CONSTITUTION OF THE PRIMEVAL WORLD. 425

easily traversed, altered, and even obliterated by igneous agency of which they afford so many indications. I regret that my limits will not permit me to enter more at large into Mr. Murchison's excellent views on geological catastrophe.

CHAP. III.-THE CONSTITUTION OF THE PRIMEVAL

WORLD,

AND THE REVOLUTIONS WHICH IT UNDERWENT, DEDUCED FROM GEOLOGICAL PHENOMENA, ON PHYSICAL PRINCIPLES,

FROM the evidence advanced in the preceding chapter it incontestibly follows, that a vast magazine of fire and explosion lies immediately within the crust of our sphere, throughout its whole terraqueous

zones.

The subject of the interior temperature of the globe has been lately investigated with all the resources of mathematical analysis by M. Fourier. He considers the heat distributed within the earth as susceptible of three distinct movements.

1. The rays of the sun penetrate the globe, occasioning diurnal and annual variations in its temperatures. These periodical changes cease to be perceptible at a certain distance beneath the surface. Beyond that depth, and even to the greatest accessible excavations, the temperature due to the sun, has long since become fixed and stationary. The whole quantity of solar heat, which regulates the periodical variations, oscillates in the exterior shell of the earth; descending further within the surface during one portion of the year, and rising up to be dissipated into space, during the opposite or winter

season.

2. The temperature of deep excavations constant for any one place, varies for localities more or less distant from the equator, so that the solar heat penetrates further at the equinoctial zones, to reascend and be dissipated at the polar regions.

3. We must however consider not only the external calorific focus of heat, but also the action of the proper or intrinsic heat of the globe. If, as the experiments about to be related seem to prove, the temperature of the deep recesses of the earth becomes perceptibly greater in proportion as we recede from the surface, it is impossible to ascribe this increase to the heat of the sun. It can proceed only from a primitive heat with which the earth was endued at its origin, and which may diminish in the course of ages with greater or less celerity by diffusion from the surface.

This hypothesis of an interior central heat has been revived by philosophers from time to time; for it naturally occurs to the mind as the true cause of several great phenomena.

Observations for the Temperature of the Earth.

In the mines of Giro-Magny, three leagues from Befort, M. Gensanne found;

At Junghohebirke, the external thermometer, 32°

At 1040 feet depth, water was

63°

INTRINSIC HEAT OF THE EARTH.

4.27

Observations made by Captain Lean in the mines of Cornwall.

M. Humboldt obtained analogous results in the mines of Freyberg in 1791, and subsequently in many mines in America.

Several persons have made objections to the above experiments; ascribing the elevation of temperature to the presence of miners, of lamps, &c. M. Arago, in order to obviate this difficulty, examined the water of an Artesian well (a spring gushing up from beneath a deep mineral stratum), and he perceived that its temperature was higher than the water near the surface of the earth. Analogous experiments have been continued, at the request of M. Arago, by M. Berges, officer of engineers, which lead to the same result. This mode of observation seems free from every fallacy.

The preceding facts, and other similar ones, concur to show that the increase is nearly a degree of Fahrenheit for 65 feet. This increase will not be always of the same amount as at the present day; it will diminish progressively, but a great many ages

must elapse before it be reduced to the half of its actual value. The extent of this diffusion of the central heat into the circumference, and of its waste into the celestial spaces, will therefore be proportional to its primitive intensity, and to the conducting quality of the investing materials.

If we apply heat to the flat bottom of a deep vessel (of iron, copper, &c.), which contains several alternate layers of sand, clay, and stony slabs, condensed as in the supermedial strata of England, and covered with water, we shall wait in vain for any distinct manifestation, at the top, of the subjacent fire. In fact, the lowest layer will become compacted by the heat into a schist impervious to liquids, so that the incumbent water will never arrive at the calorific source, and severed by bad conducting matters can never grow appreciably warm. In the great boilers of steam engines, many results to this effect daily occur which form sources of very serious annoyance. Wherever the waters of supply are calcareous, more especially selenitic, they let fall a crust of gypsum on the bottom, which progressively thickens, so as to intercept a large portion of the subjacent heat, and by separating the iron from the water, allows the metal to become ignited and to burn away. Such a deposit has been known to grow several inches thick, with a stony hardness; and till laboriously chiselled off, it has rendered the vessel quite inoperative for raising a due supply of steam.

We have merely to compare these incontestible results of art, to the kindred phenomena of nature, to recognise not merely the analogy, but identity

STATE OF THINGS AT THE COAL-EPOCHA.

429

of operation in the two cases; for difference of magnitude, constitutes no disparity of essence.

In the early epochas of the antediluvian world, soon after the granitic atlas had uplifted the primitive mountains, and before the extensive series of mineral beds, which occupy our second book, were deposited beneath the ocean, its waters resting on the nearly concentric, or slightly broken zones of gneiss and mica slate, necessarily lay in closer proximity with the interior fires, than at any subsequent period. Hence two important consequences: 1. From the thinness of the solid crust, the smallest chink or fissure in it, would be an immediate focus of submarine explosion, accompanied and followed by a commensurate comminution and dispersion of the solid rocks and organic deposits through the agitated waters. 2. The ocean would then attain its maximum temperature; a pitch certainly far higher than at present, yet not incompatible with the vital functions of fish, many of which according to Humboldt can live in water almost boiling hot. Desfontaines found the sparus thriving in tepid fountains of 100° Fahr. near the town of Cassa in the kingdom of Tunis.

From the extreme mobility of its molecules, water is the most expeditious conveyer of heat from below upwards; while, from its non-conducting quality, it is a most faithful carrier, losing none of it during its ascent. Hence any degree of warmth, however gentle, imparted to the bottom of the oceanic mass, will be transmitted unimpaired to the surface. And again, as water possesses a very high specific heat, one four times