EXTENT AND THICKNESS OF THE SAN FRANCISCO SANDSTONE. 
153 
Gate. The outcrops of Angel island, State’s prison, and Marin island are on the north, and 
have already been noticed ; hut it is probable that the exposures of the beds are greater in the 
interior. The shore of Saucelito hay, on the west of these localities, is formed of sandstone, 
similar to that of the other localities in the vicinity, except in color. It is lighter, and more 
like specimens which have been obtained from Fort Ross, many miles north, on the shore of 
the Pacific. A block of sandstone, brought down from the vicinity of the coal-beds of Belling¬ 
ham bay, closely resembles specimens from the quarries about San Francisco, not only in color, 
hut in mineral characters ; it is thus rendered probable that the foi mation is the same, although 
the evidence presented by mineralogical resemblances can never be regarded as satisfactory. 
The wide development of the formation is, however, rendered certain from the evidences pre¬ 
sented in the immediate vicinity of San Francisco. The great thickness which the series 
attains—probably over 2,000 or 3,000 feet—and the even grain of the thick beds of sandstone 
over large areas, together with the remarkable uniformity of the strata, indicate that they were 
formed in a wide-spread ocean or sea. The thick beds of shale attest the depth and comparative 
quiescence of the water. Independently of these considerations, the wide extent of the formation 
has been made known by observation ; it forms a greater part of the hills and mountains around 
the bay, and, so fir as explored, a considerable part of the mass of the Coast Mountains. It is 
believed to be the most extensive and highly developed sedimentary formation of the California 
coast, and may appropriately be known as the San Francisco or California sandstone. 
It is greatly to be regretted that, as yet, the evidences of the age of this formation are very 
few and unsatisfactory. The rocks near San Francisco, so far as I examined them, are singu¬ 
larly devoid of fossils, not a single shell having been observed in them. Masses of similar 
sandstone containing fossils are, however, thrown up by the surf upon the beach south of Point 
Lohos, and there is little reason to doubt that they are broken from a submerged outcrop of the 
formation. The rock, in color and grain, is very similar to the adjoining sandstone of the 
Point, but it is not quite so hard. It at first appeared possible that these masses were from 
strata formed out of the comminuted debris of the sandstone, and thus more modern of of recent 
origin. I am now, however, of the opinion that they are broken from the solid ledges of the 
San Francisco sandstone, under water, and that the fossils may safely be received in evidence of the 
age of the formation. The fossils are chiefly Scutellce, and represent the period of the Tertiary. 
They are firmly imbedded in the rock, and lie thickly together, three or more being often found 
within the thickness of an inch. It was found to be impossible to cleave these masses of sand¬ 
stone so as to expose fresh surfaces of the fossils. The rock is calcareous and tough, and the 
lime of the Scutellse has so completely crystallized that the fragments cleave into rhombohe- 
drons, like calc spar. The fossils appear to have determined the form of the fragments of 
rock, for the latter are generally discoid, and faced on each side by the worn surfaces of the 
Scutellae. The edges of the fossils also appear on the sides of the specimens, and it is evident 
that the lime has been less easily abraded by the surf than the granular sandstone. The struc¬ 
ture of the fossil is very beautifully shown in several of the specimens, and a figure of one of 
them is given on Plate IV, fig. 30. I propose for this species the name Scutella interlineata, 
and append a description, 1 prepared for me by Mr. William Stimpson. 
1 SCUTELLA INTERLINEATA. 
The specimens are imperfect, but the ambulacral star is apparently central, the petals equal, and closed, or nearly so, 
at their extremities. The tubercles being mostly worn off in the specimens before me, there are two rows of irregularly 
pentagonal plates in each inter-ambulacral space, which gradually increase in size toward the margin. A small portion of 
the surface upon which tubercles still remain shows these to have been sporadic, very numerous and crowded. The ambu¬ 
lacral petals are transversely lineated with impressed lines across their entire width, the thread-like lines connecting the 
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