Diluvium, Or Drift, the superficial deposits of clay, sand, gravel, and bowlders which in both hemispheres are spread more or less uniformly over the land of the polar regions and the adjacent portions of the temperate zones. Geologically this deposit is very recent, and is found overlying strata of later tertiary or pliocene age. Inasmuch as great portions of the material of which it is composed seem to have been transported or at least accumulated in their present position by some violent action, the name of diluvium was given to it by the earlier geologists. In the northern hemisphere the drift is found alike in Europe, Asia, and America, extending from the polar regions toward the equator, and disappearing on the continent of North America about lat. 38°; while in Europe all traces of it are said to be lost in the countries bordering on the Mediterranean. In South America it is recognized from Cape Horn northward into southern Chili, and according to some observers much further. The assertion that great deposits of such material occur even in tropical Brazil is denied by the more recent observers. This drifted or diluvial material is divided into diluvium proper, or unstratified drift, and stratified or modified drift, which is the result of a rearrangement of the former by water.

Unstratified drift is met with at considerable elevations over the present sea level - 3,000 ft. above the Baltic, and at a height of 4,000 ft. in the Grampians of Scotland. It is everywhere characterized by loose masses of rock, more or less rounded, which in many cases have evidently been transported for considerable distances from their parent beds. As already described in the article Bowlders, they are often of great dimensions, and increase in size as the deposit is traced toward its source to the northward. In Russia they have thus been identified with ledges more than 800 m. distant toward the north. Bowlders of the same kind of granite, easily recognized, traced from Moscow to St. Petersburg, vary from two or three feet in diameter at the former to as many yards at the latter point. Instances of these phenomena are everywhere to be seen in the northern United States. In southern Wisconsin pieces of native copper were often found in the superficial deposits long before the mines of this metal were discovered on the S. shore of Lake Superior, 300 m. to the north.

The N. shores of Long Island are strewn with bowlders of red sandstone, and of granite and other primary rocks, arranged in groups which correspond with the position of the ledges of the same rocks in Connecticut. So on the European continent, the stratified rocks of which the whole region on the S. side of the gulf of Finland is composed are covered with granitic bowlders from the primary region of Scandinavia on the other side of the gulf. These bowlders, or erratic blocks, are in some places the only evidences of diluvial action, and are found resting directly upon the solid rocks; but in such cases they have been left in this position by the subsequent washing away of the finer portion of the original diluvium or unstratified drift, in which they were included. This may be described as a heterogeneous mass of clay with sand and gravel in varying proportions, enclosing the transported fragments of rock, of all dimensions, partially rounded or worn into wedge-shaped forms, and generally with surfaces furrowed or scratched, the whole material looking as if it had been scraped together. Such is the unstratified diluvium, or bowlder clay, as it is sometimes called; while in allusion to its supposed accumulation by the agency of ice, it is often designated glacial drift.

The rocks beneath this deposit are worn smooth or polished, and more or less deeply grooved or striated in a manner which shows unmistakably that the drift has been made to move over the surface with great force, grinding, planing, and scoring the rocks beneath. Resting on this deposit are generally found accumulations of stratified clays and sands, evidently arranged by deposition from water, which are known as stratified drift, or modified drift. - The characters, relations, and distribution of these various products of diluvial action in North America have been studied with great care by Newberry and by Dawson. The latter has recently published a valuable summary under the title of "The Post-pliocene Geology of Canada." Nowhere are the phenomena of this geological period better seen than in the valley of the St. Lawrence and its tributaries. Dawson divides the deposits of this drift formation into three parts: the first, or lowest, the so-called bowlder clay; the second, or leda clay, a fine deposit in deep waters; and the third, or saxicava sand, an accumulation of shallow-water sand 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 depositsare 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 striae 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, wo 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 forami-nifera, 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 land; and that during its reemergence the arrangement of the materials which cover it was modified by exposure to the distributing and stratifying action of the waves, tides, and currents. The extent and immense number of modern icebergs seem to prove their capacity to reproduce upon the shoals and over the bottom of the Atlantic nearly all the phenomena of the drift formation.

Measured as they are by miles in length, and rising at times more than 300 ft. in height, with only one fifth of their bulk then visible, they may well float off and distribute along their track the largest bowlders which they have abstracted from the rocky cliffs down which they moved as glaciers into the sea. Of late years, however, the theory of Agassiz, that the phenomena of the drift are due not to the submarine action of floating ice, but to terrestrial glaciers, has found much favor. The vast accumulations of ice which have been so well studied in the Alps are seen in their slow and irresistible motion down the valleys to score and groove the surfaces of the rocks over which they pass, rending masses of rock from the cliffs, moving the fragments forward, and finally leaving them rolled in the shape of bowlders, and grooved and scratched by the rubbing to which they were subjected when fixed in the ice. In the Alpine regions of Europe the effects thus produced are so remarkable, and spread over such extensive districts, that many geologists who have studied them are disposed to refer all the phenomena of the unstratified drift to the action of glaciers; and in this view they are confirmed by finding unmistakable evidence that the action of the glaciers formerly extended far beyond their present limits.

But the unstratified drift is found to extend over vast regions where it is difficult to conceive that mountain glaciers could ever have found their way; and Agassiz, to account for this universal glaciation of circumpolar regions, has been led to maintain the existence of a great continental glacier, or ice cap, extending over the arctic and a great part of the temperate zone, moving downward from the polar region, and of such immense thickness as to surround and overflow the summits of our highest hills, which he supposes may have required in eastern North America a vertical thickness of two or three miles of solid ice. A similar continental glacier, according to this view, must have existed in the southern hemisphere. Prof. Dana, while adopting this notion of the origin of the glacial drift, regards the hypothesis of a central and common glacier source for each hemisphere as untenable, but supposes the existence of distinct glaciers of great magnitude. Such a one, according to him, had its origin along the watershed between the St. Lawrence and Hudson bay; but recognizing the necessity of an elevated source to give motion to the glacier, he supposes that this region, which is now not more than 1,500 ft. above the sea, was then raised many thousand feet above its present level. - In these theories of land glaciation a great depression of the surface is supposed to have succeeded the glacial period, affecting in the one case the great mountain plateau to the northward, and submerging the glaciated region so as to permit the deposition above its surface of the stratified clays and sands which overlie the bowlder clay.

But, as we have seen, this in many parts of its distribution is clearly of marine origin; and a careful study of the whole of the phenomena of the drift period in eastern North America has led Dawson to regard the operation of land glaciers in this region as of very limited extent and importance, and to maintain that the widespread glacial drift is essentially submarine in its origin. This earlier view, which as set forth by Lyell has been partially explained above, endeavors to account for the phenomena in question by causes now in operation, rather than by supposing a condition of things which it is at once difficult to conceive and to explain. As expounded by Dawson, it maintains that at the beginning of the glacial time eastern North America was already under water, and was slowly rising, though with minor oscillations of level, from the ocean, the more western portion first. Along the eastern border of the rising land, over its still submerged plains, and through its valleys, then flowed the arctic current, as it now does along the coast of Labrador and the shores of Newfoundland, bearing great quantities of floating ice, by the action of which, combined with the current, the rocky strata were eroded, and the valleys and lake basins excavated.

At an early period in this order of things, the great arctic stream, pursuing, in obedience to the force impressed upon it by the earth's rotation, a southwestern course, passed over the region of the great lakes and excavated their basins; while at a later time, diverted further eastward by the emergence of the Laurentides, it would pass along the present St. Lawrence valley, and thence southwestward to the Mississippi. To quote the language of Dawson: "The prominent southwestern striation, and the cutting of the upper lakes, demanded an outlet to the west for the arctic current. But both during the depression and the elevation of the land there must have been a time when this outlet was obstructed, and when the lower levels of New York, New England, and Canada were still under water. Then the valley of the Ottawa, that of the Mohawk, and the low countries between Lakes Ontario and Huron, and the valleys of Lake Champlain and the Connecticut, would be straits or arms of the sea; and the current, with its icebergs, obstructed in its direct flow, would set principally among these, and act on the rocks in north and south and northwest and southeast directions.

To this portion of the process I would attribute the northwest and southeast striation." As the process of elevation proceeded, and the northern current found its passage across the eastera region to the sea, by channels further and farther east, the conditions became such as to permit the deposition, from seas comparatively undisturbed, of the stratified clays and sands, which are seen in so many cases to rest upon the bowlder clay, and are found with their characteristic fossils at elevations of 500 and even 800 ft. above the sea; while others, though so far as known without organic remains, are met with at still higher levels. Portions of floating ice, however, still dropped from time the rock masses with which they were freighted, in the midst of these stratified clays. Nor are evidences wanting in the lower St. Lawrence that a second invasion of icebergs may have given rise to new accumulations of bowlder drift after the deposition of the stratified clays. The valleys among the hills, and the shores of the islands, which then rose above an icy sea, would be filled with the local glaciers, of which the traces still remain, which gave their tribute to the northern current, already charged as now with immense icebergs from the polar regions.

These in great part submerged and half stranded masses, urged by current, wind, and tide, would plough and furrow the bottom, there piling up the unstrat-ified heaps of bowlder clay, to which the earth and rocks borne by the melting ice would contribute. - The formation of the diluvium or bowlder drift is thus, according to either view, the result of the action of ice. But the glacial action, in the opinion of the land-glacialists, was limited to a definite period, and operated simultaneously over a vast area, which, according to one hypothesis, was not less than an entire hemisphere. Those, on the other hand, who restrict the action of land ice to local glaciers, and call in the aid of floating ice and the arctic current, maintain that the process of glaciation is limited rather by place than by time. Ever since the conditions of the earth have been such as to give rise to the formation of polar ice, the shores and the shallow seas to which the arctic current has borne it must have been subject to glacial action such as we have endeavored to describe.

From the days in which the glaciation of our valleys and the deposition in them of the glacial drift took place, this process has not ceased, but has been transferred to other regions; and we may suppose that the banks of Newfoundland, if now raised above the ocean's level, would present striations and glacial drift, which, but for the presence of remains showing its formation to have been in the historic period, would be indistinguishable from the ancient bowlder clays of New England and Canada.