Vol. 6] Baker: Cenozoic History of the Mohave Desert. 335 



Although the expedition was made primarily for the purpose 

 of collecting vertebrate fossils for a study of the extinct mam- 

 malian fauna of this region, and the notes on the geology are 

 consequently very fragmentary, it is hoped that the geological 

 observations may be of some interest. The value of the results 

 is very considerably lessened because of the lack of a suitable 

 map, and therefore the locations and distribution given for the 

 various rock members are only approximately correct. A de- 

 tailed petrographic study of the rock specimens will probably 

 be made at a later date. The route followed and the localities 

 visited during the reconnaissance are shown on the accompanying 

 sketch map 2 (pi. 34). 



LOCATION AND BOUNDARIES OF THE MOHAVE 

 DESERT 



The Mohave Desert region comprises the extreme southwest- 

 ern portion of the Great Basin. It lies entirely within the 

 State of California and includes within its limits portions of 

 the four counties of San Bernardino, Inyo, Kern, and Los 

 Angeles. Its boundaries on the northwest are the southern end 

 of the Sierra Nevada Mountains and the Tehachapi Range; on 

 the southwest are Sawmill Mountain, Liebre Mountain, the Sierra 

 Pelon, with their southeastern continuation to the head of the 

 Santa Clara River, and the San Gabriel Range; on the south 

 are the San Bernardino Range and the Colorado Desert ; on the 

 southeast the natural boundary is the divide between the drain- 

 age tributary to the Gulf of California and the interior drainage 

 of the Great Basin. The eastern and northern boundaries are 

 difficult to fix, for there the Mohave Desert merges into the 

 Great Basin proper with no marked drainage divides or high 

 bounding ranges. The northwestwardly directed Piute Range, 

 just inside the California border, may perhaps best be chosen 

 as the eastern boundary, north of the divide of the Colorado 

 River drainage. The northern limits of the Mohave Desert will 

 be given as an east-west line connecting Castle, High, and Clark 's 



2 For a map giving approximate areal distribution of the rocks in a 

 portion of the area herein discussed the reader is referred to Mr. 0. H. 

 Hershey's "Geological Reconnaissance Sketch Map of Southern Cali- 

 fornia,'" Univ. Calif. Pub!., Bull. Dept. Geol., vol. 3, pi. 1, 1902. 



