Vol. 6] Baker: Cenozoic History of the Mohave Desert. 347 



a section through the center of the minor axis of the Barstow 

 syncline is given in fig. 1. Faults of various degrees of dis- 

 placement are fairly common. All noted were approximately 

 parallel to the general east-west strike, and were of the normal 

 type, although the verticality of many of the fault-planes hint 

 of the possibility of some upthrusting. From the relations of 

 the faults to the folding it is probable that the folding of the 

 strata into the syncline preceded the period of faulting. The 

 structural relations in plate 38a suggest that a monocline was 

 first formed which was afterwards faulted on both sides. 



THE BLACK MOUNTAIN ROSAMOND EXPOSURE 



The Black Mountain locality is separated on the east from 

 the Rosamond exposure north of Barstow by an alluvium-covered 

 basin, near the middle of which strata lithologically similar to 

 those of the Rosamond Series are exposed in the bank of an 

 arroyo. Three members were noted in the Black Mountain 

 section which correspond closely to members of the Rosamond 

 Series in the Barstow syncline. These will be herein described 

 under the names applied to strata in the locality north of Bars- 

 tow, although the resemblances between the beds in the two 

 exposures are merely lithologic and no certain correlations can 

 be made. 



Tuff-breccia Member. — The rocks exposed in the sharp conical 

 peaks north of Black Mountain and near the head of Black 

 Canon, in the vicinity of the American Opal Company's pros- 

 pect, contain layers of interbedded lavas with flow structure. 

 Considerable agate, chalcedony, and opal occur as cavity- and 

 fracture-fillings in the tuff -breccia. In other respects the beds 

 resemble those of the tuff-breccia member in the Barstow syncline 

 and are dipping in the same direction. Their relations to the 

 other members in the Black Mountain locality are unknown, for 

 they are bounded on the south by a fault which brings them 

 in juxtaposition with beds referred to the fossiliferous tuff 

 member. 



Fine Ashy and Shaly Tuff Member. — Beds in every respect 

 similar to strata referred to this member in the Barstow syncline 

 outcrop at the foot of Black Mountain on the northwest, on the 



