The first impression which is produced upon the mind, when the phenomena of volcanic action are studied, is that here we have exhibitions of destructive violence the effects of which must be entirely mischievous and disastrous to the living beings occupying the earth's surface. A little consideration will convince us, however, that the grand and terrible character of the displays of volcanic energy have given rise to exaggerated notions concerning their destructive effects. The fact that districts situated over the most powerful volcanic foci, like Java and Japan, are luxuriant in their productions, and thickly inhabited, may well lead us to pause ere we condemn volcanic action as productive only of mischief to the living beings on the earth's surface. The actual slopes of Vesuvius and Etna, and many other active volcanoes, are abundantly clothed with vineyards and forests and are thickly studded with populous villages.

As a matter of fact, the actual amount of damage to life and property which is effected by volcanic eruptions is small. Usually, the inhabitants of the district have sufficient warning to enable them to escape with their lives and to carry away their most valuable possessions. And though fertile tracts are covered by loose dust and ashes, or by lava- and mud-currents, yet the sterility thus produced is generally of short duration, for by their decomposition volcanic materials give rise to the formation of the richest and most productive soils.

Earthquakes, as we have already seen, are far more destructive in their effects than are volcanoes. Houses and villages, nay even entire cities, are, by vibrations of portions of the earth's crust, reduced to heaps of ruins, and famines and pestilences too frequently follow, as the consequence of the disorganisation of our social systems by these terrible catastrophes.

It may well be doubted, however, whether the annual average of destruction to life and property caused by all kinds of subterranean action, exceeds that produced either by floods or by hurricanes. Yet we know that the circulation of water and air over our globe are beneficial and necessary operations, and that the mischief occasionally wrought by the moving bodies of water and air is quite insignificant compared with the good which they effect.

In the same way, we shall be able to show that the subterranean energies are necessary to the continued existence of our globe as a place fitted for the habitation of living beings, and that the mischievous and destructive effects of these energies bear but a small and insignificant proportion to the beneficial results with which they must be credited.

We have had frequent occasion in the preceding pages to refer to the work—slow but sure, silent but effective—wrought by the action of the denuding forces ever operating upon the surface of our globe. The waters condensing from the atmosphere and falling upon the land in the form of rain, snow, or hail, are charged with small quantities of dissolved gases, and these waters penetrating among the rock-masses of which the earth's crust is composed, give rise to various chemical actions of which we have already noticed such remarkable illustrations in studying the ancient volcanic products of our globe. By this action the hardest and most solid rock-masses are reduced to a state of complete disintegration, certain of their ingredients undergoing decomposition, and the cementing materials which hold their particles together being removed in a state of solution. In the higher regions of the atmosphere this work of rock-disintegration proceeds with the greatest rapidity; for there the chemical action is reinforced by the powerful mechanical action of freezing water. On high mountain-peaks the work of breaking up rock-masses goes on at the most rapid rate, and every craggy pinnacle is swathed by the heaps of fragments which have fallen from it. The Alpine traveller justly dreads the continual fusillade of falling rock-fragments which is kept up by the ever-active power of the frost in these higher regions of the atmosphere; and fears lest the vibrations of his footsteps should loosen, from their position of precarious rest, the rapidly accumulating piles of detritus. No mountain-peak attains to any very great elevation above the earth's surface, for the higher we rise in the atmosphere the greater is the range of temperature and the more destructive are the effects of the atmospheric water. The moon, which is a much smaller planet than our earth, has mountains of far greater elevation; but the moon possesses neither an atmosphere nor moisture on its surface, to produce those levelling effects which we see everywhere going on around us upon the earth.

The disintegrated materials, produced by chemical and mechanical actions of the atmospheric waters upon rock-masses, are by floods, rivers, and glaciers, gradually transported from higher to lower levels; and sooner or later every fragment, when it has once been separated from a mountain-top, must reach the ocean, where these materials are accumulated and arranged to form new rocks.

Over every part of the earth's surface these three grand operations of the disintegration of old rock-masses, the transport of the materials so produced to lower levels, and the accumulation of these materials to form new rocks, is continually going on. It is by the varied action of these denuding agents upon rocks of unequal hardness, occupying different positions in relation to one another, that all the external features of hills, and plains, and mountains owe their origin.

It is a fact, which is capable of mathematical demonstration, that by the action of these denuding forces the surface of all the lands of the globe is being gradually but surely lowered; and this takes place at such a rate that in a few millions of years the whole of the existing continents must be washed away and their materials distributed over the beds of the oceans.

It is evident that there exists some agency by which this levelling action of the denuding forces of the globe is compensated; and a little consideration will show that such compensating agency is found in the subterranean forces ever at work within the earth's crust. The effects of these subterranean forces which most powerfully arrest our attention are volcanic outbursts and earthquake shocks, but a careful study of the subject proves that these are by no means the most important of the results of the action of such forces. Exact observation has proved that almost every part of the earth's surface is either rising or falling, and the striking and destructive phenomena of volcanoes and earthquakes probably bear only the same relation to those grand and useful actions of the subterranean forces, which floods do to the system of circulating waters, and hurricanes to the system of moving air-currents.

If we ride in a well-appointed carriage with good springs, upon a railway which is in excellent order, the movement is almost imperceptible to us; and the rate of speed may be increased indefinitely, without making itself apparent to our senses. The smallest impediment to the evenness of the movement—such as that produced by a small object placed upon the rails—at once makes itself felt by a violent jar and vibration. How perfectly insensible we may be of the grandest and most rapid movements is taught us by the facts demonstrated by the astronomer. By the earth's daily rotation, we are borne along at a rate which in some places amounts to over 1,000 miles an hour; and by its annual revolution we are every hour transported through a distance of 70,000 miles; yet concerning the fact and direction of these movements we are wholly unconscious.

In the case both of the railway train and of our planet, we can only establish the reality of the movement, and its direction and rate, by means of observations upon external objects, which appear to us to have a movement in the opposite direction. In the same way we can only establish the fact of the movement of portions of the earth's crust by noticing the changing positions of parts of the earth's surface in relation to the constant level of the ocean. When this is done we find abundant proof that while some parts of the earth's crust are rising, others are as undoubtedly undergoing depression.

We shall be able to form some idea of the vastness of the effects produced by the subterranean forces, by a very simple consideration. It is certain that during the enormous periods of time of which the records have been discovered by the geologist, there have always been continents and oceans upon the earth's surface, just as at present, and it is almost equally certain that the proportions of the earth's surface occupied by land and water respectively, have not varied very widely from those which now prevail. But, at the same time, it is an equally well-established bet that the denuding forces ever at work upon the earth's surface would have been competent to the removal of existing continents many times over, in the vast periods covered by geological records. Hence we are driven to conclude that the subterranean movements have in past times entirely compensated for the waste produced by the denuding forces ever at work upon our globe. But this is not all. The subterranean forces not only produce upheaval; in a great many cases the evidences of subsidence are as clear and conclusive as are those of upheaval in others. Hence we are driven to conclude that the forces producing upheaval of portions of the earth's crust are sufficient, not only to balance those producing subsidence, but also to compensate for the destructive action of denuding agents upon the land-masses of the globe.

It is only by a careful and attentive study and calculation of the effects produced by the denuding agents at work all around us, aided by an examination of the enormous thicknesses of strata formed by the action of such causes during past geological times, that we are able to form any idea of the reality and vastness of the agents of change which are ever operating to modify the earth's external features. When we have clearly realised the grand effects produced on the surface of the globe by these external forces, through the action of its investing atmosphere and circulating waters, then, and only then, shall we be in a position to estimate the far greater effects resulting from the internal forces, of which the most striking, but not the most important, results are seen in the production of volcanic eruptions and earthquake-shocks.

Another series of facts which serve to convince the geologist of the reality and potency of the forces ever at work within the earth's crust, and the way in which these have operated during past geological periods, is found in the disturbed condition of many of the stratified rock-masses of which it is composed. Such stratified rock-masses, it is clear, must have been originally deposited in a position of approximate horizontality; but they are now often found in inclined and even vertical positions; they are seen to be bent, crumpled, puckered, and folded in the most remarkable manner, and have not unfrequently been broken across by dislocations—'faults'—which have sometimes displaced masses, originally in contact, to the extent of thousands of feet. The slate-rocks of the globe, moreover, bear witness to the fact that strata have been subjected to the action of lateral compression of enormous violence and vast duration; while in the metamorphic rocks we see the effects of still more extreme mechanical strains, which have been in part transformed into chemical action. No one who has not studied the crushed, crumpled, fractured, and altered condition of many of the sedimentary rocks of the globe, can form the faintest idea of the enormous effects of the internal forces which have been in operation within the earth's crust during earlier geological periods. And it is only by such studies as these that we at last learn to regard the earthquake and volcanic phenomena of our globe, not as the grandest and most important effects of these forces, but as their secondary and accidental accompaniments. 'Volcanoes,' it has been said, 'are the safety-valves of the globe;' and when we come to realise the real extent and nature of the internal forces ceaselessly working in the earth's crust we shall scarcely be disposed to regard the simile as an overstrained one.

The first geologist who attempted to show the exact relations existing between those subterranean forces which cause the movements of continental masses of land, and those more startling displays of energy which are witnessed in volcanic outbursts, was the late Mr. Poulett Scrope. At a somewhat later date Mr. Darwin, in his remarkable paper 'On the Connexion of certain Volcanic Phenomena in South America, and on the Formation of Mountain-chains and Volcanoes as the effect of Continental Elevations,' threw much new and important light upon the question.

While, on the one hand, we are led by recent geological investigations to reject the notions which were formerly accepted, by which mountain-ranges were supposed to be suddenly and violently upheaved by volcanic forces, we are, on the other hand, driven to conclude that without the action of these subterranean forces, the irregularities which are exhibited on the earth's surface could not have had any existence.

It is true that the actual forms of the mountain-ranges are due directly to the action of denuding forces, which have sculptured out from the rude rocky masses all the varied outlines of peaks and crags, of ravines and valleys. But it is none the less true that the determining causes which have directed and controlled all this earth-sculpture, are found in the relative positions of hard and soft masses of rock; but these rock-masses have acquired their hardness and consistency, and have assumed their present positions, in obedience to the action of subterranean forces. Hence we see that though the formation of mountain-ranges is proximately due to the denuding forces, which have sculptured the earth's surface, the primary cause for the existence of such mountain-chains must be sought for in the fact that subterranean forces have been at work, folding, crumpling, and hardening the soft sediments, and placing them in such positions that, by the action of denudation, the more indurated portions are left standing as mountain-masses above the general surface.

The old notion that mountain-chains are due to a vertical upthrust from below, finds but little support when we come to study with due care the positions of the rock-masses composing the earth's crust. On the contrary, we find that mountain-ranges are usually carved out of the crushed and crumpled edges of strata which have along certain lines been influenced by great mechanical strains, and subjected to more or less induration and chemical alteration. When we compare these folded and contorted portions of the strata with those parts of the same beds which are not so affected, we find the effects produced in the former are not such as would result from an upthrust from below, but from movements by which a tangential strain would be brought about. If we imagine certain lines of weakness to exist in the solid crust of the earth, then any movements in the portions of the crust between these lines of weakness would cause crushing and crumpling of the strata along the latter.

Recent investigations of Dana and other authors have thrown much new light upon the question of the mode of formation of mountain-chains, and the relation between the movements by which they are produced and the sudden and violent manifestations of force witnessed in volcanic outbursts. We cannot, perhaps, better illustrate this subject than by giving a sketch of the series of operations to which the great Alpine chains owe their origin.

There are good grounds for believing that the great mountain-axis of Southern Europe, with its continuation in Asia, had no existence during the earlier geological periods. Indeed, it has been proved that all the higher among the existing mountain-chains of the globe have been almost entirely formed in Tertiary times. The reason of this remarkable fact is not far to seek. So rapid is the work of denudation in the higher regions of the atmosphere, that the elevated crags and pinnacles are being broken up by the action of moisture and frost at an exceedingly rapid rate. This fact is attested by the existence of those enormous masses of angular rock-fragments which are found lodged on every vantage-ground among the mountain-summits, as well as by the continually descending materials which are borne by glaciers and mountain-torrents to the valleys below. Where such a rate of disintegration as this is maintained, no elevated mountain-crests could exist through long geological periods. It is true we find in all parts of the globe relics of many mountain-chains which were formed before the Tertiary period; but these have by long-continued denudation been worn down to 'mere stumps.' Of such worn-down and degraded mountain-ranges we have examples in the Scandinavian chains, and some of the low mountain-regions of Central Europe and North America.

Let us now proceed to illustrate this subject by briefly sketching the history of that series of operations by which the great mountain-chains of the Alpine system have been formed.

The first stage of that grand series of operations appears from recent geological researches to have consisted in the opening of a number of fissures running along a line near to that at which, in a long subsequent period, the elevation of the mountain-masses took place. This betrayal of the existence of a line of weakness in this part of the earth's crust occurred in the Permian period, and from that time onward a series of wonderful movements and changes have been going forward, which have resulted in the production of the Alpine chains as we now see them.

From the great fissures opened in Permian times along this line of weakness, great quantities of lava, scoriæ, and tuff were poured out, and these accumulated to form great volcanic mountains, which we can now only study at a few isolated spots, as in the Tyrol, Carinthia, and about Lake Lugano. Everywhere else, these Permian volcanic rocks appear to be deeply buried under the later-formed sediments, from which the Alpine chains have been carved. Few and imperfect, however, as are the exposures of these ancient rhyolite and quartz-andesite lavas and agglomerates formed at the close of the Palæozoic epoch, their greatly denuded relics form masses which are in places more than 9,000 feet in thickness. From this fact we are able to form some slight idea of the scale upon which the volcanic outbursts in question must have taken place during Permian times.

The second stage in the series of operations by which the Alpine chains have been formed, consisted in a general sinking of the surface along that line of weakness in the earth's crust, the existence of which had been betrayed by the formation of fissures and the eruption of volcanic rocks. We have already had occasion to remark how frequently such subsidences follow upon the extrusion of volcanic masses at any part of the earth's surface; and we have referred these downward movements in part to the removal of support from below the portion of the crust affected, and in part to the weight of the materials piled upon its surface by the volcanic forces.

The volcanic energy which had been manifested with such violence during the Permian period, does not appear to have died out altogether during the succeeding Triassic period. A number of smaller volcanic vents were opened from time to time, and from these, lavas, tuffs, and agglomerates, chiefly of basic composition, were poured out. The relics of these old Triassic volcanoes are found at many points along the Alpine chain, but it is evident that the igneous forces were gradually becoming exhausted during this period, and before the close of it they had fallen into a state of complete extinction.

But the great subsidence which had commenced in the Triassic period, along what was to become the future line of the Alpine chain, was continued almost without interruption during the Rhætic, the Jurassic, the Tithonian, the Neocomian, the Cretaceous and the Nummulitic periods. With respect to the strata formed during all these periods, it is found that their thiknesses, which away from the Alpine axis may be measured by hundreds of feet, is along that axis increased to thousands of feet. The united thickness of sediments accumulated along this great line of subsidence between the Permian and Nummulitic periods probably exceeds 60,000 feet, or ten miles. The subsidence appears to have been very slow and gradual, but almost uninterrupted, and the deposition of sediments seems to have kept pace with the sinking of the sea-bottom, a fact which is proved by the circumstance that nearly the whole of these sediments were such as must have been accumulated in comparatively shallow water.

By the means we have described there was thus formed a 'geosynclinal,' as geologists have called it, that is, a trough-like hollow filled with masses of abnormally thickened sediments, which had been piled one upon another during the long periods of time in which almost uninterrupted subsidence was going on along the Alpine line of weakness in the earth's crust. In this way was brought together that enormous accumulation of materials from which the hard masses of the Alpine chains were subsequently elaborated, and out of which the mountain-peaks were eventually carved by denudation.

The third stage in this grand work of mountain-making commenced in the Oligocene period. It consisted of a series of movements affecting the parts of the earth's crust on either side of the line of weakness which had first exhibited itself in Permian times. By these movements a series of tangential strains were produced, which resulted in the violent crushing, folding, and crumpling of the sedimentary materials composing the geosynclinal.

One effect of this action was the violent flexure and frequent fracture of these stratified masses, which are now found in the Alpine regions assuming the most abnormal and unexpected positions and relations to one another. Sometimes the strata are found tortured and twisted into the most complicated folds and puckerings; at others they are seen to be completely inverted, so that the older beds are found lying upon the newer; and in others, again, great masses of strata have been traversed by numerous fractures or faults, the rocks on either side of which are displaced to the extent of thousands of feet.

Another effect of the great lateral thrusts by which the thick sedimentary masses of the geosynclinal were being so violently disturbed, was the production of a great amount of induration and chemical change in these rocks. Masses of soft clay, of the age of that upon which London is built, were by violent pressure reduced to the condition of roofing-slate, similar to that of North Wales. One of the most important discoveries of modern times is that which has resulted in the recognition of the fact of the mutual convertibility of different kinds of energy. We now know that mechanical force may be transformed into heat-force or chemical force; and of such transformations we find abundant illustrations in the crushed and crumpled rock-masses of the Alpine chains.

Under the influence of these several kinds of force, not only was extreme consolidation and induration produced among the rock-masses, but chemical affinity and crystalline action had the fullest play among the materials of which they were composed. In many cases we find the originally soft muds, sands, and shell-banks converted into the most highly crystalline rocks, which retain their primary chemical composition, but have entirely lost all their other original features.

To the mass of folded, crumpled, and altered strata, formed from a geosynclinal by lateral pressure, geologists have given the name of a 'geanticlinal.' The formation of the Alpine geanticlinal was due to movements which commenced in the Oligocene period, attained their maximum in the Miocene, and appear to have declined and almost altogether died out in the Pliocene period.

The movements which resulted in the crushing and crumpling of the thickened mass of sediments along the Alpine line of weakness, also gave rise to the formation of a series of fissures from which volcanic action took place. These fissures were not, however, formed along the original line of weakness, for this had been strengthened and repaired by the deposition of ten-miles' thickness of sediments upon it, but along new fissures opened in directions parallel to the original lines of weakness, and in areas where a much less considerable amount of deposition had taken place since Permian times.

We have abundant evidence that, just at the period when those great movements were commencing which resulted in the formation of the great Alpine and Himalayan geanticlinal, earth-fissures were being opened upon either side of the latter from which volcanic outbursts took place. At the period when the most violent mountain-forming movements occurred, these fissures were in their most active condition, and at this time two great volcanic belts stretched east and west, on either side of, and parallel to, the great Alpine chain. The Northern volcanic band was formed by the numerous vents, now all extinct, in Auvergne, Central Germany, Bohemia, and Hungary, and was probably continued in the volcanoes of the Thian Shan and Mantchouria. The Southern volcanic band was formed by the numerous vents of the Iberian and Italian peninsulas, and the islands of the Mediterranean, and were continued to the eastward by those of Asia Minor, Arabia, and the North Indian Ocean. As the earth-movements which produced the geanticlinal died away, the volcanic energy along these parallel volcanic bands died away at the same time. In studying the geology of Central and Southern Europe, no fact comes out more strikingly than that of the synchronism between the earth-movements by which the geanticlinal of the Alps was formed, and the volcanic manifestations which were exhibited along lines of fissure parallel to that geanticlinal. The earth-movements and the volcanic outbursts both commenced in the Oligocene period, gradually attained their maximum in the Miocene, and as slowly declined in the Pliocene.

The fourth stage in the great work of mountain-building in the case of the Alps consisted in the operation of the denuding forces, the disintegrating action of rain and frost, the transporting action of rivers and glaciers, by which the Alpine peaks were gradually sculptured out of the indurated and altered masses constituting the geanticlinal. The action of this fourth stage went on to a great extent side by side with that of the third stage. So soon as the earth-movements had brought the submerged sedimentary masses of the geosynclinal under the action of the surface tides and currents of the ocean, marine denudation would commence; and, as the work of elevation went on, the rock-masses would gradually be brought within the reach of those more silently-working but far more effective agents which are ever operating in the higher regions of the atmosphere. It is impossible to say what would have been the height of the Alpine chain if the work of denudation had not to a great extent kept pace with that of elevation. Only the harder and more crystalline masses have for the most part escaped destruction, and stand up in high craggy summits; while flanking hills, like the well-known Rigi, are Been to be composed of conglomerates thousands of feet in thickness, composed of their disintegrated materials. It is a remarkable fact, as showing how enormous was the work of elevation daring the formation of the geanticlinal, that some of the youngest and least consolidated rocks of the Nummulitic period are still found at a height of 11,000 feet in the Alps, and of 16,000 feet in the Himalaya.

From what has been said, it will be seen that mountain-chains may be regarded as cicatrised wounds in the earth's solid crust. A line of weakness first betrays itself at a certain part of the earth's surface by fissures, from which volcanic outbursts take place; and thus the position of the future mountain-chain is determined. Next, subsidence during many millions of years permits of the accumulation of the raw materials out of which the mountain-range is to be formed; subsequent earth-movements cause these raw materials to be elaborated into the hardest and most crystalline rock-masses, and place them in elevated and favourable positions; and lastly, denudation sculptures from these hardened rock-masses all the varied mountain forms. Thus the work of mountain-making is not, as was formerly supposed by geologists, the result of a simple upheaving force, but is the outcome of a long and complicated series of operations.

The careful study of other mountain-chains, especially those of the American continent, has shown that the series of actions which we have described as occurring in the Alps, took place in the same order in the formation of all mountain-masses. It is doubtful whether the line of weakness is always betrayed in the first instance by the formation along its course of volcanic fissures. But in all cases we have evidence of the production of a geosynclinal, which is afterwards, by lateral pressure, converted into a geanticlinal, and from this the mountain-chains have been carved by denudation. Professor Dana has shown that the geosynclinal of the Appalachian chain was made up of sediments attaining a thickness of 40,000 feet, or eight miles; while Mr. Clarence King has shown that a part of the geosynclinal of the Rocky Mountains was built up of no less than 60,000 feet, or twelve miles of strata.

It has thus been established that a very remarkable relation exists between the forces by which continental masses of land are raised and depressed, and mountain-ranges have been developed along lines of weakness separating such moving continental masses, and those more sudden and striking manifestations of energy which give rise to volcanic phenomena. It is in this relation between the widespread subterranean energies and the local development of the same forces at volcanic vents, that we must in all probability seek for the explanation of those interesting peculiarities of the distribution of volcanoes upon the face of the globe which we have described in a former chapter. The parallelism of volcanic bands to great mountain-chains is thus easily accounted for; and in the same way we may probably explain the position of most volcanoes with regard to coast-lines. We have already pointed out the objections to the commonly-received view that volcanoes depend for their supplies of water on the proximity of the ocean. This proximity of the ocean to volcanic vents we are thus inclined to regard, not as the cause, but as the effect of the subterranean action. The positions of both volcanoes and coast-lines are determined by the limits of those great areas of the earth's crust which are subjected to slow vertical movements, often in opposite directions.

Terrible and striking, then, as are the phenomena connected with volcanic action, such sudden and violent manifestations of the subterranean energy must not be regarded as the only, or indeed the chief, effects which they produce. The internal forces continually at work within the earth's crust perform a series of most important functions in connection with the economy of the globe, and were the action of these forces to die out, our planet would soon cease to be fit for the habitation of living beings.

There is no fact which the geological student is more constantly called upon to bear in mind than that of the potency of seemingly insignificant causes which continue in constant operation through long periods of time. Indeed these small and almost unnoticed agencies at work upon the earth's crust are often found, in the long ran, to produce far grander effects than those of which the action is much more striking and obvious. It is to the silent and imperceptible action of atmospheric moisture and frost that the disintegration of the solid rock-masses must be mainly ascribed; and the noisy cataract and ocean-billow produce effects which are quite insignificant compared with those which must be ascribed to the slight and almost unnoticed forces. Great masses of limestone are built up of the remains of microscopic organisms, while the larger and higher life-forms contribute but little to the great work of rock-building.

In the same way it is to the almost unnoticed action of the subterranean forces in raising some vast areas of the earth's crust, in depressing others, and in bringing about the development of mountain-chains between them, that we must ascribe a far more important part in the economy of our globe than to the more conspicuous but less constant action of volcanoes.

A few simple considerations will serve to convince us, not only of the beneficial effects of the action of the subterranean energies within the earth's crust, but of the absolute necessity of the continued operation of those energies to the perpetuation of that set of conditions by which our planet is fitted to be the habitation of living beings.

We have already referred to the prodigious effects which are constantly being produced around us by the action of the external forces at work upon the globe. The source of these external forces is found in the movements and changes which are ever going on within the aqueous and atmospheric media in which the globe is enveloped. The circulation of the air, influencing the circulation of the waters in the shape of clouds, rain, snow, rivers, glaciers, and oceans, causes the breaking up of even the hardest rock-masses, and the continual removal of their disintegrated fragments from higher to lower levels. This work goes on with more or less regularity over every part of the land raised above the level of the ocean, but the rate of destruction in the higher regions of the atmosphere is far more rapid than at lower levels. Hence the circulating air and water of the globe are found to be continually acting as levellers of the land-masses of the earth.

It is by no means a difficult task to calculate the approximate rate at which the various continents and islands are being levelled down, and such calculations prove that in a very few millions of years the existing forces operating upon the earth's surface would reduce the whole of the land-masses to the level of the ocean.

But a little consideration will convince us that the circulation of the air and waters of the globe are themselves dependent upon the existence of those irregularities of the land-surfaces which they are constantly tending to destroy. Without elevated mountain ridges the regular condensation of moisture, and its collection and distribution in streams and rivers over every part of the land surfaces, could not take place. Under these circumstances the unchecked evaporation of the oceanic waters would probably go on, till the proportion of water-vapour increased to such an extent in the atmosphere as effectually to destroy those nicely-balanced conditions upon which the continued existence of both vegetable and animal life depend.

But the repeated upward and downward movements which have been shown to be going on in the great land-masses of the globe, giving rise in turns to those lateral thrusts and tangential strains to which mountain-chains owe their formation, afford a perfect compensation to the action of the external forces ever operating upon the earth's surface.

If, however, the uncompensated effect of the external forces acting on the earth's crust is calculated to bring about the destruction of those conditions upon which the existence of life depends, the uncompensated effect of the internal forces acting on the earth's crust are fraught with at least equal dangers to those necessary conditions.

In our nearest neighbour among the planets—the moon—the telescope has revealed to us the existence of a globe, in which the internal forces have not been checked and controlled by the operation of any external agencies—for the moon appears to be destitute of both atmosphere and water.

Under these circumstances we find its surface, as we might expect, to be composed of rocks which appear to be entirely of igneous origin; the mountain-masses, unworn by rain or frost, river or glacier, being of most prodigious dimensions as compared with those of our own globe, while no features at all resembling valleys, or plains, or alluvial flats are anywhere to be discerned upon the lunar surface.

But by the admirable balancing of the external and internal forces on our own globe, the conditions necessary to animal and vegetable existence are almost constantly maintained, and those interruptions of such conditions, produced by hurricanes and floods, by volcanic outbursts and earthquakes, may safely be regarded as the insignificant accidents of what is, on the whole, a very perfectly working piece of machinery.

The ancients loved to liken the earth to a living being—the macrocosm of which man was the puny representative or microcosm; and when we study the well-adapted interplay of the forces at work upon the earth's crust, both from within and without, the analogy seems a scarcely strained one. In the macrocosm and the microcosm alike, slight interferences with the regular functions occasionally take place, and both of them exhibit the traces of a past evolution and the germs of an eventual decay.