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.