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 fine-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 fine-grained slate rock. Some of the slates here appear to be made up of an aggregation of 
minute acicular crystals (? tremolite or fibrolite). Near Greenwood, three miles south of § *panish 
Dry Diggings, there is some exceedingly fine-grained argillaceous slate, with much fine- erained 
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 stratifi- 
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 American.* 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 (?) rock, and hornblendic 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 belt can be 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 be 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 limestone between the 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 hornblendic 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-roek 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 Placerville, the rocks are chiefly clay slates, exceed- 
ingly thin-bedded, — the lamination being sometimes as delicate as paper. These slates usually 
* Mr. Goodyear adds 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.” 
