The third layer seems to show, from the limestone and the fresh-water shells embedded in it, that the level land where the forest grew sank lower and lower until it formed a hollow which in time became a lake. The fourth layer, which "ends this strange, eventful history," gives evidence of the whole land having been again covered by the ocean, and again raised above the waters! If we were studying geology together, I should like to take you with me to the Museum, and we would first look at the fossils which are believed to belong to the most ancient time of life upon the earth; then we would pass on to those belonging to the second or "middle" stage, and then to the third, or "new" stage, letting these wonderful stones, taken from mountain height or deep sea bottom, or from the depths of the earth itself, tell their own eloquent story. But what I should like you to remember is that geologists of our own time tell us that the lowest layer of the earth's crust which has yet been explored appears to be made of vegetable remains, so crushed and altered by time and by the tremendous pressure of rocky layers lying above it, that though it is probably of the same material as that which forms the coal-measures, it resembles the blacklead of which pencils are made much more than the coal which you know is what has been formed by the decay of buried forests and jungles. In this layer of "graphite," geologists with the help of their microscopes have searched in vain for any trace of what once was living, but they think it may have been formed from the "flowerless" plants, or even from those still more lowly, too minute when living to be seen by the naked eye, and consisting of one tiny bag or "cell." They tell us that these "infant" plants were followed by those of larger growth, specimens of which are found in layers of rock and clay nearer the surface, and are followed by remains of the "herb yielding seed, and the fruit tree yielding fruit after its kind"--for mummies of seed vessels and fruits have been found in coal-fields in many parts of the world.