16 HAEHL AND ARNOLD— THE MIOCENE DIABASE. 



[Feb. 5, 



THE MIOCENE DIABASE OF THE SANTA CRUZ MOUN- 

 TAINS IN SAN MATEO COUNTY. CALIFORNIA/ 



BY H. L. HAEHL AND RALPH ARNOLD. 



{Bead February 5, 190 4.) 



Introduction.^ 



The presence of the basic eruptives in the Santa Cruz Mountains 

 of San Mateo County, California, and portions of Santa Clara and 

 Santa Cruz Counties, was first noted in 1865 by J. D. Whitney. 

 In discussing the geology in the vicinity of Searsville, Whitney 

 says : •' 



*' In the bed of the creek (one mile west of the ridge in which 

 the coal mine is situated) were, among the boulders of sandstone, 

 some fragments of syenitic granite and of a basaltic rock, which 

 latter is said to cap a few of the highest points of this ridge." 

 Whitney's party also passed over the divide from San Mateo to Half- 

 moon Bay, noting the Cretaceous and Miocene strata and the Mon- 

 tara granite exposed along the road, but seem to have overlooked 

 the diabase exposures on the east and south of the granite outcrop. 



W. L. Watts in a paper^ on San Mateo County says : ^'At some 

 points basaltic rocks have been observed, and on the San Gregorio 

 Rancho the Field Assistant of the Bureau noted and obtained 

 specimens of vesicular dolerite, the vesicles of which were filled 

 with petroleum." 



1 Published by permission of the Director of the U. S. Geological Survey 



2 The work of which the results are given in this paper was done while the 

 authors were post-graduate students of geology in Stanford University, under the 

 direction of Dr. J. C. Branner. The authors wish to acknowledge their indebt- 

 edness to Dr. Branner for suggestions regarding the field relations, especially of 

 the tuffs and associated rocks, and to Dr. J. P. Smith for suggestions relating to 

 the petrographical work. 



The names used in the lists of fossils in this paper are those commonly ap- 

 plied to the respective species by the West Coast paleontologists. Owinc to the 

 miperfect state of our knowledge regarding the nomenclature of the California 

 lertiary fauna, there is a probability that some of the names used are erroneous • 

 the writers, therefore, reserve the privilege of revising any or all of the names if 

 future study shall warrant it. 



^Geological Survey of California, Vol. I, p. 71, 1865. 



* Tenth Ann. Kept. Calif State Mineralogist y p. 586, 1890. 



