TWITILYORNIS.) The ill:1MM hall remains were mostly those of lower orders, such as marsupials and monotremes.
The geographic developments in North Amer ica during the Cretaceous were great. The in terior continental sea was shallowed, and finally obliterated by the uplifting of the land, so that the eastern and western portion of the United States were joined into one continent. This up lift became emphasized toward the end of the Cretaceous, when the Rocky Mountains were formed. Violent volcanic eruptions accompanied this and the lava-flows of the Yellowstone regions date from this time.
The economic products include most of the coal and lignite deposits of the Western States. Many of the gold and silver bearing fissure veins of the Rockies were formed in the Creta ceous, while other products are fire-clays, chalk, greensand, and iron ores.
In Europe the Cretaceous rocks assume great importance. According to Geikie, they may be grouped into two fairly distinct areas, of which the northern includes Great Britain, the low lands of central Europe, with portions of Silesia. Bohemia. and northern France; while the south ern embraces the central and southern part of France, the Alps, and the Mediterranean basin.
The northern area is characterized by shallow water deposits—sandstones, conglomerates, and marls—mo•e or less glauconitic, and passing into a loosely textured limestone or chalk. In the southern basin the typical rocks are massive, compact limestones, which indicate conditions of deeper water and freer communication with the open sea. The entire series of rocks is usu ally grouped by geologists as follows: Lower Cre taceous—(a) Neocomian, (b) Urgonian, (c) Aptian; Upper Cretaceons—(a) Gault or Albian, (b) Cenomanian, (c) Turonian, (d) Senonian. (e) Danian. The dividing line between the Upper and Lower Cretaceous is regarded by the French geologists as occurring at the top of the Albian. The most conspicuous member of the Cretaceous is the white chalk (Upper Creta ceous) which forms the remarkable cliffs of southeastern England and northwestern France.
Consult Dana, Manual of Geology (4th ed.. New York, 1890) ; Geikie, Tee/-Book of Geol ogy (London, 1893) ; White, "Correlation Papers, Cretaeeous," United States Geological Surrey, Bulletin S2 (Washington, 1S91).