EXTINCT FAUNAS OF THE MOHAVE DESERT 251 



the Mohave. Some of the existing types, as the bighorn, are immi- 

 grants from the Old World, and arrived very late in the history of this 

 region. Others, as the pronghorn, are evidently of American origin. 



Geologic Occurrence and Age of the Mohave Fossil Beds 



The Miocene and Pliocene faunas in the Mohave area occnr in an 

 accumulation of strata amounting to not less than 7,000 to 8,000 ft. in 

 thickness. The beds consist in large j)art of volcanic materials which 

 are interstratified with clay strata, shales, and desert conglomerates. 

 The origin of the immense quantities of ashes piled up in these forma- 

 tions is as yet unknown. They were probably derived from volcanoes 

 and other channels for extrusion of lavas and ash in or near the 

 Mohave area. In a few strata abundant remains of fresh-water mol- 

 lusks indicate deposition of these beds in fresh-water ponds or lakes. 

 At other levels the skeletons of large desert tortoises and numerous 

 remains of land mammals now characteristic of flat open country sug- 

 gest accumulation upon dry land. 



Mr. Baker considered that the Miocene and Pliocene deposits of the 

 Mohave were formed mainly under physical conditions similar to those 

 operating in the desert at the present time. As nearly as the writer 

 can judge, the climate conditions in the Mohave area through the 

 period in which the mammal beds were being laid down, were those of 

 a semi-arid region somewhat more humid than the Mohave of to-day, 

 and the climate corresponded approximately to that now obtaining in 

 the southern end of the Great Valley of California. 



Sections of the older formations containing fossils in the Mohave 

 area are most satisfactorily shown in great thicknesses of strata exposed 

 in the hills north of the town of Barstow, and in excellent exposures at 

 Eicardo between the eastern foot of the Sierras and the El Paso Eange. 

 At both localities exposures extending for many miles give unusual 

 opportunity to examine the structure of the formations, and bring to 

 view the strata containing mammalian remains. As shown in the 

 accompanying photographs, the formations at these localities are sculp- 

 tured by erosion into most fantastic shapes, like those of the famous 

 bad-land forms of the western Great Plains region. In the intricate 

 gullies and caverns of these exposures there is found a most fascinating 

 field, in which to hunt for the big game of the Mohave of ancient times. 



The oldest fossil-bearing beds of the Mohave area rest upon a 

 basement consisting in part of granite and metamorphosed or altered 

 rocks of pre-Tertiary age. They may also rest upon extruded igneous 

 rocks, presumably at least as old as Lower Miocene. 



The oldest known strata containing vertebrate fossils in the Mohave 

 area are found in the Upper Miocene near Barstow. Leaves stated to 

 be of Eocene age were collected by H. W. Fairbanks at Black Mountain 



