Thus rain-drops on greenish slates of the Coal period, with several worm tracks, such as usually accompany rain-marks on the recent mud of modern beaches, have been discovered near Sydney, in Cape Breton. As the drops resemble in their average size those which now fall from the clouds, we may presume that the atmosphere of the Carboniferous period corresponded in density with that now investing the globe, and that different currents of air varied then as now in temperature, so as, by their mixture, to give rise to the condensation of aqueous vapour.
In like manner it has been possible to detect the footprints of reptiles, even in shales as old as the Cambrian formation, and to follow their trail as they walked or crawled along.
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.
As we all know, the temperature of the atmosphere soon communicates its changes to the surface of the earth; and our meadows, which when warmed by the rays of the sun are green and covered with flowers, harden in winter into a lifeless plain. But the influence of the sun’s heat upon the soil is merely superficial, so that in the temperate zones the annual fluctuations of the thermometer are no longer perceptible at a depth of from 60 to 80 feet.
Thus, in the cellars of the Parisian observatory, a thermometer, placed many years ago 86 feet below the surface, invariably indicates +11°7 Celsius; the summer above may be ever so intensely hot, or the winter ever so cold, the column of mercury never deviates a hair’s breadth from the height it has once attained. Below these limits the warmth of the earth gradually increases—a fact placed beyond all doubt by the innumerable observations that have been made in mines, and during the boring of Artesian wells. For wherever sinkings have been made, a rising of the thermometer has always been found to take place as the auger penetrates to a greater depth below the surface. Thus, to cite but a few examples, the temperature of the Artesian well of Grenelle in Paris, which, at a depth of 917 French feet, amounted to +22°2 C., increased at the depth of 1,555 feet to +26°43, and the water, which now gashes forth from the depth of 1,684 feet, constantly maintains the same lukewarm temperature of +27°70.