n the growth of the human race, so they distinguished between
what they called the Primary period, when, as they believed, no life
stirred on the surface of the earth; the Secondary or middle period,
when animals and plants were introduced, and the land began to assume
continental proportions; and the Tertiary period, or comparatively
modern geological times, when the physical features of the earth as
well as its inhabitants were approaching more nearly to the present
condition of things. But as their investigations proceeded, they found
that every one of these great ages of the world's history was divided
into numerous lesser epochs, each of which had been characterized by a
peculiar set of animals and plants, and had been closed by some great
physical convulsion, disturbing and displacing the materials
accumulated during such a period of rest.

The further study of these subordinate periods showed that what had
been called Primary formations, namely, the volcanic or Plutonic rocks
formerly believed to be confined to the first geological ages,
belonged to all the periods, successive eruptions having taken place
at all times, pouring up through the accumulated deposits, penetrating
and injecting their cracks, fissures, and inequalities, as well as
throwing out large masses on the surface. Up to our own day there has
never been a period when such eruptions have not taken place, though
they have been constantly diminishing in frequency and extent. In
consequence of this discovery, that rocks of igneous character were by
no means exclusively characteristic of the earliest times, they are
now classified together upon very different grounds from those on
which geologists first united them; though, as the name _Primary_ was
long retained, we still find it applied to them, even in geological
works of quite recent date. This defect of nomenclature is to be
regretted, as likely to mislead the student, because it seems to refer
to time; whereas it no longer signifies the age of the rocks, but
simply their character. The name Plutonic or Massive rocks is,
however, now almost universally substituted for that of Primary.

A wide field of investigation still remains to be explored by the
chemist and the geologist together, in the mineralogical character of
the Plutonic rocks, which differs greatly in the different periods.
The earlier eruptions seem to have been chiefly granitic, though this
must not be understood in too wide a sens