It not infrequently happens that the ascending waters of hot springs
entering limestones have excavated extensive caves far below the
surface of the earth, these caverns being afterward in part filled by
the ores of various metals. We can readily imagine that the water at
one temperature would excavate the cavern, and long afterward, when at
a lower heat, they might proceed to fill it in. At a yet later stage,
when the surface of the country had worn down many thousands of feet
below the original level, the mineral stores of the caverns may be
brought near the surface of the earth. Some of the most important
metalliferous deposits of the Cordilleras are found in this group of
hot-water caverns. These caverns are essentially like those produced
by cold water, with the exception of the temperature of the fluid
which does the work and the opposite direction of the flow.

In following crevice water which is free to obey the impulses of
gravitation far down into the earth, we enter on a realm where the
rock or construction water, that which was built into the stone at
the time of its formation, is plentiful. Where these two groups of
waters come in contact an admixture occurs, a certain portion of the
rock water joining that in the crevices. Near the surface of the
ground we commonly find that all the construction water has been
washed out by this action. Yet if the rocks be compact, or if they
have layers of a soft and clayey nature, we may find the construction
water, even in very old deposits, remaining near the surface of the
ground. Thus in the ancient Silurian beds of the Ohio Valley a boring
carried a hundred feet below the level of the main rivers commonly
discovers water which is clearly that laid down in the crevices of the
material at the time when the rocks were formed in the sea. In all
cases this water contains a certain amount of gases derived from the
decomposition of various substances, but principally from the