It is not by any means an easy task to frame an estimate of the number of volcanoes in the world. Volcanoes, as we have seen, vary greatly in their dimensions—from vast mountain masses, rising to a height of nearly 25,000 feet above the sea-level, to mere molehills; the smaller ones being in many cases subsidiary to larger, and constituting either parasitic cones on their flanks, or 'puys' around their bases. Volcanoes likewise exhibit every possible stage of development and decay: while some are in a state of chronic active eruption, others are reduced to the condition of solfataras, and others again have fallen into a more or less complete state of ruin through the action of denuding forces.
Even if we confine our attention to the larger volcanoes, which merit the name of 'mountains,' and such of these as we have reason to believe to be in a still active condition, our difficulties will be diminished, but not by any means removed. Volcanoes, as we have seen, may sink into a dormant condition that may endure for hundreds or even thousands of years, and then burst forth into a state of renewed activity; and it is quite impossible, in many cases, to distinguish between the conditions of dormancy and extinction. Concerning certain small areas in Southern Europe, Western Asia, and Northern Africa, historical records, more or less reliable, extend back over periods of several thousands of years; but with regard to the greater part of the rest of the world we have no information beyond a few hundred years, and there are considerable areas which have been known only for far shorter periods, while some are as yet quite unexplored. In districts almost wholly uninhabited, or roamed over by nomadic tribes, legend and tradition constitute our only guides—and very unsafe ones they are—in the attempt to determine what volcanoes have recently been in a condition of activity.
We shall, however, probably be within the limits of truth in stating that the number of great habitual volcanic vents upon the globe, which we have reason to believe are still in an active condition, is somewhere between 300 and 350. Most of these active volcanic vents are marked by more or less considerable mountains, composed of the materials ejected from them. If we include the mountains which exhibit the external conical form, the crateral hollows, and other features of volcanoes, but concerning the activity of which we have no record or tradition, the number will fall little, if anything, short of 1,000. The mountains composed of volcanic materials, but which have lost through denudation the external form of volcanoes, are still more numerous. The smaller temporary openings which are usually subordinate to the habitual vents, that have been active during the periods covered by history and tradition, must be numbered by thousands and tens of thousands. The still feebler manifestations of the volcanic forces—such as are exhibited in 'stufas,' or steam-jets, geysers, or intermittent hot springs, thermal and mineral waters, fumaroles, emitting various gases, salses or spouting saline and muddy springs, and mud volcanoes—may be reckoned by millions. It is not improbable that these less powerful manifestations of the volcanic forces, to a great extent make up in number what they want in individual energy; and the relief which they afford to the imprisoned activities within the earth's crust may be scarcely less than that which results from the occasional outbursts at the 300 or 350 great habitual volcanic vents.
In taking a general survey of the volcanic phenomena of the globe, no fact comes out more strikingly than that of the very unequal distribution, in different districts, both of the great habitual volcanic vents, and of the minor exhibitions of subterranean energy.
Thus, on the whole of the continent of Europe, there is but one habitual volcanic vent—that of Vesuvius—and this is situated upon the shores of the Mediterranean. In the islands of the Mediterranean, however, there are no less than six volcanoes; namely, Stromboli and Vulcano, in the Lipari Islands; Etna, in Sicily; Graham's Isle, a submarine volcano, off the Sicilian coast; and Santorin and Nisyros, in the Ægean Sea.
The African continent is at present known to contain about ten active volcanoes—four on the west coast, and six on the east coast; about ten other active volcanoes occur on islands close to the African coasts. In Asia, twenty-four active volcanoes are known, but no less than twelve of these are situated in the peninsula of Kamtschatka. No volcanoes are known to exist in the Australian continent.
The American continent contains a greater number of volcanoes than the divisions of the Old World. There are twenty in North America, twenty-five in Central America, and thirty-seven in South America.
Thus, taken altogether, there are about one hundred and seventeen volcanoes situated on the great continental lands of the globe, while nearly twice as many occur upon the islands scattered over the various oceans.
Upon examining further into the distribution of the continental volcanoes, another very interesting fact presents itself. The volcanoes are in almost every case situated either close to the coasts of the continent, or at no great distance from them. There are, indeed, only two exceptions to this rule. In the great and almost wholly unexplored table-land lying between Siberia and Tibet four volcanoes are said to exist, and in the Chinese province of Mantchouria several others. More reliable information is, however, needed concerning these volcanoes, situated, unlike all others, at a great distance from the sea.
It is a remarkable circumstance that all the oceanic islands which are not coral-reefs are composed of volcanic rocks; and many of these oceanic islands, as well as others lying near the shores of the continents, contain active volcanoes.
Through the midst of the Atlantic Ocean runs a ridge, which, by the soundings of the various exploring vessels sent out in recent years, has been shown to divide the ocean longitudinally into two basins. Upon this great ridge, and the spurs proceeding from it, rise numerous mountainous masses, which constitute the well-known Atlantic islands and groups of islands. All of these are of volcanic origin, and among them are numerous active volcanoes. The Island of Jan Mayen contains an active volcano, while Iceland contains thirteen, and not improbably more; the Azores have six active volcanoes, the Canaries three; while about eight volcanoes lie off the west coast of Africa. In the West Indies there are six active volcanoes; and three submarine volcanoes have been recorded within the limits of the Atlantic Ocean. Altogether, no less than forty active volcanoes are situated upon the great submarine ridges which traverse the Atlantic longitudinally.
But along the same line the number of extinct volcanoes is far greater, and there are not wanting proofs that the volcanoes which are still active are approaching the condition of extinction. At a somewhat earlier period of the earth's history the whole line of the present Atlantic Ocean was in all probability traversed by a chain of volcanoes on the very grandest scale; but submergence has taken place, and only a few portions of this great mountain range now rise above the sea-level, forming the isolated islands and island-groups of the Atlantic. Here and there among these a still active volcano exists.
But if the great medial chain of the Atlantic presents us with an example of a chain of volcanic mountains verging on extinction, we have in the line of islands separating the Pacific and Indian Oceans an example of a similar range of volcanic vents which are in a condition of the greatest activity. In the peninsula of Kamtschatka there are twelve active volcanoes, in the Aleutian Islands thirty-one, and in the peninsula of Alaska three. The chain of the Kuriles contains at least ten active volcanoes; the Japanese Islands and the islands lying to the south of Japan twenty-five. The great group of islands lying to the south-east of the Asiatic continent is at the present time the grandest focus of volcanic activity upon the globe. No less than fifty active volcanoes occur here. Farther south, the same chain is probably continued by the four active volcanoes of New Guinea, one or more submarine volcanoes, and several vents in New Britain, the Solomon Isles, and the New Hebrides, the three active volcanoes of New Zealand, and possibly by Mount Erebus and Mount Terror in the Antarctic region. Altogether, no less than 150 active volcanoes exist in the chain of islands which stretch from Behring's Straits down to the Antarctic circle; and if we include the volcanoes on Indian and Pacific islands which appear to be situated on lines branching from this particular band, we shall not be wrong in the assertion that this great system of volcanic mountains includes at least one half of the habitually active vents of the globe.
A third series of volcanoes starts from near the last in the neighbourhood of Behring's Straits, and stretches along the whole western coast of the American continent. In this great range there are about eighty active volcanoes.
In considering the facts connected with the distribution of volcanoes upon the globe, the one which, by its striking character, seems to demand our attention in the first instance is that of the remarkable linear arrangement of volcanic vents. We have already seen that small scoria-cones are often thrown up on the flanks, or at the base, of a great volcanic mountain, along lines which are manifestly lines of fissure. In the eruption of Etna, in 1865, and again in that of 1874, Professor Silvestri, of Catania, witnessed the actual opening of great fissures on the north-east and north sides of the mountain: and along the bottom of these cracks the glowing lava was clearly visible (fig. 84, page 194). In the course of a few days, there were thrown up a number of small scoria-cones along these lines of fissure—those formed on the fissure of 1865 being seven in number, and those on the fissure of 1874 being no less than thirty-six in number. Precisely familiar phenomena were witnessed upon the slopes of Vesuvius, in 1760, when a fissure opened on the south side of the mountain, and fifteen scoria-cones, which are still visible, were thrown up along it.
We have already considered the evidence pointing to the conclusion that systems of volcanoes, like that of the Lipari Islands, are similarly ranged along lines of fissures, and there is equally good ground for believing that the great linear bands of volcanoes, which, as we have seen, stretch for thousands of miles, have had their positions determined by great lines of fissure in the earth's crust. While, however, the smaller fissures, upon which rows of scoria-cones are thrown up, seem to have been in many cases opened by a single effort of the volcanic forces, the enormous fissures, which traverse so large a portion of the surface of the globe, are doubtless the result of numerous manifestations of energy extending over vast periods of time.
The greatest of these bands along which the volcanic forces are so powerfully exhibited at the present day, is the one which stretches from near the Arctic circle at Behring's Straits to the Antarctic circle at South Victoria. The line followed by this volcanic band, which, as we have seen, includes more than one half of the active volcanoes of the globe, is a very sinuous one, and it gives off numerous offshoots upon either side of it. The great focus of this intense volcanic action may be regarded as lying in the district between the islands of Borneo and New Guinea. From this centre there radiate a number of great lines, along which the volcanic forces are exhibited in the most powerful manner. The first of these extends northwards through the Philippine Isles, Japan, the Kurile Islands, and Kamtschatka, giving off a branch to the east, which passes through the Aleutian Islands and the peninsula of Alaska. This band, along which the volcanic forces are very powerfully active, is continued towards the south-east in the New Britain, the Solomon Islands, Santa Cruz, the New Hebrides, New Zealand, and South Victoria. East and west from the great central focus there proceed two principal branches. The former of these extends through the Navigator Islands and Friendly Islands as far as Elizabeth Islands. The latter passes through Java, and then turns north-westward through Sumatra, the Nicobar Islands, the Andaman Islands up to the coast of Burmah.
The great band which we have been describing exhibits the most striking examples of volcanic activity to be found upon the globe. Besides the 150 or more volcanoes which are known to have been in a state of activity during the historical period, there are several hundred very perfect volcanic cones, many of which appear to have but recently become extinct, if indeed, they are not simply in a dormant condition. For long distances these chains of volcanic cones are almost continuous, and the only very considerable breaks in the series are those between New Zealand and the New Hebrides on the one hand, and between the former islands and South Victoria on the other.
Much less continuous, but nevertheless very important, is the great band of volcanoes which extends along the western side of the great American continent, and contains, with its branches, nearly a hundred active volcanoes. On the north this great band is almost united with the one we have already described by the chain of the Aleutian and Alaska volcanoes. In British Columbia about the parallel of 60° N. there exist a number of volcanic mountains, one of which, Mount St. Elias, is believed to be 18,000 feet in height, and several of these have certainly been seen in a state of eruption. Farther south in the part of the United States, territories drained by the Columbia River, a number of grand volcanic mountains exist, some of which are probably still active, for geysers and other manifestations of volcanic activity abound. From the southern extremity of the peninsula of California an almost continuous chain of volcanoes stretches through Mexico and Guatemala, and from this part of the volcanic band a branch is given off which passes through the West Indies, and forms a connection with the great volcanic band of the Atlantic Ocean. In South America the line is continued by the active volcanoes of Ecuador, Bolivia and Chili, but at many intermediate points in the chain of the Andes extinct volcanoes occur, which to a great extent fill up the gaps in the series. A small offshoot to the westward passes through the Galapagos Islands. The great band of volcanoes which stretches through the American continent is second only in importance, and in the activity of its vents, to the band which divides the Pacific from the Indian Ocean.
The third volcanic band of the globe is that which traverses the Atlantic Ocean from north to south. This series of volcanic mountains is much more broken and interrupted than the other two, and a greater proportion of its vents are extinct. This chain, as we shall show in a future chapter, attained its condition of maximum activity during the distant period of the Miocene, and now appears to be passing into a state of gradual extinction. Beginning in the north with the volcanic rocks of Greenland and Bear Island, we pass southwards, by way of Jan Mayen, Iceland, and the Faroe Islands, to the Hebrides and the north of Ireland. Thence by way of the Azores, the Canaries and the Cape de Verde Islands, with some active vents, we pass to the ruined volcanoes of St. Paul, Fernando de Noronha, Ascension, St. Helena, Trinidad and Tristan d'Acunha. From this great Atlantic band two branches proceed to the eastward, one through Central Europe, where all the vents are now extinct, and the other through the Mediterranean to Asia Minor, the great majority of the volcanoes along the latter line being now extinct, though a few are still active. The vol canoes on the eastern coast of Africa may be regarded as situated on another branch from this Atlantic volcanic band. The number of active volcanoes on this Atlantic band and its branches, exclusive of those in the West Indies, does not exceed fifty.
From what has been said, it will be seen that, not only do the volcanoes of the globe usually assume a linear arrangement, but nearly the whole of them can be shown to be thrown up along three well-marked bands and the branches proceeding from them. The first and most important of these bands is nearly 10,000 miles in length, and with its branches contains more than 150 active volcanoes; the second is 8,000 miles in length, and includes about 100 active volcanoes; the third is much more broken and interrupted, extends to a length of nearly 1,000 miles, and contains about 50 active vents. The volcanoes of the eastern coast of Africa, with Mauritius, Bourbon, Rodriguez, and the vents along the line of the Red Sea, may be regarded as forming a fourth and subordinate band.
Thus we see that the surface of the globe is covered by a network of volcanic bands, all of which traverse it in sinuous lines with a general north-and-south direction, giving off branches which often run for hundreds of miles, and sometimes appear to form a connection between the great bands.
These four bands of volcanic vents, running in a general north-and-south direction, separate four unequal areas within which the exhibitions of volcanic activity are feeble or quite unknown. The two grandest of the bands of volcanic activity, with their branches, form an almost complete series encircling the largest of the oceans.
To this rule of the linear arrangement of the volcanic vents of the globe and their accumulation along certain well-marked bands, there are two very striking exceptions, which we must now proceed to notice.
In the very centre of the continent formed by Europe and Asia, the largest unbroken land-mass of the globe, there rises from the great central plateau the remarkable volcanoes of the Thian Shan Range. The existence of these volcanoes, of which only obscure traditional accounts had reached Europe before the year 1858, appears to be completely established by the researches of the Russian traveller Semenof. Three volcanic vents appear to exist in this region: the active volcanoes of Boschan and Turfan or Hot-schen, and the solfatara of Urumtsi. At a point situated about half-way between these three volcanoes and the sea, another active vent, that of Ujung-Holdongi, is said to exist. Other volcanic phenomena have been stated to occur in the great plateau of Central Asia, but the existence of some at least of these appears to rest on very doubtful evidence. The only accounts which we have of the eruptions of these Thian Shan volcanoes are contained in Chinese histories and treatises on geography; and a great service would be rendered to science could they be visited by some competent explorer.
The second exceptionally-situated volcanic group is that of the Sandwich Islands. While the Thian Shan volcanoes rise in the centre of the largest unbroken land-mass, and stand on the edge of the loftiest and greatest plateau in the world, the volcanoes of the Sandwich Islands rise almost in the centre of the largest ocean and from almost the greatest depths in that ocean. All round the Sandwich Islands the sea has a depth of from 2,000 to 3,000 fathoms, and the island-group culminates in several volcanic cones which rise to the height of nearly 14,000 feet above the sea-level. The volcanoes of the Sandwich Islands are unsurpassed in height and bulk by those of any other part of the globe.
With the exception of the two isolated groups of the Thian Shan and the Sandwich Islands, nearly all the active volcanoes of the globe are situated near the limits which separate the great land- and water-masses of the globe—that is to say, they occur either on the parts of continents not far removed from their coast-lines, or on islands in the ocean not very distant from the shores.
The fact of the general proximity of volcanoes to the sea, is one which has frequently been pointed out by geographers, and may now be regarded as being thoroughly established. Even the apparently anomalous case of the Thian Shan volcanoes is susceptible of explanation if we remember the fact, now well ascertained by geological researches, that as late certainly as Pliocene times, a great inland sea spread over the districts where the Caspian, the Sea of Aral, and many other isolated lakes are now found. Upon the southern shore of this sea rose the volcanoes of the Thian Shan, some of which have not yet fallen into a state of complete extinction.
But although the facts concerning the general proximity of volcanoes to the ocean may be admitted to be thoroughly established, yet inferences are sometimes hastily drawn from these facts which the latter, if fairly considered, will not be found to warrant. It is frequently assumed that we may refer all the remarkable phenomena of volcanic action to the penetration of sea-water to a mass of incandescent lava in the earth's crust, and to the chemical or mechanical action which would result from this meeting of sea-water and molten rock. And this conclusion is supposed to find support in the circumstance that many of the gases and volatile substances emitted from volcanic vents are such as would be produced by the decomposition of the various salts contained in sea-water.
This argument in favour of the production of volcanic outbursts by the irruption of sea-water into subterranean reservoirs, involves, as Mr. Scrope long ago pointed out, a curious example of reasoning in a circle. It is assumed, on the one hand, that the heaving subterranean movements, which give rise to the fissures by which steam and other gases escape to the surface, are the result of the passage of water to heated masses in the earth's crust. But, on the other hand, it is supposed that it is the production of these fissures which leads to the influx of water to the heated materials. If it is the passage of water through these fissures which produces the eruptions, it may be fairly asked, what is it that gives rise to the fissures? And if, on the other hand, there exist subterranean forces competent to produce the fissures, may they not also give rise to the eruptions through the openings which they have originated? Nor does the chemical argument appear to rest upon any surer ground. It is true that many of the volatile substances emitted from volcanic vents are such as might be produced by the decomposition of sea-water, but, upon the other hand, there are not a few substances which cannot possibly be regarded as so produced, and, all the materials may equally well be supposed to have been originally imprisoned in the masses of subterranean lava.
The problem before us is this. Granting that it is proved that active volcanoes are always in close proximity to the ocean, are we to explain the fact by supposing that the agency of sea-water is necessary to volcanic outbursts, or by regarding the position of the coast-lines as to some extent determined by the distribution of volcanic action upon the surface of the globe? The first supposition is the one which perhaps most readily suggests itself, but the latter, as we shall hereafter show, is one in favour of which not a few weighty arguments may be advanced.
Another problem which suggests itself in connection with the distribution of volcanoes is the following. Are the great depressed tracts which form the bottom of the oceans, like the elevated tracts which constitute the continents, equally free from exhibitions of volcanic energy?
When we remember the fact that the area of the ocean beds is two and three-quarter times as great as that of the continents, it will be seen how important this question of the existence of volcanoes at the bottom of the ocean really is.
The fact that recent deep-sea soundings have shown the deepest parts of the ocean to be everywhere covered with volcanic débris is by no means conclusive upon this question; for, as we have seen, the ejections of sub-aerial volcanoes are by the wind and waves distributed over every part of the earth's surface.
Submarine volcanic outbursts have occurred in many parts of the globe, but it may well be doubted whether any such outburst has ever commenced at the bottom of a deep ocean, and has succeeded in building up a volcanic cone reaching to the surface. Most, if not all, of the recorded submarine outbursts have occurred in the midst of volcanic districts, and the volcanic cones have been built up in water of no great depth. Indeed, when it is remembered that the pressure of each 1,000 fathoms of water is equivalent to a weight of more than one ton on every square inch of the ocean-bottom, it is difficult to imagine the ordinary explosive action of volcanic vents taking place at abysmal depths. If, however, fissures were opened in the beds of the ocean, quiet outwellings of lava might possibly occur.
The solution of this problem of the probable existence of volcanic outbursts on the floor of the ocean can only be hoped for from the researches of the geologist. The small specimens of the ocean-beds brought up by deep-sea sounding-lines, taken at wide distances apart, and including but a few inches from the surface, can certainly afford but little information upon the question. But the geologist has the opportunity of studying the sea-bottoms of various geological periods which have been upheaved and are now exposed to his view. It was at one time supposed by geologists that in the so-called 'trap-rocks' we have great lava-sheets which must have been piled upon one another, without explosive action. But the more accurate researches of recent years have shown that between the layers of 'trap-rock,' in every part of the globe, traces of terrestrial surfaces and freshwater deposits are found; and the supposed proofs of the absence of explosive action break down no less signally upon re-examination; for the loose, scoriaceous materials would either be removed by denudation, or converted into hard and solid rocks by the infilling of their vesicles and air-cavities with crystalline minerals. It is not possible, among the representatives of former geological periods, to point to any rocks that can be fairly regarded as having issued from great submarine fissures, and it is therefore fair to conclude that no such great outbursts of the volcanic forces take plane at the present day on the deep ocean-floors.
In connection with the question of the relation between the position of the volcanic bands of the globe and the areas covered by the ocean, we may mention a fact which deep-sea soundings appear to indicate, namely, that the deepest holes in the ocean-floor are situated in volcanic areas. Near Japan, the soundings of the U.S. ship 'Tuscarora' showed that at two points the depth exceeded 4,000 fathoms; and the deepest sounding obtained by H.M.S. 'Challenger,' amounting to 4,575 fathoms, was taken in the voyage from New Gruinea to Japan, in the neighbourhood of the Ladrone Islands. Depths nearly as great were found in the soundings carried on in the neighbourhood of the volcanic group of the West Indian Islands. It must be remembered, however, that at present our knowledge of the depths of the abysmal portions of the ocean is very limited. A few lines of soundings, often taken at great distances apart, are all we have to guide us to any conclusions concerning the floors of the great oceans, and between these lines are enormous areas which still remain altogether unexplored. It may be wise, therefore, to suspend our judgment upon such questions till more numerous facts have been obtained.
Another fact concerning the distribution of volcanoes which is worthy of remark is their relation to the great mountain-ranges of the globe.
Many of the grandest mountain-chains have bands of volcanoes lying parallel to them. This is stinkingly exhibited by the great mountain-masses which lie on the western side of the American continent. The Rocky Mountains and the Andes consist of folded and crumpled masses of altered strata which, by the action of denuding forces, have been carved into series of ridges and summits. At many points, however, along the sides of these great chains, we find that fissures have been opened and lines of volcanoes formed, from which enormous quantities of lava have flowed and covered great tracts of country. At some parts of the chain, however, the volcanoes are of such height and dimensions as to overlook and dwarf the mountain-ranges by the side of which they lie. Some of the volcanoes lying parallel to the great American axis appear to be quite extinct, while others are in full activity.
In the Eastern continent we find still more striking examples of the parallelism between great mountain-chains and the lands along which volcanic activity is exhibited. Stretching in a more or less continuous chain from east to west, through Europe and Asia, we find the mountain-masses known in different parts of their course as the Pyrenees, the Alps, the Balkan, the Caucasus, which form the axis of the Eastern continent. These chains consist of numerous parallel ridges, and give off branches on either side of them. They are continued to the eastward by the Hindoo Koosh and the Himalaya, with the four parallel ranges that cross the great Central-Asian plateau. Now, on either side of this grand axial system of mountains, we find a great parallel band of volcanoes. The northern volcanic band is constituted by the eruptive rocks of the Auvergne, the Eifel, the Siebengebirge, Central Germany, Bohemia, Hungary, and Transylvania, few, if any, of the vents along this northern band being still active. The remarkable volcanoes of the Thian Shan range and of Mantchouria may not improbably be regarded as a continuation of the same great series.
The southern band of volcanoes, lying parallel to the great mountain axis of the Old World, also consists for the most part of extinct volcanoes, but includes not a few vents which are still active. In this band we include the extinct volcanoes of Spain and Sardinia, the numerous extinct and active vents of the Italian peninsula and islands, and those of the Ægean Sea and Asia Minor. We may, perhaps, consider the scattered volcanoes of Arabia and the northern part of the Indian Ocean as a continuation of the same series. Both of these bands may be regarded as offshoots from the great mid-Atlantic volcanic chain, and the condition of the vents, both in the principal band and its offshoots, is such as to indicate that they form parts of a system which is gradually sinking into a state of complete extinction.
There are some other volcanic bands which exhibit a similar parallelism with mountain chains; but, on the other hand, there are some volcanoes between which and the nearest mountain axes no such connection can be traced.
There is yet one other fact concerning the mode of distribution of volcanoes upon the surface of the globe, to which we must allude. It was first established by Mr. Darwin as one of the conclusions derived from the valuable series of observations made by him during the voyage of H.M.S. 'Beagle,' and relates to the position of active volcanoes with respect to the portions of the earth's crust which are undergoing upheaval or subsidence.
From the relative position of the different kinds of coral-reefs, and the fact that reef-forming corals cannot live at a depth of more than twenty fathoms beneath the sea-level, or above tide-mark, we are led to the conclusion that certain areas of the earth's surface are undergoing slow elevation, while other parts are as gradually subsiding. This conclusion is confirmed by the occurrence of raised beaches, which are sometimes found at heights of hundreds, or even thousands, of feet above the sea-level, and of submerged forests, which are not unfrequently found beneath the waters of the ocean.
By a study of the evidences presented by coral-reefs, raised beaches, submerged forests, and other phenomena of a similar kind, it can be shown that certain wide areas of the land and of the ocean-floor are at the present time in a state of subsidence, while other equally large areas are being upheaved. And the observations of the geologist prove that similar upward and downward movements of portions of the earth's crust have been going on through all geological times. Now, as Mr. Darwin has so well shown in his work on 'Coral-Reefs,' if we trace upon a map the areas of the earth's surface which are undergoing upheaval and subsidence respectively, we shall find that nearly all the active volcanoes of the globe are situated upon rising areas, and that volcanic phenomena are conspicuously absent from those parts of the earth's crust which can be proved at the present day to be undergoing depression.