Geol.— Vol. II. ] ANDERSON— STRATIGRAPHIC STUDY 183 



it is often impossible to discriminate between the rocks of the 

 several periods. In most places there is an apparent strati- 

 graphic conformity between Cretaceous and Eocene, and 

 between the latter and Miocene strata. And in most sec- 

 tions, likewise, the Pliocene (Etchegoin) rocks rest conform- 

 ably upon the older series. Dr. Becker and C. A. White ^ 

 believed that the entire collection of Cretaceous, Eocene, 

 and Miocene strata formed a continuous and conformable 

 series, and this opinion was held after observations had 

 extended over a considerable portion of the Mount Diablo 

 Range. • 



The lithological variation of the rocks is considerably 

 greater, and characteristic types are the rule in all of the 

 principal epochs. It is often possible to recognize without 

 the aid of fossils many of the typical members of the strati- 

 graphic groups. Probably the most trustworthy guide for 

 the identification of strata in all cases is that afforded by 

 paleontology, but in the later formations the persistence of 

 some of the fossil forms from the earliest Miocene to the 

 Present makes it necessary to use them with caution. With- 

 out the aid of other stratigraphic data and the recognition of 

 lithologic peculiarities it would often be difficult to distin- 

 guish between the Lower Miocene and the Coalinga Beds, 

 while both of these series contain forms that are still living 

 alonjj the west coast. However, there are a few forms that 

 have been found to be sufficiently trustworthy within provin- 

 cial limits, but it is doubtful if many of them would support 

 extensive generalizations. 



Dr. Merriam has pointed out" that Agasoma gravidum, 

 Tjirritella ocoyana, and T. hoffjnanni are characteristic of the 

 lower Miocene, and all of these have been found in the Tem- 

 blor Beds of the Mount Diablo Range, along with man\- other 

 forms occurring in the typical Lower Miocene beds of Kern 

 River. 



Similarly certain forms of Pecteii, Mactra, Scutella, and 

 Astrodopsis are believed to belong only to the Etchegoin 

 Beds, but it will require at least a reasonable degree of 



1 Bull. no. 15, U. S. Geol. Surv. pp. 14, 15 et seq. 



2 Bull. Dept. Geol. Univ. Calif, v. 3, pp. 377-38i- 



