348 



RESUME AND THEORETICAL DISCUSSION. 



Valley. On the whole, the streams in the hydraulic mining region are 

 becoming rapidly choked with tailings, so that operations are already seri- 

 ously interfered with ; while the owners of farms at the edge of the foot-hills, 

 where the larger streams debouch into the Great Valley, are much excited 

 over the prospect of constant additions of mud and sand to the already 

 extensive deposits of this kind on the adjacent land. 



To return to the specially difficult point, — the cause of the beginning of 

 the excavation of the present canons along the lines which they now occupy. 

 There seems to be no evidence to justify the falling back on any orographic 

 causes in this case. There do not appear to have been any fissures or faults 

 hi the Sierra, since the gravel was deposited, of sufficient extent to materially 

 influence the drainage, at least over by far the larger portion of the mining 

 region.* Thus we are forced, not only to accept erosion as the essential 

 agent in the formation of the existing river canons, but to admit that the 

 present streams were directed, from the beginning, into the positions which 

 they now occupy through the agency of causes influencing the drainage of 

 the region covered with gravel and volcanic materials. 



The special mode of action of these causes seems, in many cases, to be 

 involved in the greatest obscurity. Take, for instance, the Tuolumne Table 

 Mountain, to which reference has been so often made. The preservation of 

 the channel, with its well-defined rim-rock on both sides, and its softer pipe- 

 clay deposits overlying the thin stratum of pay gravel, may be ascribed, with 



ances o t e grave] by faulting are somewhat numerous in tbo extreme northern portion of the min- 

 ing regron, as described in Appendix A. In the region so closely examined by Mr. Goodyear, however, the 

 evidences of such movements of the bed-rock were but few, and the vortical displacement was never large The 

 points mentioned by him in his notes where such displacements were observed are the following : At the 

 Missouri Tunnel, the bed-rock is traversed by a fault running N. 46° W. (magnetic), and the plane of which 

 pitches to the .southwest at an angle of 60°. The gravel on the southwest side of this fault has been raised 

 from twelve to fifteen feet in vertical height; the facts at this locality arc clear and unmistakable. Again, 

 at Yankee Jim's, a fault in the bed-rock runs across the » Big Channel," in the form of an upward jump to the 

 east of about fifteen feet, with a pitch in the same direction ; the effect of this fault, however, could not be de- 

 tected in the gravel. There are indications that this dislocation is continued with similar characters in Georgia 



Hill (see Map, Plate B, for the position of the localities sneeii',,,,1! m, n i i -r i e u 



x uro iuuuiues specineu). Mr. Goodyear was also informed ol the ex- 



istence of a disturbance of the. bed-rock at King's Hill, showing itself in a displace ,t of the gravel to the 



amount of "twenty feet or more," the fault running northwesterly, and the upthrow being on its northeast side. 

 This, however, he admits that he did not discover when at the locality in question. At Flora's Canon the ex- 

 istence of a pair of parallel faults seemed to be strongly indicated by the sinking of the bed-rock so as to for 

 a deep trough with a V-shaped section, the dimensions of which are not stated. Another fault was noticed at 

 White & Co's. mine, at Castle Hill, near Georgetown. In this case, however, the displacement was only to the 

 amount of about three feet, and there was some doubt about its existence at all. The above are all the instances 

 of faults noticed by Mr. Goodyear in his investigations in the gravel region. 



ui 



/ 



