1919] Merrwm: Tertiary Mammalian Faunas of Mohave Desert 447 



The Tuff-Breccia member, or second division from the base, differs 

 from the lowest member in being composed of finer fragments con- 

 taining less granitic material and in having a much larger proportion 

 of volcanic ash in the matrix. It is considerably over a thousand feet 

 in thickness in the north limb of the Barstow syncline. The upper 

 limit of this member was considered by Baker to be defined by an 

 unconformity of unknown extent. This division is made up of 

 variegated beds ranging from cream-color through red, purple, brown, 

 and green. 



The Fine Ashy and Shaly Tuff member, numbered three above 

 the base, has a thickness of approximately 500 feet. It consists of fine 

 materials comprising ash toward the base, and dark, compact mud- 

 stone toward the top. A single fossil, a Planorbis, was found in this 

 division. 



The Resistant Breccia member, number four, may be designated 

 as a fanglomerate. It is in general coarser than the Tuff-Breccia. It 

 differs from the basal member in having a larger percentage of vol- 

 canic ash. It weathers into badland forms, and contributes some of 

 the characteristic scenic features of the region. This division is 

 approximately 1000 feet in thickness. The beds range in color from 

 gray to brown and red, but are not as brilliantly colored as in the 

 Tuff-Breccia member. A considerable collection of mammalian re- 

 mains was obtained in this division. These specimens seem to repre- 

 sent the fauna known from the typical horizon of the Barstow in the 

 fifth division immediately above. 



Of the Fossiliferous Tuff member, the typical Barstow, forming the 

 uppermost portion of the section, evidently only a portion is now 

 available for study, the remainder having been removed by erosion. 

 There is a gradual gradation from the coarser beds of the Resistant 

 Breccia member into the fine, overlying beds of the Fossiliferous Tuff. 

 The uppermost division is made up mainly of bluish-gray to yellowish- 

 brown, slightly indurated strata, composed largely of fine arkose with 

 a considerable percentage of volcanic ash. The principal deposits of 

 mammalian remains representing the Barstow fauna occur in this 

 member of the Barstow syncline section. 



Ricardo or Red Rock Canon Section. — In the extensive exposures 

 of sediments constituting the type section of the Ricardo group at 

 Red Rock Canon between the El Paso Range and the Sierra Nevada, 

 the mammalian fauna is distinctly different from that of the Barstow 

 syncline, and of a later phase. This section includes more than three 



