174 
GEOLOGY. 
is also found at Volcano Ridge, in a brown, calcareous sandstone. The specimens I have do not 
contain a sufficient number of species to indicate, with any certainty, the period to which they 
should he referred. These species are, however, all different from any yet found in the Tertiary 
of California, and appear to he much more ancient. I am led to this view by two distinct casts 
of Trigonia, a fossil which is regarded as characteristic of the Secondary period. These casts 
are imperfect; hut there is little doubt that their reference to the genus Trigonia is correct. 
Casts of Leda , Mactra , Venus , Ostrea, TurriteUa ? and other shells are also found. The 
cast of the Leda is so perfect that its specific characters can he satisfactorily given. It may he 
known as Leda subacuta . 1 
CARRIZO CREEK AND THE COLORADO DESERT. 
At several, points along the western base of the Peninsula Sierra, between the Pass of San 
Bernardino and the road to the Desert, along Carrizo Creek, we find outcrops of partly consoli¬ 
dated strata of clay and sand. These are, in most instances, nearly horizontal, or hut slightly 
inclined, hut at one point are upraised at an angle of about 20°. The exposures are on the 
margin of the Colorado Desert, and generally in the valleys reaching towards the central part 
of the mountains. The connexion of the several exposures, although not traced between the 
points mentioned, is indicated by a general similarity of lithological characters and position 
relatively to the older rocks. The outcrops do not appear in the form of a belt of hills, as at 
Ocoya Creek, hut are extremely low, hut little elevated above the general level of the surface, 
and present a flat or tabular surface, not worn away into smooth, rounded hills. 
The only point at which fossils were obtained was in the dry valley of Carrizo Creek, within 
a few miles of the open plain of the Desert. At this point the strata are nearly horizontal, and 
fill the space between two ranges of granitic and metamorphic rocks, forming a valley extending 
in a nearly northwest direction from the Desert. The valley of the creek is excavated in these 
strata, and their edges are exposed on each side in bluffs, generally less than sixty feet in 
height. These hills are generally flat, thus forming table-mounds where they are much cut by 
side ravines. Similar strata were found in slight outcrops on the Desert, near the dry gulleys of 
New River, and part way up the slope towards Carrizo Creek. They probably underlie the 
whole surface of the Desert, being-concealed from view by the thick deposits of alluvial and lacus¬ 
trine clay. 
Lithological characters . 2 —The outcrop on the slope of the Desert, near the road leading to 
Carrizo Creek, consists chiefly of a light-colored, friable sandstone, calcareous, and rather coarse. 
At the upper edge of the slope, where the road turns to descend to the level of the valley of the 
creek, the hills or bluffs are argillaceous, and of a red color. No hard sandstone was noted. 
Gypsum, in thin plates, is abundantly disseminated in these strata, and, by being washed out, 
lies all over the surface in shining masses. All these strata are soft, and easily broken down 
by a shovel. Higher up the creek, these soft and red clays, or argillaceous sands, give place to 
light-colored or yellowish sandstone containing a large per-centage of lime, being in some places 
1 Leda subacuta : Shell smooth, convex, of a height equalling half the length ; anteriorly elliptically rounded; poste¬ 
riorly acute, compressed, but with a sufficiently prominent postero-superior ridge ; ventral margin regularly convex ; beaks 
moderately prominent, placed a little before the middle. Teeth crowded, but deeply separated ; in number, nineteen before, 
and twenty-one behind the beaks ; the posterior ones immediately contiguous to the beaks are very minute. 
Length, 0.7 ; height, 0.354:; breadth or thickness, 0.25 inch. 
Locality: Volcano Ridge, California, in a brown calcareous sandstone. 
1 Reference is made to the Itinerary for observations upon the strata in detail. 
