The American Cretaceous system is likewise characterized by the presence of huge dinosaurs and other reptiles, some of them being European types, while others are peculiar. One of the most remarkable features of the American rocks is the occurrence in them of the toothed birds, ichthyornis and hesper ornis.
No break separates the Jurassic from the Cretaceous system; there is a gradual passage from the upper beds of the one into the lower beds of the other. At the beginning of Cretaceous times most of the British and Irish area existed as dry land. Over the S. E. of England lay the estuary of a large river, flowing probably from the N. The Wealden beds are the delta-deposits of that river; the English and French beds of this division covering an area of 20,000 square miles. The sea into which that river flowed occupied a considerable area in the N. of France, spread over the Low Countries into Hanover, filled the basin of the North Sea, and overflowed a portion of eastern England. Wealden beds occur in northwest Germany, and indicate the delta of a river, like that of the British area flowing from the N. While land-conditions predominated in northern and middle Europe, an open sea covered vast areas in southern Eu rope. Gradual subsidence of the sea
bottom took place during the deposition of the Wealden series, and eventually the great deltas became submerged, and a wide sea covered most of what are now the low grounds of the British area and passing E. submerged vast regions of middle Europe up to the slopes of the Ural Mountains. The depression was greatest in the W. areas where in the deep clear waters there gradually ac cumulated the calcareous matter which subsequently formed our white chalk. In the Mediterranean basin, a deep open sea would seem to have persisted all through the Cretaceous period. It was in this sea that the massive hippurite limestone was formed. Open water ap pears at this time to have extended through the Mediterranean area into Asia, covering there also vast tracts of what is now dry land, and communicat ing with the Indian Ocean. The condi tions of climate seem to have been re markably uniform over the vast regions of the earth's surface. Ferns, cycads, and conifers flourished in the lands with in the Arctic Circle, and the waters of the same region were tenanted by cuttle fish, ammonites, and huge reptiles.