UNIVERSITY OF CALIFORNIA PUBLICATIONS 

 Bulletin of the Department of Geology 

 Vol. 3, No. 17, pp. 383=396, PI. 46. ANDREW C. LAWSON, Editor 



THE ORBICULAR GABBRO AT DEHESA, 

 SAN DIEGO CO., CALIFORNIA 



BY 



Andrew C. Lawson 



CONTENTS 



PAGE 



The Discovery 383 



Geology of the Region 384 



Occurence of the Gabbro 384 



Petrographic Features of the Normal Faeies 385 



Various Local Facies 386 



Aplitic Dykes 388 



The Orbicular Faeies 389 



Microscopic Characters 390 



The Olivine 392 



The Plagioclase 393 



Chemical Composition of the Orbules 394 



Origin of the Orbicular Structure 394 



The Discover ij . — A few years ago a piece of rock was sent to 

 the University of California by Mr. H. P. Wood, Secretary of 

 the Chamber of Commerce of San Diego. The rock was of pecu- 

 liar appearance, and it was thought it might prove of exceptional 

 scientific interest. The sample forwarded was found near Dehesa 

 in the valley of the Sweetwater River by Mr. Marion Powers, by 

 whom it was supposed to be a mass of fossil coral. By corre- 

 spondence with Mr. Wood the writer secured a larger fragment 

 of the rock, a photograph of which is shown in Plate -16a. The 

 photograph shows that the supposition of its being a mass of 

 fossil coral was a natural and excusable one to make, but to 

 the petrographer it appears at once to be an igneous rock char- 

 acterized by that peculiar and interesting structure commonly 



