In some localities fissures in rocks are found to contain metallic
substances. Such fissures are frequently found partially filled with
calcareous spar which forms the matrix in which the metals are inclosed.
Metallic veins are supposed to be partially filled by mechanical means,
the particles of metallic substances being conveyed into them by the
action of water or some other power, and partly by chemical action, or
by sublimation or fumes rising from below.
Some metallic deposits appear to occur in situations where igneous rocks
have intruded themselves. Gold is supposed to be found almost invariably
under such circumstances. Such appears to be the case in the rich
deposits near the Ural mountains, and also in California and in
Australia. In all these places it is met with in quartz. It is in
pebbles or sand of the same rock that it occurs in the beds of rivers,
and in some cases is found spread over a large extent of country.
Copper, though frequently met with in veins, is also found in extensive
masses or beds, interposed between layers of rock. The same remark
applies to tin, lead, and silver. Iron is also met with in beds, and
also in nodules or rounded masses, which occur in great abundance among
some kinds of rock. The last-named is the most universally diffused of
all metals, and the most useful.