Mhkkiam. 1 



Distribution of the Neocene Sea-urchins. 



ri7 



CLASSIFICATION OF THE NEOCENE OF MIDDLE CALIFORNIA. 



The following table, giving the occurrence and geological 

 range of the echinoid species known above the middle Neocene, 

 illustrates the faunal and the probable time relations of the for- 

 mations in which they occur to each other. 



Quaternary — Echinaraclinius excentricus, Esch. 



«j , ( Echinaraclinius excentricus Esch. (?) 



| Scutella interlineata, Blake. 



( Astrodapsis WHitneyi, Rem. 

 San Pablo Astrodapsis tumidus, Rem. 



\Sciitclla (Clvpcastcr) Gabbi, Rem. 



Contra Costa f Clypeaster? {Echinarachnius) Brezuerianus, Rem. 

 County Miocene | Echinarachnius {Scutella) Gibbsi, Rem. (? ?) 



The Contra Costa Count}- Miocene represents a large portion 

 of the Lower Neocene. Whether it includes all of the Miocene 

 is uncertain. The Merced certainly includes a large portion of 

 the Upper Neocene, but how much is not yet determined. The 

 San Pablo probably represents middle Neocene. It may be 

 found to fall principally within the limits of the Miocene, and 

 probably does not represent a very large portion of the Pliocene. 

 The more exact correlation of these horizons with the Neocene 

 periods of eastern United States and Europe can be accomplished 

 only when our knowledge of the Tertiary faunas of the Pacific 

 Coast is much more complete than at present. 



The intercalation of the San Pablo formation between the 

 Contra Costa County Miocene and the Merced Pliocene adds to 

 the already long Tertiary era of California a period of sedimenta- 

 tion, during which at least fifteen hundred feet of strata were 

 deposited. The thickness of the Tertiary section and the num- 

 ber of physical and organic changes, which took place in that 

 era on the Pacific Coast, indicate that the general tendency has 

 been to underestimate rather than overestimate the relative length 

 of Tertiary time. 



