GEOLOGY OF THE HAYSTACK STOCK 205 



buff, and is very faintly pleochroic. It is twinned parallel to the 

 first pinacoid. Hypersthene is rather irregular in outline, is never 

 twinned, and is less well cleaved than augite. The pleochroism varies 

 from pale green parallel to t to reddish yellow parallel to a and b. 

 The two pyroxenes are intricately intergrown and were synchronous 

 in their crystallization which was, in some cases, contemporaneous 

 with and in others immediately followed that of plagioclase. Some 

 of the pyroxene is altered to bastite. An analysis of a mixture of 

 the two pyroxenes from the olivine-gabbro (variety E) is given on 

 p. 215. 



Biotite is present in all varieties of the rock and occurs as irregular 

 flakes of various sizes up to 2™™ in diameter. Biotite incloses small 

 grains of magnetite and more rarely minute bodies of plagioclase ; in 

 many'specimens it constitutes about 5 per cent, of the rock. Aq analysis 

 of biotite separated from variety D is given on p. 213. 



Hornblende occurs as small irregular anhedrons longer than they 

 are wide ; nearly all are fibrous, only a few sections showing the usual 

 hornblende cleavage. It is pale -yellowish green and slightly pleo- 

 chroic. A part of the hornblende is probably secondary and it may 

 be uralite, though there is nothing in its crystal form to suggest deriva- 

 tion from a previous pyroxene, and it is possible that it is secondary to 

 another hornblende. Minute inclusions of magnetite are present 

 in the hornblende and small shreds of biotite are similarly included, 

 Many of the rocks of Haystack Basin contain no hornblende at all, 

 but it is the chief ferromagnesian mineral of the quartz-diorite. 



Magnetite is present in all specimens constituting from 2 per cent, 

 to 5 per cent, of the rock. It is a little more abundant in the more 

 basic gabbros and occurs both as irregular anhedrons and idiomorphic 

 crystals. In feldspar it is present as minute dust-like particles and 

 also in larger bodies, but the greater portion of it is included in or 

 associated with the ferromagnesian minerals. The chemical analysis 

 of the rock shows that it contains considerable titanium. It crystal- 

 lized partly after the plagioclase and contemporaneously with the 

 ferromagnesian minerals. 



Olivine, for the most part allotriomorphic, is present in a few of 

 the more basic varieties of the gabbros. It is fractured and along 

 the cracks is altered to serpentine. Some of it is completely replaced 



