— 52 — 



than to ascertain their Tertiary character. The Quaternary fossils 

 near the beach, and the Pliocene obtained from the " San Diego Well," 

 are indeed very perfect, but older ones from farther inland are very 

 poor. From previous collections of these, geologists have classed them 

 as Miocene, arguing that the most inland must be the oldest. Mr. 

 Fairbanks found some very good specimens along the eastern escarp- 

 ment of the mesa, which seem to show that the whole of it may be 

 considered Pliocene, or at least as belonging to the intermediate beds 

 passing into Miocene. The slightly disturbed strata all along the coast 

 are usually found to be of later date than those much uplifted, judging 

 from the proportion of living to fossil species in them. The large fossil 

 oysters found at the eastern edge of the mesa have been hastily referred 

 to that truly Miocene species, 0. titan Conrad, but Mr. Fairbanks' 

 numerous and good specimens show that they are as near to the 

 Pliocene species described by Remond as 0. hourgeoisi, tending to show 

 a derivation of the latter from the former. I observed a similar form 

 in the Tertiary beds of Alisos Creek, Orange County, in 1872, and gave 

 it the latter name in the Catalogue of Fossils. With this, Mr. Fair- 

 banks found specimens of Pecten expansus Dall, which was first described 

 from the San Diego Well, and is also found in another " Pliocene " bed 

 at Soledad Canon, twelve miles north. There is nothing in the known 

 fossils of the mesa elsewhere to forbid the reference of the whole to the 

 Pliocene. 



Another bed of very perfect oyster shells occurs about fifty miles 

 north of the boundary line, and three east of Oceanside, above the 

 Cretaceous (B) strata. These resemble small 0. titan, but connect it 

 with Gabb's 0. tayloriana, under which name I catalogued it in 1887 

 from San Juan Capistrano, twenty-five miles farther northwest. About 

 the northern boundary of San Diego County the Tertiary begins to be 

 more uplifted, and forms short ridges along the coast, in which the 

 strata are much broken up as if by eruptive action. In these the true 

 0. titan appears, of its usual enormous size, and other characteristic 

 fossils appear also; the Miocene strata forming several lower ridges, 

 and a great part of the Santa Ana Mountains. It now appears that 

 the Ostrea tayloriana is only the young form of 0. titan. 0. hourgeoisi 

 approaches the large species of Western Mexico, which has been called a 

 variety of the Atlantic 0. virginiana. 



C. FRESH-WATER SHELLS FROM A LIGNITE BED. 



These three species were found by me several years ago, beautifully 

 preserved between the layers of lignite, to which the white shells form 

 a fine contrast. They could be easily outlined on tracing-paper, though 

 perhaps slightly deformed by pressure. 



