CHAPTER VIII.

VOLCANIC MOUNTAINS.

'Tis said Enceladus' huge frame, Heart-stricken by the avenging flame, Is prisoned here, and underneath Gasps through each vent his sulphurous breath; And still as his tired side shifts round, Trinacia echoes to the sound Through all its length, while clouds of smoke The living soul of ether choke.

VIRGIL: _Æneid iii._

In some parts of the world we meet with mountains of a very different kind from any we have yet considered,--mountains that are known at times to send forth fiery streams of glowing lava, and to emit with terrific force great clouds of steam. Such mountains have long been known, in popular but unscientific language, as "burning mountains,"[25]--a term which is unfortunate, because they do not burn in the proper sense of the word, like candles or gas-jets. They are better known as volcanoes. There are about three hundred and fifty known active volcanoes; and if we include all mountains that once were in that state, the number is about one thousand.

[25] See papers by the writer on Volcanoes and Volcanic Action in "Knowledge" for May and June, 1891, on which this chapter is partly based.

Such mountains are connected in a curious way with those upheaved ridges of the world known as mountain-chains (see chap. vi., p. 191). And not only are many mountains more or less penetrated and intersected by rocks of an igneous origin (see chap. v., p. 155), but some have been largely formed by the action of old volcanoes. In fact, there are hills in Great Britain and parts of Europe, in America, and other countries, that once were actual volcanoes (see page 277).

We must briefly consider these strange mountains so different from others, and see what we can find out about them. Let us first inquire how a volcano is made, then consider what a volcano does; that is, we must view it as a geological agent that has a certain definite part to play in the economy of the world. And lastly, we may glance at some of the old volcanoes, and see what they were doing in those long ages of the world during which the great series of the stratified rocks were formed,--which rocks are, as it were, the book in which the earth has written her autobiography.

In old days volcanoes were regarded with superstitious awe; and any investigation of their actions would have been considered rash and impious in the highest degree. Mount Etna, as Virgil tells us, was supposed to mark the spot where the angry gods had buried Enceladus, one of the rebellious giants. Volcano, a certain "burning mountain" in the Lipa Islands, was likewise called the forge, or workshop, of Vulcan (or Volcan), the god of fire. And so it comes about that all "burning mountains" take their name from this one Mediterranean island, and at the same time tell us of the mythological origin of the word. It has been said that words are "fossil thoughts;" and we have here an old and very much fossilised thought,--a kind of thought long since extinct among civilised peoples, and one which is never likely to come to life again.

A volcanic mountain consists of alternating sheets of lava and volcanic ashes, mantling over each other in an irregular way, and all sloping away from the centre. In the centre is a pit or chimney, widening out towards the top so as to resemble a funnel or cup; hence the name "crater," which means a cup. In the centre of this crater a very small cone ("minor cone") is frequently found; and it is interesting to find that many of the moon's volcanic craters possess these "minor cones." A number of cracks or fissures intersect the volcano. These frequently spread out from the centre of the mountain in all directions, like the spokes of a wheel. They generally get filled with lava that wells up from below, thus forming "dykes," which may be regarded as so many sheets of igneous rock, such as basalt, that have forced their way while still liquid in among the layers of lava and ashes. The word "ash" is used by geologists in a special sense; and volcanic ash is not, as might be supposed, a deposit of cinders, but mostly of dust of various degrees of fineness, and sometimes it is very fine indeed. Pieces of pumice-stone may be embedded in a layer of volcanic ash, and sometimes great blocks of stone that have been shot out of the volcano as from a big gun, but these only form a small part of the layer. Dykes strengthen the mountain, and tend to hold it together when violently shaken during an eruption.

The shape and steepness of a volcano depend on the nature of the materials ejected. The finer the volcanic ash, the steeper and more conical is the mountain. The building up of a volcano may be fairly illustrated by the little cone of sand formed in an hourglass as the sand-grains fall. These settle down at a certain slope, or angle, at which they can remain, instead of falling down to the bottom, as they do directly this slope is exceeded. Some volcanoes are built up almost entirely of volcanic ash and its embedded blocks. Vesuvius, Teneriffe, Jorullo, in Mexico, and Cotopaxi, in the Andes, are examples of steep volcanic cones built up in this way. Others, less steep and more irregular in shape, are chiefly formed of successive lava-flows. Little minor cones are frequently formed on the side of a volcano; and these during an eruption give rise to small outbursts of their own. They are easily accounted for by the dykes which are mentioned just now; for when molten rock forces its way through fissures, it sometimes finds an outlet at the surface, and being full of steam, as soda-water is full of gas, it gives rise to an eruption. The great opening in the centre of a volcano, with its molten lava, is like a very big dyke that has reached the surface and so succeeded in producing an eruption.

The opening of a soda-water bottle not infrequently illustrates a volcanic eruption; for when the pent-up carbonic acid cannot escape fast enough, it forces out some of the water, even when the bottle is held upright.

Every volcano has been built up on a platform of ordinary stratified rocks; and at some period _after_ these had been laid down in water and raised up into dry land, molten rock found its way through them, and so the volcano was built up by successive eruptions during many years. It is probable that earthquake shocks, preceding the first eruption, cracked up these strata, and so made a way for the lava to come up.

The main point we wish to emphasize is that _volcanoes are never formed by upheaval_. In this way they differ from all other mountains. They have not been made by the heaving up of strata, but have been gradually piled up, something like rubbish heaps that accumulate in the Thames barges as the dustmen empty their carts into them, only in the case of volcanoes the "rubbish" comes from below. It is not necessary to suppose that the reservoir down below, from which the molten rock is supplied, exists at any very great depth below the original land surface on which the volcano grows up.

The old "upheaval theory" of volcanoes, once advocated by certain authorities, instead of being based on actual evidence or on reasoning from facts, was a mere guess. Moreover, if the explanation we have given should not be sufficiently convincing, there is good proof furnished by the case of a small volcano near Vesuvius, the building of which was actually witnessed. It is called Monte Nuovo, or the New Mountain. It is a little cone 430 feet high, on the bank of Lake Averno, with a crater more than a mile and a half wide at the base. It was almost entirely formed during a single night in the year 1538, A. D. We have two accounts of the eruption to which it owes its existence; and each writer says distinctly that the mountain was formed by the falling of stones and ashes.

One witness says,--

"Stones and ashes were thrown up with a noise like the discharge of great artillery, in quantities which seemed as if they would cover the whole earth; and in four days their fall had formed a mountain in the valley between Monte Barbaro and Lake Averno, of not less than three miles in circumference, and almost as high as Monte Barbaro itself,--a thing incredible to those who have not seen it, that in so short a time so considerable a mountain should have been formed."

Another says,--

"Some of the stones were larger than an ox. The mud (ashes mixed with water) was at first very liquid, then less so, and in such quantities that with the help of the afore-mentioned stones a mountain was raised one thousand paces in height."

(The writer's astonishment led him greatly to exaggerate the height.)

These accounts are important as showing how in a much longer time a big volcano may be built up. From such small operations we learn how Nature works on a large scale. The great volcano in Mexico known as Jorullo was probably built up in a very similar way. There is a tradition among the natives that it was made in two or three days; but we can hardly believe that. Volcanoes, as they get older, tend to grow taller and bigger; but every now and then a large portion may be blown away by some great eruption, and they have, as it were, to begin again.

Let us now consider volcanoes as geological agents, and see what they do. A volcanic eruption may be described in a general way as follows: Its advent is heralded by earthquakes affecting the mountain and the whole country round; loud underground explosions are heard, resembling the fire of distant artillery. The vibrations are chiefly transmitted through the ground; the mountain seems convulsed by internal throes, due, no doubt, to the efforts of the imprisoned steam and liquid rock to find an opening. These signs are accompanied by the drying up of wells and disappearance of springs, since the water finds its way down new cracks in the rocks, caused by the frequent shocks and quiverings. When at last an opening has been made, the eruption begins,--generally with one tremendous burst that shakes the whole mountain down to its foundations. After this, frequent explosions follow with great rapidity and increasing violence, generally from the crater. These are indicated by the globular masses of steam which are to be seen rising up in a tall column like that which issues from the funnel of a locomotive. But sometimes the whole mountain seems to be more or less engaged in giving out steam, and thus to be partly enveloped in it. This is illustrated by our engraving from an instantaneous photograph of Vesuvius in eruption in the year 1872. The steam and other gases, in their violent ascent, hurl up into the air a great deal of solid rock from the sides of the central opening, after first blowing out the stones which previously stopped up the orifice.

Blocks of stone falling down meet with others coming up; and so a tremendous pounding action takes place, the result of which is that great quantities of volcanic dust and ashes are produced, generally of extreme fineness. Winds and ocean currents transport these light materials for long distances. The observations made during the famous and fruitful voyage of H. M. S. "Challenger" showed that fine volcanic dust is carried by wind and marine currents to almost all parts of the oceans. The darkness so frequently mentioned in accounts of eruptions--sometimes at a very great distance from the volcano--is entirely caused by clouds of volcanic dust hiding the light of the sun. Perhaps the best example of this is the case of the eruption of Krakatoa (in the Strait of Sunda, between Sumatra and Java) in 1883. Its explosions were heard in all directions for two thousand miles, and a perceptible layer of volcanic dust fell at all places within one thousand miles; while the finest dust and vapour, shot up fifteen or twenty miles high, were spread all over the globe, causing, while still suspended in the atmosphere, the peculiar red sunsets noticed in all parts of the world for some months after the eruption.

Again, those very curious deposits of "red clay" found in the very deepest parts of the Pacific and Atlantic oceans (at depths of about four thousand fathoms, or twenty-four thousand feet) have been shown to be chiefly composed of volcanic dust, their red colour being due to oxidised iron.

But there is another way in which a good deal of fine volcanic dust is made; and it is this: the lava is so full of steam intimately mixed up with it that the steam, in its violent effort to escape, often blows the lava into mere dust.

Another interesting phenomenon may be thus described: Portions of liquid, or half liquid, lava are caught up by the steam and hurled into the air. These assume a more or less round form, and are known as "bombs." At a distance they give rise to the appearance of flames. And here we may remark that the flaring, coloured pictures of Etna or Vesuvius in eruption, which frequently may be seen, are by no means correct. The huge flames shooting up into the air are quite imaginary, but are probably suggested by the glare and bright reflection from glowing molten lava down in the crater.

So great is the force of the pent-up steam trying to escape that it frequently blows a large part of the volcano bodily away; and in some cases a whole mountain has been blown to pieces.

Finally, torrents of rain follow and accompany an eruption,--a result which clearly follows from the condensation of large volumes of steam expanding and rising up into the higher and cooler layers of the atmosphere. Vast quantities of volcanic ash are caught up by the rain, and in this way very large quantities of mud are washed down the sides of the mountain.

Sometimes the mud-flows are on a large scale, and descending with great force, bury a whole town. It was mostly in this way that the ancient cities of Herculaneum and Pompeii were buried by the great eruption of Vesuvius in the year 79 A. D., in which the elder Pliny lost his life. The discoveries made during excavations at Pompeii are of very great interest as illustrating old Roman life. The Italians give the name _lava d'acqua_, or water-lava, to flows of this kind, and they are greatly dreaded on account of their great rapidity. An ordinary lava-stream creeps slowly along, so that people have time to get out of the way; but in the case of mud-flows there is often no time to escape. No lava-stream has ever reached Pompeii since it was first built, although the foundations of the town stand upon an old lava-flood. Herculaneum is nearer to Vesuvius, and has at times been visited by lava-streams. Mud-lavas, ashes, and lava-streams have accumulated over this city to a depth of over seventy feet.

Lava-streams vary greatly in size; in some cases the lava, escaping from craters, comes to rest before reaching the base of the slopes of the volcano; in other cases a lava-flow not only reaches the plains below, but extends for many miles over the surrounding country. Hence lava-streams are important geological agents. Let us look at some famous instances. The most stupendous flow on record was that which took place from Skaptar Jökull in Iceland, in the year 1783. In this case a number of streams issued from the volcano, flooding the country far and wide, filling up river gorges which were in some cases six hundred feet deep and two hundred and fifty feet broad, and advancing into the alluvial plains in lakes of molten rock twelve to fifteen miles wide and one hundred feet deep. Two currents of lava which flowed in nearly opposite directions spread out with varying thickness according to the nature of the ground for forty and fifty miles respectively. Had this great eruption taken place in the south of England, all the country from the neighbourhood of London to that of Gloucester might have been covered by a flood of basalt of considerable thickness.

Sometimes, when the lava can only escape at a point low down on the mountain, a fountain of molten rock will spout high into the air. This has happened on Vesuvius and Etna. But in an eruption of Mauna Loa, in the Sandwich Islands, an unbroken fountain of lava, from two hundred to seven hundred feet high and one thousand feet broad, burst out at the base of the mountain; and again in April, 1888, the same thing happened on a still grander scale. In this case four fiery fountains continued to play for several weeks, sometimes throwing the glowing lava to a height of one thousand feet in the air. Surely there can be no more wonderful or awful sight than this in the world.

The volcanoes of Hawaii, the principal island in the Sandwich Islands, often send forth lava-streams covering an area of over one hundred square miles to a depth of one hundred feet or more; but they are discharged quite quietly, like water welling out of a spring. Repeated flows of this kind, however, have in the course of ages built up a great flat cone six miles high from the floor of the ocean, to form this lofty island, which is larger than Surrey; and it is calculated that the great volcanic mountain must contain enough material to cover the whole of the United States with a layer of rock fifty feet deep.

But it is not only on the surface of the land that volcanic eruptions take place; for in some cases the outbreak of a submarine eruption has been witnessed, and it is highly probable that in past geological ages many large eruptions of this nature have taken place. In the year 1783, an eruption took place about thirty miles off the west coast of Iceland. An island was built up from which glowing vapour and smoke came forth; but in a year or less the waves had washed everything away, leaving only a submerged reef. The island of Santorin, in the Greek Archipelago, is a partly submerged volcano.

But in some cases enormous outpourings of lava have taken place, not from volcanoes, but from openings of the ground here and there, and more usually from long fissures or cracks in the rocks lying at the surface. In many cases so much lava has quietly welled out in this way that the old features of the landscape have been completely buried up, and wide plains and plateaux formed over them. Sir A. Geikie says,--

"Some of the most remarkable examples of this type of volcanic structure occur in western North America. Among these that of the Snake River plain in Idaho may be briefly described.

"Surrounded on the north and east by lofty mountains, it stretches westward as an apparently boundless desert of sand and bare sheets of black basalt. A few streams descending into the plain from the hills are soon swallowed up and lost. The Snake River, however, flows across it, and has cut out of its lava bed a series of picturesque gorges and rapids.

"The extent of country which has been flooded with basalt in this and adjoining regions of Oregon and Washington has not yet been accurately surveyed, but has been estimated to cover a larger area than France and Great Britain combined. Looked at from any point on its surface, one of these lava plains appears as a vast level surface, like that of a lake bottom. This uniformity has been produced either by the lava rolling over a plain or lake bottom, or by the complete effacement of an original, undulating contour of the ground under hundreds of feet of lava in successive sheets. The lava, rolling up to the base of the mountains, has followed the sinuosities of their margin, as the waters of a lake follow its promontories and bays."

A few further examples of mud-lavas may be mentioned here. Cotopaxi, a great volcano in Ecuador, South America, with a height of 17,900 feet, reaches so high into the atmosphere that the higher parts are capped with snow. In June, 1877, a great eruption took place, during which the melting of snow and ice gave rise to torrents of mud and water, which rushed down the steep sides of the mountain, so that large blocks of ice were hurried along. The villages around to a distance of about seventy miles were buried under a deposit of mud, mixed with blocks of lava, ashes, pieces of wood, etc.

Sometimes a volcano discharges large quantities of mud directly from the crater. In this case the mud is not manufactured by the volcano itself, but finds its way through fissures and cracks from the bed of the neighbouring sea or rivers to the crater. Thus, in the year 1691, Imbaburu, one of the Andes of Quito, sent out floods of mud containing dead fish, the decay of which caused fever in the neighbourhood. In the same way the volcanoes of Java have often buried large tracts of fertile country under a covering of volcanic mud, thus causing great devastation.

Vast quantities of dust are produced, as already explained, by the pounding action that takes place during an eruption, as portions of rock in falling down meet others that are being hurled into the air. Striking instances of this have occurred not far from Great Britain. Thus in the year 1783, during an eruption of Skaptar Jökull, so great was the amount of dust thus created that the atmosphere in Iceland was loaded with it for several months. Carried by winds, it even reached the northern parts of Scotland, and in Caithness so much of it fell that the crops were destroyed. This is remarkable, considering that the distance was six hundred miles. Even in Holland and Norway there are traces of this great shower of dust from the Icelandic volcano.

During the fearful eruption of Tomboro, a volcano in the island of Sumbawa, in the Eastern Archipelago, in 1815, the abundance of ashes and dust ejected caused darkness at midday at Java, three hundred miles away, and even there the ground was covered to a depth of several inches. In Sumbawa itself the part of the island joining the mountain was entirely desolated, and all the houses destroyed, together with twelve thousand inhabitants. Trees and herbage were overwhelmed with pumice and volcanic dust. The floating pumice on the sea around formed a layer two feet, six inches thick, through which vessels forced their way with difficulty. From such facts as these it is clear that if in past ages volcanoes have been so powerfully active as they are now, we should expect to find lava-flows, dykes, and great deposits of volcanic ash deposited in water among the stratified rocks; and such is the case. Many large masses of rock familiar to the geologist, and often forming parts of existing mountains, are to be accounted for either as great lava-flows, or dykes that have forced their way in among the strata, or as extensive deposits of volcanic ash.

But perhaps the reader would like to know what the inside of a volcanic crater is like during an eruption. Let us, then, take a peep into that fearful crater of Kilauea, in the Sandwich Islands. For this purpose we cannot do better than follow Miss Bird's admirable description of her adventurous expedition to this crater:--

"The abyss, which really is at a height of four thousand feet, on the flank of Mauna Loa, has the appearance of a pit on a rolling plain. But such a pit! It is quite nine miles in circumference, and at its lowest area--which not long ago fell about three hundred feet, just as ice on a pond falls when the water below is withdrawn--covers six square miles. The depth of the crater varies from eight hundred to one thousand feet, according as the molten sea below is at flood or ebb. Signs of volcanic activity are present more or less throughout its whole depth, and for some distance round its margin, in the form of steam-cracks, jets of sulphurous vapour, blowing cones, accumulating deposits of acicular crystals of sulphur, etc., and the pit itself is constantly rent and shaken by earthquakes. Grand eruptions occurred with circumstances of indescribable terror and dignity; but Kilauea does not limit its activity to these outbursts, but has exhibited its marvellous phenomena through all known time in a lake or lakes on the southern part of the crater three miles from this side.

"This lake--the _Hale-mau-mau_, or 'House of Everlasting Fire,' of the Hawaiian mythology, the abode of the dreaded goddess Pele--is approachable with safety, except during an eruption. The spectacle, however, varies almost daily; and at times the level of the lava in the pit within a pit is so low, and the suffocating gases are evolved in such enormous quantities, that travellers are unable to see anything. There had been no news from it for a week; and as nothing was to be seen but a very faint bluish vapour hanging round its margin, the prospect was not encouraging.... After more than an hour of very difficult climbing, we reached the lowest level of the crater, pretty nearly a mile across, presenting from above the appearance of a sea at rest; but on crossing it, we found it to be an expanse of waves and convolutions of ashy-coloured lava, with huge cracks filled up with black iridescent rolls of lava only a few weeks old. Parts of it are very rough and ridgy, jammed together like field-ice, or compacted by rolls of lava, which may have swelled up from beneath; but the largest part of the area presents the appearance of huge coiled hawsers, the ropy formation of the lava rendering the illusion almost perfect. These are riven by deep cracks, which emit hot sulphurous vapours....

"As we ascended, the flow became hotter under our feet, as well as more porous and glistening. It was so hot that a shower of rain hissed as it fell upon it. The crust became increasingly insecure, and necessitated our walking in single file with the guide in front, to test the security of the footing. I fell through several times, and always into holes full of sulphurous steam so malignantly acid that my strong dogskin gloves were burned through as I raised myself on my hands.

"We had followed the lava-flow for thirty miles up to the crater's brink, and now we had toiled over recent lava for three hours, and by all calculation were close to the pit; yet there was no smoke or sign of fire, and I felt sure that the volcano had died out for once for our special disappointment....

"Suddenly, just above, and in front of us, gory drops were tossed in the air, and springing forwards we stood on the brink of _Hale-mau-mau_, which was about thirty-five feet below us. I think we all screamed. I know we all wept; but we were speechless, for a new glory and terror had been added to the earth. It is the most unutterable of wonderful things. The words of common speech are quite useless. It is unimaginable, indescribable; a sight to remember for ever; a sight which at once took possession of every faculty of sense and soul, removing one altogether out of the range of ordinary life. Here was the real 'bottomless pit,' 'the fire which is not quenched,' 'the place of Hell,' 'the lake which burneth with fire and brimstone,' 'the everlasting burnings,' 'the fiery sea whose waves are never weary.'[26] There were groanings, rumblings, and detonations, rushings, hissings, splashings, and the crashing sound of breakers on the coast; but it was the surging of fiery waves upon a fiery shore. But what can I write? Such words as jets, fountains, waves, spray, convey some idea of order and regularity, but here there was none. The inner lake, while we stood there, formed a sort of crater within itself; the whole lava sea rose about three feet; a blowing cone about eight feet high was formed; it was never the same two minutes together. And what we saw had no existence a month ago, and probably will be changed in every essential feature a month hence.... The prominent object was fire in motion; but the surface of the double lake was continually skimming over for a second or two with a cooled crust of a lustrous grey-white, like frosted silver, broken by jagged cracks of a bright rose-colour. The movement was nearly always from the sides to the centre; but the movement of the centre itself appeared independent, and always took a southerly direction. Before each outburst of agitation there was much hissing and throbbing, internal roaring, as of imprisoned gases. Now it seemed furious, demoniacal, as if no power on earth could bind it, then playful and sportive, then for a second languid, but only because it was accumulating fresh force.... Sometimes the whole lake ... took the form of mighty waves, and surging heavily against the partial barrier with a sound like the Pacific surf, lashed, tore, covered it, and threw itself over it in clots of living fire. It was all confusion, commotion, forces, terror, glory, majesty, mystery, and even beauty. And the colour, 'eye hath not seen' it! Molten metal hath not that crimson gleam, nor blood that living light."[27]

[26] Perhaps these Scripture phrases were suggested long before the Bible was written, by the sight of some crater in active eruption.

[27] The Hawaiian Archipelago.

Continued observation of volcanoes, together with evidence derived from history, teaches that there are different stages of volcanic action. There are three pretty well-marked phases. First, the state of permanent eruption; this is not a dangerous state, because the steam keeps escaping all the time: the safety-valve is at work, and all goes smoothly. The second state is one of moderate activity, with more or less violent eruptions at brief intervals; this is rather dangerous, because at times the safety-valve does not work.

And thirdly, we have paroxysms of intense energy, alternating with long periods of repose sometimes lasting for centuries. These eruptions are extremely violent, and cause widespread destruction; the safety-valve has got jammed, and so the boiler bursts.

No volcano has been so carefully watched for a long time as Vesuvius. Its history illustrates the phases we have just mentioned. The first recorded eruption is that of A. D. 79, a very severe one of the violent type, by which Herculaneum, Pompeii, and Stabiæ were buried. We have an interesting account by the younger Pliny. Before this great eruption took place, Vesuvius had been in a state of repose for eight hundred years, and if we may judge from the Greek and Roman writings, was not even suspected of being a volcano. Then followed an interval of rest until the reign of Severus, the second eruption taking place in the year 203. In the year 472, says Procopius, all Europe was covered more or less with volcanic ashes. Other eruptions followed at intervals, but there was complete repose for two centuries; that is, until the year 1306. In 1500 it was again active, then quiet again for one hundred and thirty years. In 1631 there took place another terrific outburst. After this many eruptions followed, and they have been frequent ever since. Vesuvius is therefore now in the second stage of moderate activity.

But geologists can take a wider view than this. They can sum up the history of a volcanic region of the earth; and the result is somewhat as follows: Volcanoes, like living creatures, go through different periods or phases, corresponding roughly to youth, middle age, old age, and finally decay. The invasion of any particular area of the earth's surface by the volcanic forces is heralded by underground shocks, or earthquakes. A little later on cracks are formed, as indicated by the rise of saline and hot springs, and the issuing of carbonic acid and other gases at the surface of the earth. As the underground activity becomes greater, the temperature of the springs and emitted gases increases; and at last a visible rent is formed, exposing highly heated and glowing rock below. From the fissure thus formed, the gas and vapours imprisoned in the molten rocks escape with such violence as to disperse the latter in the form of pumice and volcanic ash, or to cause them to pour out as lava-streams.

The action generally becomes confined to one or more points along the line of action (which is a line of fissures and cracks). In this way a chain of volcanoes is formed, which may become the seat of volcanic action for a long time.

When the volcanic energies have become somewhat exhausted, so that they cannot raise up the lava and expel it from the volcanic crater, nor rend the sides of the volcano and cause minor cones to grow up on their flanks, small cones may be formed at a lower level in the plains around the great central chain. These likewise are fed from fissures.

Later on, as the heated rock below cools down, the fissures are sealed up by lava that has become solid; and then the volcanoes fall, as it were, into the "sere and yellow leaf," and remain in a peaceful, quiet state befitting their old age.

After this they begin to suffer from long exposure to the atmospheric influences of decay, and rain and rivers wash them away more or less completely.

But still the presence of heated rocky matter at no great depth below is proved by the outbursts of gases and vapours, the forming of geysers and ordinary hot springs. Gradually, however, even these signs of heat below disappear; and the cycle of volcanic phases is at an end. Such a series of changes may require millions of years; but by the study of volcanoes in every stage of their growth and decline it is possible thus to sketch out an outline of their history.

It must be confessed that in the present state of scientific knowledge no full and complete explanation of volcanic action is possible. Geologists and others are as yet but feeling their way cautiously towards the light which, perhaps before long, will illumine the dark recesses of this mysterious subject. Many theories and ideas have been put forward, but in the opinion of the writer the most promising explanation is one that may be briefly expressed as follows:

There are below the crust of the earth large masses of highly heated rock that are _kept solid_ by the enormous pressure of the overlying rocks, or otherwise they would melt,--for it is a known fact that pressure tends to prevent the melting of a solid body. But when earth-movements taking place within the earth's crust--such as the upheaving of mountain-chains--take off some of the weight, the balance between internal heat and the pressure from above is no longer maintained; and so these highly heated rocks run off into the liquid state, and finding their way to the surface through the fissures mentioned above, give rise to volcanic action. There is much to be said in favour of this view. It rightly connects volcanic action with movements of upheaval, with mountain-chains and lines of weakness in the earth's crust.

There is very good reason to believe that the earth was once in a highly heated state, and has been slowly cooling down for ages. The increase of temperature observed in penetrating mines tells us that it still retains below the surface some of its old heat. We need not therefore be surprised at the existence of heated masses of rock down below, or seek, as some have done, an entirely different source for the origin of volcanic heat than that which remains from the earth's once molten condition. It would take too long to state the reasons on which this idea of the former state of our planet is based, and moreover, it would bring us into the region of astronomy, with which we are not concerned at present.

In various parts of Great Britain and Ireland we meet with old volcanic rocks,--lavas, intrusive dykes, and sheets of basalt, etc., together with vast deposits of volcanic ash, which, sinking into the old neighbouring seas, became stratified, or arranged in layers like the ordinary sedimentary rocks. In some cases we see embedded in these layers the very "bombs" that were thrown out by the old volcanoes (see page 253). And besides these purely volcanic rocks, we often meet in these areas with great bosses of granite, which must have been in some way connected with the old volcanoes, and probably were in many cases the source from which much of the volcanic rock was derived. But more than this, in a few instances we have the site of the old volcano itself marked out by a kind of pipe, or "neck," now filled with some of its volcanic débris in the shape of coarse, rounded fragments (see page 277).

During a very ancient period, known to geologists as the Silurian Period, great lava-flows took place from volcanoes situated where North and South Wales and the Lake District now are; and by their eruptions a vast amount of volcanic ash was made, which fell into the sea and slowly sank to the bottom, so that the shell-fish living there were buried in the strata thus formed, and may now be seen in a fossilised condition.

Thus Snowdon, Cader Idris, the Arans, Arenig Mountain, and others, are very largely made up of these ancient volcanic materials. The writer has picked up specimens of fossil shell-fish near the summit of Snowdon from a bed of fine volcanic ash that forms the summit. Fig. 2 represents a section through Snowdon, from which it will be seen that we have first a few sedimentary strata, _S_, then a great lava-flow, _L_; and that volcanic ashes accumulated on the top of this, of which _A A_ are patches still left. _B_ is an intrusive dyke of a basaltic rock that forced its way through afterwards. Again, in the Lake District there is a well-known volcanic series of stratified rocks of the same age, consisting mostly of lavas and ashes, the total thickness of which is about twelve thousand feet (known as the "Green Slates and Porphyries"), so that a large part of some of the mountains there have also been built up by volcanic action; but no traces of the old volcanoes remain.

Going farther north we find abundant proof that volcanic action on a prodigious scale took place in Scotland during the very ancient period of the Old Red Sandstone, with which the name of Hugh Miller will always be associated. In Central Scotland we see lava-flows and strata formed of volcanic ash, with a thickness of more than six thousand feet, fragments of which, having escaped the destructive agents of denudation, now form important chains of hills, such as the Pentland, Ochil, and Sidlaw ranges. Nor was the volcanic action confined to this region. In the district of the Cheviot Hills similar volcanic rocks are to be seen. But here again the old volcanoes have long since been swept away, leaving us only portions of their outpourings buried in the hills.

There can be no doubt that the present area of the Grampian Hills was once the site of a considerable number of volcanoes, only at a much higher level than their present surface, elevated though that is to the region of the clouds; but in this case subsequent denudation has been so enormous that the old mountain surface has been planed away until all we can now see is a series of separate patches of granite, that were once in a fused and highly heated state far below the surface, and formed part of the subterranean reservoirs from which the volcanoes derived their great supplies of lava and steam. It is indeed difficult to imagine the enormous amount of denudation which has taken place in the Highlands of Scotland, and to realise that the magnificent range of the Cairngorms, for instance, has been for ages worn down until now they are but a remnant of what they once were.

In this region we see the once boiling and seething masses of rock which fed the old volcanoes, now no longer endowed with life-like power by the force of steam, but lying in deathlike cold and stiffness, with their beautiful crystals of mica and felspar sparkling in the sun. The volcanic fires have died out; but the traces of their work are unmistakable, among which we must not forget to reckon the beautiful minerals made by the action of heated water upon the surrounding rocks.

The beautiful cairngorm stones are still sometimes found on the mountain from which they take their name, and in all volcanic regions minerals are plentiful.

The well-known hill called Arthur's Seat, close to Edinburgh, marks the site of an old volcano. The "neck," or central opening, may be seen at the top of the hill, but choked up with volcanic rocks and débris. The crater has long since disappeared, but Salisbury Craigs and St. Leonard's Craigs are formed of a great sheet of basalt that intruded itself among the stratified rocks that had been formed there, and so belong really to a great intrusive dyke. In the Castle Rock we see the same basalt again.

During a much later age, known as the Miocene Period (see chap. x., p. 324), enormous outpourings of lava took place in Western Europe, covering hundreds of square miles. Of these the most important is that which occupies a large part of the northeast of Ireland, and extends in patches through the Inner Hebrides and the Faröe Islands into Iceland. These eruptive rocks, unlike those above referred to, must have poured out at the surface, and have taken the form of successive sheets, such as we now see in the terraced plateaux of Skye, Eigg, Canna, Muck, Mull, and Morven. These, then, are patches of what once formed a great plain of basalt. During later times this volcanic platform has been so greatly cut up by the agents of denudation that it has been reduced to mere scattered fragments; thousands of feet of basalt have been worn away from it; deep and wide valleys have been carved out of it; and in many cases it has been almost entirely stripped off from the wide areas it once covered. Where, as in the Isle of Eigg, the lava has been piled up in successive sheets, with some layers of volcanic ash between, the latter has been worn away rather faster than the hard layers of basalt, and each lava-flow is clearly marked by a terrace. These volcanic eruptions have thus had a great influence in moulding the scenery of this region. In Ireland the old basalts are well seen at the Giant's Causeway, and on the Scottish coast we see them again at the well-known Fingal's cave at Staffa. This island, like the others, is just a patch of the old lava-streams.

Its curious six-sided columns illustrate a fact with regard to the subsequent cooling of lava-flows. Some internal forces, analogous to that which regulates the shapes of crystals, have caused it to crack along three sets of lines, so placed with regard to each other as to produce six-sided columns.

In Ireland the basalts attain a thickness of nine hundred feet; in Mull they are about three thousand feet thick. It has been clearly proved that Mull is the site of one of the old volcanoes of this period, but very few others have as yet been detected. Perhaps the eruptions took place mainly from large fissures, instead of from volcanic cones, for it is known that the ground below the lava-sheets has been rent by earthquakes into innumerable fissures, into which the basalt was injected from below.

In this way a vast number of "dykes" were formed. These have been traced by hundreds eastwards from this region across Scotland, and even the north of England. In this case the molten rock was struggling to get through the overlying rocks and escape at the surface; but apparently it did not succeed in so doing, for we do not find lava-flows to the east and south. These basalt dykes are found as far south as Yorkshire, and can be traced over an area of one hundred thousand square miles.

It is thus evident that in the Miocene Period a great and extensive mass of molten basalt was underlying a large part of the British Isles, and probably the weight of the thick rocks overlying it was sufficient to prevent its escape to the surface. If it had succeeded in so escaping and overflowing, how different the scenery of much of Scotland and Northern England might have been!