86 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA, 









gray 



dip at a very high angle, sometimes inclining in one direction and sometimes in another, seeming 

 to have been considerably disturbed. In I langtown Hill, just south of Placerville, the bed-rock is 

 generally soft and decomposed ; in the Excelsior Claim much of it is apparently a sandstone, 

 sometimes fine-grained, and occasionally of coarse texture and passing into a purely silicious sand- 

 stone, the quartzose grains being rounded and mixed with scales of mica. At White Rock Point, 

 about four miles northeast of Placerville, the bed-rock is granite, much of which is soft and 

 decomposed ; it stretches southerly as far at least as Smith's Elat, distant about two miles, and 

 appears to be an isolated patch surrounded by slates and mostly covered by the volcanic and gravel 

 deposits. On the south side of Webber Hill, about two miles south-southeast of Placerville, the 

 bed-rock is a hard, dark-blue argillaceous slate, standing nearly vertical, and having a strike of 

 K 10° W. In these slates there are well-marked veins of slaty material, in the form of a 

 schist, cutting very sharply the stratification of the slates, at angles of 10° or 15°. The latter dip 

 generally to the northeast, with an inclination of 80° to 85°, while the veins pitch at about the 

 same angle in the opposite direction. The material which fills these so-called veins is finely lami- 

 nated in a direction parallel with their sides ; so that the slaty structure is nearly or quit® as well 

 marked as that of the rock in which they are enclosed. There is much pyrites disseminated through 

 the rocks in the neighborhood of Placerville, as is often the case in other localities where the argil- 

 laceous slates occur. 



In the region between Mud Springs and Shingle Springs, the bed-rock is slate, often more or 

 less " bloeky," — that is, breaking into large fragments or blocks. In some places the slate passes 

 into a semi-serpentine. A mile or two below Shingle Springs, the rocks are gneissoid and grani- 

 toid ; these continue for some distance, and gradually give place to slates and " bloeky " rocks. 

 About five miles below Shingle Springs, and a mile south of the road to Folsom, is Marble Valley, 

 on a creek of the same name, which is a branch of Deer Creek. Here is an outcrop of saccharoidal 

 and rather coarse-grained limestone, of a grayish- white or very light bluish color, which is dis- 

 tinctly stratified, with a strike of about N. 55° W., and a dip of from 80° to 85° to the northeast. 

 Half a mile farther down the Creek is another outcrop of the same rock, with a strike of N. 10 



o 



15° W., and, here also, standing nearly vertical. The belt is narrow, apparently but a few hundred 

 feet wide, and cannot be traced to any great distance in the line of its strike. There are said to 

 be several more isolated patches of limestone in a distance of two or three miles to the southeast ; 

 they are smaller, however, than those which have just been described. Along the northeast side 

 of the limestone at Marble Valley numerous large boulders of variegated jasper were noticed. 

 Below Shingle Springs, after passing the granitoid and gneissoid rooks, the bed-rock consists 

 chiefly of very fine-grained metamorphic sandstone, usually breaking into "bloeky" fragments. 

 Portions of the rock are, however, argillaceous. In the bed of Willow Springs Creek, not far 

 from where the Folsom road crosses it, the bed-rock is a fine-grained blue sandstone, which has 

 been used for building in the vicinity ; it may be seen in a wall near the north end of Willow 

 Springs Hill. Along the road between Shingle Springs and Folsom many fragments of bed-rock 

 are scattered, which are porphyritic, with crystals of white feldspar. 



All the way between Placerville and Newtown the bed-rock is much decomposed and usually 

 quite soft. It frequently passes into a talcose slate and occasionally becomes a well-marked soap- 

 stone. South of Placerville, on Squaw Hollow Creek, and all along the road to Fairplay, between 

 the North and Middle Forks of the Cosumncs, the bed-rock is granite, which continues as far as 

 Fairplay itself. From that place across the ridge to the Middle Fork of the Cosumncs, the conn 

 try is all granite, generally rather soft, and containing frequent dark-colored nodules of syenite, or 

 yenitic granite. At the Cosumnes copper mine, a heavy mass of coarsely crystalline limestone 

 crosses the river in a northeast and southwest direction, and rests against the granite on its western 

 side. In this rock is a cave of considerable, but unknown, extent. It is said to have been penetrated 

 to a distance of 800 or 900 feet, without reaching its end. In the neighborhood of drizzly Flat, 

 situated a little to the northwest of Steely's Fork, a branch of the North Fork of the Cosumnes, the 

 bed-rock is chiefly granite, usually soft and decomposed, but containing a good many hard bouldery 







