IGNEOUS ROCKS—LAND AND WATER—SPRINGS, WELLS, AND GEYSERS—SNOW AND ICE—THEIR EFFECTS.

In the foregoing pages we have chiefly considered the stratified rocks, but we are now approaching another branch of our subject—viz., “Physiography,” which, as distinguished from the usual so-called Physical Geography, will deal with the phenomena of the earth, air, and water, thus leading us to Meteorology as a conclusion.

We have arrived at a certain knowledge concerning the Earth as a planet, her place in the universe, and the composition of the “Crust,” as it is termed. We have examined the stratified rocks, which include sand and gravel, stones, and boulders equally. To a geologist they are all “rocks.” We must now examine the igneous rocks, which bear an important part in the structure of the Earth, whose surface we have now more minutely to examine. It has already been stated (p. 571) that igneous rocks have been upheaved while in a state of fusion—that is, while in a melted condition. These igneous, or fire-produced rocks, are divided into classes, just as the unstratified rocks are, and the divisions are called the Volcanic and Plutonic, including “Basic” and “Acidic,” according as they are possessed of less silica or more.

Sometimes the igneous rocks are classed as volcanic, trappean (from trappa, a stair, such as in the Giant’s Causeway), and granitic. The volcanic in such case being the modern or upper rocks, such as lava, scoria, etc., which, having been cast up by volcanoes, are of comparatively recent formation.

The Volcanic rocks, then, are of recent date, comparatively speaking; they form the constituent portions of the volcanoes of the present day, and are found as basaltic formations. They are traced as far back as the Tertiary period of the globe. Amongst the volcanic rocks we find basalt, augite, porphyry, serpentine, pumice, pitchstone, felspar, etc. But no doubt volcanic action has been going on ever since the beginning of the world as it is now, and will continue to do. It is somewhat curious that the very old igneous rocks should not be more evident.

The Plutonic rocks do not differ essentially from the foregoing. There is less quartz and more hornblende; and if the ages during which these formations have been existent in the earth-depths after they became solidified be considered, the differences will be fully accounted for. The greenstones and syenites are prominent amongst the plutonic series.

These plutonic and volcanic rocks are separated into basic and acidic, as already remarked, but the line cannot be drawn very distinctly. Granite is the chief plutonic (acidic) rock, and we frequently find it forced upwards into other strata, its essentially eruptive character being thus decided. That granite must have taken an immense time to solidify and crystallize is evident, for no new granites are ever found. We find granite in all the old mountain chains—such as the Grampians in Scotland, and the Wicklow mountains. Our chief European (active) volcanoes are, so to speak, modern, as may be supposed when their constituents are known. It may be said that granite was first deposited as sediment heated by subterranean fire, and forced up by thermal action of water to the mountains, where it is uncovered by a slow process of denudation and surface washings of the earth.

Now without at present going any farther into the causes of volcanoes we can see at a glance that the eruption of the igneous rocks must have created a marked and essential difference in the physical geography of the globe. It is to these eruptions that the dislocation and disturbance of the stratified formations are due. The igneous rocks present ridges in the mountains; sometimes they are rounded at the summits, while the aqueous and metamorphic rocks are disposed in layers.

These two classes in their varieties form the land and the crust of the earth, which is ever being acted upon by air and water. The ice, again, polishes and scratches the valleys in which it moves. The loosened boulders that tumble from the mountains are carried down by the ice, and deposited in the glacier moraine, whence flows a stream. By degrees the stone is ground up, and carried away in the water to form sediment in a “delta” at the embouchure, or to lie beneath the surface and form rock once more. The igneous rocks, composed of lavas and ashes, are volcanic rocks, deposited deep down, and then after the lapse of ages disclosed by the action of air and water.

The consideration of the land and water upon the globe shows us that they are distributed over the earth very unequally. There is nearly three times as much water in our planet as there is land, and these proportions could not be altered without giving rise to phenomena, the results of which cannot be properly estimated. Our earth has an area of 197,000,000 of square miles; about 52,000,000 of this is land, and about 145,000,000 of it water; so about three-quarters of the globe is made up of water. The first portion of our subject therefore should be directed to the examination and consideration of water, and the phenomena which arise from its presence upon the earth.

We need not go into details which every geography indicates. We will try to trace the sources, not the plain effects, which all can afterwards study from special books. In a preceding portion of this volume we have explained the chemical composition of water, and we showed by experiment that it is a fluid composed of oxygen and hydrogen gases, in the proportions of one to two volumes respectively. No matter in what form water may appear,—as water, as ice, or as steam,—these proportions never vary in pure water (see p. 352). But water on the earth is seldom, or never, pure. We know the difficulty we have to procure good drinking water, and though it may be filtered, there will remain natural salts, which are found in different degrees in all water upon the globe. We know the rain, which is perfectly pure when condensed from the clouds, absorbs carbonic acid, etc., from the atmosphere. We have shown how this water as soon as it comes upon the earth attacks the rocks, and as it progresses carries away lime. After descending deep down, it rises again in the form of Springs.

Now what are these springs? They are the result of percolation of rain-water through certain strata. When water falls it is absorbed into the ground, unless it happens to rest upon an impermeable rock, in which case it becomes a rivulet. But it can penetrate between the atoms of many rocks, and thus falls through sand and harder rocks, till it reaches a stratum which will not receive it—like clay. We then find that it will flow away in a spring, or if tapped will be an Artesian well. These water-wells are of very ancient date, but the name is more modern.31 The springs flow out, and develop, with the assistance of tributaries, into rivers. These again receive more tributaries, which swell the volume of their waters, and widen out, carrying millions of gallons hourly to the sea with sediment and gravel and stone.

Water has enormous power of disintegration. We have only to cast our eyes upon the illustrations in any volume of continental travel in Europe or America to perceive the gorges and caÑons worn out by the resistless and frequently gently-flowing river to estimate the part which water plays in Physical Geography and Meteorology.

But springs occur not only in the case mentioned; there are mineral springs, hot springs, and oil springs, all following the same rules of nature. The Artesian well has been mentioned. The Geysers of Iceland have often been portrayed, and are amongst the most wonderful phenomena of nature. These will serve as a type of the other thermal springs, of which the districts of the Yellowstone in North America afford perhaps the most extraordinary instances. These are intermittent springs, and the water rises to a great height, at intervals of about an hour and a half; and after many successive attempts, or trials, as it were, the geyser shoots up to a great height enveloped in steam.

The cause of these well-known phenomena have been explained by Bunsen, and it has already been referred to. We know that at a certain air-pressure water boils at 212° (Fahr.), but on mountains at less pressure it will boil before that degree, because the air is rarefied. So conversely, under the ground, it may reach 212° without boiling. So the surface (warm) water falls, and reaches a high temperature before it is converted into steam. When it is so converted, the vapour is formed very rapidly, and the expansive force is tremendous, shooting up the water and all the contents of the tube with terrific violence, and with a beautiful effect. Pressure therefore alters the boiling point of water.

The mineral springs of Bath and many continental towns owe their properties to the solvent power of water, which assimilates the mineral atoms and gases. They arise just in the same way as the ordinary spring, the taste and smell depending upon the soil and strata. Perhaps the oil wells are the most curious phenomena of this kind. They are excavated upon the Artesian principle. The petroleum is bored for, as we bore for water, and the oil rushes up with great force, and in enormous quantities. Gas wells are also to be found in Pennsylvania, and have supplied towns with gas for years. Both these Artesian wells are caused by the decay of vegetation. The gas is in the coal formation, and the oil has been pressed out from vegetable deposit, and as anthracite is a stony coal, petroleum is a kind of coal-tar, of natural formation.

We have alluded to the river, which emerges from the spring, which has fallen as rain. But there is another, and, to many minds, a much more interesting form of the universal fluid we call water. This is ice. Familiar as ice is, either to the stay-at-home invalid, the skater, and the traveller, there is a great deal to be said about it. It is a subject we would dwell upon had we space, for the remembrance of many a pleasant hour passed upon snow and glacier call upon us to go back again, even though only in imagination. No one who has not climbed the glacier—even the Mer de Glace to the Jardin, now such a common excursion—can fail to be struck with the beauty and grandeur of the scene presented to him, and to carry away a fond recollection of the icy regions he penetrated.

For the ordinary hard-working man there is no change, no rest so truly beneficial as a trip amongst the mountains and snowfields of Europe. He need not be a climber; that is, a climber like Tyndall or Whymper, those giants of the Alpine Club. But a stroll up to the Bel Alp, the Æggishhorn, the Riffel, the Montanvert, or the Grimsel, will give the average pedestrian some of the finest glacier scenery in Europe, and which may, we believe, compare with any in the world for beauty. These glaciers—ice-rivers—we will now consider briefly. We may take the Mer de Glace as an example (see the illustration, p. 596). That gives us a very fair idea of the ice-river, but the cut below is a good specimen of a glacier.

Suppose we start up from Chamouni, or come across from ArgentiÈres, we shall reach the Montanvert by ascending through the wood, or by the “Chapeau,” across the ice-sea. As we take the former course, we walk alongside a white-flowing and rapid river, the Arve, which unites with the Rhone below Geneva. This river divides, and if we keep alongside one (the right or south branch), we shall reach the moraine and the icy grotto, from which the water issues. It is in this way many large rivers are born. The Rhine, the Rhone, the Aar, the Ticino, have all of them their sources in the ice. The Visp and the Sass waters are other almost equally well-known examples.

There used to be a grotto or cavern, into which the tourist could enter at the source of the Arveiron, and here the beautiful blue of the ice could be studied. From this place the Chapeau is reached, up a stony path amid the trees, and from the top outside the hut we can see the Mer de Glace all broken and contorted. The frequently occurring roar of a falling rock which heat has deprived of its icy support, or the cracking and tumbling of ice-blocks, may be seen and heard in the forenoon. But the grandeur and majesty of the ice and snow-clad mountains is best enjoyed by moonlight.

On the ice we shall see huge stones and gravel and grit, which have been carried down by the ever-moving glacier, which is denuded in its course, and worn down upon the surface as it slides, scraping and grinding the valley through which it flows. By passing along a path now made easy by irons, but formerly without supports or guards, the surface of the glacier will be reached, and a man with a hatchet will cut steps for the timid traveller. We are now upon the deep ice-river, which has its springs in the snowy regions of the Col de GÉant, in the snow which is continually falling upon the heights, and draining away to water again to form a river.

Thus the circle of events is completed,—snow, nÉvÉ, ice (glacier), water, which last is again absorbed into the atmosphere, and again descends as rain or snow. And this is always going on by the action of the sun. It may here fairly be asked how snow becomes ice. Why does not the snow turn into ice at once, and form a glacier at the top of the mountain as well as at the bottom? We will endeavour to make this clear. Snow is composed of crystals, which assume certain definite forms, and when first the flakes fall they are soft and powdery. By degrees they melt a little, and when unconsolidated form what is termed nÉvÉ, the border line between ice and snow. This semi-icy snow descends under pressure, and, as it increases, the glacier is formed by huge blocks and masses being pressed together on the steep slopes of the mountains. Thus the glacier descends, rounding off rocks, and scouring as it goes, moving at a certain estimated rate daily,—about twenty inches on the average,—carrying stones and dÉbris which form the moraine, and finally when the high temperature in the valley melts the ice, it issues forth as a river into the plain, or bounds down the mountain side in a cascade. An excursion—and one by no means dangerous if a guide be taken—to the Jardin, near Chamouni, will reveal many interesting features of glacier formation, and of the glaciers themselves.

Physical Geography is therefore very much indebted to the action of water as a fluid or as a solid. In the former condition it erodes the rocks, carries down the stones and gravel and sand, forms deltas at the mouths of rivers, and elevates plains by overflowing its banks and depositing sediment. Water gives beautiful scenery, and the ever-changing features of the landscape are due to it. From the time the spring emerges to the time when it has developed into a river, bearing fine ships upon its restless waters, the universal fluid is always at its work of destruction and benefit combined. From the limpid stream we pass to the salt ocean, the reservoir of all the waters of the globe.

We have in this chapter briefly considered two very important forces which have much to do with the varying conformation of the earth—viz., fire and water in their results of volcanic action and erosion. The sea will tell us something more.