entered
the stream. In one case floating amalgam was observed on the North Fork
of the Yuba River four miles below where the tailings were dumped. A
flume (conveying water to a pump) was set above the bottom of the
stream, drawing direct without any dam. An examination of the flume
subsequent to its removal revealed the presence of about one ounce of
gold amalgam, collected at the junction of the boxes.
Commencing Work.—The
first work is started near the head of the sluice and the mine opened
from that point. As the banks are washed away the bed-rock cuts are
driven towards the face of the work and the sluices are advanced. (For
blasting see Chapter XII.)
Caving Banks.—In order
to cave a bank it is customary to use two pipes which throw streams
from opposite sides at an obtuse angle with one another, forming a
cross-fire, against the lower part of the bank. This cross-fire was
supposed to be particularly efficient, but in many cases where large
quantities of water and great pressures (2,500 to 3,000 inches with
heads of 350 to 450 feet) are employed better results have been claimed
from utilizing water in a single stream than from its subdivision
through two (or more) pipes. Any surplus water may be allowed to run
over the banks, but such surplus should be avoided as far as possible
and all the water utilized through the nozzles.
When
washing with two pipes, and the dirt caves readily, one pipe should be
employed to do the cutting while with the other the falling gravel is
washed into the ground sluices.
The
face of the bank should be kept square. Advantage should be taken of
such corners as are left, and, under all circumstances, avoid working
into what is called a " horseshoe " form. When a cut is rapidly pushed
ahead and the work is not squared, the men at the pipes become
encircled by high banks, washing can no longer be advantageously
prosecuted, and the lives of the miners are
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