PRECIOUS METAL INDUSTRY IN THE UNITED STATES. 61
The auriferous slates were at first supposed to be confined to the Sierra Nevada proper, which terminates on the north in the geological break made by the lava flows surrounding the extinct volcanoes of Lassens peak and Mount Shasta, and at the south is cut off topographically by the Mojave desert. Their geological representatives have since been traced to the northern boundary of the State and into western Oregon, and their continuation beyond the desert is found in the mountains of southern California and extends across the boundary into Lower California along the eastern side of the peninsula.
It was in the middle region of the western slope of the Sierra Nevada that the original discovery of placer deposits was made, and this region has since continued to be the greatest producer of gold. Here the topographical conditions were peculiarly favorable to the concentration of gravel and detrital material resulting from the disintegration of goldbearing rocks into bodies that could readily be worked by the aid of abundant water. The high range of the Sierra, with its remarkably long western slope, 50 miles in extent, condenses the moisture-laden currents coming from the Pacific into large and rapid streams, which become violent torrents during certain seasons. The great diurnal variation of temperature is moreover a powerful disintegrating agent. Under these conditions unusually large amounts of detrital matter are carried down by the modern streams, and in their long courses a very considerable concentration of the heavy sands rich in free gold takes place. These conditions must have prevailed to a considerable extent in an earlier geological period, for not only are rich gravels found along the beds of modern streams, but the beds of ancient and now abandoned rivers, crossing the modern ones at a considerable angle, are found to be filled with gold-bearing gravels. These ancient gravels have been in part protected from erosion by flows of lava, which constitute the many table mountains of the region, and in part remain as gravel ridges between the beds of the modern streams.
The rapid descent and considerable volume of the modern streams are more especially favorable to a system of mining which originated here when the richer modern placers were so far exhausted that it was no longer profitable to work them by former primitive methods. This is known as hydraulic mining, and consists in directing an artificial stream through a large nozzle and under the pressure of a high column of water upon a gravel bank and washing it bodily into sluice boxes, in which the gold is in part automatically caught by mercury properly disposed to come into contact with it. Under this system it has been possible to work over whole mountains of debris and extract at a profit the gold from gravel that contains only a few cents worth per cubic yard. Where, owing to the lava covering and the compacted nature of the gravel, this process is not practicable, drift mining has been resorted to, and tunnels have been run to reach the beds of the ancient streams and extract the richer concentration of gravel in the
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