In one case, where the fissure is
partially filled up with silica inclosing iron and copper pyrites, gold has also
been found in the vein-stone.
It has been remarked by M. de Beaumont, that lead and some other metals are
found in dikes of basalt and greenstone, as well as in mineral veins connected
with trap-rock, whereas tin is met with in granite and in veins associated with
the Plutonic series. If this rule hold true generally, the geological position
of tin accessible to the miner will belong, for the most part, to rocks older
than those bearing lead. The tin veins will be of higher relative antiquity for
the same reason that the "underlying" igneous formations or granites which are
visible to man are older, on the whole, than the overlying or trappean
formations.
If different sets of fissures, originating simultaneously at different levels in
the earth's crust, and communicating, some of them with volcanic, others with
heated Plutonic masses, be filled with different metals, it will follow that
those formed farthest from the surface will usually require the longest time
before they can be exposed superficially.
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