126 THE DESEKT DURING THE QUATERNARY PERIOD. 



iiTigation is practised ; this slope is a very gradual one. Southwards the desert continues into 

 Sonora, and embraces the apex of the G-ulf 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 tlie 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 Kanges 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 imder 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 oiF 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 be 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. 



