1922] Trash: The Briones Formation of Middle California 135 



Most of the field work for the present paper was done during the 

 summer of 1919, but several days were spent in the field in the fall 

 of 1920 and the spring of 1921. The paleontological evidence is based 

 both on collections made during this field study, and on previous col- 

 lections made by the University of California. 



The writer is particularly indebted to Professor B. L. Clark, whose 

 personal supervision, suggestions, and cooperation, both in the field 

 work and determination of fauna, have made possible the results here 

 presented. 



Through the kindness of Professor James Perrin Smith, the writer 

 has had access to the collections of the Briones at Leland Stanford 

 Junior University. He is particularly indebted to Mr. Frederick P. 

 Vickery of the Southern Branch of the University of California for 

 much information concerning the nature of the southern extension of 

 the Briones, and for assistance with the maps and paleontological col- 

 lections of the Stanford Geological Survey. 



The work of determining the fauna has been greatly facilitated 

 by the excellent collections made by Professors J. C. Merriam and 

 B. L. Clark. 



Acknowledgment is also due Mr. E. L. Furlong of the Museum of 

 Paleontology at the University of California for numerous suggestions 

 and friendly criticisms. 



AREAL DISTRIBUTION OF BRIONES FORMATION 



The Briones formation is so far known only from a limited area 

 near San Francisco Bay. It is particularly well developed in the 

 western part of Contra Costa County, which has been the chief field 

 of investigation. 



The most easterly known exposure of the Briones is on the south- 

 west flank of Mt. Diablo. Mr. Vickery informs the writer that he has 

 observed the Briones extending to a point a few miles south of Mt. 

 Hamilton. Twenty-five miles to the west in the Santa Cruz Quad- 

 rangle 8 and extending northerly along the San Francisco Peninsula, 

 the Briones has not been reported as occurring between the Monterey 

 and the Santa Margarita. Professor B. L. Clark has told me that he 

 has observed the Briones as far north as Carneros Creek, just west of 

 the city of Napa, some thirty-five miles north of San Francisco. 



e Branner, J. C, Arnold, Ralph, and Newsome, J. J., Santa Cruz Folio, U. S. 

 G. S. No. 163, 1909. 



