But even if the geologist confesses himself unable to establish the fact of any decline in the subterranean energies during the vast periods of which he takes cognisance, it must be remembered that such decline may really be going on; for vast as was the duration of the geological epochs, they probably constitute but a fraction of those far grander periods which are required by the speculations of the physical astronomer.
CHAPTER X.
THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE.
The first impression which is produced upon the mind, when the phenomena of volcanic action are studied, is that here we have exhibitions of destructive violence the effects of which must be entirely mischievous and disastrous to the living beings occupying the earth's surface. A little consideration will convince us, however, that the grand and terrible character of the displays of volcanic energy have given rise to exaggerated notions concerning their destructive effects. The fact that districts situated over the most powerful volcanic foci, like Java and Japan, are luxuriant in their productions, and thickly inhabited, may well lead us to pause ere we condemn volcanic action as productive only of mischief to the living beings on the earth's surface. The actual slopes of Vesuvius and Etna, and many other active volcanoes, are abundantly clothed with vineyards and forests and are thickly studded with populous villages.
As a matter of fact, the actual amount of damage to life and property which is effected by volcanic eruptions is small. Usually, the inhabitants of the district have sufficient warning to enable them to escape with their lives and to carry away their most valuable possessions. And though fertile tracts are covered by loose dust and ashes, or by lava- and mud-currents, yet the sterility thus produced is generally of short duration, for by their decomposition volcanic materials give rise to the formation of the richest and most productive soils.
Earthquakes, as we have already seen, are far more destructive in their effects than are volcanoes. Houses and villages, nay even entire cities, are, by vibrations of portions of the earth's crust, reduced to heaps of ruins, and famines and pestilences too frequently follow, as the consequence of the disorganisation of our social systems by these terrible catastrophes.
It may well be doubted, however, whether the annual average of destruction to life and property caused by all kinds of subterranean action, exceeds that produced either by floods or by hurricanes. Yet we know that the circulation of water and air over our globe are beneficial and necessary operations, and that the mischief occasionally wrought by the moving bodies of water and air is quite insignificant compared with the good which they effect.
In the same way, we shall be able to show that the subterranean energies are necessary to the continued existence of our globe as a place fitted for the habitation of living beings, and that the mischievous and destructive effects of these energies bear but a small and insignificant proportion to the beneficial results with which they must be credited.
LEVELLING ACTION OF DENUDING FORCES.
We have had frequent occasion in the preceding pages to refer to the work—slow but sure, silent but effective—wrought by the action of the denuding forces ever operating upon the surface of our globe. The waters condensing from the atmosphere and falling upon the land in the form of rain, snow, or hail, are charged with small quantities of dissolved gases, and these waters penetrating among the rock-masses of which the earth's crust is composed, give rise to various chemical actions of which we have already noticed such remarkable illustrations in studying the ancient volcanic products of our globe. By this action the hardest and most solid rock-masses are reduced to a state of complete disintegration, certain of their ingredients undergoing decomposition, and the cementing materials which hold their particles together being removed in a state of solution. In the higher regions of the atmosphere this work of rock-disintegration proceeds with the greatest rapidity; for there the chemical action is reinforced by the powerful mechanical action of freezing water. On high mountain-peaks the work of breaking up rock-masses goes on at the most rapid rate, and every craggy pinnacle is swathed by the heaps of fragments which have fallen from it. The Alpine traveller justly dreads the continual fusillade of falling rock-fragments which is kept up by the ever-active power of the frost in these higher regions of the atmosphere; and fears lest the vibrations of his footsteps should loosen, from their position of precarious rest, the rapidly accumulating piles of detritus. No mountain-peak attains to any very great elevation above the earth's surface, for the higher we rise in the atmosphere the greater is the range of temperature and the more destructive are the effects of the atmospheric water. The moon, which is a much smaller planet than our earth, has mountains of far greater elevation; but the moon possesses neither an atmosphere nor moisture on its surface, to produce those levelling effects which we see everywhere going on around us upon the earth.
The disintegrated materials, produced by chemical and mechanical actions of the atmospheric waters upon rock-masses, are by floods, rivers, and glaciers, gradually transported from higher to lower levels; and sooner or later every fragment, when it has once been separated from a mountain-top, must reach the ocean, where these materials are accumulated and arranged to form new rocks.