Proterozoic Systems.—The rocks of the Proterozoic systems are widely spread in the United States adjacent to the outcrops of the Archaean rocks, from which they are not always easily dis tinguished or separated, and in a few other places, but the areal extent of their outcrops is relatively limited. The Sudburian has been but little studied by the geologists of the United States, whereas in Canada, like the other formations of the Proterozoic, it is extensively represented and has been allotted a great deal of attention due to its relation to the great metalliferous ore bodies which have there been discovered and exploited. The Hu ronian has been much more thoroughly studied by United States geologists, particularly in the Lake Superior region where it is well exposed in the great iron-producing districts. The basal mem bers of the Huronian contain finites (consolidated deposits of glacial till or boulder clay) indicating a glacial climate in this early period of the geologic history of America. Upon this tillite lies a series of heavy quartzites and above this a marine limestone in which are found an abundance of the oldest undoubted fossils Atokania lawsoni, an algal growth of considerable size, and definite structure. The Animikian is found in the United States only in northern Michigan and Minnesota where it has been exhaustively studied in connection with the rich deposits of hematite and other iron oxides for which the region has become famous and which have their origin principally in the Animikian. In nearly all the Animikian areas small or large bodies of hematite are concen trated as a secondary enrichment from extensive deposits of cherty ferruginous carbonate or oolitic greenalite or jasper, and from the productive fields, Mesabi, Penokee-Gogebic, Menominee, and Marquette.
The Keweenawan, the uppermost of the Proterozoic systems, is not extensively represented in the United States, but in Canada it is wide-spread about the Laurentian Shield and of exceeding economic importance because of its heavy mineralization—"cop per, nickel, and silver in large amounts; cobalt, gold, platinum, and palladium in much smaller amounts; and if the iron mines are left out of account, almost all the metalliferous deposits of the southern margin of the Canadian Shield have resulted from the coming of its (Keweenawan) dikes, or sheets, or lava streams." The Keweenawan differs from the earlier Proterozoic systems in enormously thick lava beds which apparently welled up through great fissures to form a thickness of almost or quite 6 m. of igneous material, indicating a period of local volcanism unsur passed in geologic history.
The Proterozoic formations of the western cordillera in Mon tana, Idaho and British Columbia attain a thickness of over 37,000 ft., and in the Grand Canyon region over 12,000 ft. remain after an unknown amount of erosion. The Proterozoic formations else where in the continent cannot with even approximate certainty be correlated with the Lake Superior members, because the fossils are too few and indistinct to permit a correlation upon the life of the period, and the lithologic character and sequence of the series are not at all similar.
Florida lay the mountainous western edge of old Appalachia, the eastern shore of which has disappeared in the abysmal depths of the Atlantic. The south-eastern tip of Florida was included within the northernmost tip of Antillia, a land which extended eastward from Central America to the most easterly of the Lesser Antilles. Most of Texas and small parts of Louisiana and New Mexico formed the north-eastward point of ancient Columbia, and on its north-western border from west-central Nevada to Puget sound the south-westerly shore of Cascadia constituted a fifth land mass which, like the other four, has been largely sub merged or eroded. These five great land masses bounded the great interior sea, within which a complete and fairly continuous series of Palaeozoic series of sediments derived from the erosion of these bordering lands was laid down to record the geologic history in the rich development of life upon the surface of the earth during that early time.
Nowhere else in the world is the Palaeozoic record so complete or so full as in North America, and chiefly in the United States. Almost everywhere southward from the Great Lakes and the St. Lawrence the strata of the Palaeozoic outcrop occupy practi cally all of the Ohio basin, most of the Upper Mississippi and the eastern third of the Missouri. Nearly everywhere these strata are relatively horizontal and undisturbed except in the Ap palachians where they are much broken and folded, in the Cincin nati uplift where they are gently arched and in the Ozark uplift, an extensive dome where they are more steeply inclined. The Palaeozoic strata are also exposed locally in folded narrow bands from north-western Mexico to the Arctic and from the Pacific States as far east as Denver.
The Cambrian, the oldest system in the Palaeozoic, is not ex posed at the surface over large areas, but is widely distributed across the United States from California to the Appalachians, and from the St. Lawrence to Alabama, outcropping generally along the borders of the Pre-Cambrian rocks of old Laurentia, Appa lachia and Cascadia. The earliest Waucobian, the first period of the Cambrian found in the United States, has been discovered only in southern California, though the latest Waucobian extends from there to the eastward across the Sierras and northward approxi mately through Nevada, Utah, Idaho and parts of Wyoming, Montana, Oregon and Washington, and thence to the Arctic ocean, and along the Appalachians from the Gulf of Mexico to the St. Lawrence. The outcrops of the Acadian, the middle period of the Cambrian, and of the Croixian, the upper period, are more widely distributed. The Ozarkian, which has been only recently definitely established and not yet exactly delimited in vertical extent and which comprises members formerly referred to the Upper Cam brian and Lower Ordovician, is best developed in the southern Appalachians, but is rather widely distributed in the Mississippi valley region and along the borders of old Appalachia as well. These four periods, of which the first three are indisputably Cam brian, and the last possibly part of the same system, are char acterized by the development of a rich marine fauna in their rela tively shallow seas—trilobites, brachiopods and gastropods.