As a consequence of the hypothesis of central heat, it is admitted that
our planet has been agitated by a series of local disturbances; that is
to say, by ruptures of its solid crust occurring at more or less distant
intervals. These partial revolutions at the surface are supposed to have
been produced, as we shall have occasion to explain, by upheavals or
depressions of the solid crust, resulting from the fluidity of the
central parts, and by the cooling down of the external crust of the
globe.
Almost all bodies, in passing from a liquid to a solid state, are
diminished in size in the process. In molten metals which resume the
solid state by cooling, this diminution amounts to about a tenth of
their volume; but the decrease in size is not equal throughout the whole
mass. Hence, as a result of the solidification of the internal parts of
the globe, the outer envelope would be too large; and would no longer
fit the inner sphere, which had contracted in cooling. Cracks and
hollows occur under such circumstances, even in small masses, and the
effect of converting such a vast body as the earth from a liquid, or
rather molten condition, to a solid state, may be imagined. As the
interior became solid and concrete by cooling, furrows, corrugations,
and depressions in the external crust of the globe would occur, causing
great inequalities in its surface; producing, in short, what are now
called _chains of mountains_.
At other times, in lieu of furrows and irregularities, the solid crust
has become ruptured, producing fissures and fractures in the outer
envelope, sometimes of immense extent. The liquid substances contained
in the interior of the globe, with or without the action of the gases
they enclose, escape through these openings; and, accumulating on the
surface, become, on cooling and consolidating, _mountains_ of various
heights.
It would also happen, and always from the same cause, namely, from the
internal contraction caused by the unequal cooling of the globe, that
minor fissures would be formed in the earth’s crust; incandescent liquid
matter would be afterwards injected into these fissures, filling them
up, and forming in the rocky crust those long narrow lines of foreign
substances which we call _dykes_.
Finally, it would occasionally happen, that in place of molten matter,
such as granite or metalliferous compounds, escaping through these
fractures and fissures in the globe, actual rivers of boiling water,
abundantly charged with various mineral salts (that is to say, with
silicates, and with calcareous and magnesian compounds), would also
escape, since the elements of water would be abundant in the
incandescent mass. Added to these the chemical and mechanical action of
the atmosphere, of rain, rivers, and the sea, have all a tendency to
destroy the hardest rocks. The mineral salts and other foreign
substances, entering into combination with those already present in the
waters of the sea, and separating at a subsequent period from these
waters, would be thrown down, and thus constitute extensive
deposits--that is to say, _sedimentary formations_. These became, on
consolidation, the _sedimentary rocks_.
The furrows, corrugations, and fractures in the terrestrial crust, which
so changed the aspect of the surface, and for the time displaced the
sea-basins, would be followed by periods of calm. During these periods,
the débris, torn by the movement of the waters from certain points of
the land, would be transported to other parts of the globe by the
oceanic currents. These accumulated heterogeneous materials, when
deposited at a later period, would ultimately constitute
formations--that is, _transported or drifted rocks_.
We have ventured to explain some of the theories by which it is sought
to explain the cosmography of the world. But our readers must understand
that all such speculations are, of necessity, purely hypothetical.
In conformity with the preceding considerations we shall divide the
mineral substances of which the earth is composed into three general
groups, under the following heads:--
1. _Eruptive Rocks._--Crystalline, like the second, but formed at all
geological periods by the irruption or intrusion of the liquid matter
occupying the interior of our globe through all the pre-existing rocks.
2. _Crystalline Rocks._--That portion of the terrestrial crust which was
primarily liquid, owing to the heat of the globe, but which solidified
at the period of its first cooling down; forming the masses known as
Fundamental Gneiss, and Laurentian, &c.
3. _Sedimentary Rocks._--Consisting of various mineral substances
deposited by the water of the sea, such as silica, the carbonates of
lime and magnesia, &c.
The mineral masses which constitute the _sedimentary rocks_ form beds,
or _strata_, having among themselves a constant order of superposition
which indicates their relative age. The mineral structure of these beds,
and the remains of the organised beings they contain, impress on them
characters which enable us to distinguish each bed from that which
precedes and follows it.
It does not follow, however, that all these beds are met with, regularly
superimposed, over the whole surface of the globe; under such
circumstances geology would be a very simple science, only requiring the
use of the eyes. In consequence of the frequent eruptions of granite,
porphyry, serpentine, trachyte, basalt, and lava, these beds are often
broken, cut off, and replaced by others.
_Denudation_ has been another fruitful source of change. Professor
Ramsay[10] shows, in the “Memoirs of the Geological Survey,” that beds
once existed above a great part of the Mendip Hills to the extent of at
least 6,000 feet, which have been removed by the denuding agency of the
sea; while in South Wales and the adjacent country, a series of
Palaeozoic rocks, eleven thousand feet in thickness, has been removed by
the action of water. In fact, every foot of the earth now forming the
dry land is supposed to have been at one time under water--to have
emerged, and to have been again submerged, and subjected to the
destructive action of the ocean. At certain points a whole series of
sedimentary deposits, and often several of them, have been removed by
this cause, known by geologists as _Denudation_. The regular series of
rock formations are, in fact, rarely found in unbroken order. It is only
by combining the collected observations of the geologists of all
countries, that we are enabled to arrange, according to their relative
ages, the several beds composing the solid terrestrial crust as they
occur in the following Table, which proceeds from the surface towards
the centre, in descending order:--
[10] “Memoirs of the Geological Survey of Great Britain,” vol. i., p.
297.