PLIOCENE FOSSILS FROM KIRKER PASS. 397 



Some shells collected by myself at Kirker pass were referred by Mr. Becker 

 to Dr. Wm. H. Dall, who reports that he has identified the following species, 

 all of which are probably still living : 



Bittium asperum, Cpr. ; 



Tapes staleyi, Gabb ; 



Saxidomiis squalibus, Desh.; 



Solen, sp. und. ; 



3Iacoma, sp. iiud. 

 Marine organic remains are, I think, in general considered the most re- 

 liable indications of the age of strata, and this seems most reasonable, since 

 the salt waters of the world are practically one body, in which physical con- 

 ditions are more uniform than on the land or in the air, and marine forms 

 are thus less subject to change by local influences than are those of the land 

 or of fresh waters. 



There are also in the finer layers at Kirker pass numerous fossil leaves, 

 and much silicified wood is weathered out. In the proceedings of the 

 United States National Museum* there is published by Lesquereux a short 

 account of specimens collected by myself at Kirker pass. The species iden- 

 tified are as follows : 



Diospyros virginiana, L., var. turner i, Lx. ; 



Magnolia californica, Lx. ; 



LauTus, cf. canariensis, Heer. ; 



Laurus, cf. furstenbergi, Heer. ; 



Viburnum^ cf. rugosus, Pers. ; 



Vitis, sp. und. 

 These are considered to be probably Pliocene, although on page 11 of the 

 work the same collection is referred to the Upper Miocene. But there is 

 still other evidence of the Pliocene age of these beds in the character of the 

 tufas and conglomerates. These are made up chiefly of detrital material of 

 hornblende and pyroxene-andesite. So far as we know at present, the 

 andesitic eruptions took place in Pliocene times, or perhaps accompanied 

 the post-Miocene upheaval of the Coast ranges.t 



The andesitic material at Kirker pass presumably was derived from the 

 volcanic area to the north of San Pablo bay. The Railroad ranch reservoir 

 beds also consist in part of andesitic conglomerate and tufa, with fine layers 

 in which there are fossil leaves. The material is here more or less altered 

 and penetrated by many small veins of chalcedony and calcite. 



The Corral hollow beds are quite similar lithologically to those of the last 

 two localities, containing much fine shale with fossil leaves and andesitic 



*Yol. XI, 1889, p. 35. 



tFor evidence on this point, referring to the Coast ranges, see Becker, " Quicksilver Deposits: " 

 Monograph XIII, U. S. Geological Survey, 1888, pp. 222-223 ; and, with reference to the latter range, 

 Whitney's Auriferous Gravels of the Sierra Nevada, 1880, pp. 219-288. 



