pacific coast section 145 



Minutes of the Tenth Annual Meeting of the Pacific Coast 

 Section of the Paleontological Society 



By Chester Stock, Secretary 



The tenth annual meeting of the Pacific Coast Section of the Paleon- 

 tological Society was held in conjunction with the Cordilleran Section, 

 Greological Society of America, at Seattle, Washington, June 18, 1920. 

 The societies participated in the fourth annual meeting of the Pacific 

 Division, American Association for the Advancement of Science. 



The meeting was called to order by Dr. E. L. Packard in the Science 

 Hall of the University of Washington. 



As an item of business, it was moved and carried that the new officers 

 of the Paleontological Society should be voted on by mail at a later date 

 determined by the Secretary. 



reading of paleontological papers 

 The following paleontological and geological papers were then read: 



A STUDY OF THE FAUNA AND STRATIGRAPHY OF THE BRIONES FORMATION 

 OF MIDDLE CALIFORNIA 



BY PARKER DAVIES TRASK 



(Abstract) 



The Briones was formerly regarded as a part of the Monterey series (Lower 

 and Middle Miocene), but the results of recent work have indicated that it is 

 probably the lowest part of San Pablo series (Upper Miocene). The Briones 

 deposits are found in an area of some 50 miles radius in the vicinity of San 

 Francisco. Its sediments are chiefly sandstones, with some shales intercalated 

 in the upper part of the formation. The thickness of the Briones varies from 

 500 to 2,300 feet. 



There is no apparent difference in dip and strike between the Briones and 

 the formations above and below, but there .is usually a marked lithologic 

 change, and irregular contacts with Pholas borings have been found between 

 it and the over and under lying formations. 



A study of the faunal evidence indicates a closer relation to the San Pablo 

 than to the Monterey. Out of 70 determinable species found in the Briones, 

 11, or 15 per cent, extend into the Monterey, but only one of these 11 species 

 is peculiar to these tw^o formations, while 39 species, or 56 per cent, occur in 

 the San Pablo, of which 19 species are peculiar to the Briones and the San 

 Pablo. A large number of these 19 species are highly ornamented gastropods. 

 This indicates that the Briones is probably a part of the San Pablo series. 

 However, considering that there are 29 species, or 41 per cent, found only in 

 the Briones, and only 6 species peculiar to the Briones and the Lower San 

 Pablo group, and because of the stratigraphic evidence mentioned above, the 



X— Bull. Geol. Soc. Am., Vol. 32, 1920 



