Fig. 151. Plant which was living at the time coal was made, pressed in a stone and so preserved.
You must have seen in museums, or even found for yourself in stones, the remains of leaves and stems of plants which, too, are turned to stone, but which yet show the shape and form of the plant with great beauty. If you go to the north of England, where there are many coal-mines, you will have a good chance of finding pieces of stone which have been thrown out from the mines as refuse, and which have in them or on them most beautiful leaves of ferns and other plants. We know from geologists that these rocks are very old indeed, older than the valleys and downs of the south of England, yet we can see to-day what the plants which lived then looked like, because they have been turned into stone and kept for us in the rocks till the miners dig them out when digging the coal.
Fig. 152. Fern which was living at the time of the coal, pressed between sheets of stone.
But what is coal itself? You know that it is not at all like an ordinary rock, for it burns as well as wood, and has been found to be largely made of carbon. Even directly on top of the coal, and sometimes actually in the coal seams, we find plants preserved, and geologists and botanists have combined to prove that coal is really entirely composed of the crushed remains of ancient plants.
You will remember that we found that many of the plants in the peat bogs did not get decomposed entirely because of the preservative peaty acids present in the water and soil. Something of the same kind happened to the plants of the old forests which now form our coal. As they died they did not entirely decompose, but got pressed tightly together, all their living juices being squeezed away till little but the carbon in them remained. These masses of plants gradually sank beneath the sea, were covered by sandstones and limestones, and were preserved between the beds of rock, forming masses nearly as firm as the rocks themselves. These old plants, which to-day act as our fuel, are really “as old as the hills,” for they were growing in the country before the hills were made.