1915] 



Clark: Fauna of the San Pablo Group 



437 



in the San Pablo Bay section, and eastward as far as the town of 

 Walnut Creek, the change in fauna was accompanied by a change in 

 lithology. The physical evidence seems therefore to show that, with 

 local exceptions, the San Pablo conformably overlies the Scutella 

 breweriana zone. This is apparently corroborated by the close faunal 

 relationship existing between the two, which was first pointed out by 

 Professor Merriam. 40 Weaver 41 recognized this fact when he said: 

 "The Upper Monterey 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. ' ' Out of a known fauna of about sixty species 

 from the Scutella breweriana zone, about 50 per cent are also found 

 in the San Pablo. A number of species, however, appear to be char- 

 acteristic of this zone and have not been found in the San Pablo. More 

 collecting may show that the fauna from the Scutella breweriana zone 

 is more distinct than it now appears. There is a much greater differ- 

 ence between the faunas of the Scutella breweriana zone, and of the 

 Lower Monterey (Agasoma gravadum zone) as known at present, than 

 exists between the fauna of the Scutella breweriana zone and that of 

 the Lower San Pablo. 



Stratigraphic as well as faunal evidence seems to bear out the con- 

 clusion that Scutella breweriana beds should be separated from the 

 Monterey. The writer will reserve the discussion of the details of 

 this evidence for a later paper. 



Age of the San Pablo 



Two of the most important methods which at least theoretically 

 may be applied in the determination of the age of the San Pablo, with 

 reference to the different horizons of the east coast and of Europe, are 

 the "percentage method" and the identity of species. In applying 

 the "percentage method" we have a means of correlation the principle 

 of which is generally accepted, but as to the practical application there 

 is considerable disagreement. 



One of the first to use the percentage method in order to determine 

 relation of the age of the Neocene horizons of the West Coast was 

 W. H. Gabb. 42 As has already been pointed out, he placed beds that 

 are referred to in this paper as the San Pablo Group in the Pliocene. 



« Merriam, J. G, Univ. Calif. Publ. Bull., Dept. Geol., vol. 3, no. 16, p. 378, 

 1904. 



« Univ. Calif. Publ. Bull. Dept. Geol., vol. 5, no. 16, p. 267, 1909. 

 i- Gabb, W. H., Palaeontology of California, vol. 1, p. 31. 



