TERTIARY OP CARRIZO CREEK-FOSSILS. 
175 
traversed by veins of calcite. These sandstone beds differ in hardness, some being firm and 
apparently durable, and others friable and easily crumbled by the hand. In some of the harder 
beds large, spherical concretions occur. No uniform beds of conglomerate, like those found in 
the strata of Benicia and Monte Diablo Yalley, or at Bear Creek, could be found. At a point 
nearly opposite the usual camping ground, on the creek, the strata were, however, composed of 
coarse sand and gravel rudely mingled together ; but, in general, the deposits are fine, and do 
not exhibit the influence of powerful currents at the time of their deposition. 
Another extensive outcrop of similar strata was traversed on the 19th of November, and is 
described in the Itinerary. These beds were remarkable for the regularity of their stratification 
and the number of the extraordinary concretions, of all imaginable forms, which were lying in 
long lines on the outcropping edges. The materials of this series are nearly all fine, being 
clays of different shades of brown, gray, and red. Gypsum occurs with the red strata. These 
strata are upraised at an angle of about twenty degress, and trend north 25° west. There was 
no favorable section showing the dip or the thickness of each bed, but the combined thickness, 
as exposed, is not less than two thousand feet. 
At the northwestern extremity of the valley of the Desert, near the base of the Pass of San 
Bernardino, an exposure of strata, in a ravine, resembles those just described ; but they are 
harder, and not so much disturbed. They contain a great abundance of concretions ; many of 
them being true septaria, or flattened ellipsoids of clay, with internal cracks, filled with crystal¬ 
lized carbonate of lime. Nodules and concretions of peroxide of iron were also numerous, and 
some of the specimens appear to have been the remains of plants. 
The extent of the strata of Carrizo Creek, towards the south, along the mountains, is not 
known ; but the same series undoubtedly appears at intervals as far as the Gulf, and, probably, 
is extended southward along its shores. The sandy strata, which are exposed in bluffs along 
the Colorado, from Pilot Knob to and beyond Fort Yuma, and which also extend on the oppo¬ 
site shore, and along the Gila, are probably a part of the series, but have not yet been identified 
by fossils. They may be much more recent. These bluffs are the margins of the great desert 
plains, which are paved with a thick layer of pebbles—a kind of drift which has been uniformly 
spread out along the lower Colorado and Gila. A similar drift occurs on the tops of the mesas 
along Carrizo Creek. It is identified not only by a similarity in the size and general character 
of the pebbles, which are chiefly porphyry and black basaltic fragments, but by the presence of 
quantities of silicified wood, in fragments of all sizes, from an inch in length to several feet. 
The largest masses of silicified wood were, however, confined to the vicinity of Carrizo Creek. 
The pebbles, also, along the Colorado appear to be derived from a layer of conglomerate, about 
five feet thick, which overlies the sandy beds ; while on the other side of the Desert, at Carrizo 
Creek, this layer of conglomerate does not appear,'or was not observed. 
Fossils .—Fossils were obtained from one point only near the Desert and in the valley of Car¬ 
rizo Creek. They form the upper stratum of one of the flat-topped hills, and great blocks, com¬ 
posed entirely of the shells and their fragments, have broken off by their own weight and rolled 
to the foot of the short slope. The stratum is several feet thick, is very hard, and is entirely 
composed of shells, chiefly of the genera Ostrea and Pecten , and a little clay. Mr. Conrad 
recognizes, also, a species of Anomia, and regarding them all as new forms, has described them 
under the names: Pecten Deserti, Anomia subcostata, and Ostrea vespertina. (See Appendix, 
and Plate Y, figs. 34, 36, 37, 38, and 41.) Dr. Heerman, the naturalist of the Expedition, in 
passing along the dry bed of the creek, two or three miles below, picked up an Ostrea much 
