140 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
and in its mining features resembles those of the localities just described. 
It is said that the area of mining ground here was originally 200 acres in 
extent, and that it has been worked continuously since 1850, in places to a 
depth of seventy-five feet. The total yield of the Flat, up to 1868, is stated 
at $2,000,000.* The attempts to get rid of the water, which has been very 
troublesome here, making it impossible to mine to the bottom of some of the 
cavities, have not been pecuniarily successful, for the reasons which have al- 
ready been given in connection with the account of the localities on the lime- 
stone farther west. Sonora was one of the principal mining centres of Cali- 
fornia during the time of the great productiveness of the placer diggings. 
The town is situated on the junction of the limestome and the slates, in 
which latter there is in the vicinity a large amount of volcanic material, —a 
diabase. The limestone itself is intersected by numerous dykes of diorite, 
as already, mentioned. The bed of Wood’s Creek, on which the town is situ- 
ated, has been most thoroughly worked over, yielding largely.* 
The Soulsbyville district, about five miles east of Sonora, on Curtis Creek, 
is a region of considerable interest, not only on account of its gravels and 
volcanic rocks, but because there are several well-developed veins, which 
have produced.considerable gold, and which are enclosed in the granite. 
This rock occupies a large area, between the Tuolumne and the Stanislaus, 
extending almost down to Sonora. Upon it rests a lava flow which has 
descended from the Sierra, spreading out in its downward course into several 
branches, and covering considerable deposits of sedimentary materials. This 
flow follows Curtis Creek, and occupies a belt on both sides of it. The surface 
of the granite, under the volcanic masses, is somewhat irregularly rolling, and 
in the depressions rests a small quantity of gravel, which does not seem to have 
been found to contain a workable quantity of gold. Over this is a stratum 
of pipe-clay, succeeded by a mass of rhyolitic tufa or ash, fine-grained and 
white in color, enclosing fragments of pumice ; and over this, again, a deposit 
of yellow clays and sands, the whole capped by a mass of volcanic materials, 
of variable thickness, which in places is as much as 140 feet. The larger 
portion of this mass of lava is of a brecciated character, consisting of frag- 
ments of andesite, cemented together by gray volcanic sands, the whole 
* J. S. Hittell, in Report of J. Ross Browne on the Mineral Resources of the States and Territories 
west of the Rocky Mountains. Washington, 1868, p. 38. 
+ For an account, compiled by Mr. Skidmore, of the doings (social and other) at Sonora in the early 
days, see Report of United States Commissioner of Mining Statistics, for 1872, p. 65. 
