We have not time to read about the picnic, nor of the interest that the class showed before the appointed Saturday, as well as all the forenoon of that day. Nor can we tell how the children went down the shaft of the mine, and how they were at first so quiet that hardly a word was said. The teacher showed them a layer of coal in the mine which was about three feet thick. Just above it was a rock which was different from the coal. This they were told was sandstone, the hardened sand which had been heaped upon the forests so many thousand years before. Then below the coal was another rock which was entirely unlike either the coal or the sandstone. This was the seat-stone, the rock made out of the soil in which the forest had grown. Then below this they found three more layers, sandstone, coal, and seat-stone, and so on until the bottom of the mine was reached. By this time the children were ready to ask questions. "Oh, Miss Turner, what is this curious-looking thing in this part of the seat-stone?" asked one of the boys. Miss Turner replied: "That is a fossil. It is part of a root of a tree, and has retained its shape and appearance all these thousands and thousands of years." [Illustration: IN A COAL MINE.] One of the miners who had been listening to the conversation said: "If you will step this way, madam, I can show you the whole of a tree-trunk in the coal." The children eagerly crowded around as the miner showed the fossilized trunk of a tree still standing just as it grew, with its roots in the seat-stone and its top in the sandstone above the coal--for here the layer of coal was several feet in thickness. A few minutes afterward, as the children were looking carefully at the sides of the mine to see if they could find more fossils, the shy little girl said quietly to the teacher: "I think that I have found something, Miss Turner; won't you please see?" She led the way to a trunk which showed the various stages in the process of change. One end was still almost like wood, the middle part was a very soft brown coal, while the other end was true coal. "That helps us to understand more about the way in which the forests were changed to coal," said Miss Turner. "Now here is one more proof that coal was formed out of wood." The teacher picked up a piece of coal and broke it with a hammer. Then she showed on the new surface some patches of a black substance. "Does not that look like charcoal?" she asked. "You know that charcoal is wood partly burned." Thus the class learned how nature, ages and ages ago, began to prepare for the use of man a fuel which seems inexhaustible, is superior to wood in many respects, and is freely distributed in various portions of the world. This coal, which has taken the place of wood to a great extent in furnishing heat for our houses and stores, is found in large quantities in the United States, but was not mined or used here until the middle of the last century. CHAPTER VI. COAL.