This
result is accomplished in two different ways. The direct effect
arising from the imposition of weight, that derived from the mass of
stratified material, is, as we know, to bring about a down-sinking of
the earth's crust. In the measure of this falling, heat is engendered
precisely as it is by the falling of a trip-hammer on the anvil, with
which action, as is well known, we may heat an iron bar to a high
temperature. It is true that this down-sinking of the surface under
weight is in part due to the compression of the rocks, and in part to
the slipping away of the soft underpinning of more or less fluid rock.
Yet further it is in some measure brought about by the wrinkling of
the crust. But all these actions result in the conversion of energy of
position into heat, and so far serve to raise the temperature of the
rocks which are concerned in the movements. By far the largest source
of heat, however, is that which comes forth from the earth's interior,
and which was stored there in the olden day when the matter forming
the earth gathered into the mass of our sphere. This, which we may
term the original heat, is constantly flowing forth into space, but
makes its way slowly, because of the non-conductive, or, as we may
phrase it, the "blanketing" effect of the outer rock. The effect of
the strata is the same as that exercised by the non-conductive
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 w