388 PROCEEDINGS OF THE CALIFORNIA 



westerly from Copperopolis, Bear Mountain has an outlier along its southwest 

 ern base, in the form of a low but tolerably well marked hilly ridge, between 

 which and the base of the mountain is a narrow but continuous valley ; and it is in 

 this valley that the copper-bearing belt of Copperopolis is found. Southwest of 

 this outlier, and for a distance of three or four miles northwesterly from Copper- 

 opolis, Salt Spring Valley consists mainly of a region of low hills, traversed 

 by a net-work of steep and narrow gulches. Farther northwest the surface of 

 the valley for three or four miles is more uniform, and here we find the nearly 

 level acea of " Tower's Ranch," and the gently sloping basin of the " Salt 

 Spring Valley reservoir." Beyond this, the country is again hilly to the Cala- 

 veras river. Southeast and south of Copperopolis, the surface is everywhere 

 hilly. The slope of the Gopher Hills towards the southwest is rapid until we 

 reach the low rolling country which forms Ihe border of the Sau Joaquin Val- 

 ley. 



Black Creek debouches from Bear Mountain a mile or so southeast of Cop- 

 peropolis, aud flows to the Stanislaus. Littlejohn's Creek takes its rise in the 

 hilly regions of the valley west of Copperopolis, and flowing southwesterly, 

 finds its way through the hills into Rock Creek. The latter rises in Bear 

 Mountain, five or six miles northwesterly from Copperopolis, and flowing south- 

 west across Salt Spring Valley, breaks through the Gopher Hills, and contin- 

 ues its course through the lower country to French Camp Slough, a branch of 

 the San Joaquin. All these creeks become dry in the summer, though in win- 

 ter they often carry very large volumes of water. At the point where Rock 

 Creek breaks through the Gopher Hills is the substantial dam of the Salt 

 Spring Valley Reservoir. 



GEOLOGY. 



The strike and dip of the rocks are more or less variable ; but, so far as my 

 observations extend in the region described, they have everywhere the same 

 general northwesterly trend and high northeasterly dip which characterize so 

 large a portion of the gold-bearing slates of central California. The strike is 

 usually from N. 50° W. to N. 70° W., (magnetic) and the dip from 50° 

 northeast to vertical. I have seen no case here of a decided southwesterly dip, 

 nor of a low one to the northeast. It is somewhat remarkable, by the way, that 

 this high northeasterly dip should be so general as it is in the great mass of 

 auriferous slates which forms the southwestern flank of the Sierra Nevada. It 

 is towards the granite axis of the chain, instead of from it, as would seem more 

 natural. The causes of this are by no means as yet fully explained. It is a 

 circumstance, however, which would lose none of its interest in the future, if, 

 as certain facts mentioned in the Geological Report, Vol. I, p. 286, might pos- 

 sibly seem to indicate, further explorations should prove it to be in general a 

 great inversion of the strata — their upper portions having been " forced back 

 by immense pressure from above, producing a condition of things similar to 

 that so often observed in the Alps, which is known as the ' fan structure,' and 

 has so much perplexed geologists." When we take into account the enormous 

 denudation, amounting to thousands of feet in perpendicular depth, which is 



