About the middle of Cretaceous times, there extended from the Gulf of Mexico on the south to or nearly to the Arctic Ocean on the north a narrow inland ocean or sea, a few hundred miles in width, covering what is now the western part of Kansas and the eastern part of Colorado, and separating the North American continent into two distinct bodies of land. This ocean, because of its location, bordered on both sides by low-lying lands—the Rocky Mountains had not then been pushed up—doubtless was comparatively calm and placid, free from violent storms and high tides. That the climate, in the region of Kansas at least, was warm or even subtropical is fairly certain, since plants allied to those now living in warm, temperate, or subtropical regions were then living much farther to the north; and since the animals which then lived in this sea were only such as would be expected in waters of warm temperature. Its tributary rivers could have been neither large nor swift-flowing, since the sediment at its bottom was free, or nearly free, from in-brought material. This was at least the case not very far from its shores. Its slowly falling sediment was composed, almost exclusively, of microscopic shells of animals and plants, foraminifera and coccoliths. The deposits thus made are almost identical with those now forming in various parts of the world in clear but not deep waters, away from the immediate coasts of the continents, almost pure chalk. Animals dying in this inland sea fell slowly to the bottom during or after decomposition of their softer parts, and the slowly increasing sediments covered up and buried the preservable parts. The many predaceous fishes and other scavengers with which the waters abounded often tore the decomposing bodies apart, separating and displacing the bones of the skeleton; and the currents of the shallow waters washed others apart. Often the teeth of fishes and other carnivorous animals are found imbedded in the bones, and many are the scars and toothmarks observed in the fossil bones. After the ocean had dried up and the bottom had been raised far above the present level of the oceans, other deposits made in lakes and by the winds covered deeply the consolidating sediments, burying them for millions of years with all that they contained. Long-continued erosion by winds and rains has again laid bare many parts of the old ocean bottom, and has washed them out into ravines and gullies. Many hundreds of square miles of this chalk are now laid bare in western Kansas, upon which the growth of vegetation has been prevented by the arid climate. Here and there may now be discovered protruding from the sloping or precipitous surfaces of this exposed chalk bones or parts of bones of the old animals buried so long ago in the soft sediment of the ancient ocean bottom.