must have happened in order
to bring about the successive growth of forests one above another on the
same spot, the question at once arises as to how these movements of the
solid earth came about, and what was the cause which operated in such a
manner. We can only judge that, in some way or other, heat, or the
withdrawal of heat, has been the prime motive power. We can perceive,
from what is now going on in some parts of the earth, how great an
influence it has had in shaping the land, for volcanoes owe their
activity to the hidden heat in the earth's interior, and afford us an
idea of the power of which heat is capable in the matter of building up
and destroying continents. No less certain is it that heat is the prime
factor in those more gradual vertical movements of the land to which we
have referred elsewhere, but in regard to the exact manner in which it
acts we are very much in the dark. Everybody knows that, in the majority
of instances, material substances of all kinds expand under the influence
of heat, and contract when the source of heat is withdrawn. If we can
imagine movements in the quantity of heat contained in the solid crust,
the explanation is easy, for if a certain tract of land receive an
accession of heat beneath it, it is certain that the principal effect
will be an elevation of the land, consequent on the expansion of its
materials, with a subsequent depression when the heat beneath the tract
in question becomes gradually lessened. Should the heat be retained for a
long period, the strata would be so uplifted as to form an anticlinal, or
saddle-back, and then, should subsequent denudation take place, more
ancient strata would be brought to view. It was thus in the instance of
the tract bounded by the North and South Downs, which were formerly
entirely covered by chalk, and in the instance of the uprising of the
carboniferous limestone between the coal-fields of Lancashire,
Staffordshire, and Derbyshire.

How the heat-waves act, and the laws, if any, which they obey in their
subterranean movements, we are unable to judge. From the properties which
heat possesses we know that its presence or absence produces marked
differences in the positions of the strata of the earth, and from
observations made in connection with the closing of some volcanoes, and
the opening up of fresh earth-vents, we have gone a long way towards
establishing the probability that there are even now slow and ponderous
movements