It is not the element of height alone which gives mountains their significance. There are many other features, which are little studied, yet of real import. It is, however, not a matter of indifference whether a chain thrusts up its peaks 1000, 5000, 10,000, or 20,000 feet, and the height has been made and will continue to be made a subject of careful investigation. In reference to height, we distinguish what, in a general sense, we call mountains,[3] into hills, mounts, and mountains of various degrees of magnitude. Yet the height of the highest range, in comparison with the diameter of the earth, is insignificant, only about ¹⁄₁₇₀₀, and the combined mass of mountains are of no more account in comparing them with the entire mass of the globe, than the roughnesses on the rind of an apple, or perhaps more exactly still, than those on the shell of an egg. The combined mountain systems in the world would not suffice, if transferred to the North and South Pole, to fill out the earth to such an extent that the polar and equatorial diameters would be equal. In following out his profound scientific investigations, Alexander von Humboldt, in order to ascertain the center of the earth’s gravity, taking into account the existing elevations above the ocean level, was led to the conclusion that too great importance was formerly assigned to mountains in their relations not to the course of history, but to the earth as subject to mathematical laws. Very careful observations revealed the fact to him that all the mountains of France, if reduced to a level and spread out, would raise the grade of the whole country to a height not more than 816 feet above the sea line. All the mountains of Europe, distributed in like manner, would raise the level to only about 630 feet. In Asia the same process would make the vast plain only 1080 feet high, in North America 702 feet, in South America only 1062 feet; while the mountains of the entire globe would raise the level to only 947 feet above the level of the sea. So insignificant are the combined mountain systems of the earth in respect to size, in comparison with the immense body on which they stand, though their importance is great when we regard their influence on the localities where they are found. Yet in this last regard, mountains deserve careful study, for they not only exercise and have exercised a great influence over nature and man, but they serve as our best key to open to our view the internal structure of the earth. Some mountains, though of great height and broad base, like Etna, Vesuvius, Teneriffe, and many volcanoes, belong to no true mountain system; and even when they lie near together, and yet have no inner principle of unity, they are not spoken of as a chain or a range: they make merely a mountainous district. It is the repetition of the common type and the existence of a continuous valley which gives a right to use the names chain and range.