ever, now almost universally
substituted for that of Primary.

There is still a wide field of investigation 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 sense, since there are granite
formations even as late as the Tertiary period; those of the middle
periods were mostly porphyries and basalts; while in the more recent
ones, lavas predominate. We have as yet no clue to the laws by which
this distribution of volcanic elements in the formation of the earth is
regulated; but there is found to be a difference in the crystals of
the Plutonic rocks belonging to different ages, which, when fully
understood, enables us to determine the age of any Plutonic rock by its
mode of crystallization; so that the mineralogist will as readily tell
you by its crystals whether a bit of stone of igneous origin belongs to
this or that period of the world's history, as the palaeontologist
will tell you by its fossils whether a piece of rock of aqueous origin
belongs to the Silurian or Devonian or Carboniferous deposits. Although
subsequent investigations have multiplied so extensively not only the
number of geological periods, but also the successive creations that
have characterized them, yet the first general division into three great
eras was nevertheless founded upon a broad and true generalization. In
the first stratified rocks in which any organic remains are found, the
highest animals are fishes, and the highest plants are cryptogams;
in the middle periods reptiles come in, accompanied by fern and moss
forests; in later times quadrupeds are introduced, with a dicotyledonous
vegetation. So closely does the march of animal and vegetable life keep
pace with the material progress of the world, that we may well consider
these three divisions, included under the first general classification
of its physical history, as the three Ages of Nature; the more important
epochs which subdivide them may be compared to so many great dynasties,
while the lesser periods are the separate reigns contained therein.
Of such epochs there are ten, well known to geologists; of the lesser
periods about sixty are already distinguished, while many more loom up
from the dim regions of the past, just discerned by the eye of science,
though their his