In the Upper New Red Sandstone (Lower Trias), near Hildburghausen, in Saxony, a strange unknown animal, supposed to belong to the frog order, has left foot-prints bearing a striking resemblance to the impressions made by a human hand; and in the still older red sandstone of Connecticut, a gigantic bird has marked a foot four times larger than that of the ostrich. It existed long before the Ichthyosaurus was seen on earth, and yet by a singular chance its traces, printed on a foundation proverbially unstable, have outlived the wreck of so many ages. However brief and defective the foregoing review of the fossil world may have been, it has still sufficed to point out the existence on our planet of so many habitable surfaces, each distinct in time, and peopled with its peculiar races of aquatic and terrestrial beings, all admirably fitted for the new states of the globe as they arose, or they would not have increased and multiplied and endured for indefinite periods. ‘The proofs now accumulated,’ says Sir Charles Lyell, ‘of the close analogy between extinct and recent species are such as to leave no doubt on the mind that the same harmony of parts and beauty of contrivance which we admire in the living creation has equally characterised the organic world at remote periods. Thus, as we increase our knowledge of the inexhaustible variety displayed in living nature, our admiration is multiplied by the reflection that it is only the last of a great series of pre-existing creations, of which we cannot estimate the number or limit in times past.’ CHAPTER III. SUBTERRANEAN HEAT. Zone of invariable Temperature—Increasing Temperature of the Earth at a greater Depth—Proofs found in Mines and Artesian Wells, in Hot Springs and Volcanic Eruptions—The whole Earth probably at one time a fluid mass. Born neither to soar into the air, nor to inhabit the deep waters, nor to pass his life in subterranean darkness, man is unable to depart to any considerable distance from the earth’s surface. If he ascends in a balloon, he soon reaches the limits where the rarefied atmosphere renders breathing impossible; a few thousand feet limit his efforts to pierce the earth’s crust; and should he be cast out into the sea, he is soon drowned. But beyond the limits to which his body is confined, his mind soars into space, and plunging into the unknown interior of our globe, seeks to unravel the mystery of its formation. In the following pages I purpose briefly to point out the circumstances which guide him in his speculations, and enable him to roam, at least in spirit, through the profound abysses of the subterranean world.