pale yellow, 

 6= reddish brown, c= almost chestnut-brown in rather thick 

 sections. The angle 6 : c is at least 13°. Small particles of 

 the normal brown hornblende are included in the larger augite 

 grains, usually with the c axis in common, and some of the 

 hornblende prisms, on the other hand, are regularly intergrown 

 with or include small augite prisms. As a rule the horn- 

 blende prisms are less than l mm in diameter and they are 

 frequently terminated by the usual planes. The brown horn- 

 blende exhibits in nearly all crystals a more or less marked 



Mue»*> 



4- 



tendency to pass into a pale green or colorless amphibole with 

 slight pleochroism and having the angle 6 : c two or three 

 degrees greater than in the brown variety. This green min- 

 eral seems to possess the characteristics of actinolite and it 

 will be so designated hereafter. Besides the transformation 

 into actinolite there is often a similar replacement of the 

 original hornblende by a blue mineral which retains the amphi- 



