100 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 











town, Cedar and Spanish hills, near Placerville, appears to be as follows, beginning from the bed- 

 rock : 



1. Gravel proper, which is generally but a few feet in thickness. 



2. Yellow sand, of varying thickness, generally rather fine, and free from pebbles, or nearly so. 

 It has, occasionally, streaks of fine gravel running through it. Its thickness is sometimes as much 



as thirty or forty feet. 



3. A mass of finer materials, consisting of extremely fine sand, with a large proportion of clayey 

 matter. This is often very closely compacted, and is of a mottled yellow and white color. It is 

 called " pipe-clay " by the miners ; thickness, up to twelve feet. 



4. The so-called " white lava," a material which appears to be a consolidated and slightly silici- 

 fied or metamorphosed, volcanic ash. It is easily dressed by the hammer, and is in general use 

 as a building stone, for which it is, in most respects, well adapted, although rather too easily 

 crushed. It occasionally though rarely contains small and sometimes angular, but sometimes 

 smoothly worn fragments of quartz and metamorphic rock. 



5. The " mountain gravel," which is a well-washed or water-worn gravel chiefly of volcanic 

 materials, although containing a few metamorphic and granitoid pebbles. This gravel is usually 

 slightly auriferous in the neighborhood of Placerville, and is locally known as " mountain gravel." 

 This with its sandy streaks is a perfect fac-simile, so far as structure is concerned, of the ordinary 

 metamorphic gravel. There are occasional boulders of white lava all through the mountain gravel, 

 alon^ the north side of Hangtown Hill ; and there seems to be a much greater variety of texture 

 among the volcanic rocks in the mountain gravel than, there is in the black lava. This is very 

 natural, since the former shows by its water-worn character that it has been transported from a 

 great distance, and it would naturally contain a greater variety of materials. Probably fifty per 

 cent of the whole mass of the mountain gravel consists of pebbles weighing less than five pounds 

 apiece ; and there are comparatively few boulders in it too large to go through the sluices. All 

 through the Placerville district this gravel is not so solidly compacted together that it cannot be 



"piped." 



6. The " black lava," which is generally a strongly compacted volcanic breccia, containing, how- 

 ever, occasional pebbles and boulders of granitic and metamorphic rocks. 



It is often the case that one or more of these layers is wanting at particular localities. P>ut 

 where they are all present, tin; above is the order of their arrangement. 



In the Robinson Mine, on Prospect Flat, near the head of the ravine of that name, southwest of 

 Smith's Flat, and east of Placerville, a pretty well marked channel, running about S. 42° W. has 

 been worked for several hundred feet, by means of a shaft 100 feet deep to the bed-rock, through 

 white lava," which has several small streaks of gravel intercalated between its successive beds. 

 The bed-rock gravel, at the shaft, is thirteen feet thick, and the mine pays well. 



a 



In one place at White Eock Point, northeast of Placerville, the banks exhibit a thin stratum of 

 gravel lying on the bed-rock, and covered by twenty or thirty feet of compact " white lava," above 

 which comes a stratum of fine gravel three or four feet thick, consisting entirely of metamorphic 

 materials ; this last is itself overlain by a bed of horizontally and very delicately and thinly bedded 

 clay, from five to eight feet in thickness, while over all comes the "mountain gravel." A short 

 distance east of here, however, the " white lava " gradually thins out wedge-shaped towards the east, 

 while the underlying gravel grows thicker till finally the " white lava " entirely disappears and the 

 two streaks of gravel unite in one. At one locality near here also, the " white lava " was seen in 

 mass resting immediately on the surface of the soft and decomposed granite bed-rock with no gravel 



or anything else between. 



At Negro Hill, near Placerville, in the Oldfield Claim, where two or three acres have been 

 worked, they have a bank of sixty to seventy feet in height. The gravel on the bed-rock is in 

 general not very smoothly water-worn, and it ranges in thickness from twenty-five to thirty feet. 



It is immediately overlain by the " black lava," which is from fifteen to twenty-five feet thick, and 

 extremely hard, so that it breaks up into enormous blocks, many hundreds of tons in weight, thus 



