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112 and gravels. These three deposits in the valley of the St. Lawrence can often be seen in actual superposition, and the order is invariable. In some places all contain marine shells; in others these are limited to the upper part of the leda clay or the lower part of the saxicava sand. In many parts of its distribution the bowlder clay holds the remains of marine animals, stones with adhering barnacles and bryozoa being imbedded therein; and elsewhere, according to Dawson, it exhibits other but not less unequivocal evidences of a submarine origin. The true bowlder clay is spread out over the region under consideration as a somewhat widely extended and uniform sheet; yet it may be said to fill up all small valleys and depressions, and to be thin or absent on ridges or rising grounds. The bowlders which it contains are by no means uniformly dispersed. When cut through by rivers or denuded by the action of the sea, ridges of bowlders are often seen to be enclosed within it. It is to be observed, according to the same writer, that although bowlders with layers of stones occasionally occur in the leda clay, and the upper sands and gravels sometimes contain large bowlders, such deposits are readily distinguished from the true bowlder clay. Although generally resting directly on striated rock surfaces, it is sometimes underlaid by rolled gravel or by peat. It is usually destitute of stratification, but horizontal lines, indicating differences in texture and in color, can sometimes be seen, and it occasionally exhibits surfaces on which lie large bowlders, striated and polished on their upper sides, forming a sort of pavement. The rocks underneath this bowlder clay are very generally polished and striated in a manner similar to those seen beneath Alpine glaciers. The grooves in the region under consideration belong to two series, one evidently produced by a force moving to the southeast, and the other to the southwest. The southwest set prevails in the valley of the St. Lawrence, in western New York, and around Lakes Huron, Superior, and Michigan. Nearly at right angles is another set, directed to the southeast, found to the north of Lake Ontario, in the valleys of the Ottawa and Lake Champlain, and in the highlands of eastern Canada and New England. These, according to Hitchcock, are seen in Vermont at a height of 4,800 ft. above the sea. In some localities the two sets of striæ are found in the same region, and even intersecting one another. Resting upon the bowlder clay, and apparently made up from the rearrangement of its finer portions, we find the leda clay of Dawson, so called from the abundance of the shells of leda truncata which it contains. It is the Champlain clay of Dana. In many parts it also abounds in other shells, in foraminifera, and in some localities in the remains of fishes; its fauna being, alike in Canada and New England, of a somewhat arctic type, and identical with that of the gulf of St. Lawrence

at the present time. Resting upon this clay is found in many localities a stratified sand in which abound the shells of saxicava rugosa. In some cases the passage from the one into the other is gradual, while in others there seems to have been an interval marked by the denudation of the underlying clay. In some localities, however, this sand rests on the bowlder clay, and where this is wanting, directly on the rock, which in this case is often striated, and was probably once covered by the bowlder clay, which was afterward swept away. These stratified fossiliferous clays are found at heights of 500 and even 800 ft. above the present level of the sea in eastern North America. Ridges, terraces, and inland sea cliffs are also noticed over the region characterized by the deposits already mentioned, and are evidently closely connected with the rearrangement of the drift materials, and with the slow movement of elevation from the sea in which these stratified materials were deposited. The origin of the unstratified drift is, however, a question which has been much controverted; the point in dispute being whether this deposit has been accumulated by the action of icebergs under the sea, in the waters of which the stratified deposits were subsequently arranged, or whether it was the result of the action of land glaciers at a time prior to the depression of the region beneath the sea level. The iceberg theory was perhaps first formulated by Peter Dobson of Vernon, Conn., in a note in the “American Journal of Science and Arts” (vol. x., 1826), where he describes the scratched appearance of the bowlders scattered over New England as if due to “their having been dragged over rocks and gravelly earth in one steady position,” and adds: “I think we cannot account for these appearances unless we call in the aid of ice as well as water, and that they have been worn by being suspended and carried in ice over rocks and earth under water.” The transportation of masses of rock by icebergs as they drift along the currents which set from the polar regions, and the distribution of their loads over the bottom of the ocean as the bergs melt away, present, in the view of many, a repetition of the process by which in remote times the surfaces of the present continents were covered with the drift materials. Lyell supposes that the lands, with their present irregularities of surface already defined, were slowly submerged, while islands of floating ice passed along in the polar currents, grounding on the coast and on shoals, and pushing forward the loose sand and gravel spread over the bottom. Thus abraded down to the solid rock, and the surface of this grooved and striated, the shoals, by continued subsidence, passed down to great depths, where the loose materials gathering upon them were no longer disturbed. Finally he supposes the direction of the movement to have been reversed, and the bottom of the ocean to have been again raised to form dry