The succession of the members of the palaeozoic 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 palaeozoic 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 be 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 cal-ciferous 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 Me-nevian 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 preexisting 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 constituting the Erie division of the New York series, the equivalent of the English Devonian or old red sandstone, and characterized by an abundant terrestrial fauna, the precursor of that of the carboniferous series, into which it passes by such transitions that it is a matter of discussion where to draw the line.

The carboniferous series is so named because it is the earliest and most important coal-bearing series of strata, and includes great beds of fossil fuel, interstratified with sandstones and shales. At the base of the carboniferous in Michigan, Pennsylvania, and western Virginia, and also in Nova Scotia and New Brunswick, deposits of gypsum and salt are met with. In the western part of its distribution, toward the Mississippi, the carboniferous formation includes great thicknesses of marine limestone,which are wanting in the east. Overlying the carboniferous in Kansas and Iowa are beds which are the equivalent of the magnesian limestones of the north of England, and of the rocks called Permian in Russia. They are regarded as the summit of the palaeozoic series.-The palaeozoic rocks correspond to the transition rocks of Werner, to the lower part of which the name of the graywacke series was very generally given until the labors of Sedgwick and Mur-chison classified them and established the great divisions of Cambrian, Silurian, and Devonian. The thickness of these groups varies greatly in different parts of their distribution.

Thus, while the entire palaeozoic series in Pennsylvania is estimated at 40,000 ft., it is reduced to 4,000 in the valley of the Mississippi. This is due to the fact that the great sandstones, apparently derived from the erosion of rocks to the eastward, thin out in the opposite direction. In a similar manner the Cambrian and Silurian rocks, which attain in Great Britain a thickness of 30,000 ft., are represented by less than 2,000 ft. in Scandinavia.-Under the name of mesozoic or secondary rocks are included the triassic, Jurassic, and cretaceous series. The former has received its name from the threefold division of it in Europe into sandstones, overlaid by fossiliferous limestones, which are succeeded by sandstones and shales. At the base of the trias in the Tyrol, at St. Cassian and Haltstadt, occurs a series of fossiliferous beds in which the characteristic animal remains of the trias are found mingled with those of the palaeozoic, thus showing a passage between the palaeozoic and the mesozoic rocks. The trias, both in England and on the continent of Europe, is characterized by beds of rock salt and gypsum, like the Silurian and the lower carboniferous in North America. The sandstones of the trias in England are often red, and constitute what is there named the new red sandstone.