[Illustration: Fig. 14.--Diagram of vein. The different shadings show
the variations in the nature of the deposits.]

A vein deposit such as we are considering may, though rarely, be
composed of a single mineral. Most commonly we find the deposit
arranged in a banded form in the manner indicated in the figure (see
diagram 14). Sometimes one material will abound in the lower portions
of the fissure and another in its higher parts, a feature which is
accounted for by the progressive cooling and relinquishment of
pressure to which the water is subjected on its way to the surface.
With each decrement of those properties some particular substance goes
out of the fluid, which may in the end emerge in the form of a warm or
hot spring, the water of which contains but little mineral matter.
Where, however, the temperature is high, some part of the deposit,
even a little gold, may be laid down just about the spring in the
deposits known as sinter, which are often formed at such places.

In many cases the ore deposits are formed not only in the main channel
of the fissure, but in all the crevices on either side of that way. In
this manner, much as in the case of the growth of stalactitic matter
between the blocks of stone in the roofs of a cavern, large fragments
of rock, known as "horses," are often pushed out into the body of the
vein. In some instances the growth of the vein appears to enlarge the
fissure or place of the deposit as the accumulation goes on, the
process being analogous to that by which a growing root widens the
crevice into which it has penetrated. In other instances the fissure
formed by the force has remained wide open, or at most has been but
partly filled by the action of the water.