The Story of the Hills: A Book About Mountains for General Readers.

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,—

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:—

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!

CHAPTER IX.
MOUNTAIN ARCHITECTURE.

The dying splendours of the sun slowly sinking and entering the "gates of the West" may well serve as a fitting emblem of the mountains in their beautiful old age, awaiting in silent and calm dignity the time when they also must be brought low, and sink in the waters of the ocean, as the sun appears daily to do. Yes, they too have their day. They too had their rising, when mighty forces brought them up out of their watery bed. Many of them have passed their hey-day of youth, and their midday; while others, far advanced in old age, are nearing the end of their course.

But as the sun rises once more over eastern seas to begin another day, so will the substance of the mountains be again heaved up after a long, long rest under the sea, and here and there will rise up from the plains to form the lofty mountain-ranges of a distant future.

Everywhere we read the same story, the same circle of changes. The Alpine peak that proudly rears its head to the clouds must surely be brought low, and finally come back to the same ocean from which those clouds arose. It is in this way that the balance between land and water is preserved. In passing through such a great circle of changes, the mountains assume various forms and shapes which are determined by:—

1. 1. Their different ages and states of decay.
2. 2. The different kinds of rocks of which they are composed, and especially by their "joints," or natural divisions.
3. 3. The different positions into which these rocky layers have been squeezed, pushed, and crumpled by those stupendous forces of upheaval of which we spoke in chapter vi.

Let us therefore glance at some of these external forms, and then look at the internal structure of mountains.

In so doing we shall find that we have yet a good deal more to learn about mountains and how they were made; and also we shall then be in a better position to realise not only how very much denudation they have suffered, but also how greatly they have been disturbed since their rocks were first made.

Every one who knows mountains must have observed how some are smooth and rounded, others sharp and jagged, with peaks and pinnacles standing out clearly against the sky; some square and massive, with steep walls forming precipices; others again spread out widely at their base, but the sloping sides end in a sharp point at the top, giving to the mountain the appearance of a cone. Their diversities of shape are so endless that we cannot attempt to describe them all.

First, with regard to the general features of mountains. Looked at broadly, a mountain-range is not a mere line of hills or mountains rising straight up from a plain on each side, such as school-boys often draw in their maps; very far from it. Take the Rocky Mountains, for instance. "It has been truly said of the Rocky Mountains that the word 'range' does not express it at all. It is a whole country populous with mountains. It is as if an ocean of molten granite had been caught by instant petrifaction when its billows were rolling heaven high."[28]

It has often been observed by mountain climbers that when they get to the top of a high mountain, and take a bird's-eye view of the country, all the mountain-tops seem to reach to about the same height, so that a line joining them would be almost level. For this reason, perhaps, writers so often compare them to the waves of an ocean. This feature is very conspicuous in the case of the Scotch Highlands.

Sir A. Geikie has well described what he saw from the top of Ben Nevis:—

"Much has been said and written about the wild, tumbled sea of the Highland Hills. But as he sits on his high perch, does it not strike the observer that there is after all a wonderful orderliness, and even monotony, in the waves of that wide sea? And when he has followed their undulations from north to south, all round the horizon, does it not seem to him that these mountain-tops and ridges tend somehow to rise to a general level; that, in short, there is not only on the great scale a marked similarity of contour about them, but a still more definite uniformity of average height? To many who have contented themselves with the bottom of the glen, and have looked with awe at the array of peaks and crags overhead, this statement will doubtless appear incredible. But let any one get fairly up to the summits and look along them, and he will not fail to see that the statement is nevertheless true. From the top of Ben Nevis this feature is impressively seen. Along the sky-line, the wide sweep of summits undulates up to a common level, varied here by a cone and there by the line of some strath or glen, but yet wonderfully persistent round the whole panorama. If, as sometimes happens in these airy regions, a bank of cloud with a level under-surface should descend upon the mountains, it will be seen to touch summit after summit, the long line of the cloud defining, like a great parallel ruler, the long level line of the ridges below. I have seen this feature brought out with picturesque vividness over the mountains of Knoydart and Glen Garry. Wreaths of filmy mist had been hovering in the upper air during the forenoon. Towards evening, under the influence of a cool breeze from the north, they gathered together into one long band that stretched for several miles straight as the sky-line of the distant sea, touching merely the higher summits and giving a horizon by which the general uniformity of level among the hills could be signally tested. Once or twice in a season one may be fortunate enough to get on the mountains above such a stratum of mist, which then seems to fill up the irregularities of the general platform of hill-tops, and to stretch out as a white phantom sea, from which the highest eminences rise up as little islets into the clear air of the morning.... Still more striking is the example furnished by the great central mass of the Grampians, comprising the Cairngorm Mountains and the great corries and precipices round the head of the Dee. This tract of rugged ground, when looked at from a distance, is found to present the character of a high, undulating plateau."[29]

This long level line of the Highland mountain-tops may be seen very well from the lower country outside; for example, from the isles of Skye and Eigg, where one may see the panorama between the heights of Applecross and the Point of Ardnamurchan showing very clearly the traces of the old table-land.

How are we to explain this curious fact, so opposed to our first impressions of a mountain region? It is quite clear that the old plateau thus marked out cannot be caused by the arrangement or position of the rocks of which the Highlands are composed. If these rocks were found to be lying pretty evenly in flat layers, or strata, undisturbed by great earth-movements, we could readily understand that they would form a plateau. But the reverse is the case: the rocks are everywhere thrown into folds, and frequently greatly displaced by "faults;" yet these important geological features have little or no connection with the external aspect of the country. It is therefore useless to look to internal structure for an explanation. We must look outside, and consider what has been for ages and ages taking place here.

As already pointed out, an enormous amount of solid rock has been removed from this region—thousands and thousands of feet. It was long ago planed down by the action of water, so that a table-land once existed of which the tops of the present mountains are isolated fragments. No other conclusion is possible. To the geologist every hill and valley throughout the whole length and breadth of the Highlands bears striking testimony to this enormous erosion. The explanation we are seeking may therefore be summed up in one word, "denudation." The valleys that now intersect the table-land have been carved out of it. If we could in imagination put back again onto the present surface what has been removed, we should have a mental picture of the Highlands as a wide, undulating table-land; and this rolling plain would suggest the bottom of the sea. The long flat surfaces of the Highland ridges, cut across the edges of inclined or even upright strata, are the fragments of a former base-line of erosion; that is, they represent the general submarine level to which the Highlands were reduced after exposure to the action of "rain and rivers," and finally of the sea. As the sea gradually spread over it, it planed down everything that had not been previously worn away, and so reduced the whole surface to one general level like the sea-bed of the present day. But it is not necessary to suppose that the whole region was under water at the same time, and it is probable that there were separate inland seas or lakes. In these the rocks of the Old Red Sandstone were formed; and they in their turn have suffered so much denudation that only patches and long strips of them are left on the borders of the Highlands.

Before we speak of individual mountains and their shapes, it is important to bear in mind another fact about mountain-chains; namely, that they are very low in proportion to their breadth and length. The great heights reached by some mountains produce such a powerful impression on our senses that we hardly realise how very insignificant they really are. It is only by drawing them on a true scale that we can realise this. The surface of the earth is so vast that even the highest mountains are in proportion but as the little roughnesses on the skin of an orange. Fig. 2 (see chap, vii., p. 236) represents a section through the Highlands, drawn on the same scale for height as for length.

What has been said about the Highland plateau applies equally well to many other mountain-ranges. Mr. Ruskin observed something rather similar in the Alps. He says,—

"The longer I stayed in the Alps, and the more closely I examined them, the more I was struck by the one broad fact of there being a vast Alpine plateau, or mass of elevated land, upon which nearly all the highest peaks stood like children set upon a table, removed, in most cases, far back from the edge of the plateau, as if for fear of their falling; ... and for the most part the great peaks are not allowed to come to the edge of it, but remain like the keeps of castles, withdrawn, surrounded league beyond league by comparatively level fields of mountains, over which the lapping sheets of glaciers writhe and flow, foaming about the feet of the dark central crests like the surf of an enormous sea-breaker hurled over a rounded rock and islanding some fragment of it in the midst. And the result of this arrangement is a kind of division of the whole of Switzerland into an upper and a lower mountain world,—the lower world consisting of rich valleys, bordered by steep but easily accessible, wooded banks of mountain, more or less divided by ravines, through which glimpses are caught of the higher Alps; the upper world, reached after the first steep banks of three thousand or four thousand feet in height have been surmounted, consisting of comparatively level but most desolate tracts of moor and rock, half covered by glacier, and stretching to the feet of the true pinnacles of the chain."

He then points out the wisdom of this arrangement, and shows how it protects the inhabitants from falling blocks and avalanches; and moreover, the masses of snow, if cast down at once into the warmer air, would melt too fast and cause furious inundations.

All the various kinds of rocks are differently affected by the atmospheric influences of decay, and so present different external appearances and shapes, so that after a little experience the geologist can recognize the presence of certain rocks by the kind of scenery they produce; and this knowledge is often of great use in helping him to unravel the geological structure of a difficult region. Thus granite, crystalline schists, slates, sandstones, and limestones, all "weather" in their own ways, and moreover split up differently, because their joints and other natural lines of division run in different ways.

Thus granite is jointed very regularly, some of the joints running straight down and others running horizontally, so that the rain and atmosphere seize on these lines and widen them very considerably; and thus the granite is weathered out either in tall upright columns, like those seen at Land's End, or else into great square-shaped blocks with their corners rounded off, presenting the appearance of a number of knapsacks lying one over the other. In this way we can account for the well-known "Tors" of Devonshire, and the "Rocking Stones." Granite weathers rapidly along its joints, and its surfaces crumble away more rapidly than might be expected, considering how hard a rock it is; but the felspar which is its chief mineral constituent is readily decomposed by rain water, which acts chemically upon it. The deposits of China clay in Devonshire are the result of the decomposition and washing away of the granite of Dartmoor.

Granite mountains are generally rounded and "bossy," breaking now and then into cliffs, the faces of which are riven by huge joints, and present a very different appearance from those composed of crystalline schists with their sharp crests and peaks. Ben Nevis and the Cairngorms are partly composed of granite.

Gneiss is a rock composed of the same minerals as granite; namely, mica, quartz, and felspar. And yet mountains composed of this rock have quite a different aspect, and sometimes, as in the Alps, produce very sharp and jagged pinnacles. The reason of this is that gneiss splits in a different way from granite, because its minerals are arranged in layers, and so it is more like a crystalline schist.

Mica-schist is another rock very abundant in mountain regions. This rock is composed of quartz and mica arranged in wavy layers. The mica, which is very conspicuous, lies in thin plates, sometimes so dovetailed into each other as to form long continuous layers separating it from those of the quartz; and it readily splits along the layers of mica. This mineral is easily recognised by its bright, shiny surface. There are, however, two varieties,—one of a light colour and the other black.

Mica-schist and gneiss are often found in the same region, and are the materials of which most of the highest peaks in Europe are composed. We find them abounding in the district of Mont Blanc; and all the monarch's attendant aiguilles, with the splintered ridges enclosing the great snowfields in the heart of the chain, consist mostly of these two rocks. The Matterhorn, Weisshorn, Monte Viso, the Grand Paradis, the Aiguille Verte and Aiguille du Dru are examples of the wonderful forms produced by the breaking up and decay of these two rocks.

The different varieties of slate split in a very marked way. Slates are often associated with the schists, and exert their influence in modifying the scenery.

Limestone ranges, though less striking in the outlines of their crests than those composed of slates and crystalline schists, and not reaching to such heights, are nevertheless not at all inferior in the grandeur of their cliffs, which frequently extend for miles along the side of a valley in vast terraces, whose precipitous walls are often absolutely inaccessible. The beauty of limestone mountains is often enhanced by the rich pastures and forests which clothe their lower slopes. The dolomitic limestone of the Italian Tyrol, being gashed by enormous vertical joints and at the same time having been formed in rather thin layers which break up into small blocks, produces some very striking scenery. But wild as these mountainous ridges may be, their forms can never be confounded with those of the crystalline schists; for however sharp their pinnacles may appear at first sight, careful examination will always show that their outline is that of ruined masonry, suggesting crumbling battlements and tottering turrets, and not the curving, flame-like crests and splintered peaks of the crystalline schists.[30]

It has already been explained that all sedimentary rocks have been formed under water in layers or strata, and it must be obvious that the stratification of such rocks has an important influence on scenery; and very much depends on whether the strata have been left undisturbed, with perhaps just a slight slope, or whether they have been folded and crumpled; for the position of the strata, or "bedding," as it is called,—whether flat, inclined, vertical, or contorted,—largely determines the nature of the surface. Undoubtedly the most characteristic scenery formed by stratified rocks is to be seen in those places where the "bedding" is horizontal, or nearly so, and the strata are massive. A mountain constructed of such materials appears as a colossal pyramid, the level lines of stratification looking like great courses of masonry. The joints that cut across the strata allow it to be cleft into great blocks and deep chasms; so that, as in the case of the dolomitic limestone above mentioned, we find a resemblance to ruined buildings.

We cannot find a better example of this in our own country than the mountains of sandstone and conglomerate (of the Cambrian age) that here and there lie on the great platform of old gneiss in the west of Sutherland and Ross. Sir A. Geikie says,—

"The bleak, bare gneiss, with its monotonous undulations, tarns, and bogs, is surmounted by groups of cones, which for individuality of form and independence of position better deserve to be called mountains than most of the eminences to which that name is given in Scotland. These huge pyramids, rising to heights of between two thousand and four thousand feet, consist of dark red strata, so little inclined that their edges can be traced by the eye in long, level bars on the steeper hillsides and precipices, like lines of masonry. Here and there the hand of time has rent them into deep rifts, from which long 'screes' (slopes of loose stones) descend into the plains below, as stones are detached from the shivered walls of an ancient battlement. Down their sides, which have in places the steepness of a bastion, vegetation finds but scanty room along the projecting ledges of the sandstone beds, where the heath and grass and wildflowers cluster over the rock in straggling lines and tufts of green; and yet, though nearly as bare as the gneiss below them, these lofty mountains are far from presenting the same aspect of barrenness. The prevailing colour of their component strata gives them a warm red hue, which even at noon contrasts strongly with the grey of the platform of older rock.... These huge isolated cones are among the most striking memorials of denudation anywhere to be seen in the British Isles. Quinag, Canisp, Suilven, Coulmore, and the hills of Coygoch, Dundonald, Loch Maree, and Torridon are merely detached patches of a formation not less than seven thousand or eight thousand feet thick, which once spread over the northwest of Scotland. The spaces between them were once occupied by the same dull red sandstone; the horizontal stratification of one hill, indeed, is plainly continuous with that of the others, though deep and wide valleys, or miles of low moorland, may now lie between. While the valleys have been worn down through the sandstone, these strange pyramidal mountains that form so singular a feature in the landscapes of the northwest highlands have been left standing, like lonely sea-stacks, as monuments of long ages of waste."[31]

Again, the vast table-lands of the Colorado region illustrate on a truly magnificent scale, to which there is no parallel in the Old World, the effects of atmospheric erosion on undisturbed and nearly level strata. Here we find valleys and river gorges deeper and longer than any others in the world; great winding lines of escarpment, like ranges of sea cliffs; terraced slopes rising at various levels; huge buttresses and solitary monuments, standing like islands out of the plains; and lastly, great mountain masses carved out into the most striking and picturesque shapes, yet with their lines of "bedding" clearly marked out.

On the other hand, where, as is almost always the case in mountain-ranges, the stratified rocks have been folded, crumpled, twisted, and fractured by great "faults," we find a very different result. In these cases the rocks have generally been very much altered by the action of heat. For here we find crystalline schists, gneiss, granite, and other rocks in the formation of which heat has played an important part; and very often the igneous rocks have forced their way through those of sedimentary origin and altered them into what are called metamorphic rocks (see chapter v., page 156). Thus they have lost much of their original character and structure.

The repeated uplifts and subsidences of the earth's crust, by which the continents of the world have been raised up out of the sea to form dry land, have, broadly speaking, thrown the rocky strata into a series of wave-like undulations. In some extensive regions these undulations are so broad and low that the curvature is quite imperceptible, and the strata appear to lie in horizontal layers, or to slope very slightly in a certain direction. This is, in a general way, the position of the strata of which plains and plateaux are composed.

But in the longer and comparatively narrow mountain regions that traverse each of the great continents, forming, as it were, backbones to them, the undulations are very much more frequent, narrower, and higher. Sometimes the rocks have been thrown into huge open waves, or the folds are closely crowded together, so that the strata stand on their ends, or are even completely overturned, and thus their proper order of succession is reversed, and the older ones actually lie on the top of the newer ones.

As we approach a great mountain-chain we observe many minor ridges and smaller chains running roughly parallel with it, and, as it were, foreshadowing the great folds met with in the centre of the chain and among its highest peaks. These small folds become sharper and closer the nearer we get to the main chain, and evidently were formed by the same movements that uplifted the higher ranges beyond; but the force was not so great. Thus we find the great Alpine chain flanked to the north by the smaller ranges of the Jura Mountains; and on the south, side of the Himalayas we find similar smaller ranges of hills.

Ruskin thus describes his impression of the Jura ranges, which he very aptly compares with a swell on the sea far away from a storm, the storm being represented by the wild sea of Alpine mountains:—