1916] Martin: Pliocene of Middle and Northern California 227 



Ashley and others have usually been characteristic enough to 

 justify the determination as Pliocene, but they have not been suffi- 

 cient to assign any particular part of the Merced to a certain 

 division of the Pliocene. Some palaeontologists who are familiar 

 with California geology have considered the series to be upper Plio- 

 cene. The reasons for such a determination are to be found in the 

 relation which the Merced bears to formations outside of the area 

 rather than the fauna found within the Merced itself. At Aho 

 Nuevo Bay, a short distance to the south, a small area of Merced 

 is reported to occur conformably above the Purisima Formation. The 

 faunal and stratigraphic relations of the Purisima appear to indicate 

 that it is lower Pliocene in age. From the superposition of the 

 Merced upon the Purisima it is evident that the Merced is at least 

 later than lower Pliocene. 



Recent collecting over the whole area covered by the Merced 

 has revealed fossil invertebrates from the entire vertical range and 

 also from the greater portion of its lateral extent. Numerous 

 creeks and small streams cut transversely across the section from 

 its southwestern boundary to the Merced Valley. In nearly every 

 one of these streams good exposures are to be found, and fossil 

 localities, while not numerous, are usually found in all of these 

 sections. The hills immediately to the southwest of Millbrae have 

 afforded several localities from which a large number of fossil 

 leaves were obtained, as well as fossil invertebrates. Unfortunately 

 the generic and specific character of the leaves could not be de- 

 termined, consequently they give no evidence as to the age of the 

 beds in which they were found. They are of value, however, in 

 showing the character of the deposit, clearly defining it as an 

 estuarine or littoral zone. To the southeast of Millbrae good 

 exposures rarely occur on account of the alluvium and sand-dune 

 deposits which conceal the underlying beds. To the northwest the 

 outcrops become much more numerous and accessible, and the state 

 of preservation of the fossils much better as the ocean beach is 

 approached. 



The most satisfactory and accessible section for a faunal study 

 is the one exposed along the sea-cliff from Mussel Rock to the out- 

 let of Lake Merced. In general, fossil localities are not as numer- 

 ous as those found in other sections, but the excellent exposures 

 of the strata afford good opportunities for investigation. Near 

 the base, just above the contact at Mussel Rock, several localities 



