208 University of California Publications in Geology [Vol. 13 



south, a distance of three miles. It is a true granite, varying from 

 an alaskite to a biotite-hornblende granite. Orthoclase predominates 

 over plagioelase in all specimens examined. It is coarser in grain 

 than the gneissoid quartz diorite and granodiorite, and is only ex- 

 ceptionally gneissoid. At the south end of the area there is a mar- 

 ginal development of alaskite porphyry. 



Schist and quartzite inclusions, quite common in the quartz diorite 

 near Rattlesnake Valley, are entirely lacking in the Rattlesnake 

 granite. The contacts of the granite against the schist and gneissoid 

 quartz diorite generally cut across the structure of these rocks. At 

 the north end of the mass the structure seems to indicate that the 

 granite forced the schist apart to make room for itself. The walls of 

 the granite mass dip steeply away from its center in some places, at 

 others the dip is toward the center at somewhat lower angles. 



Effect of the intrusion on wall rocks. — The contacts of the Rattle- 

 snake granite were studied carefully at but two localities, its northern 

 and southern extremities. At the northern end the granite is intru- 

 sive into fissile quartz-mica schists. The only evidence of metamor- 

 phism is the presence of large muscovite flakes for some distance from 

 the granite, and a small amount of coarse contorted schist at the im- 

 mediate contact. The varying strikes of the schist indicate that the 

 granite made way for itself to some extent by forcing apart the wall 

 rocks. At the southern end of the granite mass, potash-rich granitic 

 material has been injected along the schistose planes. 



It is evident that the Rattlesnake granite is an injected body of 

 distinctly later age than the gneiss and schist which it intrudes. But 

 its relation in time to the Cuyamaca basic intrusive is indeterminate. 



Assuming that the date of the two intrusions was essentially the 

 same, the hypothesis is advanced that the gabbro-norite complex and 

 the granite of the Rattlesnake mass are complementary differentiates 

 of a single magma of intermediate composition. The exposed area of 

 the granite is small compared to that of the basic intrusive, but this 

 may have no bearing on the relative volume of the two rocks at the 

 time of their intrusion. It is interesting to note in this connection 

 that the world average chemical composition of quartz diorites, the 

 prevailing igneous rock of this region, is almost the exact mean of the 

 compositions of gabbro-norite and granite. 



