Theory of stratified rocks supposes disturbances of the earth’s crust—These disturbances ascribed by geologists to the action of subterranean heat—The existence of subterranean heat, and its power to move the crust of the earth, proved by direct evidence—Supposed igneous origin of our globe—Remarkable increase of temperature as we descend into the earth’s crust—Hot springs—Artesian wells—Steam issuing from crevices in the earth—The geysers of Iceland—A glimpse at the subterranean fires—Mount Vesuvius in 1779—Vast extent of volcanic action—Existence of subterranean heat an established fact.

In developing the modern theory of Geology, we have all along assumed that the Crust of the Earth has been subject to frequent disturbances from the earliest ages of the world. Again and again, in the course of our argument, we have talked of the bed of the sea being lifted up, and converted into dry land; and, on the other hand, of the dry land being submerged beneath the waters of the sea. We have not even hesitated to suppose that these two opposite movements of upheaval and submersion often took place by turns over the same area; nay, that there is scarcely a region on the surface of the Globe which has not been several times submerged, and several times again upheaved.

Yet all this has not been taken for granted without proof. Our readers have seen what a long array of sober reasoning may be drawn out to show that the Stratified Rocks have been, for the most part, deposited under water:—first, from the nature and arrangement of the materials which compose them; secondly, from the character of the Organic Remains they contain. And since they are now above water, it is plain that either they have been lifted or the ocean has subsided. Furthermore, if we find, as we often do, two strata in immediate succession, the one underneath, exhibiting the trees of an ancient forest still standing erect with their roots attached, the other above, abounding in the remains of aquatic animals; we must conclude that when the ancient forest flourished this portion of the Earth’s Crust was above the level of the sea; that afterward it was submerged, and a new deposit, in which the marine remains were embedded, was spread out above the earlier vegetation; and that, last of all it again emerged from the waters, and became once more dry land. Finally, when a vertical section of the Earth’s Crust exhibits a continued series of such strata alternating with each other, it affords a proof that this particular area must have been several times under water, and several times again dry land, in the long course of ages.

These conclusions are now all but universally received among Geologists. The Crust of the Earth, we are assured, is not that unyielding and immovable mass which men commonly take it to be. On the contrary, it has been from the beginning ever restless and in motion, rising here and subsiding there, sometimes with a convulsive shock capable of upturning, twisting, distorting hard and stubborn rocks as if they were but flimsy layers of pliant clay; sometimes with a gentle, undulating movement, which, while it uplifts islands and continents, leaves the general aspect of the surface unchanged, the arrangement of the strata undisturbed, and even the most tender Fossils unharmed. Disturbances of this kind have been going on in various parts of the world even within the period of history; and they may be distinctly traced to the action of subterranean Heat. In support of a theory so startling and unexpected, Geologists appeal to the direct evidence of facts: and we now propose to bring some of these facts under the notice of our readers.

At the outset, however, it is important to set forth clearly the doctrine we hope to illustrate and confirm. With the origin of the internal heat that prevails within the Crust of the Earth we have no concern. This is still an unsettled point among Geologists themselves. Some conjecture that our Globe, when first launched into space, was in a state of igneous fusion; that is to say, that all the solid matter of which it is composed was held in a molten condition by the action of intense heat; that, in course of time, as this heat passed off by radiation, the surface gradually cooled and grew hard; that an external shell of solid rock was thus formed, which has been ever growing thicker in proportion as the Earth has been growing cooler; and that the actual condition of our planet is the result of this process continued down to the present day,—a fiery mass of seething mineral within, and a comparatively thin crust of consolidated rock without. Others suppose that the internal heat of the Globe is developed by the agency of chemical changes constantly going on in the depths of the Earth; and others, again, look for a cause to the action of electricity and magnetism. But these and such like speculations are still under discussion, and not one of them can be regarded as anything more, at best, than a satisfactory hypothesis. Anyhow, it is not about the causes of internal heat that we are just now interested, but about the fact of its existence, and the nature of its effects. Is it true that an intense heat prevails very generally beneath the superficial covering of the Globe? and is that heat capable of producing those stupendous changes which are ascribed to it in our theory of Geology? These are the questions to which we mean to devote our chief attention.

It is a very significant fact, that the deeper we penetrate into the Crust of the Earth, the hotter it is. At first, no doubt, for a short distance, the reverse is the case. When we begin to descend we find it cooler below than above, because the further we depart from the surface the more we are removed from the influence of the Sun. But at a certain point—in our climate at about fifty feet below the surface—the influence of the Sun’s heat ceases to be sensibly felt. When this limit is passed, the temperature begins to rise, and thenceforth the deeper we go the hotter the earth becomes.

This broad and general fact has been tested by experiments in every part of the world, and has been found true in all countries, in all climates, in all latitudes, whether in coal-pits, or mines, or deep subterranean caves. “In one and the same mine,” says Sir John Herschel,89 “each particular depth has its own particular degree of heat, which never varies: but the lower always the hotter; and that not by a trifling, but what may well be called an astonishingly rapid rate of increase,—about a degree of the thermometer additional warmth for every ninety feet of additional depth,90 which is about 58° per mile!—so that, if we had a shaft sunk a mile deep, we should find in the rock a heat of 105°, which is much hotter than the hottest summer day ever experienced in England.” Now if the temperature continue to increase at this rate toward the centre of the Earth, it is quite certain that, at no very great distance from the surface, the heat would be sufficiently intense to reduce the hardest granite and the most refractory metals to a state of igneous fusion.

Again, every one is familiar with the existence of hot springs, which come up from unknown depths in the Earth’s Crust, and which, appearing as they do in almost all parts of the world, testify in unmistakable language to the existence of internal heat. At Bath, for instance, in England, the water comes up from the bowels of the Earth, at a temperature of 117° Fahrenheit; and in the United States, on the Arkansas River, there is a spring at 180°—not much below the boiling point. This remarkable phenomenon, however, may be more closely investigated in the case of Artesian Wells, so called from the province of Artois, in France, where they first came into use. These wells are formed artificially, by boring down through the superficial strata of the Earth, sometimes to enormous depths, until water is reached. It has been found in every case that the water coming up from these great depths is always hot; and, furthermore, that the deeper the boring the hotter the water. A well of this kind was sunk in 1834 at Grenelle, in the suburbs of Paris, to a depth of more than 1800 English feet, and the water, which rushed up with surprising force, had a temperature of 82° Fahrenheit; whereas the mean temperature of the air in the cellars of the Paris Observatory is only 53°. The water has ever since continued to flow, and the temperature has never varied. At Salzwerth, in Germany, where the boring is still deeper, being 2,144 feet, the water which rises to the surface is 91° of our scale.

Then we have, in many countries, jets of steam which issue at a high temperature from crevices in the Earth, and which tell of the existence of heated water below, as plainly as the steam that escapes from the funnel of a locomotive or from the spout of a tea-kettle. Phenomena of this kind are very common in Italy, where they are sometimes exhibited at intervals along a line of country twenty miles in length. But in Iceland it is that they are displayed in the highest degree of splendor and power. On the southwest side of that island, within a circuit of two miles, there are nearly a hundred hot springs called Geysers, from some of which, at intervals, immense volumes of steam and boiling water are violently projected into the air. The Great Geyser is a natural tube, ten feet wide, descending into the Earth to a depth of seventy feet, and opening out above into a broad basin, from fifty to sixty feet in diameter. This basin, as well as the tube which connects it with the interior of the Earth, is lined with a beautifully smooth and hard plaster of siliceous cement, and is generally filled to the brim with water of a clear azure color, and a temperature little below boiling point. The ordinary condition of the spring is one of comparative repose, the water rising slowly in the tube and trickling over the edge of the stony basin. But every few hours an eruption takes place. Subterranean explosions are first heard, like the firing of distant cannon; then a violent ebullition follows, clouds of steam are given out, and jets of boiling water are cast up into the air. After a little the disturbance ceases, and all is quiet again. Once a day, or thereabouts, these phenomena are exhibited on a scale of extraordinary grandeur: the explosions which announce beforehand the approaching display are more numerous and violent than usual; then such volumes of steam rush forth as to obscure the atmosphere for half a mile around; and, finally, a vast column of water is projected to a height of from one to two hundred feet, and continues for a quarter of an hour to play like an artificial fountain. Geysers scarcely less grand and striking are to be seen in New Zealand, from which the water is thrown up at a temperature 214° Fahrenheit, or two degrees above boiling point.

Such are the evident symptoms of subterranean heat,—hot springs, jets of steam, fountains of boiling water,—which are manifested unceasingly at the surface of the Earth in every quarter of the Globe. But it is sometimes given us to behold, as it were, the subterranean fire itself, and to contemplate its power under a more striking and awful form. From time to time, in the fury of its rage, the fiery element bursts asunder the prison-house in which it is confined, and rushes forth into the light of day; then flames are seen to issue from the surface of the Earth, yawning chasms begin to appear on every side, the roaring of the furnaces is heard in the depths below, clouds of red-hot cinders are ejected high into the air, and streams of incandescent liquid rock are poured forth from every crevice, which, rolling far away through smiling fields and peaceful villages, carry destruction and desolation in their track. These are the ordinary phenomena of an active volcano during the period of eruption; and even while we write, most of them may be witnessed actually taking place for the hundredth time, on the historic ground of Mount Vesuvius. Our typical example, however, we shall take from the eruption of that mountain in the year 1779. It was not, indeed, especially remarkable for its violence or for the catastrophes by which it was attended; but it had the good fortune to be accurately recorded by an eye-witness, Sir William Hamilton, who, at that time, represented the English Government at the Court of Naples; and we are thus more minutely acquainted with all its various circumstances than with those of any other eruption of equal importance.

For two years before, the mountain had been in a state of excitement and disturbance. From time to time rumbling noises were heard underground, dense masses of smoke were emitted from the crater, liquid lava at a white heat bubbled up from crevices on the slopes of the mountain, and through these crevices a glimpse could be had here and there of the rocky caverns within, all “red-hot like a heated oven.” But in the month of August, 1779, the eruption reached its climax. About nine o’clock in the evening of Sunday the eighth, according to the graphic description of Sir William Hamilton, “there was a loud report, which shook the houses at Portici and its neighborhood to such a degree as to alarm the inhabitants and drive them out into the streets. Many windows were broken, and, as I have since seen, walls cracked, from the concussion of the air from that explosion. In one instant, a fountain of liquid transparent fire began to rise, and, gradually increasing, arrived at so amazing a height, as to strike every one who beheld it with the most awful astonishment. I shall scarcely be credited when I assure you that, to the best of my judgment, the height of this stupendous column of fire could not be less than three times that of Vesuvius itself, which, you know, rises perpendicularly near 3,700 feet above the level of the sea. Puffs of smoke, as black as can possibly be imagined, succeeded one another hastily, and accompanied the red-hot, transparent, and liquid lava, interrupting its splendid brightness here and there by patches of the darkest hue. Within these puffs of smoke, at the very moment of their emission from the crater, I could perceive a bright but pale electrical light playing about in zigzag lines. The liquid lava, mixed with scoriae and stones, after having mounted, I verily believe, at least 10,000 feet, falling perpendicularly on Vesuvius, covered its whole cone, and part of that of Somma, and the valley between them. The falling matter being nearly as vivid and inflamed as that which was continually issuing fresh from the crater, formed with it a complete body of fire, which could not be less than two miles and a half in breadth, and of the extraordinary height above mentioned, casting a heat to the distance of at least six miles around it. The brushwood of the mountain of Somma was soon in a flame, which, being of a different tint from the deep red of the matter thrown out from the Volcano, and from the silvery blue of the electrical fire, still added to the contrast of this most extraordinary scene. After the column of fire continued in full force for nearly half an hour the eruption ceased at once, and Vesuvius remained sullen and silent.”91

The existence, then, of intense heat within the Crust of the Earth may be regarded as an established fact where-ever an active Volcano appears at the surface. Now let us consider for a moment, the very extensive scale on which these fiery engines of Nature are distributed over the face of the Globe. First, on the great continent of America. The whole chain of the Andes—that stupendous ridge of mountains which stretches along the western coast of South America, from Tierra del Fuego on the south to the isthmus of Panama on the north—is studded over with Volcanos, most of which have been seen in active eruption within the last 300 years. Passing the narrow isthmus of Panama, this line of Volcanos may still be traced through Guatemala to Mexico, and thence northward even as far as the mouth of the Columbia River. Here is a vast volcanic region extending fully 6,000 miles in length, and spreading out its fiery arms, we know not how far, to the right and to the left. At Quito, just on the Equator, a branch shoots off toward the northeast, and, passing through New Granada and Venezuela, stretches away across the West India Islands, taking in St. Vincent, Dominica, Guadaloupe, and many others; while, in the opposite direction, it is certain that the volcanic action extends westward, far away beneath the waters of the Pacific, though we have no definite means of ascertaining where its influence ceases to be felt.

Another vast train of active Volcanos is that which skirts the eastern and southern coasts of Asia. Commencing on the shores of Northwestern America, it passes through the Aleutian Islands to Kamtschatka; then, in a sort of undulating curve, it winds its course by the Kurile Islands, the Japanese group, the Philippines, and the northeastern extremity of the Celebes, to the Moluccas. At this point it divides into two branches; one going in a southeasterly direction to New Guinea, the Solomon Islands, the Friendly Islands, and New Zealand; the other pursuing a northwesterly course through Java and Sumatra into the Bay of Bengal.

There is a third great line of volcanic fires which has been pretty well traced out by modern travellers, extending through China and Tartary to the Caucasus; thence over the countries bordering the Black Sea to the Grecian Archipelago; then on to Naples, Sicily, the Lipari Islands, the southern part of Spain and Portugal, and the Azores. Besides these there are numerous groups of Volcanos not apparently linked on to any regular volcanic chain, nor reduced as yet by scientific men to any general system; Mount Hecla, for instance, in Iceland, the Mountains of the Moon in Central Africa, Owhyhee in the Sandwich Islands, and many others rising up irregularly from the broad waters of the Pacific.

From this brief outline some idea may be formed of the magnificent scale on which volcanic agency is developed within the Crust of the Earth. It must be remembered, however, that any estimate based upon the enumeration we have given, would be, in all probability, far below the truth; for we have mentioned those Volcanos only which have attracted the notice of scientific men, or which have chanced to fall under the observation of travellers. Many others, doubtless, must exist in regions not yet explored, and in the profound depths of the seas and oceans, which cover nearly two-thirds of the area of our planet. Moreover, we have said nothing at all of extinct Volcanos—such as those of Auvergne in France, and of the Rocky Mountains in America—which have not been in active operation within historical times; but in which, nevertheless, the hardened streams of lava, the volcanic ashes, and the cone-shaped mountains terminating in hollow craters, tell the story of eruptions in bygone ages, not less clearly than the blackened walls and charred timbers of some stately building bear witness to the passing wayfarer of a long extinguished conflagration.

We contend, therefore, that the doctrine of intense subterranean heat is not a wild conjecture, but is based on a solid groundwork of facts. First, there is presumptive evidence. In every deep mine, in every deep sinking of whatever kind, the heat of the earth increases rapidly as we descend. Hot water comes from great depths, and never cold. Sometimes it is boiling: sometimes it has been converted into steam. All this is found to be the case universally, whenever an opportunity has occurred for making the trial; and it seems to afford a strong presumption that if one could go still deeper, the heat would be found yet more intense, and would at length be capable of reducing to a liquid state the solid materials of which the earth is composed. Next, there is the direct testimony of our senses. A channel is opened from the depths below, flames are seen, red-hot cinders are cast up, and molten rock is poured out over the surface of the Earth in a liquid stream of fire. This evidence, however, though direct and conclusive as far as it goes, is not universal. It proves that an intense white heat prevails within the Crust of the Earth, not everywhere, but at least in those numerous and extensive regions where active Volcanos exist. So stands the case, as it seems to us, for the doctrine of subterranean heat as far as regards the fact of its existence.