Another way of observing these curious little movements is by burying microphones in the ground. The microphone is a little instrument invented of late years which is capable of enormously magnifying the very slightest sounds, such as our ears will not detect. By its means one can hear, as some one said, "the tramp of a fly's foot," if he will be so obliging as to walk over it. It has thus been proved in Italy that the earth sends forth a confused medley of sounds caused by little crackings and snappings in the rocks below our feet.

In this way it will be possible to predict a serious earthquake, because it will give warning some days before, by the increase of the little tremors and sounds; and it is to be hoped that by this simple means human lives may be saved.

Now, these disturbances are of precisely the same nature as earthquakes,—in fact, we may call them microscopic earthquakes. To the geologist they are of great interest, as they seem to afford some little insight into the difficult question of the upheaval of mountains, and to show us something of the constant working of those wonderful forces below the surface of the earth by means of which continents are raised up out of the sea, and mountain-chains are elevated thousands of feet. It is probable that both are due to the working of the same forces, and are accomplished by the same machinery.

We now pass on to consider those more violent movements of the solid land known as earthquakes. This kind of disturbance is such as might be produced by a sudden shock or blow given below the ground, from which waves travel in all directions. First comes a rumbling noise like the roar of distant artillery; then come the earthquake waves one after another, causing the ground to rise and fall as a ship does on the waves of the sea; the ground is frequently rent asunder, so that chasms are formed, into which in some cases men and animals have been hurled alive. In the case of a very violent earthquake the waves travel long distances. Thus the great earthquake by which Lisbon was destroyed in the year 1755 disturbed the waters of Loch Lomond in Scotland. In this fearful catastrophe sixty thousand human beings perished. If the disturbance takes place near the sea, great sea waves are formed, which cause fearful destruction to life and property. This happened in the case of the Lisbon earthquake; and in the year 1868, when Ecuador and Peru were visited by a fearful earthquake, a great sea wave swept over the port of Arica, and in a few minutes every vessel in the harbour was either driven ashore or wrecked, and a man-of-war was swept inland for a quarter of a mile.

Earthquakes bring about many changes on the surface of the earth. For example, on mountain-slopes forests are shattered, and large masses of soil and débris are shaken loose from the rock on which they rested, and hurled into the valleys; streams are thus choked up, and sometimes lakes formed, either by the damming up of a river or by the subsidence of the ground.

It is frequently found after an earthquake that the level of the ground has been permanently altered; and this effect of earthquakes is important in connection with the subject we are now considering,—namely, how mountains are upheaved. Sometimes, it is true, the movement is a downward one; but more generally it takes place in an upward direction. As an example of this, we may mention the Chilian earthquake of 1835, which was very violent, and destroyed several towns on that coast, from Copiapo to Chile. It was afterwards found that the land in the Bay of Conception had been raised four or five feet. At the island of Santa Maria, to the southwest of this bay, the land was raised eight feet, and in one part ten feet; for beds of dead mussels were seen at that height above high water, and a considerable rocky flat that formerly was covered by the sea now became dry land. It was also proved by means of soundings that the sea round the island was shallower by about nine feet.

Now the question arises, "How are earthquakes caused?" Various suggestions have been made; but it is pretty clear that all earthquakes are not produced in the same way. For instance, volcanic eruptions are frequently attended by earthquakes. Violent shocks of this nature generally precede and accompany a great eruption, as is frequently the case before an eruption of Mount Vesuvius.

Steam plays a very important part in all volcanic eruptions; and these earthquakes are probably caused by great quantities of pent-up steam at a high pressure struggling to escape. It is also possible that when molten rock is forcibly injected into the crevices and joints of overlying rocks earthquake shocks may be produced by the concussion. The old Roman poet and philosopher, Lucretius, endeavoured to solve this problem, and concluded that "the shakings of the surface of the globe are occasioned by the falling in of enormous caverns which time has succeeded in destroying." But though the explanation might possibly apply to a few cases of small earthquakes, it is not a satisfactory one, for it is not at all likely that many large cavities exist below the earth's surface, because the great weight of the overlying rock would inevitably crush them in.

We have already pointed out that earthquakes frequently happen in mountainous regions; and this fact alone suggests that perhaps the same causes which upheave mountains may have something to do with earthquakes. But there are other reasons for believing that the same force which causes earthquakes also upheaves mountain-chains. The reader will remember the case of the Chilian earthquake that raised part of the Andes a few feet in height.

Now, it is quite clear that the rocks of which mountains are composed have suffered a great deal of disturbance. We have only to look at the crumbled and contorted strata to see that they have been forced into all kinds of positions, sometimes standing bolt upright (see diagrams, chap. ix., p. [307]). And as we cannot believe, for many reasons, that these movements were of a very sudden or violent kind, we must consider that they took place slowly on the whole; but besides being folded and twisted, the rocks of mountains frequently exhibit clear signs of having been split and cracked. The fractures are of all sizes, from an inch or more up to hundreds or even thousands of feet. They tell us plainly that the rocks were once slowly bent, and that after a certain amount of bending had taken place, the strain put upon them became greater than they could bear, and consequently they snapped and split along certain lines. This is just what might be expected. For instance, ice on a pond will bend a good deal, but only up to a certain amount; after that, it cracks in long lines with a remarkably sharp and smooth fracture. But suppose the pressure came from below instead of from above, as when a number of people are skating on a pond. Should we not see the ice forced up in some places, so that some sheets stood up above the others after sliding past their broken edges? This is just what the rocks in different places have frequently done. After a fracture has taken place the rock on one side has slid up over the other, and the two surfaces made by the fracture—like two long walls—are no longer seen at the same level. One has been pushed up, while the other has gone down (see diagram of the ranges of the Great Basin, chap. viii., p. [273]).