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253. Besides these patches which are interwoven with the clay, there are independent accumulations of gravel, and rubbly masses of rock-fragments, which seem to have been formed contemporaneously with the boulder-clay, and by the same agency. In Britain such accumulations generally occupy the eastern extremity of longitudinal valleys, where they form curious ranges of flat-topped hillocks; abut against the base of some mountain; or gather, without regard to any order of arrangement, along the eastern flank of those trap hills which present a bold front or "crag" to the westward. They are found for the most part in more open situations than the clay, as if they had been arrested in their progress eastward by prominences and shallows, while the clays were borne to deeper and more sheltered recesses. Like the dark clays, they are destitute of organic remains, their larger pebbles are derived from primitive rocks, interspersed with fragments of sandstone, shale, and coal from the secondary formations.

254. To account for the origin of the group thus described, many theories have from time to time been advanced, of which only two deserve notice, as being at all adequate to the purpose intended. The first is that which supposes a set of powerful currents to have passed over Britain and the adjoining continent; these currents taking a course from the north and north-west towards the south and south-east, and sweeping before them clay, sand, gravel, and loose blocks, which were deposited, as the force of the waters abated, without any order or arrangement. How long the currents continued, theorists do not aver; but from the water-worn aspect of the boulders and gravel, an indefinite period is allowed. With respect to the direction of the drifting force, little doubt is entertained, for many reasons:-1. Blocks of granite, gneiss, &c. which must have been derived from the Grampians, are found scattered along the eastern lowlands of Scotland; primitive rocks from the Lammermuir and Cheviot ranges are detected in the vale of the Tweed and in Northumberland; others from the Cumberland mountains are widely dispersed over Durham and the east of Yorkshire; boulders from the Welsh range are found in the midland counties of England; while the erratic blocks of Friesland and Germany point to the Scandinavian ridge as the source from which they were derived. 2. Those hills which range east and west have, without exception, their western brows swept bare, while their eastern flanks are thickly strewed with gravel and boulders. 3. Many accumulations of gravel bear evidence of their having been piled up by a force from the north-west. 4. Blocks evidently derived from the outcrops of certain strata are often

found among the debris a few yards to the south-east, showing clearly that the transporting power passed over them from the north-west. 5. The supposed currents have been modified in their direction by ranges of hills, so as to set the volume of water with greater rapidity down the valleys which lie between them, as the greatest accumulations of drift and boulders are found at the eastern extremities of such gorges and valleys. But while no doubt is entertained either as to the agency of water in the formation of these accumulations, or as to the direction in which the waters flowed, great difficulty is felt in conceiving any current sufficiently powerful to sweep before it blocks of several tons weight, and that over heights and hollows for many hundreds of miles. Indeed it seems impossible to reconcile the theory of violent currents with the phenomena presented; for, granting the occurrence of some extraordinary cataclysm, during which the waters of the ocean were thrown over the land, the currents must have abated in velocity as they drew to a close, leaving the detritus to arrange itself more in accordance with the laws of gravity than what is exhibited in a mass of clay and boulders.

255. The second theory supposes that those portions of Europe now covered with erratic blocks were submerged after the deposition of the stratified formations; that this submergence was caused by some extraordinary revolution in the planetary relations of our earth; that it was accompanied by a change of climate, and other terrestrial conditions; that while in this state, icebergs and avalanches formed around the earlier mountains which were still left above water; and that these icebergs, as they were loosened from the shore by the heat of summer, and floated southward by the currents of the ocean, dropped their burden of boulders and gravel precisely as Captain Scoresby (page 24) found modern icebergs dropping their debris in the northern seas, and as the officers of the recent Antarctic expedition observed similar phenomena in the Southern Polar Ocean. It is further supposed, that while icebergs distributed the erratic blocks and other debris in deep waters, avalanches and glaciers were forming moraines of gravel in the valleys of the then existing land analogous to what is observed in the alpine glens of Switzerland. Again, one cannot read Mr Simpson's account of the shores of the Polar seas, and learn that the ice formed during winter over whole leagues of gravel, breaks up during summer, and is blown on the beach by winds, or piled up by the tides, where, melting, it leaves long flat-topped ridges, without perceiving a wonderful resemblance between these effects and the long singularly-shaped ridges of ❝ diluvial" gravel. According

to this theory, it is easy to account for the south-eastward direction of the drift, for the Polar Ocean still maintains its great southward current to the equatorial seas, modified, undoubtedly, in its course, by the inequalities of the bottom over which it passes. The chief difficulty to be obviated is the temporary diminution of temperature which the north of Europe must have then experienced; and this can only be accounted for by some derangement in the planetary relations of our globe.

256. Both theories are beset with many difficulties, and though the latter accounts more satisfactorily for most of the phenomena of the erratic block group, still there are many points respecting the distribution and extent of the deposit to be investigated before either can be finally adopted. All that can be affirmed in the present state of the science is the composition and nature of the clay, gravel, and boulders, as abovedescribed the course of the currents concerned in their deposition-the fact of the land having a configuration of hill and valley not differing much from what now exists—and the peculiar scantiness, if not total absence, of organic remains. If the latter theory be adopted, it is easy to perceive how the soft bottom of the ocean, as it was elevated into dry land, would be furrowed and channeled by the receding waters-here being swept bare of its mud, but retaining the boulders; there being covered by accumulations of transportable clay and gravel; while the deeper hollows being left undrained, would form lakes and morasses, which were in turn to be silted up by subsequent material.

OSSIFEROUS SANDS AND GRAVEL.

257. Next in point of antiquity to, if not contemporaneous with, the clays and boulders of the preceding group, may be ranked those ossiferous sands and gravels found scattered at intervals over the valleys of Britain, the continent of Europe, and the river plains of North America. They are termed ossiferous (Lat. os, a bone, and fero, I bear), from their containing bones of elephants, hippopotami, horses, bears, deer, and other animals, which belong to existing species, but do not now inhabit the regions where these remains occur. For instance, large portions of England, Wales, Scotland, and Ireland are covered by irregular accumulations of rounded pebbles and gravelly sands, in which are found bones of the elephant, hippopotamus, &c. none of which have been known in this country within the historic period. In similar deposits the skeletons of elephants and mammoths have been discovered in

Siberia and the north of Europe; the bones of the mammoth, mastodon, and megatherium in America; and even among the Esquimaux of the Polar seas Captain Ross and Mr Simpson observed platters fashioned from the fossil grinders of these gigantic mammalia. Neither at present, nor throughout the whole historic period of four thousand years, have any of those countries been in conditions of climate to support such huge graminivora, and therefore geologists are compelled to assign a very remote and ancient origin to the gravels in which their relics are entombed.

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258. The composition and aggregation of these sands and gravels point to the long-continued action of water by which their pebbles were rounded and smoothed like those of the rivers, lakes, and sea-shores of the present day. The mineral

character of the pebbles enables the geologist often to decide with certainty as to the quarter from whence they were drifted; and in Britain this generally corresponds with that from which the erratic blocks were derived. Like the boulders, the great mass of the pebbles are from primitive rocks, interspersed with secondary sands, rolled flints, and calcareous cement. The imbedded bones are more or less impregnated with iron and lime, are harder and heavier than recent bone, but never so much petrified as to obliterate the bony structure. The gravels have all a light ferruginous tint, and can only be distinguished in certain localities from true tertiary gravel by the recentness of their fossils, or by some circumstance of position or mode of aggregation.

259. Much uncertainty prevails with respect to the origin and aggregation of these ossiferous sands and gravels. Many of them are no doubt local, and have been formed by the action of rivers, the silting up of lakes and other extensive shallows; and could such be separated from those which appear to have been accumulated by some very powerful and extensive agency, the task were greatly simplified. Unluckily, however, this seems to remain an insuperable difficulty, so that geologists are compelled to class together all deposits of ancient ossiferous gravel into one group, without much regard to the agencies concerned in their accumulation. This grouping is rendered still more indefinite by the assertion of some eminent geologists, that ossiferous gravels have been found underneath the erratic boulder clay, containing the same kind of bones with those above it. Should this be the case, it would tend to establish the theory, that the ossiferous gravels and erratic blocks took their origin from the same set of unusual causes; that they belonged to an era which was posterior to the tertiary, and prior to the existing arrangements of nature; and that before this epoch, which was of considerable duration, many of the tertiary races had died away, and been succeeded by others, most of which still exist, though now extinct in the regions where they then flourished.

OSSIFEROUS CAVES, FISSURES, AND BRECCIA.

260. Belonging to the same era with the ossiferous gravels, and only here separated for the sake of perspicuity, occur numerous caverns and fissures filled with the bones of elephants, rhinoceroses, hyenas, bears, deers, and other animals. These caverns are found in England, France, Belgium, Germany, along the coasts of the Mediterranean, in North America, and in Australia. They are situated almost exclusively in

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