TERTIARY AND POST-TERTIARY FOSSILS—ANCIENT LAKE. 
235 
be broken out in blocks. The lines of stratification were very numerous and horizontal; this, 
with the absence of coarse materials, indicating that the water from which it was deposited was 
comparatively quiet. The fossils found in this clay, at different points over a distance of nearly 
one hundred miles, show the lacustrine origin of the greater part of it. All the shells that 
were collected were submitted to Dr. Augustus A. Gould, of Boston, for examination, and his 
descriptions accompany this report. He finds them to consist of the following species, four of them 
being new: Gnathodon Lecontei, Conrad ; Planorbis ammon, Physa humerosa, Amnicola protea , 
and Amnicola longinqua. 1 In addition to these, Anodonta Californiensis , Lea, was very abundant, 
especially in the northern part of the Desert. These show the deposit to be of fresh-water origin; 
the Gnathodon, however, is a brakish water genus, and is found in the mud of estuaries. This shell 
was not seen in the northern part of the Desert, where the others were most abundant, but was pro¬ 
cured for the first time near Salt Creek, about twenty miles north of the entrance to Carrizo Creek. 
The Tertiary is also characterized by the fossils obtained at Carrizo Creek. These are found by 
Mr. Conrad to be new species of Ostrea, Pecten, and Anomia, and he has named them as follows: 
Ostrea vespertina, 0. Heermani, Anomia subcostata, Pecten Deserti . 2 —(See Appendix, Article II, 
and Plate Y, figs. 36, 37, 38, fig. 34, and fig. 41.) They are believed to be of the age of the 
Miocene. These were the only fossils found in the strata around the Desert. There is little 
doubt that the strata containing such vast numbers of concretions outcroping north of Salt Creek, 
and those seen between the Hot Spring and the base of the Pass of San Bernardino, together 
with those forming the bank or terrace between Pilot Knob and Fort Yuma, are of similar 
age. The horizontal beds around Pilot Knob, bearing the stratum of conglomerate at the sum¬ 
mit, are much more sandy and modern in their appearance than the fossiliferous series of Carrizo 
Creek, and it is very probable that they are more recent, and superimposed upon the Miocene 
strata. There is little doubt that the Miocene underlies the alluvium of the Desert, and it will 
probably be found along the base of the mountains on the northeast side in outcrops similar to 
those on the west. 
ANCIENT LAKE. 
The foimer existence of an extensive fresh-water lake in the northern part of the Desert is 
shown not only by the extended deposit of alluvial or lacustrine clay, containing fossils, but by 
the existence of extended shore-lines and beaches along the sides of the bordering mountains. 
The great depression of the surface below the level of the Colorado, and probably below the level 
of the Gulf, shows also the probability that the region was once submerged. The extent of this 
submergence, or the limits of the former sheet of water, is now indicated to us by the fine clay 
which must have subsided from it. The extent of this clay is shown upon the map. In the 
northern part of the Desert it is seen to reach nearly from one side of the valley to the other, 
while on the southeast it reaches to the head of the California Gulf. There is no barrier in that 
direction, and it is impossible to resist the conviction that the waters of the Gulf once occupied 
the whole space, and extended up to the base of the San Bernardino Mountain. The valley of 
the Desert is, in fact, but the northern end of the great valley occupied by the Gulf, and the 
probability that it was once submerged is exceedingly strong. When, however, we consider the 
fact of the former existence of a great lake, and the peculiar configuration of the valley and its 
relation to the Colorado Biver, it becomes almost certain that the waters of the Gulf did cover 
the region, extending nearly one hundred and seventy miles further inland. Thus, if the 
1 See Plate XI and Appendix, Article IN. 
s Descriptions of these were published in connexion ■with the Preliminary Report, H. Doc. 129, 1855. 
