thermal springs is derived from uncooled lavas, perhaps not far
below the surface. But when hot springs occur at a distance of
hundreds of miles from any volcano, as in the case of the hot
springs of Bath, England, it is probable that their waters have
risen from the heated rocks of the earth's interior. The springs
of Bath have a temperature of 120 degrees F., 70 degrees above the
average annual temperature of the place. If we assume that the
rate of increase in the earth's internal heat is here the average
rate, 1 degree F. to every sixty feet of descent, we may conclude
that the springs of Bath rise from at least a depth of forty-two
hundred feet.

Water may descend to depths from which it can never be brought
back by hydrostatic pressure. It is absorbed by highly heated
rocks deep below the surface. From time to time some of this deep-
seated water may be returned to open air in the steam of volcanic
eruptions.

SURFACE DEPOSITS OF SPRINGS. Where subterranean water returns to
the surface highly charged with minerals in solution, on exposure
to the air it is commonly compelled to lay down much of its
invisible load in chemical deposits about the spring. These are
thrown down from solution either because of cooling, evaporation,
the loss of carbon dioxide, or the work of algae.

Many springs have been charged under pressure with carbon dioxide
from subterranean sources and are able therefore to take up large
quantities of lime carbonate from the limestone rocks through
which they pass. On reaching the surface the pressure is relieved,
the gas escapes, and the lime carbonate is thrown down in deposits