288 FINLAY 



Intergrowths with hornblende have been observed but they are 

 rare. Sections approximately parallel with (ooi) show the 

 emergence of an optic axis at one side of the field. 



Phenocrysts of hornblende are associated with the porphy- 

 ritic crystals of augite. This mineral is in eight-sided sections, 

 .9 mm. by .6 mm., which have ///(no), ^(010) and a(ioo), 

 in the prism zone, very sharply developed. The pleochroism is 

 strong, a = pale yellowish-brown, h = deep earthy brown, c = 

 light brown. The crystals are often composed of two or more 

 shells of varying chemical composition, the inner core being the 

 darker. The whole individual may be surrounded by a thin 

 rim of grains of magnetite, although embayments resulting 

 from magmatic resorption are rare. The extinction angle is 

 low, 4°. Alteration at the center of the larger crystals leads 

 to the formation of calcite. 



Both the above minerals occur in a second generation in the 

 ground-mass. The hornblende is in long and very narrow 

 clear brown blades which fray out at the ends. They are no- 

 ticeably fibrous. Their extinction angle is low. They attain a 

 maximum size of .7 mm. by .03 mm. but usually they are but 

 half as large. They make up about 10 per cent, of the rock. 

 Augite occurs in the ground-mass in rough grains and stubby 

 prisms .2 mm. by .05 mm., gray or nearly colorless. It is 

 perhaps twice as abundant in the ground-mass as is the horn- 

 blende. Much magnetite is associated with it and square sec- 

 tions of this mineral are very common. Needles of apatite 

 which are large and perfect are found near the augite. 



The feldspar of the ground-mass is commonly xenomorphic. 

 It seems to have been the last mineral to crystallize out of the 

 magma. Occasionally well-defined minute plagioclase individ- 

 uals are found which are evidently secondary. Their extinc- 

 tion angle is uniformly one or two degrees, indicating oligoclase. 



Alteration products are very common in this rock. Calcite 

 is everywhere disseminated through it in granular masses. The 

 zeolitic minerals have frequently formed in small cavities, and 

 at the center analcite is generally found. In this rock the 

 mineral is colorless, with very few cleavage cracks. It is com- 



