were to be made more fruitful, her gulfs to be filled with the rich detritus poured into them by the rivers, her whole surface to be rendered more habitable for the higher races who were to possess it. We left America at the close of the Carboniferous epoch. A glance at the geological map will show the reader that during the Permian, Triassic, and Jurassic epochs little was added to the United States, though here and there deposits belonging to each of them crop out. In the Cretaceous epoch, however, large tracts of land were accumulated, chiefly in the South and West; and during the Tertiaries the continent was very nearly completed, leaving only a narrow gulf running up to the neighborhood of St. Louis to be filled by modern detritus, and the peninsula of Florida to be built by the industrious Coral-Workers of our own period. The age of the Alleghany chain is not yet positively determined, but it was probably raised at the close of the Carboniferous epoch. Up to that time, only the Laurentian Hills, the northern side of that mountainous triangle which now makes the skeleton, as it were, of the United States, existed. The upheaval of the Alleghanies added its eastern side, raising the central part of the continent so as to form a long slope from the base of the Alleghanies to the Pacific Ocean; but it was not until the Tertiary Age that the upheaval of the great chain at the West completed the triangle, and transformed that wide westerly slope into the Mississippi Valley, bounded on one side by the Alleghanies, and on the other by the Rocky Mountains. It is my belief, founded upon the tropical character of the Fauna, that a much milder climate then prevailed over the whole northern hemisphere than is now known to it. Some naturalists have supposed that the presence of the tropical Mammalia in the Northern Temperate Zone might be otherwise accounted for,--that they might have been endowed with warmer covering, with thicker hair or fur. But I think the simpler and more natural reason for their existence throughout the North is to be found in the difference of climate; and I am the more inclined to this opinion because the Tertiary animals generally, the Fishes, Shells, etc., in the same regions, are more closely allied in character to those now living in the Tropics than to those of the Temperate Zones. The Tertiary age may be called the geological summer; we shall see, hereafter, how abruptly it was brought to a close.