Nothing is more difficult than to write a chronological history of the
revolutions and changes to which the earth has been subjected during the
ages which preceded the historic times. The phenomena which have
concurred to fashion its enormous mass, and to give to it its present
form and structure, are so numerous, so varied, and sometimes so nearly
simultaneous in their action, that the records defy the powers of
observation to separate them. The deposition of the sedimentary rocks
has been subject to interruption during all ages of the world. Violent
igneous eruptions have penetrated the sedimentary beds, elevating them
in some places, depressing them in others, and in all cases disturbing
their order of superposition, and ejecting masses of crystalline rocks
from the incandescent centre to the surface. Amidst these perturbations,
sometimes stretching over a vast extent of country, anything like a
rigorous chronological record becomes impossible, for the phenomena are
so continuous and complex that it is no longer possible to distinguish
the fundamental from the accidental and secondary causes.
In order to render the subject somewhat clearer, the great facts
relative to the progressive formation of the terrestrial globe are
divided into epochs, during which the sedimentary rocks were formed in
due order in the seas of the ancient world, the mud and sand in which
were deposited day by day. Again, even where the line of demarcation is
clearest between one formation and another, it must not be supposed
there is any sharply defined line of separation between them. On the
contrary, one system gradually merges into that which succeeds it. The
rocks and fossils of the one gradually disappear, to be succeeded by
those of the overlying series in the regular order of succession. The
newly-made strata became the cemetery of the myriads of beings which
lived and died in the bosom of the ocean. The rocks thus deposited were
called _Neptunian_ by the older geologists.
But while the seas of each epoch were thus building up, grain by grain,
and bed by bed, the new formation out of the ruins of the older, other
influences were at work, sometimes, to all appearance, impeding
sometimes advancing, the great work. The _Plutonic rocks_--the _igneous
or eruptive rocks_ of modern geology, as we have seen above, were the
great disturbing agents, and these disturbances occur in every age of
the earth’s history. We shall have occasion to speak of these eruptive
formations while describing the phenomena of the several epochs. But it
is thought that the narrative will be made clearer and more instructive
by grouping this class of phenomena into one chapter, which we place at
the commencement, inasmuch as the constant reference to the eruptive
rocks will thus be rendered more intelligible. To these are now added
the section “Metamorphic Rocks,” from the fifth edition of the French
work.
The rocks which issued from the centre of the earth in a state of fusion
are found associated or interstratified with masses of every epoch, more
especially with those of the more ancient strata. The formations which
these rocks have originated possess great interest; first, because they
enter into the composition of the terrestrial crust; secondly, because
they have impressed on its surface, in the course of their eruption,
some of the characteristics of its configuration and structure; finally,
because, by their means, the metals which are the objects of human
industry have been brought nearer to the surface. According to the order
of their appearance, or as nearly so as can be ascertained, we shall
class the eruptive rocks in two groups:--
I. The _Volcanic Rocks_, of comparatively recent origin, which have
given rise to a succession of trachytes, basalts, and modern lavas.
These, being of looser texture, are presumed to have cooled more rapidly
than the Plutonic rocks, and at or near the surface.
II. The _Plutonic Rocks_, of older date, which are exemplified in the
various kinds of granites, the syenites, the protogines, porphyries, &c.
These differ from the volcanic rocks in their more compact crystalline
structure, in the absence of tufa, as well as of pores and cavities;
from which it is inferred that they were formed at considerable depths
in the earth, and that they have cooled and crystallised slowly under
great pressure.