CHAPTER III

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THICKNESS OF THE EARTH’S CRUST—CENTRAL HEAT.

The question arises, since upon geological grounds it is demonstrable that the crust of the earth has been repeatedly upheaved and broken, have we reason to conclude that similar states of paroxysm and convulsion may not again return? This brings us to the consideration of two very interesting problems, namely,—THE THICKNESS OF THE EARTH’S CRUST—And THE DOCTRINE OF CENTRAL HEAT. Have we any means of determining either of these points? The doctrine of the igneous origin of granite and other rocks proceeds upon the assumption of a vast reservoir of heat existing somewhere within the interior; and the question to be solved is—What is the thickness of the solid crust beneath which the molten rocks have their origin? and what the cause of their fusion?

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.

But astronomy gives a different and more comfortable solution of the problem. The influence of the moon alone, it would appear, acting upon our planet, requires a thickness of crust of at least ONE THOUSAND MILES, to prevent the fabric of the globe from being severed into fragments. The earth, considered in connection with its own planetary system, has three distinct motions in space, a fact in science usually illustrated by the movements of the common spinning-top. A more striking illustration may be seen in the steam-vapor which has aided you onward, that living cloud of light and heat which towers and floats away in these beautifully curling wreaths. Like the trail of the comet, how gracefully it sweeps over the plains in its forward movement: then it turns to the right or left in the direction of the wind: and then, in a third convolution, every globule of the airy mass is twirling on an axis of its own. Equally buoyant is the earth, hung upon nothing, and cleaving the liquid firmament. It turns on its axis, causing the vicissitude of day and night; it moves through its orbit, making the circuit of the sun and the diversity of the seasons; and, in addition, there is an oscillatory motion like the unsteady zig-zag twistings of the carriage-train, occasioned by the excess of the equatorial over the polar diameter. This excess amounts to about a three-hundredth part. But, small as it is, it exerts an assignable influence over the cohesion or attraction of the solid framework. Now, by a nice mathematical demonstration, resting on the sun and moon’s attraction, Mr. Hopkins infers, as indicated by the phenomena of _precession and nutation_, that the minimum thickness of the earth’s crust cannot be less than one-fourth or one-fifth of the earth’s radius. The theorem is of too abstract a nature to be here introduced; but it appears from it that the observed amount of _precession_ requires this degree of solid matter, which gives a clear depth of solid arch over either vacuum, resplendent light, or fiery fluid, of from eight hundred to a thousand miles. This may well allay the fears of the most timid as to the stability of the ground beneath his feet, whatever be the state of the interior, or under whatever modifications the materials therein may exist.

II. But if this thickness of crust is required now, it must have been equally required in all past time: hence, it may be argued, no security is thereby afforded against the bursting out of the pent-up fires, or disruption of the outer crust? Now, it has been questioned whether there be such a thing as a CENTRAL fluid heat at all, while the solidity of the earth throughout has been maintained as more in unison with the principles of established science.

The doctrine of a central heat is as old as the days of Bishop Burnet, who imagined that the internal fire, pre-existent in the bowels of the earth, was the agent employed in breaking up the fountains of the deep for the production of the deluge. Leibnitz and Buffon regarded the earth as an extinguished sun or vitrified globe, which, according to the calculations of the latter, required seventy-five thousand years to cool down to its present temperature; and that, in ninety-eight thousand years more, the heat will be utterly exhausted, and productive nature extinguished. Whiston fancied that the earth was created from the atmosphere of one blazing comet, and deluged by the humid tail of another. And Whitehurst, one of the oldest of modern geologists, regarded all the strata of every formation, as concentrically arranged over the surface of the globe, and then employed the expansive agency of internal fire to account for their upheaved disrupted condition. These cosmogonies, it is needless to remark, all now rank with the speculations of the alchemists, and that Behmen and their authors are considered as of equal authority in the sciences of chemistry and geology.

The searching tests of experiment, as already noticed, have been brought to illustrate the subject of internal heat, but the results have not been decisive, nor very satisfactory. It has been stated, as a general rule in the mines examined, that, in proportion to their depth, the heat increases as we descend; and the mean result of all the best observations, as given by Cordier, amounts to one degree of heat for every forty-five feet of depth. Different mines, however, it has been ascertained, vary in their degrees of downward temperature; as in the Durham and Newcastle coal-pits, the increase is estimated at one degree for every forty-four feet,—in the Cornwall iron-mines at one in seventy-five feet,—and in Saxony some of the mines give an increase of only one degree in one hundred and eighty or one hundred and ninety feet of perpendicular descent. Much of this difference may, indeed, be readily accounted for by the nature of the contents of the mines themselves, their position in the system, and the quality of the rocks among which they are situated. But a difficulty will remain, how to dispose of the increasing ratio of temperature, and the changes that must necessarily result from it in the bowels of the earth. Thus, assuming it to be a uniformly increasing ratio in proportion to the depth, it will follow from this law of increase that we reach a point, about twenty-four miles down, hot enough to melt iron: at double that distance, such a heat as will fuse every substance with which we are acquainted: and at a hundred miles, that a temperature will exist, of whose resolving powers we have no experience, and cannot even conjecture. The astronomical theory of a thousand miles of crust melts into airy nothing in its presence, and the formula of Fourier, before it has reached the required hundred, will have found a nucleus in complete fusion, acting intensely upon the thin external crust, and seeking through every crevice of these “flagrantia mœnia mundi,” to issue forth in torrents of fire.

It has been farther objected to a central fluidity, that such a fluid must be in constant circulation by the cooling of its exterior—a fact ascertained in the case of all fused metals. Tides too, it has been argued, would be produced in the fluid matter, however deeply seated, through the influence of the sun and moon, and which _tides_ would necessarily occasion such oscillatory and expansive movements as astronomy has neither noticed nor accounted for. Again, the supposition of a central heat of the earth, prevailing from the beginning and through all the phases of its history, implies that its cooling is still going on; and that, in consequence, a contraction in the mass or bulk of the earth will follow the law observed by all other bodies in parting with their heat. Hence this contraction might lead to the shortening of the day and other mechanical results. But Laplace satisfied himself, by reference to ancient astronomical records, that there had been no alteration in the length of the day, even to the smallest calculable degree or point of a second; and that thence, the hypothesis of a fluid, or even primitive heat of the earth, had here no confirmation.

An objection to the theory has likewise been urged, from the well-known property and tendency of heat to become equally diffused through all the

## particles of any body in which it exists. Thus, it is an established

principle, that heat not only diffuses itself on all sides, but passes continually from bodies in which the temperature is greater to those in which it is less; and that if a body be placed in a medium having a temperature different from its own, the momentary variations of its temperature will be as the differences between the temperature of the body and of the medium. Hence, when the heat beneath the surface of the earth, at whatever depth, becomes of sufficient intensity to melt iron, it cannot pass beyond this until the whole surrounding mass is heated to the same degree of intensity. The law of increment and transmission of caloric, it is argued, must be the same below as above; and, assuming the nucleus of the earth to be fluid, no solid crust could thus be formed upon the surface until every particle of the heated fluid mass was cooled down to the point of consolidation. This principle is well understood in the formation of a crust of ice upon water. If extendible to other bodies, and to subterranean distances, then the simple fact of an existing outer crust, solid and cooled down to the existing temperature, militates against the probability of a central fire, and is utterly repugnant to the hypothesis of a liquid central mass.

It is maintained, however, on the other hand, that the central caloric, however intense at any depth, has long ago arrived at the point at which the conducting power of the rocky crust has either entirely ceased or permits no further sensible decrease; that this point was reached some time before the creation of man, when the process of cooling had acquired a maximum or stationary condition, and that it formed a part of the processes by which the earth was adapted to its high destination among the works of God. But may not the adaptation have been effected by the gradual conduction of the heat outwardly, not by suffering it to remain and glow in opposition to its known properties in the inner regions? It may be maintained as a safe principle in physical science, that if there be heat in the center of the globe, it must have the properties of heat and none other. No geologist hesitates to admit, upon evidence amounting to demonstration, that a vast source of heat exists in the interior of the earth, widely spread beneath the stony pavement, and that it has existed at all times. But whether that heat is local or generally diffused—whether it is central or infra-superficial—whether it is constantly maintained, or is excited at intervals by certain combinations—are questions as yet of mere speculation, and for the solution of which we have no data to lead us, beyond probable inferences.

Upon the whole, as the known density of the earth is considerably greater than that of a solid sphere, composed of any such rocks as we are acquainted with, the presumption is, that heavier materials, in an increasing ratio, than any constituting the superficial crust, enter into the composition and structure of the central parts. All the great mountain systems in the different zones of the globe are the product of palæozoic times. The fires which cast them out have gradually diminished by every succeeding effort. A steadier equilibrium betwixt the conflicting elements of the upper and the lower world, appears to have taken place—once only disturbed, at the deluge, since man’s occupation—and for the repetition of which there exists no preparation in the established course of nature. The very convulsions which have shattered the earth to its foundations, while they are evidences of benevolent wisdom, furnish, at the same time, the best guarantee against blind fortuitous derangements to come; the result, as they are, of periodical causes, acting in a way and with an intensity of which we have no experience, and for which, indeed, we have no expression in any of the sciences.

GENERAL PRINCIPLES.

## PART IV.

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