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University of California Publications in Geology [Vol. 9 



is separable into at least two zones exclusive of the shale facies. The 

 following statement is made : 



The fauna of the lower division is much more characteristic than the upper; 

 that is to say, it differs more decidedly from that of the beds immediately above 

 and below it. . . . The most characteristic species are Agasoma gravida, Dosinia 

 mathewsoni, Chione mathewsoni, and Mytilus mathewsoni. . . . The lower fauna 

 is recognized at many places in Contra Costa County, and always near the base 

 of the series. This faunal division might appropriately be called the zone of 

 Agasoma gravida. 



In the San Francisco Folio, 13 which was published very recently 

 by Professor A. C. Lawson, the beds containing the fauna of the 

 Agasoma gravidum zone, as defined by Dr. J. C. Merriam, are included 

 in the Miocene as a part of the Monterey group. For this the writer 

 is in part responsible. 



AGASOMA GRAVIDUM BEDS IN THE VICINITY OF THE TOWN OF 



WALNUT CREEK 



The unconformity between the Agasoma gravidum beds and the 

 Area montereyana beds in certain localities in Contra Costa County 

 is in the Sobrante sandstone, the basal member of the Monterey group 

 of Lawson. It is best seen in the section of the Monterey a little to 

 the south of the town of Walnut Creek and only a few miles west of 

 Mount Diablo. Here the Agasoma gravidum beds outcrop on two 

 sides of a southeasterly plunging, tightly appressed syncline, resting 

 upon the Tejon (Eocene) and being overlain un conformably by beds 

 containing the fauna of the Area montereyana zone. 14 



On the west side of the syncline, to the southwest of the town of 

 Walnut Creek and on the west side of San Ramon Valley, the beds of 

 the Agasoma gravidum zone are about 525 feet in thickness. They 

 consist for the most part of a fine gray tuffaceous sandstone, which 

 in some localities has a blueish cast. A little above the middle of the 

 section there is a thin bed of a siliceous gray shale. Excellent ex- 

 posures of these beds may be seen about two miles to the south of the 

 town of Walnut Creek. 



Relation to Tejon. — No heavy conglomerates were found in this 

 section at the base of the Agasoma gravidum beds. The exact line 

 of contact between it and the Tejon (Eocene) is uncertain. So far, 

 no direct evidence has been obtained in any of the sections studied 



is San Francisco Folio, U. S. Geol. Surv., no. 193, 1914. 



I* See Geologic Map of Concord Quadrangle, S. F. Folio, U. S. Geol. Surv., 

 no. 193, 1914. 



