But while Mr. Glacier made the drumlins of the stuff he brought with him, he enjoyed himself (at least let us hope so) tobogganing on hills he found ready made. These hills are real mountains; usually the granite heart of the mountain, because only a very strong rock could stand having one of these playful giants riding over him and live to tell the tale. Such glacier "slides" are referred to as "domes" or "round tops" or "bald mountains." Mr. Agassiz, the great scientist who spent so many years studying the motion of glaciers, could tell from the height of one of these bald and rounded hills how high the glacier was that rode over it. For instance, the glaciers rode over what is known as Blue Mountain in Pennsylvania, which is 1,500 feet high. "Then," Mr. Agassiz would have said, "the glaciers that did that must have been at least 2,000 feet thick; for a glacier can only flow over a rocky mass when it is half as tall again as the rock." You see it is the mass of it, the pressure of its own weight, that boosts the glacier up the slide. It seems almost like lifting oneself by one's boot-straps, doesn't it? III. The Ants and the Volcanoes Beside all the hills we have mentioned there are several others, well worth looking into; ant-hills, for example, not only because ants are so interesting in themselves but because the ants helped to answer what for a long time was one of the puzzles of science, "How are volcanoes made?" When your mother's mother went to school--or it may have been back in your mother's mother's mother's time--a little girl, on being asked in the geography class, "What is a volcano?" was expected to say something like this: "Please, teacher, it's a mountain with a hole in it." [Illustration] [Illustration] _From a photograph. Copyright by W. P. Romans_ SACRED FUJIYAMA AND ITS COUNTERPART FOUR THOUSAND MILES AWAY On the top is the famous Fujiyama, the sacred mountain of Japan, and on the bottom Mount Rainier in the State of Washington. Although they are more than four thousand miles apart, the two volcanoes look as if they had been cast in the same mould, owing to the uniform system by which volcanoes are built up. THE WISE MEN AND THE ANT CRATERS