But this process only takes place in the deeper parts of our seas, far removed from land, where the supply of land-derived materials fails,--for even the finest mud supplied by rivers probably all settles down before travelling two or three hundred miles from its native shores. Thus we learn that when one agency fails, Nature makes use of another to take up the important work of rock-building. How the other rocks which we mentioned in our list were formed,--such as granite, basalt, and the metamorphic rocks,--we must explain in a future chapter dealing with volcanoes and their work. CHAPTER VI. HOW THE MOUNTAINS WERE UPHEAVED. The notion that the ground is naturally steadfast is an error,--an error which arises from the incapacity of our senses to appreciate any but the most palpable, and at the same time most exceptional, of its movements. The idea of _terra firma_ belongs with the ancient belief that the earth was the centre of the universe. It is, indeed, by their mobility that the continents survive the increasing assaults of the ocean waves, and the continuous down-wearing which the rivers and glaciers bring about.--PROFESSOR SHALER. We have found out the quarries which supplied the rocky framework of mountains, and have learned how the work of transporting these vast quantities of stone was accomplished by the agency of ever-flowing glaciers, rivers, and streams. We must now consider the second stage of the work, and inquire how the mountains were raised up. Referring back to our illustration of the cathedral (see pages 143-147), it will be remembered that this work was included under the head of _Elevation_. But perhaps some one might ask: "How do you know that the mountains have been elevated or upheaved? Is it not enough to suppose that they owe their height entirely to the fact that they are composed of harder rock, and so have been more successful in resisting the universal decay and destruction?" Now, such an objection contains a good deal of truth, for mountains _are_ formed of hard rocks; but at the same time we know that the agents of denudation are more active among them than on the plains below, so that, in the higher mountain regions at least, the work of demolition may actually proceed faster than it does on low ground. Mountains are higher than the rest of the world, not merely because they are built of more lasting material, but also because they have been uplifted for thousands of feet above the level of the sea; and the evidence of their upheaval is so plain as to be entirely beyond doubt.