58 CALIFORNIA ACADEMY OF SCIENCES. 



much difference between the two horizons. The lower fossiliferous strata are distin- 

 guished by Pecten exj)nnsus, Pecten stearnsii, Oj^inlia anomala, and OjmHci varicostata, 

 Pecten expansus being very numerous. The upper beds have few Pecten expansus, 

 but are characterized by Pecten hemphilli, which completely replaces the Pecten 

 stearnsii of the lower layers. Echinarachnius excentricus and Grepiduln grandis are 

 also common in the upper layers. 



The series of Pliocene strata exposed at Pacific Beach has been named the 

 San Diego formation by Dall,^ who places it below the Merced formation. At another 

 place in this paper the wrfter brings forward reasons for placing the San Diego form- 

 ation above the Merced. Besides the reasons given at that place (page 17) there 

 is other evidence showing that the San Diego formation is part of the latest Pliocene. 

 This is the occurrence in it of EcMnarachnius excentricus. This species of echinoderm 

 is not found in the Merced series or in any of the underlying formations, so far as 

 known, but is found in the lower San Pedro series (lowest Pleistocene) and in all of the 

 .succeeding formations, and is still living. Considering the short geologic range 

 usually covered by species of echinoderms, this evidence strongly favors the theory 

 that the San Diego formation is at the top of the Pliocene, and probably takes in a 

 part of the lower San Pedro series. 



The lithologic and stratigraphic similarity of the San Diego formation to the 

 Deadman Island Pliocene is marked. Both consist of rather fine, brownish yellow 

 sand strata only moderately consolidated, except locally, with low dip, and showing 

 the effect of only a slight movement since their deposition. Both formations are 

 overlain unconformably by strata of Pleistocene origin. The faunal aspects of the 

 two formations are somewhat different. Pecten expansus of the San Diego formation 

 replaces Pecten caurlnus, which is abundant in the Pliocene of Deadman Island, and 

 numerous species are found in the Deadman Island Pliocene which do not occur in 

 the Pacific Beach Pliocene deposits. Pecten stearnsii is common to both formations, 

 and as its geologic range is very limited, it is strong evidence of the contemporaneity 

 of the two formations. Correlating more accurately, the Deadman Island Pliocene 

 is probably contemporaneous with the lower (Pliocene) horizon at Pacific Beach, for 

 Pecten stearnsii is limited to that horizon in the San Diego formation, so far as is known. 

 Pacific Beach — Pleistocene. — The upper six to ten feet of the Pacific Beach 

 bluff is composed of a gravel and sand formation which rests unconformably upon 

 the Pliocene strata. The line of demarkation between the Pliocene and the upper 

 Pleistocene formation is generally distinct, as the lower layer of the Pleistocene is of 

 coarse, fossiliferous gravel, while the underlying Pliocene strata are of fine sand. 

 Well preserved fossils are found at many points in this Pleistocene layer along Pacific 

 Beach, while at other places in the same stratum the fossils are so brittle that it is im- 

 possible to remove them from the soft matrix. The fauna of this Pleistocene stratum 

 is very similar to that of the upper San Pedro deposits exposed at the north point of 

 the San Pedro bluff. The break between the faunas of the Pacific Beach Pliocene 

 and the overlying Pleistocene is much greater than that between the Deadman 

 Island Pliocene and its overlying Pleistocene (lower San Pedro series) layer. 



1 North American Tertiary Horizons. By W. H. Dall. I8th Ann. Eept. U. 8. Geol. Survey, Part II, 1898, p. 336. 



