_Stereo Copyright, Underwood & U._ _London and New York_
A Curious Rock greatly revered by the Natives
This is the Dance Rock of the Walpi Indians of Arizona. Its curious shape is the result of weathering.]
What goes on along the coast finds a parallel in the interior of continents where, as in Arizona, in America, or by the desert of Gobi, in Asia, or in the Karroo of South Africa, or in Central Australia and Africa, there is great dryness of climate and a continual disintegration of the surface rocks. Sometimes the dust or sand remains and gradually consolidates or hardens. More often it is only a temporary visitor. Wind and rain are continually removing it, sometimes in vast quantities, into the sea; and in the course of time the most astounding changes are wrought in the surface and appearance of the land. The softer rocks are worn down; the harder ones are left sticking out. Gradually the surface is carved out into heights and hollows. The harder rocks become the hills and ridges; the softer rocks are worn into valleys and plains. If there were no water left on the earth's surface a great deal of this process would still go on. In some respects it might become more violent, for owing to the absence of moisture the winds of the earth would always be laden with fine particles; and every one who has seen a "sand-blast" at work, or even the modified sand-blast which is sometimes used for cleaning the stonework of some of our cities, will appreciate what a tornado laden with sand grains might do in the way of destroying the surface of any rock on which it was playing. But, as a matter of fact, the action of water in carving the surface of the earth is the most important of all the factors we have at present to consider.
As rain falls from the clouds it absorbs the gases of the air, including oxygen and carbonic acid. Now both these are what we call corroding agents. If water is allowed to fall on a steel knife the knife rusts; but it has been shown by Dr. Gerald Moody, during the last few years, that if there were no acid gas present, the rusting would not take place. Oxygen and carbonic acid will rust other things beside metal; they will rust stone. Moreover, when the rain reaches the earth it absorbs any other acids of the soil which rotting vegetation may afford, and reinforced by these it goes on to attack the stones over which it flows. When it rolls along as a brook or a river it is no doubt attacking in this way the rocks and stones of its channel, though this action is not very strikingly shown. But sometimes the rusting or dissolving action of water is very evident. When it issues from a peat bog, for example, and is consequently highly charged with acid, it will make a very great impression on any limestones it may encounter; for as any schoolboy knows who has ever put a piece of chalk in vinegar, or in any of the stronger acids of the school laboratory, all the limestones are peculiarly susceptible to this form of chemical attack. Peat-water eats into limestone rapidly, while the limestone above the stream escapes, though it is a little (and much more slowly) dissolved by rain. Hence arise some curious features in the scenery of limestone districts. The walls of limestone above the water are not eaten away so fast as their base over which the water flows. Consequently they are undermined and are sometimes cut into tunnels and caverns and caves.