Vol. 5] 



Weaver. — Scut Pablo Formation. 



269 



of living' forms makes it Pliocene. The fact that a time interval 

 is represented between the Monterey and San Pablo gives also 

 additional weight to the argument. Considering the sum total 

 of evidence it seems best to regard the San Pablo formation as 

 being of lower Pliocene age. 



SUMMARY. 



The San Pablo formation as represented in middle California 

 is composed of coarse, thick-bedded sandstones, shales, and con- 

 glomerates, with a large admixture of volcanic tuff and ash. It 

 varies in its physical characteristics at different localities and 

 ranges in thickness up to over three thousand feet. Its charac- 

 teristic color is a bluish-gray which upon weathering changes to 

 a buff color. At San Pablo Bay it lies unconformably upon the 

 Monterey, but at the other localities in this part of the state no 

 break between them can be seen. The total number of marine 

 species contained in the formation is seventy-three, and many of 

 these are characteristic forms. Twenty-one, or 28.7 per cent, of 

 them range down into the Monterey and 56 per cent, are still 

 living in the waters of the Pacific Coast today. From the large 

 per cent, of living species it has seemed best to consider the San 

 Pablo formation as Pliocene, and as representing the lower por- 

 tion of that system. 



Issued January 21, 1909. 



