154 GEOLOGY. 
These fossils occur in considerable numbers along the beach, and it is probable that a gtratum 
of these interesting relics may be found, in place, along the shore further south. 
The block of sandstone from Bellingham bay contains two large Pectens of the age of the 
Tertiary, and this furnishes additional evidence of the Tertiary age of the San Francisco sand- 
stone. The fossils of Benicia—the Trochus, Turritella, and shark’s tooth, Lamna ornata— 
point to the same conclusion, the only doubt being as to the connexion of the strata. The 
fossiliferous stratum may be much higher in the series than the rocks about San Francisco. It 
is not impossible that a portion of the Upper Cretaceous is represented, although there is noth- 
ing to indicate it in the lithological characters of the strata. The occurrence of Cretaceous 
strata in the northern part of the State! renders this more probable.? 
The formation throughout its extent, so far as explored, has been uplifted and thrown into 
wave-like flexures. A body of the strata in a horizontal position is not known. The dips are 
generally at angles of from twenty to sixty degrees, and in such directions as to indicate the 
existence of a series of anticlinals and synclinals. The trends of the strata conform so nearly 
with the direction of all the principal ridges and headlands as to show that the flexures of the 
rocks have determined the relief of the region. This direction is nearly northwest and south- 
east, and is clearly shown on the maps of the Bay by the parallel lines of coast. The promon- 
tory extending northwest from Angel Island finds its continuation in this island, and beyond it, 
in the island of Yerba Buena, indicating the presence of a long anticlinal axis. This is 
rendered more probable by the dip of the strata at the island, and the apparently reverse dip of 
the beds on the opposite shore at San Francisco. It is probable that the channel between the 
city and Yerba Buena occupies a synclinal depression. It was not possible to follow out the 
flexures of the strata from North Point westward to the serpentine ridge of Fort Point, but the 
dips that were exposed were sufficient to warrant the conclusion that the strata were much flexed 
and folded between the two places, and indicated that the two principal hills of the city were 
formed by the summits of flexures. The folded condition of the strata is represented in the 
little section on page 155, drawn to show the relations of the rocks between the city and the shore 
of the Pacific. "The curved lines show the probable bending of the great mass of the'strata. 
Indications of an anticlinal fold of the strata were apparent at Benicia. The strata exposed 
on the opposite shore of the little bay, or cove, which terminated the section on the east, 
‘appeared to dip in a direction opposite to the others. Such foldings were, quus more dis- 
tinct further south, in the hills west of Mount Diablo. 
of the two inner rows, as well as those of the inner and outer ones. The position of the anus could not be made out; 
it is probably infra-marginal. The ambulacral furrows of the inferior surface are apparently but little ramified. 
Diameter of the largest specimen 474 inches; height at the middle 0.48 inch. This specimen was presented to me by 
Dr. C. F. Winslow, formerly of San Francisco 
1 Discovered and announced by Dr. J. B. Trask. 
3 A short paper on the un characters and probable age of the San Francisco sandstone was read by the T 
with the permission of the War Department, at the meeting of the American Association for the Advancement of Scienc 
Providence, 1856. See سب زار‎ 9th meeting. 
