58 lAUFUKMA AlAlH-MV dl Si IKNfKS. 



iiuicli iliffereuce between the two horizons. Tlie lower fo-ssiliferoii.s strata are distin- 

 ■^nislied by I^eclen e.rjxiuaJDi, /'cr/cn titennixii, 0/nj/in anomulii, and Opalia vnricostata, 

 /'ectcn e.ij)itii!iiis beiiij; very nnmeroiis. Tlie upper l)eds liave few Pecien expaiisus, 

 but are characterized l)y /\-clcu hetnjihilli, wliieli eoin|)letely replaces the Pecien 

 sltarttsii of the lower layers. /uhinarnchnius excentricKK ami CrcpUhiln (jrnndis are 

 also common in the npper layers. 



The serii's of IMioecne .strata exposed at I'aeilic Hcaeii lias been nanud tlie 

 Sail Diejio forination by Dall,' who plaees it below the IMereed forinatioii. At aimtber 

 place in this paper the writir liriiii^s forward reasons for placing the San J)iego lonii- 

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

 is other evidonee showiiii; that the San Diego formation is part of the latest Pliocene. 

 This is the occurrence in it of h'c/iiiiarnchnius edcc)i(ricHt>. This species of echinoderin 

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

 known, but is found in llic lower San Pedro series (lowest Pleistocene) ami inalloflbe 

 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 forination 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 fauna! aspects of the 

 two formations are somewhat different. Pcclen expnnsHs of the San Diego formation 

 replaces Pccicn cnurinus, 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 Bench — Pleistocene. — The upper six to ten feet of the Pacific Beach 

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

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

 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. • 



> North AnMrtcmn TertUry HorUona. Bj V. B. DM. 18tb Ann. Sept. U. S. Geol. Snrrsy, Put U, 18B8, p. 335. 



