Clark: The San Lorenzo Series of Middle California 83 



been found only in the lower portion of this series of beds, while a 

 number have been found only in the middle and upper beds, the 

 Kirker tuffs and Concord formation. The collecting in the lower 

 part of the San Ramon formation of the different sections is much 

 better, the fossils are better preserved and more work has been put 

 on that part of the series than in the other formations ; for these 

 reasons the known species in these lower beds are more numerous 

 than they are higher up. Thus, we may expect that a considerable 

 number of the species listed here as having been found only in the 

 lower beds of the San Lorenzo will eventually, with further collecting, 

 be found to extend into the higher horizons. However, it may well be 

 that the species found in the upper beds which have not been found 

 in the lower are good horizon markers, provided that the difference 



Pig. 3. A north and south section to the north of Mount Diablo, including the 

 San Lorenzo series. Ttj, Tejon; Tmk, Markley formation; Tk, Kirker forma- 

 tion ; Tsp, San Pablo ; Tp, Pinole tuff ; Tor, Orinda formation ; Quat, Quaternary. 



in the faunas may not be due to a difference in the conditions of 

 environment. The solution of this problem must be left for future 

 work. 



THE SAN LOEENZO SERIES TO THE NORTH OF MOUNT DIABLO 



General Statement 



The San Lorenzo beds to the north of Mount Diablo, which were 

 studied by the writer in detail, lie in an area to the south and south- 

 east of the town of Pittsburg, the nearest point being a little over 

 three miles distant ; here the outcrops are found along a strip extend- 

 ing from a little over a mile west of Kirker Creek to about three miles 

 east of Markley Canon (see Mount Diablo topographic sheet, United 

 States Coast and Geodetic Survey) ; the width of this strip is about 

 a mile. The Oligocene formations extend to the northwest of this 

 area onto the Napa Quadrangle, where, in the vicinity of the town 

 of Bay Point, they disappear below the alluvium; to the southeast 

 these beds, together with those of Miocene and Pliocene age, form the 

 low, rolling hills which lie to the south and southeast of the town of 

 Antioch ; farther east, all these formations are covered by the alluvium 

 of the San Joaquin Valley. 



