Chapter 12

There is, however, yet another source of heat, if indeed solar heat be not a mere case of its general action, far more general and universal, which has its origin in the bodies themselves, and has no reference to any extrinsic cause. All bodies are sensibly heated when condensed, and lose sensible heat when they expand, so that their temperatures vary with the greater or less distance of their particles. The atmosphere of the earth furnishes a marked illustration of this fact. Of nearly uniform chemical composition throughout, its elastic nature, conflicting with its gravity, renders it more dense in its lower than in its higher regions. The former are in consequence warmer than the latter, and the mean temperature of our climates is in fact due to this character of our atmosphere. But this mean temperature could not be maintained, were not that of the earth itself in harmony with it. The surface might, no doubt, be cooled or heated by the adjacent air, but the heat, if given out from an earth warmer than the atmosphere, would be rapidly replaced from within, and a constant accumulation ensue in the air, while, if the earth were cooler, a diminution, equally constant, of the temperature of the atmosphere, must take place. The earth is, however, itself subject to the same law. All the materials of which it is composed, are capable of compression, in a greater or less degree, and of being heated by compression. The tendency of all material substances to the centre of attraction, loads the parts nearest to that centre with the whole weight of the superincumbent mass. And in the depth of four thousand miles, which intervenes between the centre and the surface, the heat must be far more than equal to that obtained by the compound blow-pipe or galvanic deflagrator, under whose intense energies the most refractory substances liquefy. Hence it may be inferred as a fact, as certain as any in physical science, that the interior of the earth is at present in a state resembling igneous fusion, not produced, however, by any of the more familiar sources of heat, but by the intense pressure the upper masses exert upon those nearer to the centre.

Here, then, we find the reason of the earth's having assumed a figure consistent with the equilibrium of a fluid mass, whose particles are endued with a mutual attraction, and which has a motion around an axis.

Let us suppose all the particles which now constitute the earth, to have been originally disseminated throughout a vast space, and to have approached their common centre of gravity by the force of mutual attraction; the consideration thus caused would have produced the state of intense heat that is now kept up within by pressure; and the conducting power of the bodies would have propagated the heat nearly equal throughout the mass. The surface would then have existed in a liquid state as well as that beneath. But as the radiation from the surface of a heated body is in exact proportion to its temperature, this cause of cooling would have been intense, and a crust must soon have formed upon the outer surface; this crust would have increased in thickness so long as the heat thrown off by radiation exceeded that received from the sun. When this state of equilibrium was finally attained, all the great phenomena which a body thus heated could exhibit, would cease, and the subsequent changes would become due only to forces such as we now see acting upon the surface, or would be the completion of actions commenced during the previous state.

We know, from astronomical investigations, that this state of equilibrium has existed for upwards of twenty centuries, while analogy would lead us to infer that it must have been attained at no long period after the last great catastrophe to which our planet was subjected.

Let us now see whether the fact of the interior of the globe being more intensely heated than its surface, can be inferred in any other manner than from the course of reasoning whose principles are here cited. The feeble power of man, feeble at least compared to the size of the globe he inhabits, has been able to penetrate to but small depths in its outer shell, but even at these small depths, an increase of temperature has been remarked, and so frequently and carefully observed, as to leave no doubt of its being a general law. This increase, too, appears exactly consistent with that which it might be inferred ought to take place. But we, even to the present day, occasionally see the igneous fluid from beneath forced up to the surface, and spreading from volcanic craters over great regions. Observation shows us that at remote epochs such phenomena were much more frequent than at present. We want no more positive proofs that the interior of the earth is still intensely heated, and that the bed of the ocean and the solid land are mere crusts formed upon the surface of a mass in a state analogous to that of igneous fusion.

Were the surface, as we have inferred it must have been, ever itself intensely heated, the volatile and gaseous matters which now constitute our atmosphere and oceans, must have united to form an atmosphere of far greater extent than it is at present. The aqueous matter rising into regions where the rarity of the air would cause cold sufficient to condense it, would have been in a state of constant motion, boiling in the lower regions, being precipitated in the higher, and acting most energetically to promote the general cooling. And so soon as the surface became cooler than 212 deg., the water would begin to settle upon its surface, forming at first lakes in its basins or cavities, and finally extending itself into one vast ocean, covering the whole or parts of the solid crust according to its greater or less degree of uniformity.

The conversion of the igneous liquid surface into solid matter, could only have taken place in successive shells or concentric layers; hence would arise a stratified character. And as the cooling proceeded, lowering the mean temperature of the whole mass, a consequent diminution of bulk must have taken place, according to the well known law of expansion by heat and contraction on cooling. Such diminution in bulk must have broken the strata into fragments, through the fissures of which, according to the laws of hydrostatics, the fluid mass beneath would rise until the equilibrium of rotation would have been obtained, and the strata, originally concentric, would be dislocated and turned in every possible direction, pierced with veins and dikes of all possible magnitude, from slender threads to mountain masses, caused by the cooling and consolidation of the rising fluid, and occasionally spreading in overlying currents, congealed and fixed in ridges and chains. These veins and dykes would present different characters, according to the dates of their elevation. If raised at a period when the surface was still of high temperature, they must have crystallized slowly, and in a perfect manner; at diminished temperatures, the crystallization would be less complete; if raised into the mass of ocean, they would assume one character; if coming in contact with air, another. A breaking of the bed of the ocean, and bringing its waters in contact with the liquid mass beneath, might produce consequences extending in their action to districts of the globe, the most remote from those in which the convulsion occurred; for the water, rising into vapour, would tend to extend itself in one uniform atmosphere over the whole surface of the globe, and might be precipitated in unusual abundance wherever causes of condensation existed. Thus, partial, or even total deluges, may have occurred, great portions of the ocean being hurried in vapour from its bed, and precipitated upon the land whose temperature is not affected by the distant catastrophe.

The waters might, in some cases, flow directly back to the ocean, in others might accumulate in basins and form lakes, fresh at first, and gradually becoming saline. These in turn might burst their bounds, carrying ruin and devastation in their course, or might by evaporation be dried up, and be again filled by a recurrence of the original cause of supply.

Such violent and rapid action would finally be exhausted by the gradual cooling of the earth, but the outer crust would still press on the igneous fluid beneath, and although far less liable to rupture, its fluid action might yet enable it to force its way occasionally to the surface, but at distant intervals, and with diminished energy. Now, a new series of phenomena must occur, similar to the more familiar of those we see acting at present; at first more intense, but finally, when the state of equilibrium of temperature is reached, exactly such as we now find them both in kind and in energy.

To see how far such a view of what might have occurred, under the action of well known causes, in case of a certain original order of things, is correct, let us examine the appearances our globe actually presents.

To a systematized and general examination, it presents the appearance of a great ocean, covering about three-fourths of its whole surface, and surrounding two great, and a number almost infinite of smaller islands. The two great islands are the old and the new continents; the largest of those that remain is New-Holland. To exhibit this great ocean in its most general aspect, take an artificial globe, raise the south pole 50 deg. above the horizon, and bring New-Zealand to the meridian. The hemisphere above the horizon will now be wholly of water, with the exception of the southern part of South America on the one side, and New-Holland, with the Indian archipelago, on the other. These bear, when united, but a small proportion to the entire hemisphere. The opposite hemisphere contains more land than water; and when it is in its turn placed above the horizon, the Atlantic will be seen lying almost wholly on the western side of the meridian, and forming, with the Arctic ocean, a species of channel, narrowing from the latitude of the Cape of Good Hope towards the northern pole, and communicating with the great ocean which lies principally in the opposite hemisphere by Behring's straits. On this hemisphere are also seen parts of the Pacific and Indian oceans, which are considerably more than equal in surface to the lands which project into the opposite one.

If we turn our attention to the land, we find it unequal in its surface; and although compared with the whole diameter of the earth, the inequalities be very small, yet, compared with our own stature, they often present an imposing magnitude. These greater elevations are mountains; and we find them sometimes united in chains, sometimes isolated, and at other times uniting to form elevated plains or table lands. These table lands sometimes slope outwards, at others they are surrounded by eminences that prevent the efflux of the waters, or only admit them to pass through apertures made by their own action. Upon our continent, table lands of the latter description are to be found of great magnitude, entering as parts of the great system of the Cordilleras or Andes; in Europe they are rare, but in Tartary, Persia, and in central Africa, they occur, forming regions of great extent. In general, the greater part of the mountains of a continent appear to have a connexion more or less obvious; it has even been conceived that they form the skeleton upon which the rest of the land has been deposited, and which has determined the form of the continent. Thus we speak habitually of chains of mountains. Mountains, however, do not always present a continuous ridge, from which the peaks or more elevated summits rise, but occasionally, the groups we call chains, are composed of separate mountains divided by valleys; such are the mountains of Scotland, of Sweden, and Norway; and such is the general structure of the chain of mountains called in the state of New-York the Highlands, of whose connexion and grouping we shall hereafter speak.

This being understood, namely, that by a chain or ridge of mountains we do not necessarily intend a continuous elevation, the term may be conveniently used in order to express the configuration of mountains. These chains surround or border upon greater or less basins, which are each distinguished by the name of the principal stream that conveys its surface waters to the ocean, or they may, as has been stated, envelop a table land, whence there is no issue for the waters, or no more than a mere passage sufficient to afford them an outlet. Even if a map contain no expression of the position of mountains, we can, by mere inspection of the courses of rivers, determine the lines in which the chains are directed, and, from the size of the rivers, judge in some measure of the elevation of the district. Thus, on inspection of the map of Europe, we find four of its greatest rivers rising at no great distance from each other, the Rhine, the Rhone, the Danube, and the Po; here, then, we might infer a great elevation, and here we accordingly find its highest mountains, the Alps. In another part of this continent, we see the Dwina, the Nieper, and the Volga, diverge from points not far distant from each other, and here accordingly we find an elevated table land, two hundred miles in length by fifty in breadth, marked however by no mountain summits. In central Asia, we see a vast space inclosed by lines joining the sources of a number of mighty rivers, the Indus, the Ganges, the Barrampooter, the Irrawaddy, the Houng Ha, and Kiang Ku, the Amour, the Lena, the Yermisir, and the Oby; accordingly, here we find the greatest table land surrounded by the highest mountains of the globe. Still, however, the instance we have cited of the rivers of Russia shows, that the land whence great rivers take their rise, is not necessarily mountainous; in this case the ascent is almost imperceptible, and the summit offers the aspect of a level and marshy plain. Such also occurs in the famous boundary between the United States and Canada, where the highlands that figured in two successive treaties have disappeared, and in their supposed place has been found a series of swamps.

Attempts have been made to arrange the chains of mountains into connected systems. Of these the most successful is that of Malte-Brun.

"If we draw a line from the centre of Thibet, across Chinese Mongolia towards Ochotsk, and thence towards Cape Tchutscki, the eastern promontory of Asia, this line will in general coincide with a great chain of mountains which runs from the south-west to the north-east, and which every where descends rapidly towards the Indian and Pacific oceans, while on the contrary, it extends itself towards the Frozen ocean in high plains and secondary hills. It is probable that we may some day refer to the same rule the chain of Lapata, called the backbone of the world, in Africa; at any rate this chain runs from the Cape of Good Hope to that of Gardafui, in a direction south-east and north-west, and therefore in nearly the same direction as the great chain of Asia, but we are ignorant of the disposition of the slopes of these mountains. We may regard the mountains of the Happy Arabia, which are both steep and lofty, as the link that connects the mountains of Lapata with the table lands and mountains of Persia, which proceed from the mountains of Thibet.

"If we follow the western coasts of America, from Behring's straits, which hardly form a sensible interruption, to Cape Horn, we find an uninterrupted chain of mountains. From time to time this chain retires a little into the interior, but more frequently it immediately borders upon the great ocean, in immense cliffs, and often by frightful precipices. On the other side of it, the manner in which the lakes discharge themselves, and the direction of the great rivers, show sufficiently, that the surface of America inclines gently towards the Atlantic ocean.

"It results from a combination of these observations, that the greatest chains of mountains on our globe, are ranged in an arc of a circle around the great ocean, and the sea of India; that they seem to present rapid descents towards the immense basin they surround, and gentle slopes on their opposite sides; in fine, from the Cape of Good Hope to Behring's straits, and thence to Cape Horn, the eye of the most timid observer cannot fail to see some trace of an arrangement, as surprising from its uniformity, as from the vast extent of ground which it embraces.

"Let us pause for an instant to consider this great fact of physical geography. If we conceive ourselves placed in New South Wales, with our face turned towards the north, we have America on our right hand, Africa and Asia on our left. These continents, which we hardly before ventured to approach in our imagination, considered in this point of view, form a consistent system, whose structure, as far as we are acquainted with it, presents in its great features an astonishing symmetry. A chain of enormous mountains surrounds an enormous basin; this basin, divided into two by a vast collection of islands, often bathes with its waves the feet of this great primary chain of the earth."

In this chain lie the greatest mountains of the globe. One peak of the Himmalayah rises nearly five miles above the level of the sea; another has a height of 25,500 feet; and a third of 22,217 feet. In South America are Soratu, in height 25,250 feet.

Illimani, 24,000 Chimborazo, 21,400

not to mention Antisana, Mauflos, Chillau, Cotopaxi, all of which exceed in height any mountains that do not lie in this great system. Nay, did not the great Volcano of Owyhee enter into the order with a height of 18,000 feet, the list of those surpassing the other mountains of the globe, might be very much extended.

We shall have occasion hereafter to speak of the volcanic energies still exerted in this vast stony girdle, and shall therefore confine ourselves strictly to mere external form.

The arms and branches of mountain chains enclose as has been seen, basins marked by rivers which convey their surface waters to the ocean. The rains which fall on the sides of mountains and hills, unite in torrents and streams, which follow the lines of most rapid slope in their course to the sea.

The greater rivers mark the lowest part of a principal basin, on each side of which, at a greater or less distance, are to be found rising grounds, themselves hollowed out into lateral secondary basins, containing courses of water less considerable than the first, into which they cast themselves, and whose branches they are. The borders of these secondary basins are again hollowed out into basins of a third order, whose slopes also contain water courses less considerable than the preceding, into which they in turn discharge themselves. This ramification continues until we reach the smallest ravines of the boundary mountains, and the map appears, as it were, covered with a net work of rivers and lesser streams. The great valley of the Mississippi and Missouri, forms perhaps the most striking instance of this sort, upon the surface of our globe.

Rivers and streams are constantly exerting a mechanical action on the surfaces over which they run; abrading and tearing off fragments even of the hardest rocks, they roll them in their course until the velocity becomes insufficient to transport them farther. At diminished velocities they move fragments of less size, down to the smallest pebbles; at still less velocities, they transport sand, and finally earthy matter, in the most minute division. These are deposited in succession in positions corresponding to the rapidity of the stream, and hence the beds of rivers present at each of their different sections, materials of magnitude and quality corresponding to the rate at which the stream usually flows. The increase in the magnitude of streams, due to violent rains and the melting of the snows, changes the position of the substances that compose their bed, and the more easily suspended materials are often held until the stream actually meets the ocean. In such sudden increases, the streams often overflow their usual banks, and make their deposits laterally, until the constant succession of such deposits raises the adjacent ground high enough to set bounds to the further spreading of the stream. This deposit is remarkable for its taking place in greatest quantity close to the usual bed of the stream; and thus it speedily opposes natural dykes to its own redundant waters. This action is most conspicuous at points where marked changes take place either permanently or periodically in the rapidity of running water: when streams descend from mountains into lines of less descent, a deposit uniformly takes place, forming _flats_ or _intervals_, as they are styled in the United States, of which we have such beautiful instances in the valleys of the Connecticut and Mohawk, and that part of the Hudson near Albany; again, where rivers meet the sea, they are interrupted in their course by the rise of the tides of the ocean, and here again deposits take place, sometimes forming shoals and banks in the ocean itself; at other times, bars and obstructions at their own mouths; and again, deltas of solid land, constantly encroaching upon the sea. This action, which is continually going forward, is called alluvial. The delta of greatest fame, and from which the others have derived their generic name, is that of the Nile; this we have evidence, almost historic, to prove to be wholly the gift of the river. And if it no longer increase as rapidly as in former ages, the cause is obvious, for the alluvion has been pushed so far forward as to meet a strong current that sweeps along the African coast, and must carry off much of the earth the Nile discharges into the Mediterranean. The great rivers of Asia and of America carry still greater quantities of solid matter, but we have not the same distant traditions to refer to for the amount of the increase they have caused; still, however, we know that the mouth of the Mississippi has been advanced into the Gulf of Mexico several leagues since the settlement of Louisiana; and that islands of great extent are frequently formed, in the course of a single year, by the deposits of the Ganges.