1922] Hudson: Geology of the Cuyamaca Region of California 195 



found in certain of the border faeies, differs in no way in its optical 

 properties from that of the gneisses. Its pleochroism is yellow to 

 almost colorless, parallel to the principal axis, deep brown to black, 

 parallel to the cleavage. The quartz seems to be identical with that 

 of the more siliceous rocks. 



Description of typical rocks. — Fifteen types of basic igneous rocks 

 have been recognized in this region. In order to give an idea of the 

 mineralogic composition and texture of these rocks a detailed petro- 

 graphic description of ten specimens, typical of as many varieties of 

 rock, will be presented. The five varieties not described — augite dio- 

 rite, brown-hornblende gabbro, gabbro-norite, olivine gabbro-norite, 

 and quartz gabbro-norite — differ not at all in texture from the types 

 described. As their names indicate, their discrimination from the 

 other types depends either on some minor, though characteristic, con- 

 stituent, or on a difference in the relative proportion of common con- 

 stituents, or on a difference in the kind of plagioclase. 



Norite (no. 151, pi. 9, fig. 1), from the small area of basic rock one and one- 

 half miles east of Pine Hills. 



This rock has the following composition: 



Anorthite 40% 



Hypersthene 31% 



Augite 25% 



Brown Hornblende 2% 



Pyrrhotite 2% 



The maximum size of grain is 2.3 mm., the average about 0.5 mm. 



The plagioclase and pyroxenes have mutually interfering, irregular boun- 

 daries, made up of a series of curves. There is little tendency toward euhedral 

 outline on the part of any of the constituents. 



The brown hornblende occurs both as rims on the hypersthene individuals, 

 lying between the hypersthene and the feldspar, and as separate patches within 

 the hypersthene. In both cases it is clear and compact. An individual of brown 

 hornblende, shown in plate 9, figure 2, at one place has irregular, branching arms 

 between several feldspar grains. Optically it is one individual throughout. 



The pyrrhotite occurs in grains up to 0.8 mm. in diameter. These grains are 

 frequently quite irregular in outline, having two or more branches from their 

 main body. However, many of the sulphide grains have extremely simple form, 

 being circular or slightly oval in section. The most irregular grains invariably 

 are found filling spaces at the meeting of several silicate individuals, quite like 

 the hornblende, mentioned above. The grains with circular and oval sections, 

 on the other hand, are always completely inclosed within a single silicate indi- 

 vidual, generally a pyroxene. 



The pyrrhotite occurs most commonly at the edges of pyroxenes, extending 

 thence into feldspar. Sometimes the pyrrhotite is separated from the pyroxene 

 by brown hornblende.' This, however, is not the result of any reaction between 

 the ore mineral and the pyroxene, but is part of an ordinary hornblende "rim" 

 which on either side of the grain of ore lies between the pyroxene and feldspar. 



Olivine norite (no. 190, pi. 9, fig. 3), from summit of hill south of Pine Hills. 



