are by
no means limited to those early periods, they are nevertheless very
characteristic of them, and are indeed the great foundation-stones on
which the physical history of the globe is built.

Starting from this landmark, the earlier geologists divided the world's
history into three periods. As the historian recognizes as distinct
phases in the growth of the human race Ancient History, the Middle Ages,
and Modern History, 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 aspect of the earth as well as its inhabitants was 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, that disturbed and displaced
the materials accumulated during such a period of rest. The further
study of these subordinate periods showed that what had been called
Primary formations, 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, how