that they do not send up the water by the action
of gravitation, but under the influence of gaseous pressure.

Where, as in the case of upturned porous beds, the crevice water
penetrates far below the earth's surface or the open-air streams which
drain the water away, the fluid acquires a considerable increase of
temperature, on the average about one degree Fahrenheit for each
eighty feet of descent. It may, indeed, become so heated that if it
were at the earth's surface it would not only burst into steam with a
vast explosive energy, but would actually shine in the manner of
heated solids. As the temperature of water rises, and as the pressure
on it increases, it acquires a solvent power, and takes in rocky
matter in a measure unapproached at the earth's surface. At the depth
of ten miles water beginning as inert rain would acquire the
properties which we are accustomed to associate with strong acids.
Passing downward through fissures or porous strata in the manner
indicated in the diagram, the water would take up, by virtue of its
heat and the gases it contained, a share of many mineral substances
which we commonly regard as insoluble. Gold and even platinum--the
latter a material which resists all acids at ordinary
temperatures--enters into the solution. If now the water thus charged
with mineral stores finds in the depths a shorter way to the surface
than that which it descended, which may well happen by way of a deep
rift in the rocks, it will in its ascent reverse the process which it
followed on going down. It will deposit the several minerals in the
order of their solubilities--that is, the last to be taken in will be
the first to be crystallized on the walls of the fissure through which
the upflow is taking place. The result will be the formation of a vein
belonging to the variety known as fissure veins.

[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 p