Clark: The San Lorenzo Series of Middle California 81 



of the Monterey beds, in which the shells of Ostrea n. sp. aff. titan 

 Conrad occur in abundance. A sharp line of contact was found 

 separating the Agasoma gravidum beds from the Tejon below. 



Relation to Monterey {Miocene). — The San Ramon beds are absent 

 only a few miles to the southeast of the section just described; here 

 the basal beds of the Area montereyana zone (Lower Monterey) rest 

 directly upon the Tejon. These basal beds, in places, are composed 

 of fairly heavy conglomerates similar to those seen on the west side 

 of the syncline. At a number of localities tuff boulders were found 

 in the conglomerate, similar to the Kirker tuffs of the Sobrante anti- 

 cline and to the tuft's found in the Kirker formation, the upper mem- 

 ber of the Oligocene to the north of Mount Diablo in the vicinity of 

 Kirker Creek. It seems probable that these boulders, found in the 

 conglomerates of the basal Monterey, were derived from the Kirker 

 tuffs of the San Lorenzo, as tuff's of this character are not known to be 

 present in this general region in any of the formations below that 

 horizon. 



San Lorenzo Series in the Paciteco Syncline 



Occurrence and Lithology. — The third general section of the Con- 

 cord Quadrangle in which beds containing the San Lorenzo fauna 

 occur is to the east of Muir Station and to the southeast of the town 

 of Martinez; here they outcrop on either side of what Lawson has 

 designated the Pacheco syncline. The highest beds in this syncline 

 belong to the "Monterey Group." The three faunal zones of the 

 Monterey, as outlined in the Folio, are present, though the entire 

 thickness is only a few hundred feet. 



The San Lorenzo fauna is found on both sides of the syncline in 

 fine gray sandstones immediately above the Tejon. The beds con- 

 taining this fauna have a thickness of only about one hundred feet, 

 They are overlain by heavy conglomerates which in some localities 

 are as much as a hundred feet thick. These conglomerates yield the 

 fauna of the Area montereyana zone (Lower Monterey) ; as at Wal- 

 nut Creek, the species Ostrea n. sp. aff. titan Conrad is found in 

 abundance in the conglomerate. 



Fauna of the San Lorenzo Series to the West of Mount Diablo 



Opposite this page is a complete list of the invertebrate species 

 obtained from the San Lorenzo beds to the west of Mount Diablo with 

 the localities at which they were found indicated in the columns at the 



