1917] Clark: Geology and Ore Deposits of the Lcona Rhyolite 363 



Pre-Recent Erosional Surface. — In the vicinity of Blmhurst there 

 is evidence of an old erosional surface whose remnants form part of 

 the present topographical surface. The general direction and possible 

 extent of the stream which flowed on this surface is shown by the 

 sketch map in figure 1. It is not known how extensive this former 

 surface may have been, but the evidence used in making the map 

 consists of the presence of stream pebbles composed mostly of Oakland 

 conglomerate and boulders of Chico sandstone. The pebbles found 

 lying loose upon the surface are probably the last trace of the former 

 stream deposits. The small area north of Lake Chabot, mapped as 

 Knoxville, is partially occupied by these pebbles imbedded in alluvium 



Fig. 1. Sketch, in part, hypothetically drawn, showing the pre-recent stream 

 system and its relation to the present topography. The dotted lines represent 

 the pre-recent stream valleys. The branching full lines represent the present 

 streams. The heavy lines represent faults. 



derived from the formations to the east. North of the F. C. Talbot 

 ranch about a mile, along the road leading to the country club, there 

 is a considerable thickness of the old river gravels. Here cross- 

 bedding is well exposed on both' sides of the former stream valley. 

 To the south of the road just referred to, in an old stone quarry, the 

 stream gravels are exposed in vertical section. The deposit is about ten 

 feet thick and rests on an uneven eroded surface of the rhyolite. This 

 old stream valley has a course transverse to the present streams. The 

 presence of pebbles on the tops of some of the hills indicates that the 

 former stream system antedates that of the present by a considerable 

 period of geologic time. 



The direction of the present stream valleys being at such variance 

 with those of the pre-Eecent indicates that the former were superim- 



