e, 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 clew 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 may enable 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 history is not yet unravelled.

Before proceeding further, I will enumerate the geological epochs in
their succession, confining myself, however, to such as are perfectly
well established, without alluding to those of which the limits are
less definitely determined, and which are still subject to doubts and
discussions among geologists. As I do not propose to make here any