182 GEOLOGY. 
The rock of some of them is like semi-opal in density and lustre. The fracture is also similar, 
being curved ; and the surface and edges vitreous in their appearance, like glass. Their color 
varies from white to drab, or fawn color, and is sometimes pink. Thin layers or lines of strati- 
fication are visible in all of them. Silica is the chief constituent, and it was probably deposited 
in an impalpable or gelatinous form. Many of the specimens resemble, in their texture, the 
water-worn masses which were picked up on the beach at San Pedro; but none of the layers 
were so dark-colored or highly charged with bitumen. These dark silicious masses of San Pedro 
are, without doubt, from a sirnilar series of infusorial strata, and the deposits are doubtless con- 
nected or synchronous in their origin. This is rendered almost certain by the discovery of a 
deposit of the white earth at San Luis Obispo, which contains forms almost identical with those 
of Monterey. A specimen, obtained from the locality by a friend, has been forwarded to Pro- 
fessor Dailey. 
T'he strata near the town, which are quarried into for building stone, are also fossiliferous ; 
the surfaces of many of the slabs, when they are split parallel with the numerous lines of strati- 
fication, being found to be covered with innumerable impressions and casts of small bivalve 
shells. "These lie thickly together, and sometimes one within the other, as if they had fallen to 
the bottom in quiet water, or rather, had been left bare upon a shelving beach of fine clay. The 
most abundant fossil is the small Tellina, which Mr. Conrad describes as 7. congesta. It occurs 
in a similar rock at San Carlos, or in the valley of Carmello creek, and is associated with a 
Lutraria, L. Traskie,' (Plate III, fig. 23,) and impressions of very small crabs, too obscure to be - 
identified. ~ 
A stratum of the Monterey rock, similiar in its texture to that used for buildings, but differing 
in color, being dark olive-brown, also contains the casts of Tellina Congesta in great numbers. 
They cover square yards of the surface of flat layers.—(See figs. 21 and 14 a Plate III.) This 
abundant fossil occurs also at San Diego with the Miocene fossils. 
In addition to the Tellina and Lutraria, these rocks are charged with innumerable and beau- 
tiful Polythalamia, lying in thin layers through the stone and becoming visible by their numbers 
in white lines on the edges of the slabs. They may be seen by the unassisted eye, but their 
forms cannot be traced without the microscope. "They are very white and perfectly preserved, 
and are exceedingly beautiful objects for microscopic examination. The clay in which they 
occur is so firm and indurated that it is very difficult to detach them for study; every cross- 
fracture of the rock presents innumerable sections, and occasionally the full form of a shell is 
visible on the surface. It is this dark-colored rock which gives off a bituminous odor ; but it is 
not like the bituminous masses of San Pedro, which are composed chiefly of silex. 
GREAT BASIN. 
The stratified formations of sandstone and sandy clays, observed at different points within the 
limits of the southern part of the Basin, are doubtless Tertiary, but, from the absence of a sufi- 
cient number of fossils, it is not possible to affirm positively on this point, or to indicate the 
probable division of the Tertiary to whieh they may be referred. 
The only fossils found were the silicified stems of plants described in the Itinerary, Chapter 
V., page 36. They occur in the upraised sandstone strata of the eastern end of the Cañada de 
las Uvas, where it opens out upon the slope of the Basin. The fossils are sufficient to show the 
comparatively recent origin of the strata, but, being entirely new, and of undetermined affinities, 
! By permission of Mr. Conrad, this species is dedicated to Dr. J. B. Trask, who collected it. 
