The gold region is practically confined to the first belt, along the west slopes of the Sierra Nevada, intersected by nearly parallel rivers, and broken by deep canons. An intrusive core of granite forms the central feature, which becomes gradually more exposed and extensive, till, in latitude 36-7°, it reaches almost from crest to plain. The core is flanked by metamorphic slates of Triassic and Jurassic age, much tilted, often vertical, the strike being generally parallel with the axis of the range, and in the south dipping toward the east. This so-called auriferous slate formation consists of metamorphic, crystalline, argilaceous, chloritic, and talcose slates. In the extreme north-west it appears with though subordinate to granite. Gradually it gains in importance as the superimposed lava in Butte and Plumas counties decreases, and north of the American River it expands over nearly the entire slope; but after this it again contracts, especially south of Mariposa; beyond the junction of the ranges it reappears in connection with granite. To the same formation are confined the payable veins of gold quartz, chiefly in the vicinity of crystalline and eruptive rocks. They vary in thickness from a line to two score feet or more, and follow a course which usually coincides with that of the mountain chain, that is, north-northwest with a steep dip eastward. The most remarkable vein is the extensive mother lode of the Sierra Nevada, which has been traced for over 60 miles from the Cosumnes to Mariposa.
The slate formation is covered by cretaceous, tertiary, and post-tertiary deposits, of which the marine sedimentary, chiefly soft sandstone, made up of granite debris, occurs all along the foothills, conspicuously in Kern county. The lava region extends through Plumas and Butte northward round the volcanic cones headed by mounts Lassen and Shasta, whose overflows have hidden the gold formation of so large an area. The wide-spread deposits of gravel are attributed to a system of tertiary rivers long since filled up and dead, which ran in nearly the same direction as the present streams, and with greater slope and wider channels. Eroding the auriferous slates and their quartz veins, these river currents spread the detritus in deposits varying from fine clay and sand to rolled pebbles, and boulders weighing several tons, and extending from perhaps 300 or 400 feet in width at the bottom to several thousand feet at the top, and from a depth of a few inches to 600 or 700 feet. The whole mass is permeated with gold, the larger lumps remaining near their source, while the finer particles were carried along for miles. The most remarkable of these gravel currents is the Dead Blue River, so called from the bluish color of the sand mixed with the pebbles and boulders, which runs parallel to the Sacramento some fifty miles eastward, with an average width of a quarter of a mile. The depth of detritus averages three hundred feet, and is very rich in the lower parts, where the debris is coarser and full of quartz. Although the so-called pay dirt, or remunerative stratum, lies in alluvial deposits nearly always within ten feet of the bedrock, and frequently permeates this for a foot or so in the slate formations, yet the top layers often contain gold in payable quantities, even in the upper portions of high banks, which can be washed by the cheap hydraulic process.
Rich surface deposits and few participants did not tend to advance mining methods; but as the easily worked alluvia became scarcer, and the number of miners increased, attention was turned to less remunerative auriferous strata, to be found, not alone in the shallow river bar and gulch diggings which so far had been merely skimmed, but extending through benches above the level of the streams and ravine hollows, and through flats and gravel hills, the deposits of ancient rivers. With these were connected rich beds difficult of access, as in the bottom of rivers, or at a great depth beneath layers of soil of little or no value. All of which required a combination of hands and capital, for removing barren surface, sinking shafts, and driving tunnels, and for machinery with which to perform this wasteful work in the most expeditious manner, and to better extract a compensating amount of gold. Numbers of experiments were introduced by thoughtful immigrants, but nearly all devised without practical knowledge, and utterly useless. Many excellent ideas were, however, obtained from men conversant with the methods of other countries, and these suggestions assisted in unfolding one method after another. In 1850 the long-tom began to supplant the cradle, of which it formed practically an extension, with a capacity fivefold and upward greater. Complementary to it was the quicksilver machine for saving fine gold. Both were replaced within two or three years by the more effective and permanent sluice, an extension of the two, and either constructed of boards, or as a simple inclined ditch, with rocks instead of wooden riffles for retaining the gold.
Operations on river bars soon led to explorations of the bed itself, to which end the stream was turned into artificial channels to lay bare the bottom. The cost and risk of deviating the river course caused the introduction of dredgers with fair success. Along the northern coasts of California the auriferous bluffs, worn away by the surf, deposit very fine gold in the beach sand, which is carried away on mule-back and washed at the nearest stream. To the sluice and its coordinates are due the immense increase in the production of gold during the early mining period; for without their aid the industry would have failed to provide remunerative employment for more than a small proportion of the mining force, as shown by the rapid deviation of poorer laborers to other pursuits after 1852. The saving effected by the rocker, as compared with the pan, was about fourfold. The tom gained an equal advance upon the rocker, and the sluice was found to be three times cheaper than the tom, for about 35 cents per cubic yard of mining dirt. Even this price, however, was too heavy to permit the mining of the largest auriferous deposits, in the gravelly banks and hills, which had moreover to be removed before richer underlying strata could be profitably worked.
The sluice process permitted them to be cheaply washed, so that in the excavation or removal lay the chief cost. To this end was invented in 1853 the hydraulic process, to undermine and wash down banks by directing against it a stream of water through a pipe, under great pressure. The same stream did the work of a host of pick men and shovellers, and supplied the washing sluice; so that in course of time, with cheaper labor and machinery, the cost of extracting gold from a cubic yard of gravel was reduced as low as half a cent, while the cost under the old rocker system of 1848-9 is estimated at several dollars. After many checks from lack of experience the hydraulic system acquired here a greater expansion than in any other county, owing to the vast area of the gravel beds, and the natural drainage provided by the Sierra Nevada slopes; but an immense preliminary outlay was required in bringing water through flumes, ditches, and tunnels, sometimes for several score of miles, through mountains, over deep ravines, and along precipitous cliffs, by means of lofty aqueducts hung sometimes by iron brackets; large reservoirs had also to be provided, and outlets and extensive places of deposit at a lower elevation for
the washed debris.
Early Day Hydraulic Mining
