192 University of California Publications in Geology [Vol. 13 



gneissoid structure is generally best developed in the vicinity of con- 

 tacts of the quartz diorite with the schist and is lacking at a distance 

 from schist bodies. For instance, immediately east of the schist body 

 south of the Friday Mine the rock is distinctly foliate. To the east 

 of this point the gneissic structure becomes less prominent until one 

 mile to the east, on approaching the main schist body, the rock again 

 becomes quite gneissoid. The same change occurs on going west from 

 the west boundary of the main schist belt north of Wynola. Again, 

 the irruptive rock in the vicinity of the schist mass south of Cuya- 

 maca Reservoir is gneissoid, while in rock of the same composition in 

 Stonewall Peak and along the basic intrusive contact to the southwest 

 the foliate structure is nearly or completely lacking. 



The strike of the gneissoid structure is always essentially parallel 

 to that of the neighboring schist. The dip of the foliation in the 

 intrusive rock often conforms to the dip of the schist. It is inferred 

 that the gneissic structure is parallel to the walls of the schist bodies. 

 An inspection of the map will show that some of the schist bodies have 

 curving courses, notably those in the vicinity of Pine Hills. The struc- 

 ture of gneiss and schist conforms even in these cases. It is therefore 

 concluded that the gneissoid structure is not the result of the dynamic 

 metamorphism of solid rock, but that it represents the flow lines of a 

 partially consolidated magma. 



Age of the quartz diorite. — The intrusive nature of the contacts of 

 the quartz diorite and the Julian schist is evident from the occurrence 

 of injection gneisses, paragneisses, and contact minerals along the con- 

 tact. The Cuyamaca Basic Intrusive cuts across the structures of 

 schist and quartz diorite gneiss and is undoubtedly younger than 

 either. 



To the northwest of the Cuyamaca region, in the Santa Ana Range, 

 and to the south, in the mountains of Lower California, Cretaceous 

 rocks rest upon the eroded surface of great granitic masses. No in- 

 stances of post-Cretaceous granite are known in California. We can 

 then with reasonable assurance say that the quartz diorite is pre- 

 Cretaceous. From Mendenhall's work in the Santa Ana Mountains 

 we know that at least part of the granite of the Peninsular Range is 

 post-Triassic. This leads to the conclusion that the Stonewall quartz 

 diorite is pre-Cretaceous and post-Triassic, probably corresponding in 

 age to the post-Mariposa granitic masses of the Sierra Nevada. 



Type of intrusion. — The wide zone on the west side of the main 

 belt of Julian schist, in which injected material and contact minerals 



