Mer kiam. | 



Distribution <>/ the Neocene Sea-urchins. 



related to the Merced as it should be it the two belong to the 

 same formation or the same period, it is in fact not so closely 

 related to the Merced as to the Contra Costa County Miocene. At 

 San Pablo Bay the formation shows more than one thousand feet 

 of very fossiliferous sandstones containing an abundance of echi- 

 noids, all of which are different from those of the Merced. The 

 Merced section south of San Francisco shows several thousand 

 feet of fossiliferous sandstones with echinoids but not a single 

 specimen of any of the San Pablo species has been found among 

 them. The molluscan faunas of the two formations, though not 

 mutually so exclusive as the Echinoderms, are very different. Not 

 more than one-fifth of the fifty species known from the San Pablo 

 occur in the Merced. It is difficult to conceive of any way in 

 which two shallow-water, marine faunas so different as these could 

 exist within a few miles of each other, through the length of time 

 required in forming more than one thousand feet of sandstones, 

 without their becoming mingled to some extent, and mingling of 

 their most characteristic forms we do not find. 



So far as faunal proof is concerned, it is not easy to believe 

 that the Merced and San Pablo have been deposited at the same 

 time, and any stratigraphic evidence which may later tend to 

 indicate their contemporaneity will show the existence of biolog- 

 ical conditions in Middle California, during that period, radically 

 different from any which have ever been met with elsewhere. 



Age of San Pablo. — If the Merced and San Pablo are not of 

 the same age, both faunal and stratigraphic evidence would point 

 toward the latter as the older of the two. The fauna of the San 

 Pablo is more closely related to that of the Miocene of Contra 

 Costa County than to the Merced fauna. Stratigraphically the for- 

 mation shows a much greater amount of deformation than the 

 Merced, and where the upper beds are overlaid by the Quaternary 

 a much more pronounced unconformity is seen than has so far 

 been shown to exist between the Merced and the Quaternary. 

 Lithologically there is a noticeable difference between the two, as 

 the great beds of ash and tuff so characteristic and widespread in 

 the San Pablo appear to be entirely absent from the Merced. The 

 only distinctly volcanic deposit in the whole cliff section at Seven- 



