198 University of California Publications in Geology [Vol. 13 



The mineral composition is as follows: 



Olivine 73% 



Augite 14% 



Brown hornblende 10% 



Pyrrhotite 3% 



Average grain is 1.2 mm. 



The olivine occurs in equant or slightly elongated grains, meeting in irreg- 

 ular curving contacts. The augite is in large very irregular individuals including 

 the olivine poikilitically. 



Pyrrhotite occurs in irregular masses, in part at the meeting point of three 

 or more olivine grains, in part within single olivine grains. No sulphides occur 

 in the augite or hornblende. 



Troctolite (no. 70), from point eight feet north of body of massive ore, 170- 



foot level, Friday Mine. 



The mineral composition is as follows: 



Bytownite 56% 



Olivine 36% 



Pyrrhotite 0.6% 



Minerals of kelyphitic zones .... 7.0% 



No euhedral tendency is exhibited by either olivine or feldspar. The olivine 

 occurs in part as separate, rounded grains, surrounded by feldspar, and also as 

 strings of coalescing grains which give the rock a banded appearance (see pi. 9, 

 fig. 6, and pi. 10, fig. 1). The olivine crystallized before the feldspar. 



The olivine grains are separated from the inclosing feldspars by rims or 

 zones. These are of three kinds: (1) compact, faint brown or green, horn- 

 blende. These are clear and often optically continuous for a considerable frac- 

 tion of the circumference of the olivine grain. (2) Aggregates of green fibers, 

 set at right angles to the periphery of the olivine. The fibers show inclined 

 extinction and are probably a fibrous amphibole. (3) Compound zones, com- 

 posed of compact hornblende within and fibrous mineral without. Green spinel, 

 pleonaste, occurs in sharply angular grains in these zones. 



Pyrrhotite is associated with both the feldspars and olivines. Where lying 

 entirely within single crystals its outlines are relatively simple. 



Hypersthene diorite (no. 121), from area of hypersthene diorite, two miles 

 north of North Peak. 



The mineral composition is as follows: 



Andesine 70% 



Hypersthene 9% 



Green Hornblende 19% 



Magnetite 1.7% 



The average grain is 0.3 mm., the maximum 2.5 mm. 



There is little tendency toward euhedral outline on the part of any of the 

 constituents. They meet in curving lines, as is shown in plate 10, figure 2. 



The green hornblende is not fibrous but compact. It appears to have been 

 formed at the expense of the hypersthene by alteration working inward from 

 the edges; that is, by change of the edges of a preexistent pyroxene body, and 

 not as rims implanted on the edges by outward growth. 



Magnetite is found in all the silicates as rounded grains. Its outlines are 

 simpler than those generally shown by pyrrhotite. 



