Читать книгу The Story of the Hills: A Book About Mountains for General Readers - H. N. Hutchinson - Страница 8

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The valleys only feed; the mountains feed and guard and strengthen us.—Ruskin.

It is not an exaggeration to say that there are no physical features of the surface of the earth which render such a variety of services as mountains. The operations which they perform involve such far-reaching consequences that it is difficult to say where their effects cease. Indeed, it might almost be maintained that they are the mainspring of the world,—as far as its surface is concerned,—for it would fare ill with mankind if they were removed or in some way destroyed. Things would then very soon come to a standstill. The soil would become exhausted; streams would cease to flow; and the world would become a kind of stagnant pool.

The three main services of the hills are these:—

1  I. Mountains help to condense water-vapour from the atmosphere, thus bringing back to the earth moisture which it loses continually by evaporation.

2 II. Mountains are elevated reservoirs of water in one form or another, and thus not only feed the streams and rivers, but give them force and direction as well.

3 III. Mountains suffer themselves to be slowly worn away in order that the face of the earth may be renewed; in other words, they die that we, and all created things, may live.

I. Mountains help to condense water-vapour from the atmosphere, thus bringing back to the earth the moisture which it loses continually by evaporation. Every one knows that there is abundance of water-vapour in the atmosphere, but the question arises, How does it get there? The answer to this lies in the simple fact that every surface of water exposed to the air undergoes loss by evaporation. If you wish to satisfy yourself on this point, place a saucer of water in your room, and in a few days it will all be gone. We hang clothes out to dry, and so avail ourselves of this curious power that air has of taking up water in the form of vapour. Steam, or water-vapour, is really invisible, though we frequently talk of seeing the steam issuing from a locomotive; but what we really see is a cloud of condensed steam, and such clouds,[7] like those that we see floating in the air, are really masses of little tiny particles of water which can reflect or throw back the light which falls upon them, and thus they become visible. Again, a kettle of water, if left too long on the fire will entirely boil away. It is all turned into steam, and the steam is somehow hidden away in the air, though a little of it will be condensed into slight clouds by the colder air outside the kettle.

But how can water stow itself away in the air without being seen or felt?

An illustration may help to explain this. Suppose you scatter a spoonful of small shot over a carpet or a dark-coloured table-cloth; you would probably not be able to see them at a little distance. Now, gather them together in a heap, and you see them at once. The heap of shot in some ways resembles a drop of water, for in a drop of water the tiny particles (or molecules) of which it is composed are close together; but by heating water you cause them to fly asunder and scatter themselves in various directions. They are lost to sight, and moreover have no power of attracting each other or of acting in concert; each one then takes its own course, whereas in the drop of water they were in some wonderful way bound together by mutual attraction. They dance in groups; but the rude force of heat will scatter these little dancing groups, and break them up into that state which we call a state of vapour.

The forces of heat and cohesion are directly at variance; and it is just a question of degree whether the one or the other gets the mastery in this "tug of war." The more you heat the water, the faster the little groups of molecules break up and disappear in the air. They must in some way go moving between the particles of air, and collisions keep taking place with inconceivable rapidity.

And now another question arises; namely, how much water-vapour can the air take? That depends chiefly on its temperature. Air when heated will take up a great deal of steam; and the more you heat air, the more it can take up. When air at a given temperature can take up no more, it is said to be saturated for that temperature; but if the temperature be raised, it will immediately begin to take up more. For each degree of temperature there is a certain amount of water-vapour which can be absorbed, and no more. But suppose we take some air which is already saturated and lower its temperature by giving it a sudden chill, what will happen? It will immediately give up part of its steam, or water-vapour; namely, the exact amount which it is unable to contain at the lower temperature.[8]

There are various ways in which you can test this matter for yourself. For instance, take a hand-glass, and breathe on it. You know what will happen: a film of moisture forms upon it; and you know the reason why. It is simply that the cold glass gives a chill to one's breath (which being warm is highly charged with water-vapour from the lungs), and so some of the vapour is at once condensed. Now, this serves very well to explain how mountains catch water-vapour, and condense it. They are, as it were, a cold looking-glass; and the hot breath of the plains, as it strikes their sides, receiving a sudden chill, throws down part of the vapour it contains. On the higher parts of mountain-ranges the cold is so great that the water assumes the form of snow.

CLOUDS ON BEN NEVIS

Mountains, as every one knows, are colder than the plains below. No one cares to stay very long on a mountain-top, for fear of catching cold. It may be worth while to consider why they are cold. Perhaps you answer, "Because they are so high." That is true, but not a complete answer to our question. We must look at the matter a little more closely. The earth is a warm body surrounded by space in which the cold is inconceivably intense; but just as we protect our bodies against cold with garments, so the earth is wrapped up in an atmosphere which serves more or less to keep in the heat. All warm bodies give out heat as luminous bodies give out light; but the rays of heat, unlike those of light, are quite invisible to our eyes, so that we are unaware of them. These "dark heat-rays," as they are called, do not make any impression on the retina, because our eyes are not capable of responding to them as they do to the ordinary rays of light. But there is a delicate little instrument known as the thermopile, which responds to, and so detects these invisible rays; and if our eyes were sensitive to such vibrations as these, we should see heat-rays (which like light and sound are due to vibrations) streaming from every object, just as light does from a candle-flame.

Those parts of the earth which are least covered or protected by the atmosphere lose heat most rapidly,—in the same way that on a frosty day one's fingers become cold unless covered up. Now, there is less air over mountains; and in those higher regions above the peaks what air there is, is more rarefied, and therefore less capable of stopping the heat-rays coming from the earth. Professor Tyndall has shown that water-vapour in the air has a great power of stopping dark heat-rays; and the lower regions, which contain more vapour, stop or absorb a good deal of heat which would otherwise escape into space.

Look at a map of any continent, and you will see the rivers streaming away from the mountains. All those vast quantities of water come from the atmosphere; and mountains do a large share of the work of condensing it from the state of vapour to that of water. Take the map of India, and look at the great range of the Himalayas. At their feet is the hot valley of the Ganges, which meets that of the Brahmapootra River. An immense amount of evaporation takes place from these mighty rivers, so that the air above them becomes laden with water-vapour. Farther south is the tropical Indian Ocean, from which the direct rays of the sun draw up still vaster quantities of water. And so when south winds blow over India, they are full of water-vapour; and presently they strike the flanks of the Himalayas, and at once they are chilled, and consequently part with a large amount of the vapour which they contained. This is best illustrated by the case of the southwest monsoon wind of the summer season, which sets in during the month of April, and continues to blow steadily towards the northeast till October. After leaving the Bay of Bengal, this warm wind, laden with vapour, meets ere long with the range known as the Khasi Hills, and consequently throws down a large part of its vapour in the form of rain. The rainfall here in the summer season reaches the prodigious total of five hundred inches, or about twenty times as much as falls in London during a whole year. After passing over these hills, the monsoon wind presently reaches the Himalayas; and another downpour then takes place, until by the time it reaches the wide plains of Thibet, so much water has been given up that it becomes a very dry wind instead of a moist one.

It must not be supposed, however, that the condensation effected by mountains is entirely due to this coldness. They have another simple and effective way of compelling the winds to give up rain: their sloping sides force the winds which strike them to ascend into higher regions,—wedging them up as waves run up a sloping stony bank on the seashore,—and when the winds reach higher regions of the atmosphere they must (as explained above) suffer loss of heat, or in other words, have their temperature lowered. They also expand considerably as they rise into regions where the atmospheric pressure is less; and as every gas or vapour loses heat in the act of expansion, they undergo a further cooling from this cause also.

We have now learned that the cooling process is brought about in three different ways: (1) By contact with the cold body of the mountains; (2) By giving out heat into space; (3) By expansion of the air as it reaches into the higher regions of the atmosphere. The "cloud-caps" on certain mountains and promontories are to be explained by all these causes combined.

The west coast of Great Britain illustrates the same thing on a smaller scale. There the warm waters of the Gulf Stream, travelling in a northeasterly direction straight away from the Gulf of Mexico, strike the west coast of Ireland, England, and Scotland; and as most people are aware, the mild climate of Great Britain is chiefly due to this fact. If you contrast for a moment the east and west coasts of Britain, you will see that the latter is much more rocky and mountainous than the east coast. Mountains run down nearly all our western coasts. Now, it is this elevated and rocky side of Britain which catches most of the rain. Very instructive it is to compare the annual rainfall in different parts of Britain. On Dartmoor about 86 inches of rain fall every year, while in London only about 24 inches fall annually; but then London has no range of mountains near, and is far away from the west coast. Again, while people in Ambleside have to put up with 78 inches of rain, in Norfolk they are content with the modest allowance of 24 inches or so. At a place called Quoich on the west coast of Scotland, about 117 inches fall every year. These differences are chiefly due to the different contour of the land down the west side of Britain, which is mountainous, while the east side is flat, and also to the fact that while easterly winds, which have come over the continent, are dry, our prevailing winds are from the west and southwest, and are consequently heavily laden with vapour from the Atlantic Ocean. These winds follow the direction of the Gulf Stream, driving it along before them; and in so doing they take up large quantities of vapour from its surface. When these warm winds touch our western coasts, they receive a chill, and consequently are no longer able to contain all the vapour which they bring with them, and so down comes the rain.

II. Mountains are elevated reservoirs of water in one form or another, and thus not only feed the streams and rivers, but give them force and direction as well. It is very important that the mountains should not allow the waters they collect to run away too fast. Try to think for a moment what would happen if instead of being, as it were, locked up in the form of snowfields and glaciers, the water were all in the liquid form. It would soon run away, and for months together the great river-valleys would be dry and desolate. When the rain came, there would be tremendous floods; dire destruction would be wrought in the valleys; and very soon the great rivers would dwindle down to nothing. Vegetation too would suffer seriously for want of water during the summer months; and the valleys generally would cease to be the fertile sources of life which they are at present. The earth would become for the most part like a stagnant marsh.

But in the higher mountain regions there is a beneficent process going on which averts such an evil. The precious supplies of water are stored up in the solid forms of snow and ice. Now, we all know that snow and ice take a long time to melt; and thus Nature regulates and like a prudent housewife economises her precious stores. The rivers which she feeds continually, from silent snowfields and glaciers among her mountain-peaks, are the very arteries and veins of the earth; and as the blood in our bodies is forced to circulate by pressure from the heart, so the rivers are compelled to flow by pressure from the great heart of the hills,—slow, steady, continuous pressure, not the quick pulses which the human heart sends through the body.

And again, as the blood, after circulating through the body in an infinite number of life-giving streams, returns to the heart once more on its journey, so the thousand streams which wander over the plains find their way back to the heart of the mountains, for the water is brought there in the form of vapour and clouds by the winds.

When we build water-towers, and make reservoirs on high ground to give pressure to the water in our pipes, and make it circulate everywhere,—even to the tops of our houses,—we are only taking a hint from Nature. The mountains are her water-towers, and from these strong reservoirs, which never burst, she commands her streams, forcing them along their courses in order that they may find their way to the utmost bounds of continents.

But there is another way in which mountains regulate the supply of water, and prevent it from running away too fast,—one not so effective as the freezing process, but still very useful, because it applies to the lower hills below the line of perpetual snow. This may be well illustrated by the state of some of the Scotch hills in the middle of summer or autumn, when there is little if any snow resting upon them.

Any one familiar with these hills will have noticed how full of water their sides are. Tiny threads of streams trickle slowly along everywhere; peat-beds are saturated with dark-brown water; even the grass and soil are generally more or less wet, especially under pine forests. One can generally get a cup of water somewhere, except after a long dry summer, which is exceptional. Then there is the dew forming every night. Forests with their undergrowth of soil—moss and fern—also help very considerably to check the flow of water. We have often asked ourselves when watching some swift-flowing river, "Where does all this water come from? Why does it not dry up in hot weather?" The answer came fully after we had climbed several mountains, and seen with our eyes the peat-beds among the hills, and heard the trickling of the tiny rivulets hurrying along to feed some neighbouring burn, or perhaps to run into some mountain tarn or loch, and noticed the damp, spongy state of the soil everywhere,—not to mention the little springs which here and there well up to the surface, and so contribute their share.

The rivers and streams of Scotland assume various tints of amber and dark-brown, according to the amount of rain which has recently fallen. These colours are due to organic matter from the peat. Compare Scott's description of the Greta:—

"In yellow light her currents shone,

Matching in hue the favourite gem

Of Albion's mountain diadem."

The waters of some Scotch rivers after heavy rain look as black as pitch.

Nor must we omit the lakes which abound in most mountain regions, and serve as natural reservoirs for the rivers, besides giving a wonderful charm to mountain scenery.

The largest lakes in mountainous regions are found on the courses of the rivers; and there is good reason to believe that they were formed, not by any process of subsidence, but by the same operations that carved out the valleys. In many cases they are due to the damming up of a stream. But in some countries the streams dry up during summer,—in Palestine or Sinai, where there is but little soil on the hills, and consequently hardly any vegetation. Such barren hills cannot hold the continual supplies which pour gently forth from the mountains of higher latitudes.

The Alps feed four of the principal rivers of Europe. We cannot do better than quote Professor Bonney, whose writings on the Alps are familiar to all geologists. In his "Alpine Regions of Switzerland" the following passage occurs:—

"This mass of mountains, the great highlands of Europe, is therefore of the utmost physical and geographical importance. Rising in places to a height of more than fifteen thousand feet above the sea, and covered for an extent of many thousand square miles with perpetual snow, it is the chief feeder of four of the principal rivers in Europe,—the Po, the Rhone, the Rhine, and the Danube. But for those barren fields of ice, high up among the silent crags, the seeming home of winter and death, these great arteries of life would every summer dwindle down to paltry streams, feebly wandering over stone-strewn beds. Stand, for example, on some mountain-spur, and look down on the Lombardy plain, all one rich carpet of wheat and maize, of rice and vine; the life of those myriad threads of green and gold is fed from these icy peaks, which stand out against the northern sky in such strange and solemn contrast. As it is with the Po, so it is with the Rhine and the Rhone, both of which issue from the Alps as broad, swelling streams; so, too, with the Danube, which, although it does not rise in them, yet receives from the Inn and the Drave almost all the drainage of the eastern districts."

A very little reflection will serve to convince any one how vastly important and beneficial is the slope of the mountains, and how it gives force and direction to streams and rivers. Without this force, due to universal gravitation, by which the waters seek continually lower levels, the supplies in the hills would be useless. Mere lakes on flat surfaces would not answer the purpose; and so the sources of water are elevated in order that it may pour over the world below.

No writer has given such fascinating descriptions of mountains as Mr. Ruskin; and no one has more eloquently described the functions they perform. In the fourth volume of his "Modern Painters," which every one who cares for mountains should read, we find the following beautiful passage:—

"Every fountain and river, from the inch-deep streamlet that crosses the village lane in trembling clearness, to the massy and silent march of the everlasting multitude of waters in Amazon or Ganges, owe their play and purity and power to the ordained elevations of the earth. Gentle or steep, extended or abrupt, some determined slope of the earth's surface is of course necessary before any wave can so much as overtake one sedge in its pilgrimage; and how seldom do we enough consider, as we walk beside the margins of our pleasant brooks, how beautiful and wonderful is that ordinance, of which every blade of grass that waves in their clear waters is a perpetual sign,—that the dew and rain fallen on the face of the earth shall find no resting-place; shall find, on the contrary, fixed channels traced for them from the ravines of the central crests down which they roar in sudden ranks of foam to the dark hollows beneath the banks of lowland pasture, round which they must circle slowly among the stems and beneath the leaves of the lilies; paths prepared for them by which, at some appointed rate of journey, they must evermore descend, sometimes slow, and sometimes swift, but never pausing; the daily portion of the earth they have to glide over marked for them at each successive sunrise; the place which has known them knowing them no more; and the gateways of guarding mountains opened for them in cleft and chasm, none letting them in their pilgrimage, and from afar off the great heart of the sea calling them to itself: 'Deep calleth unto deep.'"

Geologists, however, do not in these days teach that the present paths of rivers were made for them, but rather that the rivers have carved out their own valleys for themselves. The old teaching before the days of Lyell and Hutton, the founders of modern geology, was that valleys were rents in the rocks of the earth's crust formed by some wonderful convulsion of Nature, whereby they were cracked, torn asunder, and upheaved. But a careful study of rivers and their valleys for many years has shown that there is no evidence of such sudden convulsions. The world is very old indeed, and rivers have been flowing much as we see them for ages and ages. A few thousand years is to the geologist but a short space of time; and there can be no doubt that a stream can in the course of time carve out for itself a valley. The operations of Nature seem slow to us because our lives are so short, and we can see so little change even in a generation; but the effects of these changes mount up enormously when continued through a long space of time. Nature works slowly; but then she has unlimited time, and never seems in a hurry. It is like the old story of the hare and the tortoise; and the river, working on steadily and quietly for hundreds or thousands of years, accomplishes far more in the end than sudden floods or violent catastrophes of any sort.

III. Mountains suffer themselves to be slowly worn away in order that the face of the earth may be renewed; in other words, they die that we, and all created things, may live. The reader will find a full account of the methods by which these results are accomplished in chapters v. and vii., and therefore we must not anticipate this part of the subject. Let it suffice for the present to say that this destruction of the hills is brought about by the action of heat and cold, of rain and frost, of snow and ice, and the thousand streams that flow down the mountain-sides. It is with soils that we are chiefly concerned at present. Try to think for a moment of the literally vital consequences which follow from the presence of good rich soils over different parts of the earth, and ask whether it would be possible for civilised races of men to flourish and multiply as they do if it were not for the great fertile valleys and plains of the world. Mountain races are neither rich nor powerful. Man exists mainly by cultivation of the soil; and among mountains we only find here and there patches that are worthy of the labour and expenditure of capital involved in cultivation. But in the great plains, in the principal river-valleys of the world, and among the lesser hill-ranges it is different. The lowlands are the fertile regions. All great and powerful nations of the world are children of the plains. It was so in the past; it will be so in the future, unless men learn to feed on something else than corn, milk, and flesh, which is not very likely.

The Egyptians, the earliest civilised race of which we have satisfactory records, dwelt in the fertile valley and delta of the Nile. They clearly perceived the value of this great river to themselves, and worshipped it accordingly. They knew nothing of its source in the far-away lakes of Central Africa; but they knew truly, as Herodotus tells us, that Egypt was "the gift of the Nile," for the alluvial soil of its delta has been formed by the yearly floods of that great river, as its waters, laden with a fine rich mud, spread over its banks, and for a time filled the valley with one sheet of water. The Assyrians and Babylonians had their home in the valley of the Euphrates and Tigris. The Chinese, too, have their great rivers. Russia is well watered by powerful rivers. The most populous parts of the United States of America are watered by the great Mississippi, and the other rivers which flow into it. England, Germany, and France are furnished with well-watered plains.

Soils are the chief form of national wealth. Minerals, such as coal and iron, are of course extremely valuable, and help to make an industrious race rich; but the land is the main thing, after all, and by land we mean soil. The two words are almost synonymous. But since the soil is formed chiefly of débris brought from the mountains, it would be more true to say that these are the real sources of wealth. Soils contain besides a large amount of valuable organic matter (that is, decayed matter which has once had animal or vegetable life) different kinds of minerals, which are necessary to the support of plant life: potash, soda, carbonate of lime, silica, magnesia, iron, phosphorus, and manganese in their various compounds are all present in the rocks of which mountains are composed. We must again fall back upon "Modern Painters" for an effective description of the forming of soil by destruction of the hills:—

"The higher mountains suffer their summits to be broken into fragments and to be cast down in sheets of massy rock, full, as we shall presently see, of every substance necessary for the nourishment of plants; these fallen fragments are again broken by frost, and ground by torrents into various conditions of sand and clay,—materials which are distributed perpetually by the streams farther and farther from the mountain's base. Every shower that swells the rivulets enables their waters to carry certain portions of earth into new positions, and exposes new banks of ground to be moved in their turn.... The process is continued more gently, but not less effectively, over all the surface of the lower undulating country; and each filtering thread of summer rain which trickles through the short turf of the uplands is bearing its own appointed burden of earth down on some new natural garden in the dingles beneath."

It may be laid down as a simple economic truth, that no nation can be powerful, rich, or prosperous, unless it possess in the first place a good soil. Other conditions, such as large navigable rivers, a good seaboard for harbouring ships, are also important; but unless the land will yield plenty of food, the population cannot be very great, for people must be fed. Foreign supplies of corn at a low price, meat and provisions of various kinds, supplement what is grown in England; but without a good soil we could not have become a powerful nation.

A high state of civilisation is in a large measure to be traced to climate and soil. The sequence is somewhat as follows:—

Mountains collect rain.

Rain fills the rivers.

Rivers make rich alluvial plains.

Agriculture follows; and food is produced.

Abundant food maintains a large population.

The population works to supply its various wants; such as roads, railways, ships, houses, machinery, etc. Then follows exchange with other countries. They send us what they can best produce, and we send them what we can best and most easily produce, and so both parties gain.

Thus towns spring up. Education, refinement, learning, and the higher arts follow from the active life of towns, where more brain-work is required, and the standard of life is higher.

And thus we may, in imagination, follow step by step the various stages by which the highest phases of civilisation are brought to pass, beginning at the mountains and ending with human beings of the highest type,—the philosopher, artist, poet, or statesman, not omitting the gentler sex, who are often said to rule the world.

The following lines of Milton possess, in the light of these facts, a deeper meaning than the poet probably intended to convey:—

"Straight mine eye hath caught new pleasures

Whilst the landscape round it measures:

Russet lawns and fallows grey,

Where the nibbling flocks do stray;

Mountains on whose barren breast

The labouring clouds do often rest;

Meadows trim with daisies pied,

Shallow brooks and rivers wide;

Flowers and battlements it sees

Bosomed high in tufted trees,—

Where perhaps some beauty lies,

The cynosure of neighbouring eyes."

With a little rearrangement of the lines, the sequence we have indicated above would be well illustrated. The mountains must come first; then the clouds, ready to bring forth their rain; then the brooks and rivers, then "russet lawns and fallows grey,"—with their "nibbling flocks." Then come the human elements in the scene,—the "towers and battlements," containing armed warriors, well fed, no doubt, and ready to do their master's bidding; lastly, the lady who adorns the home of her lord, and, let us hope, makes it worth fighting for.

For commercial purposes, large navigable rivers are of great use. And in spite of the modern railway, rivers still exert an influence in determining the routes followed by trade. London, Liverpool, Glasgow, and other busy centres of life owe their importance to the rivers which flow through them, especially since they are tidal rivers. Heavily laden barges may be seen from London Bridge going up and down with the tide every day.

Since the direction as well as the existence of large rivers is regulated by mountains, it is clear that mountains have a very direct influence on the trade of the world.

Mountains supply many of our wants. Besides water and soil, how many useful things come from the hills! Their slopes, watered by the clouds, frequently support an abundant growth of pine forest; and thus we get wood for the shipwright and joiner. Again, mountains are composed of harder rocks than we find in the plains, and that is one reason why they stand out high above the rest of the world. Their substance has been hardened to withstand for a longer time the destruction to which all rocks are subjected. They have been greatly compressed and generally more or less hardened by subterranean heat. We bake clay and make it into hard bricks; so Nature has baked and otherwise hardened the once soft strata of which mountains are chiefly composed, converting them into slates, schist, gneiss, and other kinds of rock called "metamorphic" by geologists, because they have been altered or metamorphosed from their original condition (see chapter viii., page 277). Again, granite, basalt, and other rocks known as "igneous," which once existed in a molten condition, have forced their way up from subterranean regions into the rocks forming mountain-chains; and a good deal of the hardening just alluded to is due to the presence of these fiery intruders, which have baked and hardened the rocks around them to a considerable extent, altering at the same time their mineral composition. The same causes which led to the injection of granite, basalt, and other igneous rocks in mountain-ranges brought other consequences in their train. Whatever the causes, they were closely connected with volcanic eruptions, so that highly heated water and steam found their way through cracks and other fissures in the rocks; and in the course of time the chemical actions thus set up led to the deposition of valuable metallic ores within these fissures. In this way mineral veins were formed; and volcanic action seems to be largely responsible for the production of minerals. Thus we find around Vesuvius, and in fact in all volcanic regions, large and varied supplies of minerals. Now, the geologist discovers that many mountain-chains—such, for example, as the Grampians, Alps, and Carpathians—have in past geological periods been the seats of volcanic action on a grand scale; and so we need not be surprised to learn that mountainous countries yield large supplies of valuable gems and metallic ores (see chapter viii., page 277). Even in the days of Solomon, the active and business-like Phœnicians were carrying on trade with Great Britain; and the tin came from Cornwall. Besides tin, gold, silver, lead, copper, zinc, and other metals come from our hills. Now, however, we get our copper mostly from the Andes, and our gold from Australia or South Africa, because it can be got more cheaply from these countries, to which many of our Cornish miners have emigrated.

Precious stones also come chiefly from the hills, for the same reason; for they were formed at the same time and by the same causes. Cairngorms, agates, chalcedony, jasper, onyx, topaz, diamonds, and many other gems are silent but certain witnesses to the action of subterranean heat, acting long ago on the rocks which we now see standing up high above the general surface of the ground, though once they were buried deep down below the surface. Diamonds as well as gold are often got from the beds of streams, but this is easily accounted for; the streams have washed them out and brought them down from the hills.

The following words from the Book of Job (xxviii. 5) might well be applied to the hills.

"As for the earth, out of it cometh bread:

And underneath it is turned up as it were by fire.

The stones thereof are the place of sapphires,

And it hath dust of gold."

We have thus explained the three principal services rendered by mountains, but some others remain to be mentioned.

Mountains have an important influence on climate. The climate of highlands everywhere has certain peculiarities which distinguish it from that of adjacent lowlands. The air resting on mountains is less dense than that of the lowlands, and hence has fewer molecules to obstruct the entering sunbeams by day, or to stop the outward radiation at night. Therefore mountain air must be cooler; and so we find that on mountains the mean, or average, annual temperature is lower. This rarity of the air causes the ground to become hotter by day and colder by night than the ground of the plains; and so the extremes of temperature are greater. These extremes are injurious to vegetation in the higher regions, and the want of moisture still more so. But mountain-slopes up to a certain height usually have a moist climate; that is, they have more clouds and rain than the surrounding lowlands. Below the region of snow there is generally a heavy growth of forest; and forests in their turn exercise an important influence, helping to collect moisture, and in various ways to prevent extremes either of heat or cold.

The earth is divided into three well-marked zones or belts of climate: (1) The torrid zone within the tropics, where the sun is vertical twice a year, and days and nights are nearly equal; (2) The temperate zones, where the sun's rays come more obliquely, and so are less powerful, and where the length of day and night varies considerably; and (3) The frigid zones, round each of the poles, regions of intense cold, where for six months of the year the sun is never seen. Now, these broad divisions, so familiar to school children, are considerably interfered with by the height of various districts above the sea-level, or, as geographers say, by altitude. High ranges of mountains bring somewhat arctic conditions with them, even in low latitudes, where one would expect great heat. Thus the climate of the plains is very different from that of their neighbouring mountain-ranges, although their latitudes are practically the same. Travellers in Switzerland know how hot it can be in the Rhone Valley or in the plain of Lombardy, and how much cooler it is when you get up among the glaciers and the snowfields. Or to take an illustration from Great Britain: a hot summer would be somewhat trying in Edinburgh, Glasgow, or even Inverness, because they lie low, while among the Grampians, on Speyside, or Braemar, it would be very pleasant.

Vegetation follows climate. The sultry plains of the Ganges show a luxuriant tropical vegetation, while on the middle slopes of the Himalayas the climate is temperate, like that of Europe, and consequently the vegetation resembles that of a temperate region; and the highest parts of this great range are like polar latitudes in their climate, and partly also in their vegetation.

The arctic character of the climate of high mountain regions shows itself in the flora; for on the High Alps and the Highlands of Scotland and Norway, we find no small number of truly arctic plants whose home is much farther north. A very long time ago, when the climate of the whole of Northern Europe was extremely severe, and when great glaciers descended from the mountains into the plains, so that the aspect of the country was somewhat similar to that of Greenland at the present day, arctic plants and animals came down from their northern home, and flourished abundantly. This was during the Great Ice Age, which has left behind unmistakable evidences which the geologist can interpret as if they were written records. Then for some reason the climate became milder, the glaciers melted away, in Great Britain at least; but these arctic plants were left behind, and flourished still on the cool mountains, though they died out on the warm plains (see chap. iv., pp. 123-124).