[Illustration: Fig. 34.--Map showing the relations of land and water in North America during Middle Ordovician time. Lined areas represent land. (Principal data from a map published by Willis in the Journal of Geology.)] As Cambrian time went on, the marine waters gradually spread from south to north across most of the Mississippi Valley area, causing the eastern and western arms of the sea to be connected, thus forming an interior continental sea. Otherwise the relations of land and water were much as in early Cambrian time. We know that the sea transgressed northward across the Mississippi Valley district because, on the south, the whole Cambrian system of strata (lower, middle, and upper) is present, while, farther north, only middle and upper Cambrian are present, and, farthest north, only upper Cambrian strata occur. This progressive northward overlap of younger and younger (later) Cambrian strata upon the old rock floor proves, that the Cambrian sea steadily spread farther and farther northward over the Mississippi Valley area. That this spreading sea was shallow is amply demonstrated by the deposits it left, such as shales, conglomerates (i.e., consolidated gravels) and sandstones, often ripple-marked. The Cambrian strata of North America vary in thickness from less than 1,000 feet to about 12,000 feet. In the Mississippi Valley the Cambrian strata are unaltered and almost undisturbed from their original horizontal position. In the Appalachian Mountains of the east, and the Rocky Mountains of the west, the strata are commonly notably folded and faulted. In some places, as in western New England, the strata have been notably altered (metamorphosed). The best estimates for the duration of the Cambrian period range from 2,000,000 to 3,000,000 years. It is a remarkable fact that, during this great lapse of time, North America was unaffected by any great physical disturbances such as mountain making, emergence of large tracts of land, or igneous activity. The one great physical event of the Cambrian was the gradual submergence of a considerable portion of the continent. That the climate of the earliest Cambrian was at least locally favorable for the existence of glaciers, is proved by the occurrences of true glacial deposits in rocks of that age in China, Norway, and Australia. It is a remarkable fact that the glacial materials of China occur along the Yangtse River, thus demonstrating that conditions for glaciers then existed at a latitude as far south as New Orleans. These evidences of glaciation directly refute the old idea, based upon the nebular hypothesis, that the climate of the Paleozoic was distinctly warmer than now. The glacial evidence, added to our knowledge of the character and world-wide distribution of many identical species of animals, leads us to conclude that early Paleozoic climate was not essentially different from that of very recent geologic time, but that the climate was then much more uniform than at present.