1922] Hudson: Geology of the Cuyamaca Region of California 181 



GEOLOGY 



Summary Statement 



The rocks exposed at the surface over a large part of the moun- 

 tainous region of San Diego County are quartz-hearing plutonic rocks, 

 varying from quartz diorite to true granite in composition. The re- 

 lation of these rocks to the older rocks which they have intruded shows 

 that the intrusion was of batholithic nature. Most of the cover has 

 been stripped from this batholith, the older rocks being found as 

 remnants, surrounded by granite. These older rocks are schists, the 

 residt of metamorphism of shales and sandstones, with subordinate 

 layers of lava. 



In the Cuyamaca region the schists are present in more than their 

 usual proportion and the complex of schist and granitic rock, here 

 generally a quartz diorite, often gneissic, has been intruded by younger 

 igneous rocks. The younger intrusives are of two distinct types, 

 acidic and basic. The acidic rock is a true granite. It occupies an 

 oval area in the region of Rattlesnake Valley. The basic rock varies 

 from basic diorite to ultra basic types, but the predominant varieties 

 are gabbro and norite. These basic rocks occupy a considerable area 

 which includes the three peaks of the Cuyamaca Mountains, and they 

 are also found in several small outlying areas. The younger intrusives 

 solidified at moderate depths in the earth's crust. The granite mass 

 is apparently a laccolith, while the main mass of basic rock may be 

 termed a chonolith. Dikes of basic composition cut the gabbro-norite 

 mass and pegmatite is found in intrusive relationship to all the rocks 

 mentioned above. Dikes of soda aplite occur cutting both quartz 

 diorite and schist. 



The schist series is of uncertain age ; it may be Triassic or late 

 Paleozoic, or may include rocks of both ages. The quartz-diorite 

 batholith was developed in post-Triassic time and is probably equiva- 

 lent to the post-Mariposa intrusions of the Sierra Nevada. The 

 younger intrusives are without much doubt pre-Cretaceous and fol- 

 lowed closely on the batholithic intrusion. 



Along the summit of the Cuyamaca Mountains there are several 

 prominent open valleys with flat or gently rolling surfaces. Evi- 

 dence afforded by stream cuts into the surface of the valleys shows 

 that they are underlain by considerable thicknesses of bedded, uncon- 

 solidated, sandy alluvium. A mile southwest of the Friday Mine 



