1922] Hudson: Geology of the Cuyamaca Region of California 207 



north and south and vertically. According to the usage of many 

 geologists, this mass might be termed a laccolith. According to the 

 classification proposed by Daly, 17 it would be a chonolith. 



Age of the intrusion. — The Cuyamaca basic igneous mass is younger 

 than the schists and quartz diorite. The evidence for this is: (1) it 

 generally cuts across the structure of schist and quartz diorite; (2) 

 the invaded rocks are schistose, while the intrusive is massive and 

 only exceptionally shows flow banding; (3) dikes, small laccoliths, 

 and plugs of gabbro and norite cut the quartz diorite and schist. 



It has been shown that the Stonewall quartz diorite is probably 

 of post-Triassic, pre-Cretaceous age. The basic intrusives are thus 

 post-Triassic. It was concluded that the Cuyamaca igneous mass 

 cooled at intermediate depths. If the Cuyamaca area represents the 

 site of an ancient volcano, it might be possible to find remnants of 

 its effusive rocks, which by their relation to sedimentary rocks would 

 give a clue to the age of this intrusion. 



Volcanic rocks of probable early Tertiary age are exposed in a 

 belt along the western edge of the mountains of San Diego County. 

 Determinations of specimens of these rocks by E. S. Larsen, quoted 

 in a recent paper by Ellis, class them as quartz latites. 18 It can hardly 

 be supposed that quartz latites were erupted by a volcano whose eroded 

 neck contains only basic rocks. Andesitic lavas underlie Miocene 

 sediments in Coyote Mountain, in Imperial County. 10 No accurate 

 description of these rocks is available. It does not seem at all likely, 

 however, that andesites could have been erupted in any quantity from 

 the hypothetical Cuyamaca volcano. Late Tertiary or Quaternary 

 basalts occur in northwestern San Diego County. These might con- 

 ceivably have come from a " gabbro-norite volcano," but their recency 

 argues against their assignment to the Cuyamaca igneous period. The 

 question of the exact age of the Cuyamaca intrusive must remain an 

 open one. The writer's opinion, however, is that the intrusion oc- 

 curred in pre-Cretaceous time, following closely on the development 

 of the quartz diorite batholith. 



The Rattlesnake Granite 



A mass of granitic rock outcrops in the region of Rattlesnake 

 Valley from five to six miles east of Cuyamaca Peak. Its area is 

 roughly lenticular, with rounded ends. The greatest extent is north- 



17 Daly, E. A., Igneous rocks and their origin (New York, 1914), p. 84. 

 is Ellis, A. S., U. S. Geol. Surv., W. S. P., 446, 1919, pp. 72-73. 

 io Merrill, F. J. H., op. ext., p. 12. 



