126 
THE DESERT DURING THE QUATERNARY PERIOD. 
irrigation is practised ; this slope is a very gradual one. Southwards the desert continues into 
Sonora, and embraces the apex of the Gulf of California. The whole area embraced within 
these limits bears unmistakable evidence of having been an extensive sea bottom at a com¬ 
paratively recent geological epoch—an extensive gulf, whose only representative now is that of 
California. The only present evidences of volcanic force are the frequent earthquakes, and the 
existence of the mud volcanoes south of New river. 
Into this sea rolled the Gila, the Colorado, Santa Maria, and Virgin rivers, and it is to the 
wash of these rivers, delivering their fine matters to be drifted and deposited, that the extensive 
and numerous beds of clay (alluvium) may be attributed ; while from the western shores of the 
bay, the Cordilleras, were derived the immensely thick deposits of rolled and stratified loose 
material, gravels and sand, which underlie the alluvium. At this time were formed the fawn- 
colored unconsolidated sandstone of the Mojave slope of the Cordilleras. The granitic con¬ 
glomerates at Carrizo belong to this period also, as may also be included the loose conglomerates 
of the Mojave, near Soda lake. The Cordilleras rose by successive elevations. The first 
upheave being the protrusion of the mass of granite which carried before it the gneiss and 
metamorphic rock lying above it; these it broke up, contorted, and, in part, even included in 
its not yet fully solidified mass. Then followed the period of rest, in which were deposited the 
Miocene tertiaries of the west flanks of the sierra, no corresponding beds of which are found to 
the east. This period of rest was followed by the upheaval of the felspar, porphyry, protogine, 
and trachytic rocks. Then a second calm, in which a conglomerate, including these volcanic 
rocks with syenite, occurred. Then a third uplift, raising these conglomerates at an angle. 
The trappean rocks at San Pasquale and on the west of sierra may be connected with this 
uplift. Previous to the last uplift were deposited the sands and gravels of the desert, with the 
loose conglomerates, and, posterior to it, when as yet the sea water had not wholly retired, the 
clay silt beds of the surface subsided. At this period, likewise, may have been formed the 
terrace extending from Fort Yuma to Pilot Knob. The elevation of the red felspar, porphyry, 
and trachytic rocks of the Mojave valley are, judging from their lithological character, coeval 
with the most western uplifts of the Coast Ranges of California, and, therefore, much later than 
the mass of the Cordilleras, which may be looked upon as occurring at the close of the 
Miocene period. That any portion of this country was under water recently, or within 
traditional record, is unlikely. The old Spanish belief that Alta (?) California was an island, 
is but an instance of erroneous information, rather than a proof; and the tradition of the 
Cohuilla Indians, who relate the expulsion of their ancestors from off the plains by the 
rising of the waters, can scarcely be credited, since such irruption should destroy all traces of 
the water, now cut so deeply in the sands and gravels ; recent falls of rain in a dry country 
cannot account for excavations made so deeply within so short a period as a few centuries, for 
tradition can go no higher. Some shadow of support has been afforded these conjectures by the 
barometrical readings on Lieutenant Williamson’s survey, which shows that Cohuilla springs 
and Salt creek are, respectively, 90 and 42 feet below sea level; but the correctness of the inference 
derived from these readings may be doubted. The comparison of thermometers, one of which 
was at Benicia, and the other some hundred miles distant, is liable to grave errors. The 
altitude of Fort Yuma is found to he nearly 130 feet above former calculations, and a like error 
may vitiate the readings on the desert. While, then, assuming the general level of the desert 
to be unusually depressed, we are scarcely warranted in saying that extensive levels are lower 
than the surface of the sea. 
