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University of California. 



[Vol. i. 



larger individuals are for the most part of irregular prismatic form, 

 but are without definite boundaries. They possess a distinct 

 cleavage parallel to the greater length of the prism, to which the 

 extinction is parallel. An imperfect parting at right angles to this 

 cleavage is also present. Numerous black rod-like inclusions occur 

 in irregular patches, the longer axes of the rods strictly parallel to 

 the prismatic cleavage of the crystal. Where most numerous the 

 rods give the hypersthene almost a black color, but immediately 

 adjoining such an area may be one entirely free from inclusions. 

 The fine granular aggregates of hypersthene do not exhibit any 

 inclusions, but in all other, respects agree with the larger individ- 

 uals. About most of the larger hypersthene crystals are zones of 

 fibrous green secondary hornblende. 



The monoclinic pyroxene is a colorless variety agreeing in 

 character with malacolite. It occurs in irregular grains and in 

 large individuals with more or less definite crystal boundaries. 

 Twins were observed formed according to the usual augite law 

 where the twinning plane is the orthopinacoid. The prismatic 

 cleavage is distinct, but the planes of cleavage are somewhat dis- 

 continuous, especially as observed in a basal section. There is also 

 a strong cleavage parallel to the orthopinacoid, which in some cases 

 has almost the character of the pinacoidal parting of diallage. In 

 a section which appeared to be approximately parallel to the clino- 

 pinacoid, the angle of extinction on the trace of the cleavage plane 

 was 38 . Being colorless there is no appreciable pleochroism. 

 The pyroxene contains many inclusions in the form of short black- 

 rods, which were not identified but may be magnetite. These rods 

 are arranged in zones parallel to the crystallographic planes of the 

 host, the rods lying in two or more intersecting systems in these 

 zones, giving rise in the section to a minute lattice work. There 

 are also irregular grains of magnetite included in the hornblende. 

 The alteration of pyroxene to hornblende, giving rise to borders of 

 hornblende about the pyroxene crystal, has already been described. 

 Alteration also takes place to a fine granular aggregate of brightly 

 polarizing minerals whose nature was not determined. 



Magnetite, besides forming- inclusions in the other constituents 

 of the rock, occurs in large, irregular grains. Ilmenite is also 



