thin
and numerous rather than massive, and lie near the surface, they are
considered most profitable. Few veins can be worked with profit very
far down. As traces of gold may be found almost everywhere, no one
should indulge in speculation before calculating the precentage and the
cost of extraction. Gold-hunting, after all, is a lottery with more
blanks than prizes.
The substances most frequently mistaken for gold are iron pyrites, copper pyrites and mica. The
precious metal is easily distinguished from these by its malleability
(flattening under the hammer) and its great weight, sinking rapidly in
water.
Searching for Silver.—This
metal is usually found with lead ore and native copper. Slates and
sandstones intersected by igneous rocks as trap and porphyry, ate good
localities. Pure silver is often found in or near iron ores and the
dark brown zinc blende. The Colorado silver lodes are porous at the
surface and colored more or less red or green. Any rock suspected of
containing silver should be powdered and dissolved in nitric acid.
Pour off the liquid and add to it a solution of salt. If a white powder
falls to the bottom which upon exposure turns black, there is silver in
it. Silver mines increase in value as in depth, whereas gold diminishes
as we descend.
Searching for Copper.—The
copper ores, after exposure, or after being dipped in vinegar, are
almost invariably green on the surface. They are moit abundant near
trap dykes. The pyrites is generally found in lead mines, and in
granite and clay-slate. Copper very rarely occurs in the new
formations, as along the Atlantic and Gulf borders, and in the
Mississippi Valley south of Cairo.
Searching for Lead.—Lead
is seldom discovered in the surface soil. It is also in vain to look
for it in the coal region and along the coast. It must be sought in
steep hills, in limestone and slate rocks. A surface cut by frequent
ravines or covered by vegetation in lines, indicates mineral crevices.
The galena from the slate is said to contain more silver than that from
the limestone. The purest specimens of galena are poorest in silver;
the small veins are richest in the more precious metal. A lead vein is
thickest in limestone, thinner in sandstone and thinnest in slate.
Searching for Iron.—Any
heavy mineral of a black, brown, red or yellow color may be suspected
to be iron. To prove it, dissolve some in oil of vitriol and pour in an
infusion of nut-gall or oak-bark; if it turns black, iron is present.
If a ton of rich magnetic ore cost more than $4 at the furnace good
hematite more than $3, and poor ores more than fl'50 or $2, they are,
too expensive to pay unless iron is unusually high. Deep mining for
iron is not profitable. Generally speaking, a bed of good iron ore, a
foot thick, will repay the cost of stripping it of soil, etc., twelve
feet thick. Red and yellow earths, called ochres, contain iron.
Magnetic ore is easily found by a compass.
Assay of Ores.
One
of the first questions asked after the discovery of a metallic ore,
is—" will it pay ?" We propose to state in plain words a method of
determining the character] and value of the principal ores, so that
any intelligent man, however unscientific, may answer his own question.
The chemical analysis or exact assaying of ores is too complicated, and
must be left to professional assayers.
"Will
it pay?" is an important query; for many ores of even precious metals,
are not "paying." Whether an ore is profitable depends not so much upon
the relative value of the metal as upon the ease of separating it from
the rock or "gangue" as it is called. Thus the minimum percentage of
metal, below which the working of the ore ceases to be profitable is—
