ORBICULAR GABBRO-DIORITE 301 



hornblende is in slight excess. Diallage occurs in rather large irregu- 

 lar forms without crystal boundaries, exhibiting strong development 

 of the orthopinacoidal cleavage. It is usually of faint greenish color 

 and without sensible plcochroism. Minute microscopic interposi- 

 tions are abundant, and it further incloses irregular grains and per- 

 fect idiomorphic crystals of hornblende. 



The hornblende is principally of the green uralitic kind, approxi- 

 mating parallel columnar and fibrous forms, which usually show 

 cleavage only in direction of elongation, but it entirely fails in some 

 of the more irregular aggregates. Both the hornblende and dial- 

 lage indicate some alteration (leaching) to nearly colorless forms. 

 The angle of extinction measured on many pieces of the hornblende 

 varies from 13° to 20°. Pleochroism is confined chiefly to green 

 tones, with the ray vibrating parallel to A appearing slightly yellow, 

 and that vibrating parallel to B is occasionally tinged a faint brown. 

 Compact hornblende in rhombic cross-sections, bounded by the 

 prism and usually the pinacoid faces, and showing the intersecting 

 prismatic cleavages, is distributed through the sections, with much 

 of it inclosed by the diallage. 



Basic plagioclase is the predominant feldspar. Its substance is 

 never fresh, but is largely altered to muscovite and calcite, and some 

 zoisite which, with few exceptions, has completely destroyed the 

 polysynthetic twinning lamelhe. It occurs almost entirely as irregu- 

 lar, large grains without idiomorphic outline. Perfectly fresh micro- 

 cline as irregular grains, and exhibiting the characteristic twinning 

 structure, is present in every thin section studied. It sometimes 

 occurs as inclusions of moderate-sized grains in the plagioclase. 

 Abundant prisms of apatite, usually of fairly large size, are included 

 in the feldspar. Some of these are in cross-sections which show 

 perfect hexagonal shape; others are in longitudinal sections, and 

 occasionally some of the apatite forms grains of irregular outline. 



Titanite is abundant in comparatively large idiomorphic crystals, 

 yielding characteristic, sharply rhombic cross-sections. Some of it 

 occurs in irregular grains without crystal outline. In thin section 

 the color is pale to moderately deep brown, with slight absorption in 

 the deeper-colored crystals. It is usually free from inclusions of 

 other minerals. Cleavage is rather pronounced in much of it. 



