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University of California Publications. [Geology 



of shiny black oxide of manganese. Thin interbedded layers of 

 impure fibrous gypsum, ranging up to about an inch in thickness, 

 with the long axes of the mineral prisms at right angles to the 

 bedding planes, occur in the mudstone. At the top of this mud- 

 stone a few small pieces of lignite were found. The member 

 has a thickness of approximately five hundred feet. 



Resistant Breccia Member. — Above the mudstone the beds 

 rapidly become coarser and more resistant. This member is 

 about intermediate in composition between the basal breccia and 

 the tuff-breccia members. It is in general coarser than the tuff- 

 breccia member and, unlike it, it weathers into badland forms 

 with perpendicular faces and sharp angles. In texture it re- 

 sembles the basal breccia member, but differs from that member 

 in having a larger percentage of volcanic ash. It is character- 

 istically a series of beds composed of coarse arkoses. It outcrops 

 in the trough of the Barstow syncline, overlying in normal suc- 

 cession the fine ashy and shaly tuff member in the south limb 

 of the syncline, while in the north limb its northern limit is 

 defined by a fault. Between this fault and the unconformity 

 marking the upper limit of the tuff-breccia member is several 

 hundred feet of very coarse granodiorite breccia, containing 

 angular boulders with sizes varying up to four feet in diameter, 

 with a matrix of arkosic material composed of disintegrated 

 granodiorite fragments. So well consolidated and so homogen- 

 eous in its material is this breccia that in places care must be 

 used to determine its secondary fragmental origin. 



The resistant breccia member is approximately 1000 feet in 

 thickness, although, again, its full thickness cannot be deter- 

 mined because of the strike-faults which cut its strata on both 

 sides of the trough of the syncline. It is greenish and grayish 

 in color in its lower portion, but becomes brownish and reddish 

 higher up. The colors are softer and of more subdued shades 

 than those of the tuff-breccia member. Angular granitic boul- 

 ders two to three feet in diameter are common. The beds higher 

 up become in general successively finer than those in the lower 

 half. Near the middle of the member in the south limb of the 

 syncline were found remains of a merycodont, a horse, and a 

 large camel in a bed of greenish-gray soft ash, at least fifty feet 



