Historical Geology.—­This third part of the volume, and that which peculiarly characterizes the work, opens with some general remarks upon the divisions in Geological History, and the announcement of certain important principles to be kept in view while considering the subject.  The progress of life is then described as the basis of subdivision into Geological Ages; and the subdivisions of geological time are presented as follows:—­

  I. Azoic Time or Age.

  II.  Palaeozoic Time.
    1.  The Age of Mollusks, or Silurian.
    2.  The Age of Fishes, or Devonian.
    3.  The Age of Coal Plants, or Carboniferous.

  III.  Mesozoic Time.
    4.  The Age of Reptiles.

  IV.  Cenozoic Time.
    5.  The Age of Mammals.

  V. Era of Mind.
    6.  The Age of Man.

And in connection with this is given a table of the further subdivision of this history into Geological Periods, and a map showing the distribution of the rocks of each of these periods over the surface of the United States.

The great divisions above given are, as stated, essentially the same as proposed by Professor Agassiz, who, however, made the era of Fishes to embrace the first and second ages of Palaeozoic Time, the Silurian and the Devonian, instead of restricting it, as now done, to the latter, and calling the former the Age of Mollusks.

Following these general considerations, each great division of geologic time is successively taken up, commencing with the Azoic.  Each period of the several divisions is treated of in order; and the rocks of each epoch and their distribution described, first, as they exhibit themselves in America,—­then, more briefly, as they appear in Europe.  A full account of the life that manifested itself in each epoch, both vegetable and animal, is likewise given in the same order.  The igneous and other disturbing agencies are then considered, and general remarks added upon the geography, the character of the surface, and various phenomena of the period.

The whole of this portion of the work is abundantly illustrated with well-executed figures of all the characteristic species that distinguish the several periods, mostly drawn from American examples.

Dynamical Geology.—­This particular branch of the subject is made less prominent than usual in geological works, but it will not be found lacking in any point.

The subject is presented in the following order:—­

1.  Life as an agent in protecting, destroying, and making rocks.

2.  Cohesive Attraction.

3.  The Atmosphere as a mechanical agent.

4.  Water as a mechanical agent.

5.  Heat as an agent in volcanic phenomena, igneous eruptions, metamorphism, veins, etc.

6.  Movements of the earth’s crust, plication of strata, origin of mountains, earthquakes, etc.

7.  Chemistry of Rocks.