134 University of California Publications in Geology [Vol. 13 



INTRODUCTION AND ACKNOWLEDGMENTS 



This paper presents the results of a study of the faunal and strati- 

 graphic relations of the Briones formation to the adjacent formations 

 in an effort to determine its position in the geologic scale. The results 

 indicate : ( 1 ) that the Briones probably should be classified with the 

 San Pablo group (Upper Miocene) rather than with the Monterey 

 group (Lower and Middle Miocene) ; (2) that it is a minor cycle of 

 deposition distinct from the Cierbo formation. 1 



Hitherto in its relationship to other strata, the Briones formation 

 has occupied a more or less uncertain position. The term Briones was 

 first used by Professor A. C. Lawson 2 in 1914 in the San Francisco 

 Folio. It was applied to the "Scutella breweriana beds," which at 

 that time were regarded as forming the upper faunal member of the 

 Monterey group in the region east of San Francisco Bay. Dr. J. C. 

 Merriam 3 in 1898 was the first person to indicate the faunal distinct- 

 ness of the ' ' Scutella breweriana beds. ' ' He included them, however, 

 in the undifferentiated Miocene beneath the San Pablo. Very little 

 detailed work has been done on the Briones since then. A few writers 4 

 have mentioned a faunal similarity between the Briones and the San 

 Pablo, lint they have included the Briones in the Monterey group. 

 However, Chirk"' indicated that perhaps the Briones might have a 

 closer relationship to the San Pablo than to the Monterey. 



1 The recognition of the Briones as a part of the San Pablo group causes the 

 "Lower San Pablo" formation to become the middle member of the group. In 

 order to overcome this ambiguity, Professor B. L. Clark, in his recent paper on 

 the Marine Tertiary of the West Coast of the United States, in the Journal of 

 Geology, volume 29, page 601 (1921 ), applies the name Cierbo formation to the 

 "Lower San Pablo," and he uses the term Santa Margarita for the Upper San 

 Pablo. He retains the name San Pablo for the entire group consisting of the 

 Briones, Cierbo, and Santa Margarita. 



In the present paper, unless otherwise stated, the term San Pablo refers to 

 that part of the San Pablo group above the Briones, viz., the Lower San Pablo 

 (Cierbo) and the Upper San Pablo (Santa Margarita). In other words it refers 

 to the San Pablo group as understood previous to the application of the evidence 

 here presented. 



2 Lawson, A. C, San Francisco Folio, U. S. G. S. No. 193, p. 11, 1914. 



s Merriam, J. C, Distribution of the Neocene sea-urchins of Middle Cali- 

 fornia, Univ. Calif. Publ. Bull. Dept. Geol., vol. 2, p. 112, 1898. 



* Merriam, J. C, A note on the fauna of the Lower Miocene of California, 

 Univ. Calif. Publ. Bull. Dept. Geol., vol. 3, p. 378, 1904. 



Weaver, C. E., The stratigraphv and palaeontology of the San Pablo forma- 

 tion of Middle California, Univ. Calif. Publ. Bull. Dept. Geol., vol. 5, p. 251, 1909. 



Clark, B. L., Fauna of the San Pablo group of Middle California, Univ. Calif. 

 Publ. Bull. Dept. Geol., vol. 8, p. 436, 1915. 



^ Loc tit., p. 436. 



