Vol. 6] Baker: Cenozoic History of the Mohave Desert. 343 



general a much larger proportion of volcanic ash in its matrix. 

 It does not weather into badland forms, hut produces rounded 

 hills or cliffs of rather bold, shaggy outline. It is considerably 

 over a thousand feet in thickness in the north limb of the Bars- 

 tow syncline, with its upper limit defined by an unconformity 

 of unknown extent. The member is made up of highly varie- 

 gated thick bands of whitish, cream-colored, pink, red, lavender, 

 purple, brown, and green tuff -breccia. The colors are very bright 

 and their frequent alternations present some striking contrasts. 

 The breccia fragments are of white and variously colored tuffs 

 and lava with colors ranging in almost all gradations from white 

 to black. The lava fragments exhibit various degrees of alter- 

 ation and in some fragments flow structure was noted. Locally 

 fragments of granitic rock occur and the base of the member 

 rests upon granitic rock. 



Fine Ashy and Shaly Tuff Member. — (PI. 36a.) This mem- 

 ber outcrops in the south limb of the Barstow syncline where 

 its southern limit is defined by a strike-fault of unknown dis- 

 placement which separates it from the basal breccia member. 

 The lower beds are mainly composed of a greenish-gray rather 

 fine unconsolidated ash, with thin, more resistant layers. A 

 single specimen of Planorbis, a fresh-water gasteropod at the 

 present day inhabiting ponds and lakes, was found by Professor 

 John C. Merriam near the base of the member. Platy selenite 

 and clay "mud-ball" concretions were found abundantly in the 

 thinner-bedded less-resistant lower portion. The clay and iron- 

 stone concretions resemble very tuberous potatoes ; they measure 

 from one to two inches in long diameter and are flattened parallel 

 to the bedding. The more resistant ironstone is strewn on the 

 slopes, giving them a brown or deep purple color when viewed 

 from a distance. The weathered surfaces of the beds have a 

 very marked soft, velvety appearance and the surface film is 

 marly, probably because of the deposition of soluble salts leached 

 from the underlying beds. 



Near the top of the member is a considerable thickness of 

 dark brownish-gray compact mudstone, with rather massive bed- 

 ding and apparently formed of fine clay. The surfaces of this 

 mudstone along cracks and joints are covered with a thin film 



