Under these heads we propose to examine the successive transformations to which the earth has been subjected in reaching its present condition; in other words, we propose, both from an historical and descriptive point of view, to take a survey of the several _epochs_ which can be distinguished in the gradual formation of the earth, corresponding with the formation of the great groups of rocks enumerated in the preceding table. We shall describe the living creatures which have peopled the earth at each of these epochs, and which have disappeared, from causes which we shall also endeavour to trace. We shall describe the plants belonging to each great phase in the history of the globe. At the same time, we shall not pass over entirely in silence the rocks deposited by the waters, or thrown up by eruption during these periods; we propose, also, to give a summary of the mineralogical characters and of the fossils characteristic of, or peculiar to each formation. What we propose, in short, is to give a history of the formation of the globe, and a description of the principal rocks which actually compose it; and to take also a rapid glance at the several generations of animals and plants which have succeeded and replaced each other on the earth, from the very beginning of organic life up to the time of man’s appearance. ERUPTIVE ROCKS. Nothing is more difficult than to write a chronological history of the revolutions and changes to which the earth has been subjected during the ages which preceded the historic times. The phenomena which have concurred to fashion its enormous mass, and to give to it its present form and structure, are so numerous, so varied, and sometimes so nearly simultaneous in their action, that the records defy the powers of observation to separate them. The deposition of the sedimentary rocks has been subject to interruption during all ages of the world. Violent igneous eruptions have penetrated the sedimentary beds, elevating them in some places, depressing them in others, and in all cases disturbing their order of superposition, and ejecting masses of crystalline rocks from the incandescent centre to the surface. Amidst these perturbations, sometimes stretching over a vast extent of country, anything like a rigorous chronological record becomes impossible, for the phenomena are so continuous and complex that it is no longer possible to distinguish the fundamental from the accidental and secondary causes.