Neither must it be forgotten that, under the hypothesis admitted by those who assume the existence of a central fire, our planet is to be considered as actually a liquid mass, as the external crust is in comparison but a thin skin. Under these conditions, it would be difficult to believe that this great ocean of lava is not, like the watery ocean, agitated by the alternating motion of tides, and that it does not move twice every day the raft, as it were, which is floating on its surface. It is difficult to understand how it is that the earth is not much more depressed at the poles than it now is, and has not been transformed into a real disk. This flattening of the poles is not more considerable than the mere superficial inequalities in the equatorial zone between the summits of the Himalayas and the abysses of the Indian Ocean. M. Liais attributes the slight flattening of the two poles to the erosion which the water and ice in those parts, irresistibly drawn as they are toward the equator, incessantly cause, year after year and century after century, by the enormous quantity of _débris_ torn away from the surface of the soil, which they bear with them. The principal argument of those who look upon the existence of a central fire as a demonstrated fact is that, in the external strata of the earth, so far as they have been explored by miners, the heat keeps on increasing in proportion to the depth of the excavation. In descending the shaft of a mine we invariably pass through zones of increasing temperature; only the rate of increase varies in different parts of the earth, and according to the strata through which the shaft is sunk. The heat increases more rapidly in schist than in granite, and in metallic veins more even than in schist; in lodes of copper more than in those of tin, and in beds of coal more than in metallic veins. M. Cordier, being struck by all the objections which presented themselves to his mind as to the thinness of the terrestrial crust, has admitted that this covering could not be stable without having at least from 75 to 175 miles of thickness. CLASSES OF ROCKS --SIR CHARLES LYELL