Vol. 6] Baker: Cenozoic History of the Mohave Desert. 351 



Heavy deposits of gravel occur on the northern slope of the Tertiary 

 ridge. They rest upon the lake beds in such a manner as to suggest that 

 the northward slope of the surface is a structural feature due to the 

 tilting of a block of strata in that direction. If that is true, these lake 

 beds probably underlie the Mohave Kiver Valley in this vicinity. 



Tuff-breccia Member. — Tuff-breccia is widely exposed in the 

 Calico Mountains, being' found as a basement rock in the midst 

 of the hills and exposed on the western flanks. The beds exam- 

 ined correspond closely in character with the strata of the tuff- 

 breccia member in the Barstow syncline from which they are 

 separated on the west by a narrow alluvium-covered basin. 

 Chalcedony and opal form seams and irregular masses which 

 are rather abundant just north of the southeast corner of Sec. 

 36, Township 11 N, Range 1 E. Milky white opal forms botry- 

 oidal and mammillary masses incrusting chalcedony, and is found 

 as layers in chalcedony, tuff-breccia, and lava. Hyalite was 

 found as an incrustation upon opal, while quartz was the last 

 mineral to be formed in the cavities. The tuff-breccia probably 

 underlies the Borate member. 



Borate Member. — At the mouth of Wall Street Canon, on the 

 southwestern base of the Calico Mountains, there is a dark red 

 volcanic breccia made up of large-sized boulders of red lava. 

 This is followed toward the north by reddish, brownish, and 

 purplish breccias resembling the resistant breccia member in the 

 Barstow syncline. Next come strata resembling the fine ashy 

 and shaly tuff member of the exposures north of Barstow, which 

 are composed of whitish, light bluish-gray, light green, and light 

 yellowish-brown soft velvety ashy beds with layers of "paper 

 shale," mudstone, limestone, fibrous gypsum, and colemanite. 

 Underlying these beds and conformable with them are dark 

 brown, fine gravelly beds which show ripple marks. Below the 

 latter the succession is uncertain because of the intense crum- 

 pling and faulting of the strata, but it is probable that the tuff- 

 breccia member with intercalated lava flows is the next lowest 

 stratigraphic member. 



In the finer beds is found considerable black-banded chert. 

 The darker brown, thicker, more resistant layers intercalated 

 with the shales show rain prints and sun cracks. The thicker, 

 more compact and ripple-marked strata of the borax-bearing 



