coatings which are put on steam boilers. A more familiar comparison
may be had from the blankets used for bedclothing. If on top of the
first blanket we put a second, we keep warmer because the temperature
of the lower one is elevated by the heat from our body which is held
in. In the crust of the earth each layer of rock resists the outflow
of heat, and each addition lifts the temperature of all the layers
below.

When water-bearing strata have been buried to the depth of ten miles,
the temperature of the mass may be expected to rise to somewhere
between seven hundred and a thousand degrees Fahrenheit. If the depth
attained should be fifty miles, it is likely that the temperature will
be five times as great. At such a heat the water which the rocks
contain tends in a very vigorous way to expand and pass into the state
of vapour. This it can not readily do, because of its close
imprisonment; we may say, however, that the tendency toward explosion
is almost as great as that of ignited gunpowder. Such powder, if held
in small spaces in a mass of cast steel, could be fired without
rending the metal. The gases would be retained in a highly compressed,
possibly in a fluid form. If now it happens that any of the strain in
the rocks such as lead to the production of faults produce fissures
leading from the surface into this zone of heated water, the tendency
of the rocks containing the fluid, impelled by its expansion, will be
to move with great energy toward the point of relief or lessened
pressure which the crevice affords. Where rocks are in any way
softened, pressure alone will force them into a cavity, as is shown by
the fact that beds of tolerably hard clay stones in deep coal mines
may be forced into the spaces by the pressure of the rocks which
overlie them--in fact, the expense of cutting out these in-creeping
rocks is in some British mines a serious item in the cost of the