Why shouldn't the caves and the boulders live happily together just like other people? The answer is simple. The glaciers of the Ice Age, with their enormous weight, crushed in the roofs of caves in every region over which they flowed; and it was these same glaciers that left the boulders. Since the glaciers went away the underground rivers that hollow out the caves have not had time to make new ones. It takes ages and ages to make a nice big cave. II. The Train of Thought These widely scattered boulders furnished the students of the subject with the very best evidence that there was once an Ice Age. First, the geologists noticed, just as the Indians did, that the boulders were of a different kind of rock from that of the regions in which they were found. Up in Wisconsin, running southwest from Waterloo is a train (as it is called) of boulders sixty miles long. The boulders are of a very hard rock called quartzite, while all the rock deposits in that region are of limestone or sandstone. [Illustration: MR. BOULDER ON HIS PERCH This is what is called a "perched boulder." Being a harder kind of rock than that on which it was left by the glaciers, it has held out against the winds and weather, while the stone under it has been worn away. ] In eastern Wisconsin, along with these stones, have been found pieces of copper, although there are no copper deposits near by. To the northeast of where the fragments of copper were found are the great copper deposits of what is now Michigan, and from this region the glaciers brought the copper and scattered it about as they moved south and southwest. So these mysterious stones and other things kept pointing toward the north, in a kind of dumb show. In mountain rain storms you can see the torrents driving great stones before them, so one of the first theories about the stranded boulders was that, at some time in the earth's history, there had been great floods covering whole continents, sweeping away rocks from the mountains and carrying them here, there, and everywhere. That theory also accounted for the rounded shape of the boulders, for if you have a volume of water big enough and swift enough you can roll boulders wherever you like. WHAT A QUEER HOBBY-HORSE! But why should the boulder trains all lead to the north? And how could water carry boulders right across a deep mountain valley and pile them high up on the mountains on the other side? How could water perch one boulder on another or on a flat ledge of rock or on the summits of the cliffs? Boulders so perched are very common, and often they are so nicely balanced that a man can set them rocking; and sometimes a small boy can do it. Every young man who goes to Dartmouth College knows about the rocking stone some half mile east of the college. In the town of Barre is a big boulder with a small boulder on its back, and the small boulder can be set rocking like a child's hobby-horse.