Centuries ago, so many thousand centuries that even the most learned men can only guess at their number, strange things were coming to pass. The air was so moist and cloudy that the sun’s rays had hard work to get through. It was warm, nevertheless, for the crust of the earth was not nearly so thick as it is now, and much heat came from the earth itself. Many plants and trees grow best in warm, moist air; and such plants flourished in those days. Some of their descendants are living now, but they are dwarfs, while their ancestors were giants. There is a little horse tail growing in our meadows, and there are ferns and club mosses almost everywhere. These are some of the descendants; but many of their ancestors were forty or fifty feet high. They grew very fast, especially in swamps; and when they died, there was no lack of others to take their places. Dead leaves fell and heaped up around them. Stumps stood and decayed, just as they do in our forests to day. Every year the soft, black, decaying mass grew deeper.