Page:Walcott Cambrian Geology and Paleontology II.djvu/19

 in the Grand Canyon, Llano, and Belt series, and in formations correlated with them [see Van Hise and Leith, 1909, pp. 45, 46], were deposited in fresh- or brackish-water seas to which the marine life of the extra-continental seas very rarely had access. Such access occurred in mid-Beltian time when a protozoan, a crustacean, and a few annelids penetrated and became adapted to the conditions of the Montana sea, and more or less similar forms to the Arizona sea. Other and different forms may have lived in these and other interior bodies of water, but as yet we have no knowledge of them.

On the eastern side of the continent the unconformity between the oldest Cambrian and the known Algonkian rocks is so marked that there is no question of a great stratigraphic and time break between the two systems. The same is true of the Lake Superior, Hudson Bay, and Cordilleran areas. The Algonkian sedimentary rocks of the Atlantic coast region and the central interior continental areas, like most, if not all, of those of the western interior or Cordilleran areas, are of terrigenous origin, and in the absence of a marine fauna are considered as having been deposited in epicontinental seas or lakes of fresh or brackish water.

With our present information it appears that toward the close of Archean time a period of diastrophism ensued, resulting in an uplift of the North American, and probably other continental masses. It was accompanied by local disturbances that resulted in the profound folding and metamorphism of the Archean complex and the formation of high mountains and uplands. Large areas of low lands existed between the higher lands, and in these the terrigenous sediments began to accumulate in inland seas and lakes and in the marine waters along the shores. Great quantities of eruptive matter were extruded, the agencies of diastrophism continued to exert their influence, but with decreasing energy, and during the latter part of Algonkian time they were still less active. When the continental area that had largely been a land surface since the first great uplift at the close of the Archean began to admit the Cambrian sea, or what is more probable, the sea began to rise, the latter found a surface of relatively low relief, and in some districts great areas of sediment that had been deposited in the inland lakes, so situated that the Cambrian sediments were laid down almost conformably superjacent to them.