We do know that the temperature increases as we go farther down from the surface, but the increase is very different in different districts--in some places being five times greater than it is in others at an equal depth, and it is always greatest in localities where volcanoes have been active not long before. Now if there were an ocean of melted rock at a certain distance down below the crust all over the globe, there could scarcely be such a great difference between one place and another, and for this and many other reasons geologists are inclined to think that, from some unknown cause, great heat is developed at special points below the surface at different times. This would account for our finding volcanic rocks in almost all parts of the world, even very far away from where there are any active volcanoes now.

But, as I have said, we do not clearly know why great reservoirs of melted rock occur from time to time deep under our feet. We may perhaps one day find the clue from the fact that nearly all, if not all, volcanoes occur near to the water's edge, either on the coast of the great oceans or of some enormous inland sea or lake. But at present all we can say is, that in certain parts of the globe there must be from time to time great ma.s.ses of rock heated till they are white-hot, and having white-hot water mingled with them. These great ma.s.ses need not, however, be liquid, for we know that under enormous pressure white-hot metals remain solid, and water instead of flashing into steam is kept liquid, pressing with tremendous force upon whatever keeps it confined.

But now suppose that for some reason the ma.s.s of solid rock and ground above one of these heated spots should crack and become weak, or that the pressure from below should become so great as to be more powerful than the weight above, then the white-hot rock and water quivering and panting to expand, would upheave and burst the walls of their prison.

Cannot you picture to yourselves how when this happened the rock would swell into a liquid state, and how the water would force its way upwards into cracks and fissures expanding into steam as it went. Then would be heard strange rumbling noises underground, as all these heavily oppressed white-hot substances upheaved and rent the crust above them.

And after a time the country round, or the ground at the bottom of the sea, would quake and tremble, till by and by a way out would be found, and the water flashing into vapour would break and fling up the ma.s.ses of rock immediately above the pa.s.sage it had made for itself, and following after these would come the molten rock pouring out at the new opening.

Such outbursts as these have been seen at sea many times near volcanic islands. In 1811 a new island called Sabrina was thrown up off St.

Michael's in the Azores, and after remaining a short time was washed away by the waves. In the same way Graham's Island appeared off the coast of Sicily in 1831, and as late as 1885 Mr. Shipley saw a magnificent eruption in the Pacific near the Tonga Islands when an island about three miles long was formed.

Another very extraordinary outburst, this time on land, took place in 1538 on the opposite side of the Bay of Naples to where Vesuvius stands.

There, on the sh.o.r.es of the Bay of Baiae, a mountain 440 feet high was built up in one week, where all had before been quiet in the memory of man. For two years before the outburst came, rumblings and earthquakes had alarmed the people, and at last one day the sea drew back from the sh.o.r.e and the ground sank about fourteen feet, and then on the night of Sunday, September 29, 1538, it was hurled up again, and steam, fiery gases, stones, and mud burst forth, driving away the frightened people from the village of Puzzuoli about two miles distant. For a whole week jets of lava, fragments of rock, and showers of ashes were poured out, till they formed the hill now called Monte Nuovo, 440 feet high and measuring a mile and a half round the base. And there it has remained till the present day, perfectly quiet after the one great outburst had calmed down, and is now covered with thickets of stone-pine trees.

These sudden outbursts show that some great change must occur in the state of the earth's crust under the spots where they take place, and we know that eruptions may cease for centuries in any particular place and then begin afresh quite unexpectedly. Vesuvius was a peaceable mountain overgrown with trees and vines in the time of the Greeks till in the year A.D. 79 occurred the terrific outburst which destroyed Herculaneum and Pompeii, shattering old Vesuvius to pieces, so that only the cliffs on the northwest remain and are called Somma (see Fig. 37), while the new Vesuvius has grown up in the lap, as it were, of its old self. Yet when we visit the cliffs of Somma, and examine the old lava streams in them, we see that the ancient peaceful mountain was itself built up by volcanic outbursts of molten rock, and showers of clinkers or scoriae, long before man lived to record it.

Meanwhile, when once an opening is made, we can understand how after an eruption is over, and the steam and lava are exhausted, all quiets down for awhile, and the melted rock in the crater of the mountain cools and hardens, shutting in once more the seething ma.s.s below. This was the state of the crater when I saw it in 1864, though small streams still flowed out of two minute cones; but since then at least one great outburst had taken place in 1867, and now on this November night, 1868, the imprisoned gases rebelled once more and forced their way through the mountain-side.

At this point we can leave off forming conjectures and really study what happens; for we do know a great deal about the structure of volcanoes themselves, and the history of a lava-flow has been made very clear during the last few years, chiefly by the help of the microscope and chemical experiments. If we imagine then that on the day of the eruption we could have seen the inside of the mountain, the diagram (Fig. 39) will fairly represent what was taking place there.

[Ill.u.s.tration: Fig. 39.

Diagrammatic section of an active volcano.

_a_, Central pipe or funnel. _b_, _b_, Walls of the crater or cup.

_c_, _c_, Dark turbid cloud formed by the ascending globular clouds _d_, _d_. _e_, Rain-shower from escaped vapour. _f_, _f_, Shower of blocks, cooled bombs, stones, and ashes falling back on to the cone.

_g_, Lava escaping through a fissure, and pouring out of a cone opened in the mountain side.]

In the funnel _a_ which pa.s.ses down from the crater or cup _b_, _b_, white-hot lava was surging up, having a large quant.i.ty of water and steam entangled in it. The lava, or melted rock, would be in much the same state as melted iron-slag is, in the huge blast-furnaces in which iron-rock is fused, only it would have floating in it great blocks of solid rock, and rounded stones called bombs which have been formed from pieces of half-melted rock whirled in air and falling back into the crater, together with clinkers or scoriae, dust and sand, all torn off and ground down from the walls of the funnel up which the rush was coming. And in the pipe of melted rock, forcing the lava upwards, enormous bubbles of steam and gas _d_, _d_ would be rising up one after another as bubbles rise in any thick boiling substances, such as boiling sugar or tar.

In the morning before the eruption, when only a little smoke was issuing from the crater, these bubbles rose very slowly through the loaded funnel and the half-cooled lava in the basin, and the booming noise, like that of heavy cannon, heard from time to time, was caused by the bursting of one of these globes of steam at the top of the funnel, as it brought up with it a feeble shower of stones, dust, and scoriae.

Meanwhile the lava surging below was forcing a pa.s.sage _g_ for itself in a weak part of the mountain-side and, just at the time when our attention was called to Vesuvius, the violent pressure from below rent open a mouth or crater at _h_, so that the lava began to flow down the mountain in a steady stream. This, relieving the funnel, enabled the huge steam bubbles _d_, _d_ to rise more quickly, and to form the large whitish-grey cloud _c_, into which from time to time the red-hot blocks, scoriae, and pumice were thrown up by the escaping steam and gases. These blocks and fragments then fell back again in a fiery shower _f_, _f_ either into the cup, to be thrown up again by the bursting of the next bubble, or on to the sides of the cone, making it both broader and higher.

Only one feature in the diagram was fortunately absent the evening we went up, namely, the rain-shower _e_. The night, as I said, was calm, and the air dry, and the steam floated peacefully away. The next night, however, when many people hurried down from Rome to see the sight they were woefully disappointed, for rain-showers fell heavily from the cloud, bringing down with them the dust and ashes, which covered the unfortunate sight-seers.

This was what happened during the eruption, and the result after a few days was that the cone was a little higher, with a fresh layer of rough slaggy scoriae on its slopes, and that on the side of the mountain behind the Hermitage a new lava stream was added to the many which have flowed there of late years. What then can we learn from this stream about the materials which come up out of the depths of the earth, and of the manner in which volcanic rocks are formed?

The lava as I saw it when coming first out of the newly-opened crater is, as I have said, like white-hot iron slag, but very soon the top becomes black and solid, a hard cindery ma.s.s full of holes and cavities with rough edges, caused by the steam and sulphur and other gases breaking through it.[1] In fact, there are so many holes and bubbles in it that it is very light and floats on the top of the heavier lava below, falling over it on to the mountain-side when it comes to the end of the stream. Still, however, the great ma.s.s moves on, so that the stream slides over these fallen clinkers or scoriae. Thus after an eruption a new flow consists of three layers; at the top the cooled and broken crust of clinkers, then the more solid lava, which often remains hot for years, and lastly another cindery layer beneath, formed of the scoriae which have fallen from above (see Fig. 40).

[1] For the cindery nature of the surface of such a stream see the initial letter of this chapter.

[Ill.u.s.tration: Fig. 40.

Section of a lava-flow. (J. Geikie.)

1, Slaggy crust, formed chiefly of scoriae of a gla.s.sy nature. 2, Middle portion where crystals form. 3, Slaggy crust which has slipped down and been covered by the flow.]

You would be surprised to see how quickly the top hardens, so that you can actually walk across a stream of lava a day or two after it comes out from the mountain. But you must not stand still or your shoes would soon be burnt, and if you break the crust with a stick you will at once see the red-hot lava below; while after a few days the cavities become filled with crystals of common salt, sulphur or soda, as the vapour and gases escape.

Then as time goes on the harder minerals gradually crystallise out of the melted ma.s.s, and iron-pyrites, copper-sulphate, and numerous other forms of crystal appear in the lower part of the stream. In the clinkers above, where the cooling goes on very rapidly, the lavas formed are semi-transparent and look much like common bottle-gla.s.s. In fact, if you take this piece of obsidian or volcanic gla.s.s in your hand, you might think that it had come out of an ordinary gla.s.s manufactory and had nothing remarkable in it.

[Ill.u.s.tration: Fig. 41.

A slice of volcanic gla.s.s showing the lines of crystallites and microliths which are the beginnings of crystals.[1] (J. Geikie.)]

[1] This arrangement in lines is called _fluidal structure_ in lava.

But the microscope tells another tale. I have put a thin slice under the first microscope, and this diagram (Fig. 41) shows what you will see.

Nothing, you say, but a few black specks and some tiny dark rods. True, but these specks and rods are the first beginnings of crystals forming out of the ground-ma.s.s of gla.s.sy lava as it cools down. They are not real crystals, but the first step toward them, and by a careful examination of gla.s.sy lavas which have cooled at different rates, they have been seen under the microscope in all stages of growth, gradually building up different crystalline forms. When we remember how rapidly the top of many gla.s.sy lavas cool down we can understand that they have often only time to grow very small.

[Ill.u.s.tration: Fig. 42.

A slice of volcanic gla.s.s under the microscope, showing well-developed microliths. (After Cohen.)]

The smaller specks are called _crystallites_, the rods are called _microliths_.[1] Under the next microscope you can see the microliths much more distinctly (Fig. 42) and observe that they grow in very regular shapes.

[1] _Micros_, little; _lithos_, stone.

Our first slice, however (Fig. 41), tells us something more of their history, for the fact that they are arranged in lines shows that they have grown while the lava was flowing and carrying them along in streams. You will notice that each one has its greatest length in the direction of the lines, just as pieces of stick are carried along lengthways in a river. In the second specimen (Fig. 42) the microliths are much larger and the stream has evidently not been flowing fast, for they lie in all directions.

This is what we find in the upper part of the stream, but if we look at a piece of underlying lava we find that it is much more coa.r.s.e-grained, and the magnifying-gla.s.s shows many crystals in it, as well as a number of microliths. For this lava, covered by the crust above, has remained very hot for a long time, and the crystals have had time to build themselves up out of the microliths and crystallites.

Still there is much gla.s.sy groundwork even in these lavas. If we want to find really stony ma.s.ses such as porphyry and granite made up entirely of crystals we must look inside the mountain where the molten rock is kept intensely hot for long periods, as for example in the fissure _g_, Fig. 39.

Such fissures sometimes open out on the surface like the one I saw, and sometimes only penetrate part of the way through the hill; but in either case when the lava in them cools down, it forms solid walls called d.y.k.es which help to bind the loose materials of the mountain together. We cannot, of course, examine these in an active volcano, but there are many extinct volcanoes which have been worn and washed by the weather for centuries, so that we can see the inside. The d.y.k.es laid bare in the cliffs of Somma are old fissures filled with molten rock which has cooled down, and they show us many stony lavas; and Mr. Judd tells us of one beautiful example of a ruined volcano which composes the whole island of Mull in the Hebrides, where such d.y.k.es can be traced right back to a centre. This centre must once have been a ma.s.s of melted matter far down in the earth, and as you trace the d.y.k.es back deeper and deeper into it, the rocks grow more and more stony, till at last they are composed entirely of large crystals closely crowded together without any gla.s.sy matter between them. You know this crystalline structure well, for we have plenty of blocks of granite scattered about on Dartmoor, showing that at some time long ago molten matter must have been at work in the depths under Devonshire.

We see then that we can trace the melted rock of volcanoes right back--from the surface of the lava stream which cools quickly at the top, hurrying the crystallites and microliths along with it--down through the volcano to the depths of the earth, where the perfect crystals form slowly and deliberately in the underground lakes of white-hot rock which are kept in a melted state at an intense heat.

[Ill.u.s.tration: Fig. 43.

A piece of Dartmoor Granite, drawn from a specimen.]

But I promised you that we would have no guesswork here, and you will perhaps ask how I can be certain what was going on in the depths when these crystals were formed. A few years ago I could not have answered you, but now chemists, and especially two eminent French chemists, MM.

Fouque and Levy, have actually _made_ lavas and shown us how it is done in Nature.

By using powerful furnaces and bellows they have succeeded in getting temperatures of all degrees, from a dazzling white heat down to a dull red, and to keep any temperature they like for a long time, so as to imitate the state of a ma.s.s of melted rock at different depths in the earth, and in this way they have actually _made_ lavas in their crucibles. For example, there is a certain whitish rock common in Vesuvius called _leucotephrite_,[1] which is made up chiefly of crystals of the minerals called leucite, Labrador felspar, and augite. This they proposed to make artificially, so they took proper quant.i.ties of silica, alumina, oxide of iron, lime, potash, and soda, and putting them in a crucible, melted them by keeping them at a white heat. Then they lowered the temperature to an orange-heat, that is a heat sufficient to melt steel. They kept this heat for forty-eight hours, after which they took out some of the mixture and, letting it cool, examined a slice under the microscope. Within it they found crystals of _leucite_ already formed, showing that these are the first to grow while the melted rock is still intensely hot. The rest of the mixture they kept red-hot, or at the melting-point of copper, for another forty-eight hours, and when they took it out and examined it they found that the whole of it had been transformed into microliths of the two other forms of crystals, Labrador felspar and augite, except some small eight-sided crystals of magnet.i.te and picot.i.te which are also found in the natural rock.

[1] _Leucos_, white; _tephra_, ashes.

There is no need for you to remember all these names. What I do want you to remember is, that, at the different temperatures, the right crystals and beginnings of crystals grew up to form the rock which is found in Vesuvius. And what is still more interesting, they grew exactly to the same stages as in the natural rock, which is composed of _crystals_ of leucite and _microliths_ of the two other minerals.

This is only one among numerous experiments by which we have learnt how volcanic rocks are formed and at what heat the crystals of different substances grow. We are only as yet at the beginning of this new study, and there is plenty for you boys to do by and by when you grow up. Many experiments have failed as yet to imitate certain rocks, and it is remarkable that these are usually rocks of very ancient eruptions, when _perhaps_ our globe may have been in a different state to what it is now; but this remains for us to find out.

Meanwhile I have still another very interesting slide to show you which tells us something of what is going on below the volcano. Under the third microscope I have put a slice of volcanic gla.s.s (Fig. 44) in which you will see really large crystals with dark bands curving round them.

These crystals have clearly not been formed in the gla.s.s while the lava was flowing, first because they are too large to have grown up so rapidly, and secondly because they are broken at the edges in places and sometimes partly melted. They have evidently come up with the lava as it flowed out of the mountain, and the dark bands curving round them are composed of microliths which have been formed in the flow and have swept round them, as floating straws gather round a block of wood in a stream.

Such crystals as these are often found in lava streams, and in fact they make a great difference in the rate at which a stream flows, for a thoroughly melted lava shoots along at a great pace and often travels several miles in a very short time; but an imperfectly melted lava full of crystals creeps slowly along, and often does not travel far from the crater out of which it flows.

[Ill.u.s.tration: Fig. 44.