384 



University of California. 



[Vol. 3. 



called orbicular. The writer made a preliminary petrographic 

 examination of the material and reported the results of his 

 examination in a brief note to the Cordilleran Section of the 

 Geological Society of America at its third annual meeting in 

 December, 1901, and the specimens of the rock were then 

 exhibited and were commented upon by the Fellows of the Society. 

 The writer deferred publishing an account of the rock till he 

 should have an opportunity of seeing its occurrence in the field. 

 Various circumstances, however, prevented his visiting the 

 locality until last summer, when, being in San Diego, he made a 

 trip to Dehesa, looked over the ground, and found the orbicular 

 rock in place. 



Geology of the Region. — Dehesa is a store and Post-office 

 situated a few miles east of El Cajon Valley on the Sweetwater. 

 In reaching it from San Diego one passes over the broad mesa 

 lands which border the ocean front of San Diego County, and at 

 El Cajon Valley encounters the underlying surface of granitic 

 rocks upon which the stratified sediments of the mesa repose. 

 These granitic rocks are of quite varied character and have as 

 yet not been subjected to much geological investigation. We 

 have, however, the reconnaissance work of Fairbanks* from 

 which it appears that the region where these granitic rocks 

 abound is an extensive complex of igneous intrusives and crys- 

 talline metamorphics, the former greatly preponderating. The 

 igneous rocks comprise, according to Fairbanks, granite, gneiss, 

 syenite, diorite, gabbro and diabase. Near Dehesa there appears 

 in the midst of the granite a large mass of coarse textured basic 

 igneous rock concerning which Fairbanks says : " At Dehesa the 

 granite is replaced by norite which forms a high mountain on 

 the north side of the river and extends southeasterly three or 

 four miles forming two high peaks. "t It is on the slopes of this 

 mountain on the north side of the Sweetwater that the orbicular 

 rock occurs which forms the subject of this paper. 



Occurrence of the Gabbro. — The area occupied by this basic 

 rock to the northeast of Dehesa has an extent from east to west 



* Geology of Sari Diego County; also of portions of Orange and San Bernardino 

 Counties. Calif. State Mining Bureau, 11th Rept. State Mineralogist, 1893, p. 76. 

 tOp. Cit, p. 85. 



