was at that time relatively homogeneous in
consistence, and, in virtue of the circulation which takes place in
heated fluids, must have been comparatively homogeneous in temperature;
and it must have been surrounded by an atmosphere consisting partly of
the elements of air and water, and partly of those various other
elements which are among the more ready to assume gaseous forms at high
temperatures. That slow cooling by radiation which is still going on at
an inappreciable rate, and which, though originally far more rapid than
now, necessarily required an immense time to produce any decided change,
must ultimately have resulted in the solidification of the portion most
able to part with its heat--namely, the surface. In the thin crust thus
formed we have the first marked differentiation. A still further
cooling, a consequent thickening of this crust, and an accompanying
deposition of all solidifiable elements contained in the atmosphere,
must finally have been followed by the condensation of the water
previously existing as vapour. A second marked differentiation must thus
have arisen; and as the condensation must have taken place on the
coolest parts of the surface--namely, about the poles--there must thus
have resulted the first geographical distinction of parts. To these
illustrations of growing heterogeneity, which, though deduced from known
physical laws, may be regarded as more or less hypothetical, Geology
adds an extensive series that have been inductively established.
Investigations show that the Earth has been continually becoming more
heterogeneous in virtue of the multiplication of sedimentary strata
which form its crust; also, that it has been becoming more heterogeneous
in respect of the composition of these strata, the later of which, being
made from the detritus of the earlier, are many of them rendered highly
complex by the mixture of materials they contain; and further, that this
heterogeneity has been vastly increased by the actions of the Earth's
still molten nucleus upon its envelope, whence have resulted not only
many kinds of igneous rocks, but the tilting up of sedimentary strata at
all angles, the formation of faults and metallic veins, the production
of endless dislocations and irregularities. Yet again, geologists teach
us that the Earth's surface has been growing more varied in
elevation--that the most ancient mountain systems are the smallest, and
the Andes and Himalayas the most modern; wh