Vol. 6] Baker: Ccnozoic History of the Mohave Desert. 



345 



thick. This is the lowest horizon at which mammalian fossils 

 were found. 



Fossiliferous Tuff Member. — It is as yet difficult to define the 

 limits of this the highest member of the Rosamond Series in the 

 Barstow locality. Only what is probably a remnant of the lower 

 portion of the member has been preserved from erosion. There 

 is a gradual gradation from the coarser and more resistant beds 

 of the resistant breccia member into the overlying finer tuffs 

 of the fossiliferous member, but at the contact of the two there 

 is an alternation of layers of coarser and finer material. In 

 one of the lower finer layers the mammalian remains mentioned 

 above were found. Some coarser layers occur in the fossiliferous 

 tuff member, and the fragments of a part of these show the 

 effects of water wear. 



This member is made up essentially of light yellowish-brown 

 and light bluish-gray beds of comparatively unindurated ma- 

 terial. The light yellowish-brown beds are composed mainly 

 of fine granitic arkose with interstratified layers of coarser gran- 

 itic and lava breccia. The grayish beds are mainly finer in 

 texture and contain a larger percentage of volcanic ash, as well 

 as a large majority of the mammalian fossils found. Some 

 finer gray layers are interstratified with the lower light-brown 

 portion. The lower portion and the overlying mantle of allu- 

 vium have the same structure, texture, and composition of ill- 

 assorted material, their color is very nearly the same shade, 

 although that of the fossiliferous tuff member is a trifle lighter; 

 and although this member may be said to possess in general a 

 higher degree of induration than the more superficial capping 

 of alluvium, the frequent absence of well-defined bedding-planes 

 in the older beds make their separation from the younger debris 

 mantle locally a matter of considerable difficulty. Some of the 

 less indurated layers in the lower portion of the fossiliferous 

 member are very susceptible to gully erosion. The major drain- 

 age courses in the upper light-gray member are transverse to 

 the strike of the beds, but the tributary gullies show a notable 

 arrangement parallel to the strike. The upper light-gray mem- 

 ber contains many local, comparatively resistant bands which 

 may or may not be made up of coarser material. 



