Page:The American Cyclopædia (1879) Volume VII.djvu/708

696 Montalban series suggests that they are remaining fragments of great formations once widely spread over an ancient floor of granitic (Laurentian) gneiss; but that these four series mentioned include the whole of the stratified crystalline rocks of North America is by no means certain. How many more formations may have been laid down over this region and subsequently swept away, leaving only isolated fragments, we may never know; but it is probable that a careful study may establish the existence of many besides the four series above enumerated.” Notwithstanding the distinction which has been drawn between crystalline and uncrystalline rocks, there is probably to be found somewhere a series of beds marking the passage from these crystalline schists to the uncrystalline sediments of the palæozoic, although, so far as yet studied, the oldest known strata hitherto referred to the latter are completely uncrystalline, and rest unconformably upon crystalline eozoic rocks. There appears to be a close similarity between the latter in widely separated countries, the great series already indicated being recognized with their typical characters in remote parts of the globe.—The palæozoic rocks have been divided into five great groups, sometimes called systems; but these divisions, as already remarked, are local, and the breaks in stratification and in the succession of organic remains are in some parts filled by beds of passage. As will be seen in the table, there is some difference in the nomenclature of the lower palæozoic rocks, a portion of the Cambrian of Sedgwick being included by Murchison in the Silurian. In the present account we shall use these terms in the sense in which they were applied by the former. The lower portions of the palæozoic show no evidence of terrestrial forms of life, their vegetable remains consisting of algae, and their animals of mollusks, corals, and crustaceans. At the summit of the Silurian, however, fishes and amphibians appear, while an abundant land vegetation of acrogens and gymnosperms begins to make its appearance. The palæozoic rocks are of especial interest to the student of American geology, as they form the surface of the greater portion of the United States east of the Rocky mountains. The succession of the members of the palæozoic series in this country was first clearly defined by the geological survey of New York, which in its reports in 1842 included under the name of the New York system the whole of the known palæozoic rocks to the base of the coal formation. The subdivisions then established have since been generally adopted in the United States, and their relations to those recognized in Great Britain will he seen in the table. The names Cambrian, Silurian, and Devonian found their way into American nomenclature some years later. For an account of the progress of discovery in these rocks, the reader is referred to the third part of a paper on “The History of Cambrian and Silurian,” by Dr. Hunt, in the “Canadian Naturalist” for July, 1872. The lower and middle Cambrian is represented in the New York series by the Potsdam sandstone, and the calciferous sand rock, having a combined thickness of less than 1,000 ft. To the eastward along the confines of New England, and thence northeastward along the base of the Green mountain range, however, a series of 10,000 ft. or more of sandstones, argillites, and limestones (including the Levis formation), is regarded as the representative of the lower and middle Cambrian, and has received the names of the Taconic system and the Quebec group. Still further east, along the E. coast, in Massachusetts, New Brunswick, and Newfoundland, are found strata of lower Cambrian age, referred to the Menevian of Great Britain. Between the middle and the upper Cambrian in New York is a break marked by a change in the fauna, and in some localities by a want of conformity between the strata. The Chazy limestone, which in some places is wanting, shows the passage between the two. The upper Cambrian is represented by the limestones of the Trenton group, followed by the Utica slates and the shales and sandstones of the Hudson river group; the last three divisions being known in Ohio as the Cincinnati group. Succeeding this occurs the Oneida conglomerate, followed by the Medina sandstone rocks, which are in part derived from the ruins of the underlying formations, and which mark a period of disturbance and a break in the succession. They are succeeded by the Clinton, Niagara, and Onondaga formations. The latter, sometimes known as the Salina formation, is characterized by beds of rock salt and of gypsum, and is succeeded by the water-lime beds, which, as well as the other strata of this division, from the Medina sandstone upward, consist chiefly of dolomite or magnesian limestone. This upper part of the American Silurian represents the deposits in a basin separated from the open ocean, and depositing by its gradual evaporation strata of salt and gypsum, the strata associated with which are almost destitute of organic remains. They attain a considerable thickness in Ontario and in central New York, but thin out to the eastward and disappear before reaching the Hudson river. To this division succeed the lower Helderberg limestones, characterized by an abundant fauna, and marking by their distribution a change in the geographical conditions of the region, by which a deposit of marine limestone was spread alike over all the preëxisting rocks, to the eastward, resting unconformably upon the Cambrian and the eozoic rocks, and attaining in eastern Canada a thickness of 2,000 ft. or more, where it is overlaid by a great series of sandstones, representing the Oriskany and the subsequent Devonian. This, in the New York series, is marked by but a small amount of sandstones, followed by the corniferous limestone and the Hamilton group, which together make up the upper Helderberg, and are succeeded by a series of sandstones, the whole