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University of California. 



IVOL. 2. 



sandstones which occupy the north side of the creek. This mass 

 of aporhyolite is the northern end of a belt of such rock, which is 

 traceable along the front of the range for many miles to the south- 

 east, and its abrupt termination, with a thickness of over 300 feet, 

 in the creek line of Hamilton Gulch, clearly indicates that it stops 

 against a fault. The eastern limit of the mass is a nearly vertical 

 plane, the trace of which is best seen in the canon immediately to 

 the south of the map. This limit is, therefore, also a fault. On 

 the map the aporhyolite is represented as reposing directly upon 

 the Franciscan. This is its relation in portions of its extension to 

 the southeast, and the conglomerate, which apparently intervenes 

 between the two formations, is omitted from the section at the 

 head of Dwight Way. Considerable doubt exists as to the actual 

 sequence in this section below the base of the aporhyolite, owing to 

 the obscurity of the exposures, and the presence of the Franciscan 

 as mapped is inferred from the relations observed in other sec- 

 tions. It seems probable that whatever Shasta-Chico may intervene 

 between the aporhyolite and the Franciscan must be of very limited 

 volume. 



In the midst of the aporhyolite at its base in Hamilton Gulch is a 

 small area of serpentine less than half an acre in extent. This 

 serpentine extends only to the fault line, and is not seen on the 

 north side of the Gulch. It represents a serpentinized peridotite, 

 which must either be intrusive in the aporhyolite or be a portion of 

 the surface upon which it was extruded. In the absence of any 

 evidence of intrusion, the latter view has been adopted as consistent 

 with the relations of the same rock to serpentine in the hills back 

 of East Oakland, and this interpretation of the relation of the ser- 

 pentine to the aporhyolite supports the view that the aporhyolite 

 reposes directly, in part at least, upon the worn surface of the 

 Franciscan. If this be so, then the two faults which bound the 

 aporhyolite on the north and east have affected the upthrust of the 

 latter against the Shasta-Chico sandstones. The extent of this 

 upthrust on the east fault can not be less than 400 feet. This fault 

 is traceable across the hills to Telegraph Canon, and shows in 

 various places evidence of intense disturbance along the plane of 

 dislocation. The east and west fault crosses Skyline Ridge 

 obliquely, and descending into Strawberry Canon, abuts upon 



