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University of California. 



[Vol. 2. 



portions it is exceedingly fresh, but in the coarser rocks and perido- 

 tites it shows all stages of the alteration to diallage. The alteration 

 begins by the creation through the center of the crystal of fine 

 cleavage planes parallel to the orthopinacoid. Associated with 

 these parallel cracks in the center of the crystal are long rectangular 

 or spindle-shaped alteration products, which polarize brightly when 

 the remainder of the crystal is dark. Slender magnetite rods are 

 sometimes developed in the alteration. Inclusions of hornblende 

 are quite common, being arranged either irregularly or along the 

 cleavage planes. Occasionally the augite shows an approximation 

 to an idiomorphic form, but in general it is allotriomorphic. 



The rhombic pyroxene present is chiefly hypersthene, a colorless 

 non-pleochroic enstatite being seen in only a few instances. The 

 hypersthene is remarkably widespread, occurring in nearly all facies 

 of the rock except some of the purely non-feldspathic ones. Its 

 presence seems to have no relation to the presence or absence of 

 olivine. It is strongly pleochroic, showing the usual colors, d = red, 

 b = yellow, c = green. Sections parallel to the macropinacoid are 

 often devoid of pleochroism, remaining green in all positions, and 

 show the emergence of an optic axis. The axial angle is small, for 

 both axes appear in the field in convergent light. The specific grav- 

 ity, which is 3.315, is rather low for this mineral, and shows that it 

 is not rich in iron. As a rule the crystals are exceedingly fresh and 

 free from inclusions, which are so characteristic of hypersthene. No 

 schillerization was seen, but in those individuals which had become 

 slightly affected by alteration there has been produced a system of 

 fine parallel cracks. 



Hornblende is fully as widely distributed through the different 

 facies of the rock as hypersthene, save that it is less common in the 

 peridotites. It generally plays a less important role than the hyper- 

 sthene, except in the ophite diorites. In the dikes of diabasic 

 structure it has clearly replaced the pyroxene in some cases, while 

 in others it must be considered primary. In the gabbros it some- 

 times occurs in independent individuals, but more commonly as 

 broken or discontinuous rims about the augite grains, and occasion- 

 ally about the olivine. It is quite strongly pleochroic, giving 

 brownish, greenish brown, and pale yellow tints. It is what is 

 termed basaltic hornblende. 



