PRELIMINARY INVESTIGATIONS.
In the
investigation of all hydraulic-mining enterprises the first problem
which presents itself to the engineer is the ascertaining of the value
of the gravel deposits. This involves the determining of the course of
the channel; the depth and position of the bed-rock, generally covered
by hundreds of feet of detritus; the available area for washing; and
the estimates of the yield of the ground, with the calculations of the
cost of the work. Accurate information on these points is necessary.
But without the assistance of underground explorations few o£ them can
be definitely determined. A study of the geology and topography of the
deposit and ot its surroundings aids in determining the course of the
channel, the depth of the bed-rock, and the facilities for dump. The
value of the gravel can be approximated by sinking small pits, washing
the material obtained from them and from such other places as good
judgment dictates.
Where
the prosecution of an enterprise involves the expenditure of large
sums, it is advisable to thoroughly explore the ground by means of
prospecting shafts and drifts. Should the results of this work warrant
the opinion that the ground would pay to hydraulic, then the
water-supply and the facilities for dump should be accurately
determined, with close estimates of the costs.
Indications.—The
colors red and blue, with their varying shades, as seen in gravel
deposits, are generally considered by miners to be good indications of
gold in the different localities. While it is true that these different
colored sands often accompany gold, it by no means follows that gold
always accompanies them.
Ferruginous colored spots, so well marked in " upper
or top gravel," are not, as a rule, so productive of gold in
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