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Cretaceous

period, chalk, close and time

CRETACEOUS, a name applied by geolo gists to the latest period of the Mesozoic Era, and to the system of rocks then formed. It follows the Jurassic and precedes the Tertiary. The name conies from the Latin for chalk and in the type localities the formation is usually characterized by white, soft chalk, but some times, more especially in Italy and the south of France, this chalk is replaced by compact, solid limestones. In England and France the Cretaceous rocks consist chiefly of carbonate of lime, but usually abound with silica, in the shape of nodules, plates and veins, and with iron pyrites in nodules and radiated cylinders. The organic remains in the chalk are, with few exceptions, eminently marine, and from the fine texture of the substance in which they have been embedded are usually well preserved. They embrace seaweeds, sponges, corals, echino derms, mollusks, crustacea, fishes and reptiles. Deep-sea dredgings in the Atlantic have re vealed the fact that a fine, white, organic ooze, resembling the chalk, is still in process of formation in the oceanic abysses at the present day. In America, the system is divided into Lower or Comanchean, and Upper or Creta ceous. Some writers give Comanchean the rank of a period, making it co-ordinate with Juras sic, and restrict the term Cretaceous to the Upper Cretaceous as above defined. During

Lower Cretaceous the continent of North America was not very different in outline from now, except that the Gulf of Mexico was ex panded, and that the Pacific still covered the site of much of the present Coast Ranges. During Upper Cretaceous time, a great arm of the sea encroached on the land, finally extend ing frorn the Gulf of Mexico to the Arctic. In this a great thickness of marine sediments was laid down. At the same time much of the Atlantic and Gulf Coastal Plain was under water. Toward the close of the period (Lara mie Epoch) the interior sea again retreated leaving a large area of low swamp, in which much coal was formed. The period, and the Mesozoic Era as well, were brought to a close by folding which produced the Rocky Moun tains. The Andes were probably folded at the same time.

Life of the The great groups of reptiles which developed during Triassic and Jurassic (q.v.) reached their climax in Cretaceous, and Dinosaurs, Pterosaurs, Moso saurs, Ichthyosaurs and Plesiosaurs all became extinct at the close. The Ammonite Cephalo pods also died out completely. The most notable addition was the development of the modern types of flowering plants, which became dominant before the close of the period.