1922] Hudson: Geology of the Cuyamaca Region of California 197 



no case, however, so irregular as that of the augite mentioned above. On the 

 other hand, there are several sulphide grains, entirely enclosed in single feld- 

 spars which have almost perfectly circular outlines. 



The spinel occurs in minute, irregular grains. The magnetite generally shows 

 irregular forms, with rounded outlines, much like those of pyrrhotite. The re- 

 lations between the spinel and magnetite, shown in plate 9, figure 5, point to a 

 close genetic connection. 



Olivine gabbro (no. 130), from point about one mile north of North Peak. 

 On road northeast of sawmill. 



The mineral composition is as follows: 



Bytownite 50% 



Olivine 21% 



Augite 19% 



Brown hornblende 9% 



Pyrrhotite- 1% 



The average grain is 0.4 mm., the maximum 1.0 mm. 



All the constituents show mutually interfering, curving outlines. 



Many of the olivine grains are rimmed with hypersthene, in just the same 

 way that the augite individuals are rimmed with brown hornblende. The 

 hypersthene rims often show optical continuity for more than one-quarter the 

 circumference of the olivine grains, in one case for more than one-half the cir- 

 cumference. The brown hornblende sometimes makes up over one-half of a 

 crystallographically continuous, compound individual with augite. 



The pyrrhotite occurs within the mineral individuals, rather than at the 

 contacts of dissimilar minerals. The pyroxenes contain more pyrrhotite than 

 does the olivine, the feldspar least of all. When entirely inclosed in individual 

 minerals the pyrrhotite shows extremely simple outlines, circular or oval. 



Brown-hornblende norite (no. 108), from north center of northwest quarter of 

 section 20, township 13 south, range 4 east. 



The mineral composition is as follows: 



Labradorite 46% 



Hypersthene 20% 



Brown Hornblende 25% 



Augite 6% 



Magnetite 2.4% 



Pyrrhotite - 0.6% 



The average grain is 0.3 mm., the maximum 0.8 mm. 



The hypersthene, augite and plagioclase have mutually interfering, curving 

 boundaries. The brown hornblende is seen as euhedral sections, entirely enclosed 

 in feldspar, and also occurs in completely anhedral fashion, molded against 

 crystallographic planes of the feldspar. It is generally found implanted on the 

 ends of hypersthene individuals. 



Some of the hornblende then seems to have crystallized before at least part 

 of the plagioclase, while some is apparently later than all the plagioclase. 



The pyrrhotite is found generally at the boundaries between pyroxenes and 

 feldspars, and the grains in such situations have rather complicated outlines. 

 The few grains of ore that occur within single silicate individuals (in this rock, 

 within feldspars) have simple circular or oval outlines. 



Periodotite (no. 60), from east wall of north cross-cut, 170-foot level, Friday 

 Mine. 



