Lyell showed also the great age of the world by the changes which have taken place in climate. In Greenland are a multitude of fossil plants, which show that it formerly enjoyed a mild and genial climate. Fossil tulip and walnut trees have been found within the Arctic circle. "On the North American continent, between the Arctic circle and the forty-second parallel of latitude," said Lyell, "we meet with signs of ice-action on a scale as grand, if not grander than in Europe." The drift covered from the Atlantic border of New England and Labrador westward to Dakota and Lake Winnipeg, and farther north, across the continent. Some stones in this bed of ice were thirty feet square, weighing over four million pounds. Some boulders from the Alps, weighing three thousand tons each, are now found on the Juras. "It must, I think," said Lyell, "be conceded that the period required for the coming-on of the greatest cold, and for its duration when most intense, and the oscillations to which it was subject, as well as the retreat of the glaciers and the 'great thaw,' or disappearance of snow, from many mountain-chains where the snow was once perpetual, required not tens, but hundreds, of thousands of years." In Arctic Siberia herds of elephants must have roamed, as their bodies, covered with hair and flesh, have been dug up in recent years. Great Britain and Europe have been much warmer than now. Our own immense coal fields show a former tropical climate, with their great tree-ferns and tree-rushes, while the remains of reindeers have been found in Connecticut. No wonder Lyell became fascinated with the history of the changes of this planet, and the life of man before historic times. A great book seemed open to him, and he studied it by night and by day: the Archæan Time--no life; Paleozoic Time, including the Silurian Age, with its shells and trilobites; the Devonian, with its fishes; Carboniferous, with its coal plants; Mesozoic Time, including the Reptilian Age with its reptiles; Cenozoic Time, including the Mammalian or Tertiary, with its mammals, and Quaternary, or age of man. Paleozoic means "ancient life;" Mesozoic, "middle life;" Cenozoic, "recent life." Lyell divided the Tertiary strata into three groups: Eocene, recent dawn; Miocene, less recent; Pliocene, more recent. In the Eocene Age Great Britain was sub-tropical, and, in North America, Vermont was like North Carolina in temperature. Then came the Glacial Period, with ice probably five thousand feet thick over New England. Then the Champlain Period, with its floods, continents depressed, and climate warm, followed in Europe by a second Glacial Period.