Just before the opening of the Paleozoic era practically all of North America appears to have been dry land, which had undergone so much erosion that it was low and far less rugged in relief than at present. This we know, because the rather widespread early Paleozoic (Cambrian) strata almost everywhere rest upon deeply eroded rocks of either Archeozoic or Proterozoic age. Considering both the time involved and the wide area affected, we have no record of anything like such a profound erosion interval since the beginning of the Paleozoic era. It seems that the constructive or upbuilding forces within the earth were then remarkably quiescent, while the destructive forces (erosion) were almost unhampered in their work of cutting down the land. Have we any definite idea of the relations of land and water in North America during the first or Cambrian period of the Paleozoic era? In the affirmative answer to this question, certain principles will be brought out which the reader should keep in mind as we trace out the succeeding great physical changes in the history of North America. It should, however, be remembered that, in the brief space at our disposal, only the most general, or the most significant localized, physical changes in the long and intricate known history of the continent since the opening of the Paleozoic era can be brought out. In early Cambrian time a narrow arm of the sea (like a strait) extended from the Gulf of St. Lawrence southward across eastern New York and over the site of the present Appalachian Mountains connecting with the Gulf of Mexico on the south. On the west, a much larger and broader arm of the sea (like a mediterranean) extended from Alaska southward over the site of the Rocky Mountains of Canada and across the sites of the Columbia Plateau to Great Basin of the western United States. All the rest of the continent was land, apparently almost or wholly devoid of high mountains. By what process of reasoning do we conclude that arms of the early Cambrian sea reached across eastern and western North America? First, wherever marine strata of definitely determined early Cambrian age now occur, the early Cambrian sea must have existed because those strata were obviously deposited in that sea. Second, to those areas we must add others from which it can be demonstrated that early Cambrian marine strata have been removed by erosion. Enough field work along these lines has been done in North America to render it practically certain that the relations of land and water during early Cambrian time were essentially as above outlined. [Illustration: Fig. 33.--Map showing the relations of land and water in North America during early Cambrian time, at least 25,000,000 years ago. Lined areas represent land. (Principal data from a map by Willis published in the Journal of Geology.)]