Let us first inquire into the effects produced by the atmosphere. The air around us is composed mainly of two well-known gases; namely, oxygen and nitrogen. There is also a small proportion (about one in ten thousand) of carbonic acid gas; a variable quantity of water-vapour, and in the neighbourhood of towns, traces of other noxious gases, such as sulphurous acid and chlorine.
Now, the nitrogen plays a very unimportant part, as it merely serves to dilute the powerful gas, oxygen, which has such important life-sustaining properties. We live by breathing oxygen; so do all animals; and the more pure air we can contrive to get into our lungs, the better. But undiluted oxygen would be too strong for us, and so its strength is diminished by being mixed with four parts of nitrogen; that is to say, the air only contains about one fifth by volume, or bulk, of oxygen and four fifths of nitrogen.
Now, oxygen, being always ready to combine chemically with some other element, is a great agent of change and decay. It attacks all the metals except gold and platinum. Iron, we all know, oxidises, or rusts, only too quickly; but copper, lead, silver, and other metals are more or less attacked by it. So it is with all the rocks exposed at or near the surface of the earth. Oxygen will, if it can, pick out something to combine with and so bring about chemical changes which lead to decay. But a much more powerful agent is the carbonic acid gas in the atmosphere; although there is so little of it, there is enough to play a very important part in causing rocks to crumble away, and in some cases to dissolve them entirely. The supply of this gas is continually being renewed, for all living animals breathe out carbonic acid, and plants give it out by night. Under the influence of sunlight plants give out oxygen, so that gas is supplied to the air by day.
Both oxygen and carbonic acid gas are dissolved by rain as it falls through the air; and so we cannot separate the effects of the dry air by itself from those of rain and mist, which are more important agents. The action of rain is partly mechanical, partly chemical, for it not only beats against them, but it dissolves out certain mineral substances that they contain.
All rocks are mixtures of two or more kinds of minerals, the particles of each being often invisible to the naked eye. Thus granites are essentially mixtures of felspar, quartz, and mica; ordinary volcanic rocks ("trap-rocks") of felspar and augite; sandstones consist mainly of particles of silica; limestones of carbonate of lime; shales and slates of silicate of alumina, the principal substance in clay. These grains are usually joined together by a cement of some mineral differing more or less from the other particles. Lime is found in many of the rocks as the cement that binds their particles together; while oxide of iron and silica serve this purpose in many other instances. Now, if the lime or iron or silica is dissolved by water, the rock must tend to crumble away. Any old building shows more or less manifold signs of such decay, and this process is called "weathering." All this applies merely to the surfaces of rocks; and if there were no other forces at work, their rate of decay would be very slow.
But there are other forces at work. In the first place, sudden changes of temperature have a destructive influence. If the sun shines brightly by day, the rocks—especially in higher mountain regions—are considerably expanded by the heat they receive; and if a hot day is followed by a clear sky at night, the free radiation of heat into space (see chap. ii., p. [39]) causes them to become very cold, and in cooling down they contract. In this way an internal strain is set up which is often greater than they can bear, and so they split and crack. Thus small pieces of rock are detached from a mountain-side. An Alpine traveller told the writer that one night when sleeping on a mountain-side, he heard stones rattling down at frequent intervals. Livingstone records in his journal that when in the desert he frequently heard stones splitting at night with a report like that of a pistol. But sometimes the expansion by day is sufficient to cause fragments of rock to be broken off.
Frost, however, is responsible for a vast amount of destruction among rocks. When water freezes, it expands with tremendous force; and this is the reason why water-pipes so frequently burst during a frost, though we don't find it out until the thaw comes,—followed by long plumbers' bills. Rocks, being traversed in several directions by cracks, allow the water to get into them, and this in freezing acts like a very powerful wedge; and so the rocks on the higher parts of the mountains are continually being split up by Nature's ice-wedge.
The amount of rock broken up in this way every year is enormous. Stone walls and buildings often suffer greatly from this cause during a long frost, especially if the stone be of a more than usually porous kind, that can take up a good deal of rain water.
Where trees, shrubs, etc., grow on rocks, the roots find their way into its natural divisions, widened by the action of rain soaking down into them; and as they grow, they slowly widen them, and in time portions are actually detached in this manner. Moreover, the roots and rootlets guide the rain water down into the cracks, or joints, as they are called. Even the ivy that creeps over old ruined walls has a decidedly destructive effect.
At the base of every steep mountain may be seen heaps of loose angular stones; sometimes these are covered with soil, and form long slopes on which trees and shrubs grow. Every one of the numerous little gullies that furrow the mountain-sides has at its lower end a similar little heap of stones. Sometimes a valley among the mountains seems half choked with rocky fragments; and if these were all removed, the valley would be deeper than it is. In some hot countries, where the streams only flow in winter, this is especially the case; for example, every valley, or "wady," in the region of Mount Sinai and Mount Horeb is more or less choked up with boulders and stones of every size, because the stones come down faster than they can be carried away.