1922] Hudson: Geology of the Cuyamaca Region of California 193 



are found, makes it probable that the intrusive contact dips at a low 

 angle to the east beneath the schist body. The contact zone on the 

 east side of the schist belt, while narrower than that on the west, is 

 still so wide that it appears likely that the contact here also dips to- 

 ward the schist. It is thus probable that the schist masses wedge out 

 downward. The widespread occurrence of quartz diorite in the Cuya- 

 maca and Ramona regions suggests that we have to deal with a great 

 batholithic mass. 



The Cuyamaca Basic Intrusive 



The main basic intrusive mass of the Cuyamaca Mountains, to- 

 gether with the smaller outlying masses, presents many petrographical 

 variations, but the rocks of which it is composed, with a few exceptions, 

 may be grouped as gabbros, norites, and basic diorites. These rocks 

 are of medium to fine grain, specimens having their largest mineral 

 grains over 5 mm. in size being rare. In general the rocks have a 

 maximum grain of from 1.5 to 4.0 mm. Despite the fine grain, the 

 rocks never exhibit diabasic structure, and the porphyritic habit is 

 exceptional. 



The rocks of the mass have been classified as diorites on the one 

 hand and norites and gabbros on the other hand, according as their 

 plagioclase feldspar contains more or less than 50 per cent of albite 

 molecule. No attempt was made to mark off the norites from the 

 gabbros during the field work. Even had the discrimination been 

 possible in the field, the complex intermingling of the two types and 

 the heavy soil cover would have rendered their separate mapping im- 

 practicable. It can, however, be said that norites are more abundant 

 than gabbros in the immediate vicinity of the Friday mine, and also 

 in the embayment in sections 16 and 17, township 13 south, range 4 

 east, and in the outlying area to the northwest of that embayment. 

 Gabbro predominates over norite in the region west of Middle Peak 

 and Cuyamaca Peak. As a result of microscopic study two areas of 

 diorite have been roughly delineated. One of these, lying along Cedar 

 Creek, two miles north of North Peak, is predominantly hypersthene 

 diorite, but augite diorite and brown-hornblende norite are also pres- 

 ent. The other area of diorite occupies the lobe of the basic intrusive 

 mass that extends east, on the east flank of Cuyamaca Peak. It is 

 characterized by augite diorite. 



Constituent minerals. — The various rock types are mixtures of a 

 limited number of minerals, in varying proportions. 



