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University of California Publications in Geology [Vol.13 



The mineral composition is as follows: 



Bytownite 54% 



Hypersthene 24% 



Augite 4% 



Olivine 16% 



Pyrrhotite 2% 



The average grain is 0.7 mm., the maximum 2.0 mm. 



Both hypersthene and augite are in some instances molded against crystal- 

 lographic boundaries of the feldspar. Some of the pyroxenes are then probably 

 later than part of the feldspar. There is, however, little tendency to diabasic 

 texture, most of the constituents meeting in curving contacts. 



The olivine occurs in nearly equant grains with irregular rounded outlines. 

 These grains are found both within the pyroxene and the feldspar. In the latter 

 situation a kelyphitic zone separates the olivine from the feldspar. These 

 zones, as in specimen no. 70, are composed of compact amphibole within and a 

 fibrous mineral without. 



The olivine appears to be the earliest silicate. Its crystallization was fol- 

 lowed by that of the feldspar and pyroxene, whose periods overlapped, so that 

 most of the pyroxene is later than at least part of the feldspar. 



The pyrrhotite is found in association with all the silicate minerals. Its 

 grains have not very complex outlines, those entirely inclosed in single olivines 

 having particularly simple, circular sections. 



Gabbro (no. 202), from a canon running northwest from Cuyamaea Peak, at 

 5000 feet elevation. 



The mineral composition of this rock is as follows: 



Bytownite 46% 



Olivine 7% 



Hypersthene 13% 



Augite 27% 



Green Hornblende 4% 



Magnetite 3% 



Pyrrhotite 0.37% 



Spinel Very small amount 



The average grain is 0.5 mm., the maximum 1.5 mm. 



The augite occurs in large individuals with irregular rounded outline. In 

 some cases branching masses extend out from the main mass of an individual, 

 the whole being an optically continuous body. This is illustrated by plate 9, 

 figure 4. The augite frequently includes grains of feldspar poikilitically. 



The hypersthene, like the augite, shows ophitic texture, but the external 

 outline of its individuals are much more regular. Both pyroxenes generally 

 have mutually interfering, curving boundaries against the feldspar, but in a few 

 cases the hypersthene is molded against crystallographic planes of the feldspar. 

 Apparently all of the pyroxene crystallized after at least a portion of the feld- 

 spar. 



Compact, light green hornblende forms rims on the augite, sometimes so 

 extensive as to leave very little augite. The hornblende and augite appear to 

 be crystallographically continuous. 



The pyrrhotite occurs in grains up to 0.2 mm. in diameter. The largest grains 

 are within, or in contact with, pyroxenes, but much of the pyrrhotite is entirely 

 inclosed in feldspar. Most of the grains have rounded irregular outlines, in 



