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.

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 history is not yet unravelled.