Ages passed. Great changes came about-- too
many changes even to name here. Mountains became plains, and plains
became mountains. Sea beds became dry land, and dry land became sea beds.
Great ice sheets spread down from the North. Some of the mud and sand
over the buried forests hardened into rock. In some places the weight of
ice was added to the weight of rock over the layers of plant material.
That weight grew very great. The pressure squeezed the layers of plant material. It drove out some of the water. Other changes took place in the plant material. In time it became coal. In some places it became lignite or brown coal. In other places longer time and greater pressure made bituminous or soft coal. In still other but fewer places very great and long-continued pressure made anthracite or hard coal.
Cannel coal is believed to have been formed from different plant materials, perhaps under somewhat different conditions. Fern spores are among the important materials of cannel coal. (Fern spores are the tiny, brown, dustlike particles you may have seen on the underside of fern leaves. They serve the ferns as seeds.) Algae, or seaweeds, are found in cannel coal, too.
As has been said, there was no human being on earth during the Great Coal Age. Then how do we know all this? The coal itself kept the record-- the coal and the rocks between the coal layers! In the coal itself there are prints of leaves, stems, tree trunks, and even of the giant cockroaches and dragonflies of the Great Coal Age. In the rocks between the coal layers there are fossils of the fishes that swam above the drowned forests.