





104 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA 





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generally are, when found resting immediately upon the bed-rock, or at no great distance from it. 

 The course of this ridge is northeast and southwest, and a line of detached hills of similar character 

 extends for several miles to the southwest, appearing to cross the railroad a mile or more below 

 Colfax. 



On the south of Forney's, on Pilot Creek, the country around, within a radius of two or three 

 miles, spreads out into a broad, gently-undulating tract, where the gulches are shallow, and most of 

 which seems to drain towards Pilot Creek, although its general surface is not very far from level. 

 This whole area, so far as could be seen, is covered with volcanic cement. At a point not far from 

 two miles, in a direction S. 75° E. (magnetic) from Forney's, is a high timbered peak, the summit 

 of which is about 800 feet higher than the level of Forney's. The volcanic cement extends to the 

 foot of this peak and a very short distance up its flank. But the top and the upper 300 or 400 

 feet are of bed-rock, which is here a quartzite, more or less stained with iron, and containing 



occasionally little seams of quartz. The law which generally holds good, in the region to the 

 north, in the basin of the Middle Fork of the American, — that the highest crests are all capped 

 with volcanic matter, — seems to be here reversed, the more elevated ridges being of bed-rock, and 

 the volcanic deposits not extending above a certain zone. From this it appears probable that the 

 present depth of the volcanic matter in the central portions of the broader crests in the basin of 

 the Middle Fork is a fair indication of its maximum depth in the past, and that it has never 

 been much deeper there than it now is. In that case, it can never have extended much farther 

 up the sides of the highest projecting peaks of bed-rock than it now does ; or, at least, these peaks 

 have never been covered by it. 



Along Hangtown Hill and Cedar Hill, near Placerville, the whole crest of the ridge consists of 

 "black lava," i. e., volcanic breccia, the thickness of which in places is perhaps a hundred feet, 

 and which contains many fragments or boulders of enormous size, some weighing from fifteen to 

 twenty tons. Many of these are very angular and unworn, although they are generally somewhat 

 more rounded on the corners than is usually the case with the boulders seen on the surface of lava 

 streams. This breccia is underlain by smoothly washed volcanic gravel, here known as "moun- 

 tain gravel," and under this again is the ordinary metamorphic auriferous gravel. 



In the Franklin Claim on the south side of Little Spanish Hill, as well as at Negro Hill, near 

 Placerville and elsewhere, the " black lava " or volcanic breccia is traversed by horizontal planes of 

 stratification, proving the occurrence of successive flows of brecciated matter over the same ground. 

 In the Franklin Claim the lowest stratum of " black lava " is four or five feet thick. Above it is 

 a layer of sand and fine gravel a foot to eighteen inches thick, said to contain some fine gold, and 

 over this again are several successive layers of "black lava." Along the southern side of Little 

 Spanish Hill the gravel is generally immediately overlain, by "black lava." There is no "moun- 

 tain gravel " in this hill. 



At Smith's Flat, about three miles east of Placerville, the " white lava " occupies most of the 

 surface. The bed ranges from twenty to thirty feet or more in thickness. The gravel beneath it 

 is worked by inclines. At thirty feet below the bottom of the white lava, the gravel still contains 

 boulders of this rock intermingled with the other materials. These boulders must have come from 

 some considerably older deposit of the same kind of rock higher up in the mountains. 



Near the Toll House, about one and a half miles N. 8° E. from the Try Again Tunnel, there is 

 a high bluff of the "white lava," in which rude columnar forms are well developed; the top of 

 this bluff is not less than 200 feet above the Toll House, and the bed-rock at the house is buried 

 beneath some sixty feet of the same material. In the hill back of the face of the bluff, therefore, 

 there cannot be less than 250 feet in thickness of this material. It is overlain, on the narrow crest 

 of the spur immediately east of Smith's Flat, by a shallow bed about half-way in character be- 

 tween the "black lava " and the "mountain gravel." 



f.he "white lava " here contains a good 





many small cavities filled with a substance which has much resemblance to little fragments of 

 fossilized wood. The material does not show in its internal structure, at this point, any distinctly 

 horizontal bedding ; but the weathered faces of the columns indicate it by a corrugation of their 



