Vol. 5] 



Wea/ver. — San Pablo Formation. 



267 



Astrodapsis tumidus, Ostrea titan, Pecten crassicardo, Pseudo- 

 cardium gabbi, and Trochita filosa. The fauna seems to be suffi- 

 ciently characteristic to warrant its being correlated with the 

 San Pablo beds but whether all of it should represent the San 

 Pablo or only a part, is not certain. 



In San Mateo County in the vicinity of Purisinia Creek beds 

 known as the Purisima formation are composed largely of con- 

 glomerates, fine sandstones, and sandy shales and have a thick- 

 ness of about eight hundred feet. A very large number of 

 fossils have been collected. Many of these are common 

 in the San Pablo, among which are Macoma nasuta, Modiola 

 recta, Tapes staleyi, Yolclia cooperi, Chrysodomus tabulatus, 

 Nassa calif ornica, and Neverita. rechiziana. Arnold considers 

 the Purisima as Pliocene and possibly a little younger than 

 the San Pablo. A large number of species present in the Puri- 

 sima occur in the Merced. Arnold estimates that about 30 per 

 cent, of the fauna is extinct. At the present time it is impos- 

 sible to say whether the two formations are equivalent or not. 



The relations of the San Pablo to the Monterey Miocene are 

 clearly brought out in the San Pablo Bay section from detailed 

 areal mapping. The Monterey lies unconformably beneath the 

 San Pablo and gives evidence of a period of deformation and 

 erosion between the close of Monterey deposition and the begin- 

 ning of the San Pablo. In the other sections in middle California 

 no such relations between the San Pablo and Monterey can be 

 made out. It is only upon a palaeontological basis that they can 

 be distinguished. In the Salinas Valley and other localities in 

 the southern part of the state the unconformable relations are 

 exceptionally well marked. The upper Monterey Miocene faunal 

 zone is very closely allied to the San Pablo, and where the two 

 lie in contact it becomes very difficult to draw the line of 

 separation. There are several characteristic fossils from the 

 upper Monterey zone, among which are Trochita costellata, 

 Scutella breweriana and Mulinea densata. The latter is also 

 very abundant in the San Pablo. Out of the total San Pablo 

 fauna twenty-one species occur in the Monterey, or 28.7 per cent. 



The next formation situated geologically above the San Pablo 

 in middle California is the Merced. At no locality are the two 



