Starting from this landmark, the earlier geologists divided the world’s history into three periods.  As the historian recognizes Ancient History, the Middle Ages, and Modern History as distinct phases in 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 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 clew to the laws by which this distribution of volcanic elements in the formation of the earth is regulated;