The beds of old lakes, often consisting of layers of Shelly marl, with bones of existing or extinct quadrupeds, the surfaces of silt which lie along the actual and ancient testuaries of rivers, and often conceal buried forests and subterranean peat, present no difficulty as to their origin. For the processes by which peat grows and trees are buried, and marshy land is saved from the sea, and lakes are filled up, are at this day in action. To all such peat or turf moors, subterranean forests, marsh and fen land, drained lake-beds, and sand and gravel, the title of alluvial deposits is very commonly given. Generally, they require no supposition of extensive changes of physical geography produced by violent disturbances of nature, but seem to be clearly and perfectly explicable by causes still in action, though perhaps not in the same situations on the earth's surface. But there are other gravels, sands, and clays, to which this explanation cannot be applied without calling in aid great changes of physical geography, or physical processes not seen in daily operation ; such as extensive displacement and change of level of land and sea ; unusual floods of water ; surprising altera tions of climate, or movement of glaciers in situations where now the snow and ice of the coldest winter melt with the first breath of spring. These phenomena were classed under the title of dilurial deposits, at a time when their origin was very generally ascribed to violent floods of water, and the title was retained even by geologists who did not admit this hypothesis. They are now generally included by geologists under the designations of boulder-clay, boulder formation, and northern drift ; the latter term having been applied to them, because, in Europe and in North America, where only they have yet been distinctly recognised, they have evidently been brought from regions to the north of those in which they are now found. At the same time, according to Sir C. Lyell, " the bulk of the mass in each locality consists of the ruins of subjacent or neighbouring rocks; so that it is red in a region of red sandstone, white in a chalk country, and gray or black in a district of coal or coal-shale." They belong both to the newer pleiocene or pleistocene and to the post-pleiocene series of present geological nomenclature.
These so-called diluvial deposits are commonly admitted or assumed to be of older date than those called alluvial, and, taken in a collective sense, they are so, but this is the least important circumstance characteristic of their history. The conditions of their accumulations are remarkable.
1. It is often seen that thick deposits of clay, sand, and pebbles, or large fragments of rock, lie on the very summits of hills (as abun dantly on the hills which adjoin the valley of the Thames).
2. Fragments of rocks quite unlike those of the vicinity lie in valleys, on hills (as on the Saleve near Geneva), and even on islands (as on 3. These fragments are found solitary, or buried in clay, sand, or gravel, and sometimes in enormous abundance, as in Huntingdonshire, near Birmingham, in Holderness, and other parts; and they are such that no stones of like nature occur anywhere in the natural drainage of the country where the gravel is accumulated, nor within 20, 50, or even 100 miles of the spot.
4. The fragments (often called boulders, and also erratic blocks) appear thus in several cases to have been transported from particular parts of the country, over elevated ground, across the natural valleys and ranges of hills, but yet are, in some cases, distributed in a manner which manifests a decided dependence on some of the greater features of physical geography. Thus the abundantly spread detritus from the C berland mountains crosses the island to Tynemouth, and reaches the coast of Yorkshire, but does not cross the Pennine chain of mountains, except at one point (Staimnoor), though it spreads along the western side of it as far south as Manchester and the plains of Cheshire and Staffordshire. In like manner the detritus from the Wester Alps has been carried on to the Jura, and lies in a strange manner in all parts of the hollow of the Lake of Geneva, and on the insulated Sal&e Mountains ; yet it has been observed that the lines followed by the boulders are theme of the great valleys, so that each great valley has been the direction in which were carried the blocks from the head of that valley.
5. It is observed that often the largest blocks contained in a mass of this detritus lie at the top, resting on the smaller gravel and sand ; and that below the whole mass the hard rocks are scratched by parallel distinct small grooves or strife, marks of the dragging movement to which the stones were subject in their passage.
G. Though in some cases successive deposition can be traced in the parts of such a mass, it is very often seen that the materials are entirely unarranged, mere heaps of stones, and sand or mud ; the stones being often indiscriminately stuck in clay, large and small, heavy and light, absolutely without any stratification, such as long suspension in water must certainly have produced.
7. Finally, amidst such confused masses, bones of land quadrupeds, mostly or entirely of extinct species, and even of extinct genera, occur, and locally even in abundance. These are, however, more common in laminated lacustrino deposits resting upon the diluvial masses, or perhalat covered by them.
With the mammalian remains alluded to are not unfrequently associated, as has recently (1S61) been shown, flint implements of human work ; indicating, if not proving, the contemporaneity of some of the now and long since extinct mammals with certain races of man. (Prestwich, in Phil. Trans.,' 1860.) It was thought possible to explain these characteristic phenomena by many local inundations, or one general and overwhelming flood, capable of overcoming many of the lesser inequalities of surface-level, but modified in its course by the larger ranges of mountains and valleys. And as in the northern zones of the world (which have been much investigated in this respect) there is a very frequently observed direction of the boulders to the south or south-east, it has been pro posed for consideration whether some great change of the level of land and sea in the circumpolar regions might account for what seems a general fact. But further, as the most abundant deposits of this nature have been drifted from particular chains of mountains, as the Cumbrian group in England, the primary mountains of Norway, the Alps, kc., all which districts have undergone elevation at some time, it has been thought that their upward movement may have been the cause of the displacement and transport of the blocks. (Buckland, Diluvianfe ;' Elie de Beaumont, Sur les Revolutions dui Globe.') It has, however, been proposed to account for the distribution of the boulders by a more gradual action of the waters of the sea. If the regiou of Cumbrian rocks, for example, and a very large portion of the north of England, were supposed to be raised from the sea, by a continual or intermitting movement, so as to bring successively under the action of the breakers the whole country to the east and southeast of the area now occupied by the Cumberland mountains, this would allow of a continual drifting of the boulders to the east and Routh, by the continual tendency of the tides and currents of the sea. (Phillips, in Treatise on Geology, &c.; ' Whewell in 31urchison's Silurian System.') Floating ice has been represented as adequate to carry off from the shore where it was formed masses of mud and fragments of rocks, and, by melting or turning over, to spread them on the bed of the sea. This sea-bed raised would show the accumu lations from such icebergs, often in narrow bands or insulated patches, such as really occur, and have been long celebrated, among the heaps of Norwegian detritus which lie on the sandy plains of North Germany. (Lyell, Principles of Geology,' `Manual of Elementary Geology ;' Murchison, Silurian System.') Finally, ice in another form has been appealed to for the explanation of these phenomena. The formation of glaciers In mountain valleys is such as to permit of their forward movement clown a slope, and their carrying with them in their progress fragments of rocks and heaps of gravel and mud which by any cause fall upon their surface. These heaps of "moraine" accumulate along the sides and at the lower termi nation of the glacier, and the arrangement, or rather confused aggregation, of the materials in them resembles very much that of the masaes to be accounted for. The surface of the rocks below a glacier is scratched, as we have before stated to happen in places where boulders are noticed ; and as in the Alps It is certain that glaciers have formerly been extended much farther from the mountain summits than now they are [Snow, PERENNIAL], it has been con jectured that anciently, in the times coincident with or preceding the boulder or drift period, they were very much more extended, so as oven to have reached from the Alpe to the Jura, from the mountains of Norway to those of Bohemia, and from Shap Fells in Westmoreland to the mouth of the Humber. Upon the subsequent contraction of these glaciers, the moraines they had left would experience some changes by the action of water (melted ice), which might then run in lines impossible for watery currents after the ice was fully removed.