52 University of California Publications in Geology [Vol.7 



Of the species listed above, Dosinia whitneyi does not go 

 higher than the Monterey. Marcia, cf. oregonensis is one of 

 the most common forms in the Upper Monterey. Nucula (Acila) 

 (lalli was first described from the San Lorenzo formation (Oli- 

 gocene). It occurs in the Miocene of Oregon and Washington. 

 Mactra, n.sp., is a form found in the Monterey near the town 

 of Pinole. Devtalium conradif has been found only in the Lower 

 Miocene of this region. Dentalium petricola and Calyptraea in- 

 ornata are both common forms in the Monterey, and are not 

 found in the San Pablo. All of the others, excluding the new 

 species, are common forms in the Monterey. 



San Pablo 



General Discussion. — Lying unconformably above the Mon- 

 terey is the San Pablo. Previous writers have referred to these 

 beds as a formation ; further study shows that they may well 

 be classed as a series. This is true, not only for the section to 

 be described in this paper, but apparently also for the type sec- 

 tion on San Pablo Bay, and for that on the south side of Mount 

 Diablo. Lithologically there are great variations in the beds, 

 which can be mapped as distinct units. On the south side of 

 Mount Diablo two distinct brackish water zones are found inter- 

 calated with marine beds, and at one locality, in Sycamore 

 Canon to the northeast of the town of Danville, an unconformity 

 is to be seen near the middle of the series. More work needs 

 to be done before it can be determined whether this uncon- 

 formity may be correlated with an unconformity on the north 

 side of the mountain, described in this paper. Whether either 

 of these unconformities is of more than local significance is not 

 yet determined. In the type section at San Pablo Bay a brackish 

 water zone also appears. 



In the description of the San Pablo to the north of Mount 

 Diablo the writer has used the name "series" for these beds, 

 dividing them into two divisions, an Upper Division and a 

 Lower Division. The correlation of these divisions with the 

 upper and lower parts of the section on San Pablo Bay seems 

 justified on palaeontological evidence. 



