14 GEOLOGY — SHOKES OF SAN PABLO BAY. 



Conrad Pecten Nevadanus, and is, perhaps, identical with it. It seemed to me, however, in the 

 widely expanded alae, in the unusual curvature of both valves, and in the very angular and 

 striated costae and intercostal spaces, to exhibit differences from any species known. Beside 

 the Pecten, I found in this rock numbers of a large Mactra, of which I could obtain no entire 

 specimens, but which closely resembles M. densata, from the Miocene at Santa Barbara, and 

 described and figured (PI. Ill, fig. 12) in this report. 



Lyicg immediately upon the pecten bed, are several strata of a soft, yellowish-brown rock, 

 composed of nearly equal parts of clay and sand, .and containing large numbers of shells ; the 

 smaller species of Pecten mentioned, (P. Pabloensis) with Mactra, Natica, Nucula, and Tellina. 

 These beds are each 5-10 feet in thickness, and compose, perhaps, 50 feet of strata. Upon them 

 lies a thicker stratum of coarse, but soft, non-fossilif'erous sandstone, which is succeeded by other 

 fossiliferous strata resembling, in fossils and lithological characters, those below it. Specimens 

 of all these beds containing fossil shells, when examined under the microscope, exhibited no 

 traces of infusoria. 



Tufas and marls. — These fossiliferous strata, which have an aggregate thickness not exceeding 

 100 feet, are followed by a succession of relatively thin and perfectly conformable beds of 

 conglomerate, soft, harsh, coarse sandstones, containing masses of scoria and pumice, con- 

 cretes, tufas and marls, white, cream, bluish, greenish, yellow, and brown in color, in which I 

 was able to detect no fossils, and which have an aggregate thickness of 1,500 to 2,000 feet. 



These strata are evidently of volcanic origin, consisting of ashes, sand, mud, pumice, and 

 scoria, which have been apparently discharged into water, perhaps of considerable depth, and 

 accurately stratified by sedimentary deposition. 



To strata of similar character I shall have frequent occasion to refer, as they are found in a 

 great number of widely separated localities, and sometimes stretch continuously over large areas, 

 constituting one of the most remarkable features in the geology of not only those parts of Cali- 

 fornia and Oregon which were visited by our party, but, as I learn from various sources, of a 

 large part of the region lying west of the Rocky Mountains. 



These beds uniformly overlie, and are more recent than the tertiary deposits, and bring the 

 geological history of the western coast down to, and, as it seems to me, probably through the 

 drift. 



Oyster beds. — A very interesting feature in the section exposed on the south shore of San 

 Pablo bay, is a bed of recent shells which lies horizontally upon the edges of the inclined strata 

 which I have described, at an elevation of some 20 feet above the present level of the water in 

 the bay. This bed is about four feet in thickness, and composed of shells of species now living 

 on the neighboring Pacific coast. They consist principally of Ostrea, with great numbers of 

 Mytilus, Lithodomus, Pholas, &c. 



The Lithodomi are found in the holes which they excavated in the rocky bottom of the water 

 in which they lived. The shells are generally very perfectly preserved ; the Mytili and Litho- 

 domi having lost the epidermis in all cases, and the shells being tender, and somewhat chalky ; 

 but the Ostrea, in many instances, retain the colors which characterize the living specimens. 



This bed of recent shells affords a striking proof of the disturbances which this volcanic, and 

 earthquake-shaken coast has experienced. 



The cause which produced the difference between the present and former relative levels of the 

 surface of the water in the bay and its shores was not merely local in its action ; for a similar 

 bed of shells occurs at something like the same altitude around the north and western shores of 



