bulk until the expiration of the Pleiocene age! Such are the mighty agencies contemplated by the geologist in the various later changes which have affected the surface of our globe. The rill, the river, the torrent, the glacier, the earthquake, the volcano, are still in operation, but only as faint images of the enormous powers which in the more ancient times have been at work. That the earth has been repeatedly encroached upon by the waters every principle of his science goes to establish; but out of every convulsion he sees a better and more stable condition of things to have emerged. If the bowlder drift and the cold plastic clay formation point to a continuance of sunless, lifeless seasons, he forgets not, as the products of the period, that two-thirds of the soil of Great Britain and of the grain-bearing lands of the continent, have been derived from these accumulations—the industrial monuments of their invasion in every quarter of the world. CHAPTER III. THICKNESS OF THE EARTH’S CRUST—CENTRAL HEAT. The question arises, since upon geological grounds it is demonstrable that the crust of the earth has been repeatedly upheaved and broken, have we reason to conclude that similar states of paroxysm and convulsion may not again return? This brings us to the consideration of two very interesting problems, namely,—THE THICKNESS OF THE EARTH’S CRUST—And THE DOCTRINE OF CENTRAL HEAT. Have we any means of determining either of these points? The doctrine of the igneous origin of granite and other rocks proceeds upon the assumption of a vast reservoir of heat existing somewhere within the interior; and the question to be solved is—What is the thickness of the solid crust beneath which the molten rocks have their origin? and what the cause of their fusion?