shale and sandstone there are thousands of feet, even where the sediments are fine and their accumulations therefore probably slow. For, as we have said, this was a period of great change, in which the forests were always sinking and rising again, being submerged by lakes, being covered by the sea, and again emerging as islands, to be overrun by vegetation. As sinks some sylvan scene in all its pride Changed to lagoons of overflowing tide, Assiduous labours Land to win again Her leafy breadths, invaded by the main. Down bring the rivers to the flooded shore Cargoes of shale and silt that slow restore The sunken glebes, till they again can hold Thick ferny brakes, and forests as of old. (H. R. Knipe.) It would hardly seem unreasonable to suppose that these depositions, and the changes that brought them about, might have occupied as much time as the formation of the coal beds. This would double the figures, and make this period last something between two million and five million years. In the coal as we know it are the remains of great forest trees; gigantic tree-ferns for the most part, and of many small plants forming a close thick sod, partially buried in whole countries of marsh land. Every one knows those marsh plants, which bear the vulgar name of "mare's-tail." These humble plants were represented during the coal period by trees from twenty to thirty feet high and four to six inches in diameter. Their trunks have been preserved to us: they bear the name of _Calamites_.