GEOLOGY SHORES OF SAN PABLO BAY. 



SHORES OF SAN PABLO BAY. 



As we go north from San Francisco, passing through the straits leading into San Pablo bay, 

 we find the shore on either hand composed of sandstone, forming elevated ridges, which have 

 been produced by subordinate lines of upheaval, having the same general trend as those which 

 include them. In the channel stands &quot; Red Rock,&quot; a mass of jasper, to which allusion has 

 already been made, at the entrance to San Pablo bay, Bird island is a mass of sandstone, as 

 are the points on both sides. 



Passing the low land above point San Pablo, and going eastward toward the straits of Car- 

 quines, along the south shore of the bay, a most interesting section of strata is exposed, having 

 a thickness of at least 3,000 feet. These strata have an inclination of from 30 to 35, dipping 

 to the east, apparently from the axis which forms the eastern shore of the straits connecting 

 San Francisco and San Pablo bays. 



Sandstone. The most westerly and lowest member of the series is a somewhat massive, but 

 softish sandstone, similar to that of Bird island, the straits, and San Francisco ; apparently 

 here, as below, destitute of fossils. This is succeeded by a series of finer grained, soft sand 

 stones and shales, which contain great numbers of fossil shells. Above these fossiliferous beds 

 ensues a great thickness of conglomerates and tufas, extending to the Straits of Carquines. 



SECTION OF STRATA EXPOSED ON SOUTH SHORE OF SAN PABLO BAY. 



a- 1? I c c c a a. 



a a a a, Stratified tufas, bb, Sandstones and shales, ccc, fossiliferous strata, d d t Sandstones, e e, Oyster shells. 



Fossils. The fossiliferous shales and sandstones to which I have referred, if they are, as 

 seems probable, the equivalents of those associated with the sandstone of San Francisco, afford 

 us the first satisfactory evidence which we have of the age of the group to which they belong ; 

 and they therefore become of special interest in the study of the geology of this region. The 

 circumstances under which I visited the locality did not permit me to give as much time to the 

 examination of these beds, nor to collect so full a suite of their fossils as I thought their impor 

 tance demanded ; it is, therefore, to be hoped that some one who shall hereafter have the time 

 and opportunity may make these strata the subject of special study. 



Pecten bed. My attention was first called to the fossiliferous beds by noticing a stratum, 

 which may, perhaps, be called a shell-limestone, about 4 feet in thickness, which was composed 

 almost exclusively of Pectens. From its superior hardness it had resisted the action of the 

 waves, which had cut deeply into the softer strata, and stood out like a wall, breast high, across 

 the beach, and running, like a reef, far out into the shallow waters of the bay. The strike of 

 these strata, as indicated by the direction of the projecting edge of this stratum, is northwesterly, 

 and generally accordant with the trend of the adjacent axes of elevation. 



The Pectens contained in this bed include at least two species : one, a small one, of which I 

 was able to obtain numerous specimens, since described by Mr. Conrad under the name of 

 P. Pabloensis, (PL III, fig. 14;) the other is very large; some individuals being more 

 than six inches in diameter, but so involved in the rock that I was unable to bring away more 

 than fragments, not enough from which to describe it. It considerably resembles a large species 

 procured from the Miocene deposits of southern California, by Mr. Blake, and called by Mr. 



