I. An opinion has long prevailed among geologists of a certain school, that the crust of the earth is of very limited dimensions. A thin coating of primary crystalline rock is interposed betwixt the sedimentary strata above, and the intensely incandescent mass of which the interior is composed. The experiments of Fourier establish a formula of increasing temperature of the strata in a descending series, and from the rate of this increase, it is inferred, that about one hundred miles below the surface the entire nucleus is in a state of complete fusion. Some have even assumed the melting point to be less than thirty miles, when “the next contiguous matter is in a state of fusion, at a temperature probably higher than any that man can produce by artificial means, or any natural heat that can exist on the surface.”[11] Sir John Leslie attempted a demonstration of the ultimate resolution of the materials into light, as the only element capable of resisting the vast pressure of the outer crust; and, erroneously assuming the _modulus_ of compressibility of air, water, the metals, and all known earthy substances to be invariable, however greatly the pressure may be increased, this ingenious philosopher came to the conclusion, that, instead of Tartarean darkness, the offspring of superstition, the inner chambers of the earth are filled with luminous ether, the most pure, concentrated, and resplendent. Darwin believes that much of the vast continent of South America is suspended over an inner sea of liquid fire, and says, that, “daily it is forced home on the mind of the geologist, that nothing, not even the wind that blows, is so unstable as the level of the crust of the earth.” With the fires of Etna and Vesuvius raging on the one side, and the recent though extinct volcanoes of Auvergne and the Cantal seated so near on the other side, what security is there, amidst so many undoubted facts of the mobility of the land, that these vast piles of Alpine mountains may not again, through mere mechanical weight, break through the film of crust on which they rest, and sink into the abyss from which they so lately emerged! The doctrine of central heat, it may be replied, does not necessarily imply the universal _fluidity_ of the central mass, an opinion supported by Lyell, Poisson, and other eminent philosophers; while there is reason to infer, as repeatedly stated, that there is no identity of scale and mechanism between volcanoes now active, and the igneous causes which gave birth to these and other stupendous mountain-chains.