

--.v.- 



BED-ROCK: EL DORADO COUNTY. 



85 



On the road between Auburn and Colfax, at a point nearly opposite Clipper Gap, the bed-rock 

 is porphyritic, containing large crystals of feldspar, and it is locally known as " China rock." 

 At a point two or three miles below Colfax a little serpentine occurs, and along the road from that 

 place to Iowa Hill, on the north side of the river (the North Fork of the American), there are out- 

 crops of metamorphic conglomerate and imperfect serpentine. 



At Spanish Dry Diggings, south of the Middle Fork of the American River, the bed-rock con- 

 sists of slates and very line-grained sandstones, generally filled with crystals of iron-pyrites, and to 

 a greater or less extent traversed by small irregular seams of quartz. Immediately east of the Grit 

 Claim is a heavy mass of semi-serpentine, apparently the result of incomplete metamorphic action 

 on a line-grained slate rock. Some of the slates here appear to bo made up of an aggregation of 

 minute acicular crystals (I treinolito or librolite). Near Greenwood, three miles south of Spanish 

 Dry Diggings, there is some exceedingly fine-grained argillaceous slate, with much fine-grained 

 sandstone, and some coarser grit-rock, which has once been, apparently, a sort of fine breccia, or con- 

 glomerate. There is also considerable porphyritic schist, locally known under the name of " China 

 rock," as mentioned with regard to a locality near Colfax.* This is a hard, fine-grained, schistose 

 rock, filled with large feldspar crystals. In the St. Lawrence Claim, near Greenwood, the si ratifi- 

 cation of the rocks seems to have been greatly disturbed, the decomposed slates striking and dipping 

 in various and very different directions. It is said that the "China rock " forms a continuous 

 belt from Greenwood across the North Fork of the xlmerican.* At White & Co.'s Claim, on the 

 tunnel, the bed-rock is thoroughly decomposed and very soft, and has occasional little seams of quartz 

 running through it. At Pilot Hill, or Centreville, six miles south-southeast of Auburn, on the 

 Georgetown Divide, between the Middle and South Forks of the American River, there is a large 

 quantity of dioritic (1) rock, and hornblcndic slates ; portions of this bear some resemblance to 

 gneiss. The summit of Pilot Hill has been much metamorphosed, and consists chiefly of a very 

 hard rock through which there is distributed considerable quartz, in the form of little irregular 

 seams of chalcedony. There is also on the summit of the hill a good deal of gossan, and the quan- 

 tity of iron contained in the rock seems to be very large. At a point about half a mile, a little 

 east of south, from the summit of Pilot Hill, there is a belt of crystalline and highly metamor- 

 phosed limestone, which at that place is not over a hundred feet in width. It is said that this 

 limestone bolt can bo traced, at intervals, all the way to the South Fork of the American, in a 

 direction of S. 34° E. " Alabaster Cave " is said to bo about five miles nearly west of Centreville ; 

 while Cave Valley, on the road from Georgetown to Auburn, is above five miles north of Pilot 

 Hill. There appears to be no continuous belt of limestono between tire two localities, but only a 

 narrow outcrop at each place. Here, as elsewhere in the region north of the Mokelumne River, 

 the limestone outcrops are small, and cannot be easily connected with each other, so as to 

 form a continuous line. At Powningville, three or four miles east-northeast of Pilot Hill, on the 





road to Georgetown, there is some granite, and some tough hornblcndic rock, and this locality 

 seems to be about on the line of demarcation between the slates and the granite, which latter then 

 stretches on up the river as far as to the point when the road forks to go to Johntown, between 

 Michigan Flat and Coloma. Near Alabama Flat, on Johntown Creek, the bed-rock is serpentine ; 

 and between Alabama Flat and Johntown the porphyritic material called " China rock " occurs. 

 Just below Johntown, the bed-rock passes into thin-bedded slates which strike northwesterly and 

 dip at a high angle, probably 80°, to the east. At Crane's Gulch, between Johntown and George- 

 town, the "seam-diggings" are in slate which is considerably decomposed, and much of it talcose. 

 its strike is northwesterly and its dip nearly vertical ) but the stratification is much contorted, and 

 it contains irregular seams of quartz, which, however, follow pretty nearly the lines of bedding. 



On the line of the road from Georgetown to Plaeerville, the rocks are chiefly (day slates, exceed- 

 ingly thin-bedded, — the lamination being sometimes as delicate as paper. These slates usually 



* Mr. Goodyear acids in his notes, " My experience has been, that there are no continuous belts of any 

 thing, for any great distance, in this section of country." 



