GEOLOGY — STRAITS OF CAEQUINES. 15 



the bay ; and the shallows of the southern and eastern parts of San Francisco bay contain a 

 similar bed of dead oyster shells. 



It might be supposed that this change in the relative level of the land and water had followed 

 the opening of the Golden Gate to a greater depth than before, and by this means depressing 

 the water level without seriously disturbing the general level of the surrounding country. 

 This theory is, however, untenable, for it would require that the bays of San Francisco and San 

 Pablo should be bodies of fresh water, to which the waters of the Pacific would scarcely have 

 access ; while we know, from the fact that this extensive bed of shells is composed of marine 

 species, that the water in which they lived was -salt. We may go still further and say that 

 these beds of Ostrea, Mytilus, Pholas, &c, could scarcely be formed in San Pablo bay at present, 

 from the relatively large admixture of fresh water derived from the Sacramento and San Joaquin 

 rivers. It seems scarcely possible that a sufficient amount of salt water could ever have been 

 supplied to these oyster beds through the narrow channel of the Golden Gate, and it is probable 

 that the oysters of San Francisco bay, which are still submerged, were destroyed rather by a 

 want of salt water than a change of level. 



The presence of these shells above the level of San Pablo bay is, therefore, due to an elevation 

 of the land, and not to a subsidence of the water. Dr. Trask (Beport on Geol. Cal., 1854, jp. 27) 

 mentions this oyster bed as occurring on the north shore of San Pablo bay at the height of 30 

 feet above high tide. On the south shore it is not over 20 feet above high water mark. In San 

 Francisco bay it is still submerged. Although oysters are not necessarily always found at a fixed 

 distance below the surface, we may infer, with considerable certainty, that the center of action 

 and the greatest elevation was north of San Pablo bay. 



On the west side of San Pablo bay occurs an interval of shore, which is very low, and seems 

 connected with a depression in the general surface, which extends far to the westward. It is 

 said this depression is continuous to Bodega bay, and that no part of it rises 50 feet above the 

 ocean level. If this is true, San Pablo bay doubtless communicated directly with the ocean 

 through this channel — a channel subsequently closed by the elevation which has been described, 

 but which, when open, would have afforded a more abundant supply of salt water than they 

 now have to the marine molluscs of the bay. 



STRAITS OF CARQUINES. 



The Bay of San Pablo is, in fact, but a continuation of San Francisco bay, and occupies a 

 portion of the same great trough lying between the most distant of the nearly parallel axes of 

 elevation which have been mentioned as determining the outline of the valley of San Francisco 

 bay, viz : the Mount Diablo axis, and that forming the immediate coast of the Pacific. A 

 subordinate anticlinal, a kind of shoulder of the Contra Costa range, crowds itself down into 

 this great basin, and narrows a portion of the bay into the strait which connects the wider 

 expanses of water which have received, for convenience, distinct names. As we traverse the 

 bay of San Pablo, and approach its eastern boundary, we reach another barrier, which has 

 opposed the drainage of the great Californian valley, through which, as at the Golden Gate, 

 the waters of the Sacramento and San Joaquin have forced or found a passage. This barrier 

 is formed by a continuation north of Mount Diablo of the Mount Diablo range, which, though 

 greatly reduced in elevation, and considerably interrupted and divided, is still plainly dis- 

 tinguishable, trending about northwest, till it loses its identity in the numerous parallel ranges of 

 the coast mountains. I was not able to examine the geology of the Straits of Carquines so closely 



