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sand of the upper erratics. At greater heights, which may be about two thousand feet, both classes of phenomena disappear; and we have peaks of bare rocks surrounded by masses of their own ruins, in the form of angular blocks. Blocks of the same kind, having their surface more or less scored, are arranged at intervals in terraces between these naked peaks and the upper limits of the upper erratics.

The transport of erratic blocks having a distant origin over great irregularities of surface-one of the most remarkable peculiarities of the erratic tertiaries-is well exemplified in the Lake region of the north of England, where the marked characters of some of the rocks render their detritus easy of identification, while the limited and well-defined area from which they have been derived precludes mistake as to the direction in which they have been transported. The easily recognised granite and syenite of Shap Fells and Carrock Fell have been transported chiefly along the lines of depression between the Penine chain and the mountains from which the blocks have been derived, though they have crossed the former at one point. In their course along the low grounds they have in some cases followed the present lines of drainage, but independently of the present levels; more frequently, however, crossing them. They have travelled northwards down the Vale of Eden to Carlisle, where they are mixed with boulders which have come southwards across the Solway Firth. They have gone westward, along the depression at the northern extremity of the Penine chain, caused by the Tynedale fault, to the mouth of the Tyne, though that river and its tributaries have not their sources in the Cumbrian Mountains. The north and south range of the Penine chain has presented, in general, an obstacle to their passage eastward, and has caused them to be drifted in immense quantities southwards, by Lancaster and the narrow tract between the mountains and the sea, into the plain of the new red sandstone, over which they have spread into the Valley of the Severn at Worcester and the Valley of the Trent near Stafford, crossing in their passage the lines of the Lune, Ribble, Weaver, Mersey, and Dee. Those blocks which have been borne, not in the direction of the valleys, but across them, have first crossed the ridge of Orton and the Vale of Eden, and this valley must have existed not only before their dispersion, but before the formation of the new red sandstone, because horizontal strata of that rock occupy its bottom. The point at which they have crossed the Penine chain is the pass of Stainmoor, the lowest portion of that ridge opening directly to the west, and facing the Cumbrian Mountains. From this summit, which is 1500 feet above the sea, as from a new centre, they have pursued their course in various directions, descending the eastern slopes of the Pen

ine chain into the Valley of the Tees, which they have followed to the coast at Redcar and into the Vale of York, which they have traversed southwards to the Humber. The oolitic ridge of the Eastern Moorlands, and the chalk ridge of the Wolds, have opposed, on a minor scale, obstacles to their passage eastward similar to those presented at the outset of their course by the Penine Chain. These have been surmounted, in a similar manner, at the lowest points of those ranges. Blocks of Shap Fells granite, which have thus reached the German Ocean at high levels, and lie on the oolite at Scarborough and the chalk at Flamborough Head, attest, like those in the vales of Tees and York at lower levels, an interlacing of lines of drift from the north and west, with others from the north and east, similar to that which has been described in the case of the gravel of the Midland counties.

In Wales the northern and western erratics and marine shells are confined to the skirts of the chain. They have not been transported into the valleys of the interior. In them, however, there are considerable accumulations of local detritus, with scratched and polished fragments, and rock surfaces similar to those associated with the boulder clay of the flanks of the chain, which is undoubtedly of marine origin, from the presence of marine shells.

In whatever manner these detrital masses of the interior may have been formed, they bear evident marks of having been arranged under water, and of transport outwards. There are also on the skirts of the chain, at the mouths of the great valleys, situations from which the boulder clay appears to have been removed, and replaced by detritus carried outwards. It is important that these facts should be borne in mind, because a period of subsidence, under a sea having the power of transporting detritus derived from great distances, would be the pericd of the accumulation of the erratic deposits, and of their transport inwards, while the subsequent period of elevation would be that of denudation, and of outward transport.

The eastern coast of Ireland is fringed with deposits similar to those of Wales, containing detritus which proves transport from the north; but nothing which can be identified as having an eastern origin. Among the fragments indicating transport from the north, are those of the peculiar hard chalk of the county of Antrim, which has been traced in the erratic deposits from its source in the north, to Wexford in the south. It has also been found in the boulder clay of the extreme point of Carnarvonshire, and in South Wales near St. David's Head. The whole of Ireland is covered, more or less, with deposits of detritus, borne from north to south, and presenting the same mode of distribution, and the same general characters, which have been

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described in England. Pleistocene marine shells have been found in many parts of these Irish tertiaries; the most remote from the sea being in the heart of the county of Cavan. The most southern points of Kerry exhibit those grooves and scratches, which some geologists, who do not consider this part of Ireland to have been submerged during the pleistocene era, refer, nevertheless, to the action of ice, either terrestrial or marine.

Such are the erratic tertiaries of Britain, long known by the name of diluvium, and more recently by that of northern drift. By comparing them with the condensed description of the erratic deposits of North America, given by Professor H. Rogers, in one of the works which we have placed at the head of this Article, an almost perfect identity will be perceived, as regards the state of the rocky surface beneath them; the distribution of the bouldered materials; the condition of the land as to level at the time of their dispersion; and the epoch and duration of the causes which produced it.

The progress of opinion respecting these wide-spread and comparatively modern deposits, is worth tracing, as affording evidence that no department of geology has been made the sport of so much crude speculation, and hasty generalisation; and that in none are more difficult and interesting questions remaining unsolved, which demand for their solution long and patient investigation.

Hutton taught that these detrital deposits were the results of atmospheric erosion on a subaerial surface; and that their transport to situations beyond the influence of existing streams, had been effected by the shifting of river beds, and the bursting of lakes. Sir James Hall was the first to observe the furrows on the surface of the rocks beneath them, which he attributed to the action of currents of extraordinary energy, the direction of which he inferred from the direction of the furrows. Smith was the first to point out the distinction between the loose covering of gravel, sand, clay, and boulders so named, and the regular fossiliferous strata. The latter had been long regarded as proofs of the Noachian deluge; and when the discoveries of Smith, which proved that they represented a number of successive sea-bottoms, and a number of successive organic creations, gave a death-blow to these views, it was not surprising that the deposits so extensively distributed as those of the erratic period, and attributed, with much apparent probability, to extraordinary marine action, should be regarded as monuments of an event recorded in Scripture, and of which the memory remains in the traditions of all nations. This form of the diluvial hypothesis acquired popularity from the "Reliquiæ Diluviana" of Buckland, with whom.

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the term diluvium originated; and received the sanction of Cuvier. Its popularity, however, was of short duration. the tertiary strata became better known, it was discovered that between the most recent with which Cuvier was acquainted, and that deposit which he attributed to a transient irruption of the sea, a long series of strata intervened, now known as the tertiaries of the miocene and pliocene epochs, representing a considerable lapse of time, and testifying, by their organic contents, both of molluscs and mammals, a gradual passage from the eocene to the existing fauna and flora. It became evident, also, that the tertiary deposits of this age, both marine and fluviatile, with all gravel beds in all parts of the world, had been erroneously included in the so called diluvium.

In the face of these facts, the most strenuous supporters of the diluvial origin of the superficial deposits, rapidly abandoned it. By some they were considered to belong, not to one, but to many epochs, and to have been shot off the flanks of mountain chains, at successive periods of elevation; those of different epochs having become so blended as to render their discrimination impossible. By others, attempts were made to return to the Huttonian doctrine, respecting the subaerial origin of these deposits, together with the sounder views of that school, which explained the phenomena of the stratified rocks, their association with unstratified and crystalline masses, their consolidation and elevation, to the combined operations of the aqueous and igneous forces now in operation.

About this time marine shells, nearly all of existing species, began to be discovered in the superficial deposits; first in North Wales, at an elevation of nearly 1392 feet above the sea, and subsequently, in the various parts of England and Ireland, which have been already indicated. The diluvialists had often been asked, why, if the diluvium had been formed by an irruption of the ocean, it contained no marine remains, and why its fossils should be exclusively those of the land? They had been reminded, too, that in a deposit, which they supposed to have been formed within the human period, no human remains or works of art had ever been discovered; that the bones of land animals contained in it were chiefly those of extinct species, and that in the Scriptural account of the deluge, there is nothing to warrant the belief that it was of that violent character, or produced such a change on the surface of the earth, as their hypothesis required.

The discovery of marine remains, belonging to a very recent epoch, in the erratic deposits produced, therefore, a modified diluvial hypothesis, among those who were aware of the peculiar characters by which they are distinguished. The diluvium was

Diluvial and Glacial Hypotheses.

409 supposed to have resulted from marine action, of a violent and transient kind, upon a terrestrial surface, but prior to the existence of the human race. About the same time evidence was collected, that in the British islands and many other parts of the world, a considerable elevation of the land had taken place, not only since the neighbouring seas were inhabited by molluscs of existing species, but since they were inhabited by the same groups of existing molluscs as those now established in the vicinity. With these raised beaches which belong to an epoch still more recent than the erratic, the opponents of the modified, as well as of the original diluvial hypothesis, confounded those erratic deposits which contain marine remains. They all now became raised beaches, and instead of a universal ocean without shore-pontus undique undique mare-we had now a universal shore without any deep water deposits, and without sea bottoms possessing the characters generally appealed to as proofs of ordinary marine action of long duration. Both parties were in some measure right, and in some measure wrong. The diluvialists were right in maintaining the peculiar characters which distinguish the erratic tertiaries from raised beaches and the tertiary beds of other epochs, and in pointing out the evidence of a previous terrestrial surface over which they had been spread ; but they were wrong in the nature of the agencies to which they ascribed those peculiarities. Their opponents were wrong in denying those peculiarities, in dwelling exclusively on the characters which the erratic deposits possess in common with other tertiary strata, and in shutting their eyes to the evidence of the pliocene terrestrial surface which they covered.

To the diluvial succeeded the glacial hypothesis of Agassiz, founded on the study of the glaciers of the Alps, their powers as transporters of detritus, their effects in grinding and polishing the rocks over which they pass, and the evidence which they have left of former extension beyond their present limits. Playfair appears to have been the first, as long ago as 1816, to ascribe the transport of the erratic blocks on the Jura, to the agency of glaciers, which once stretched across the Lake of Geneva and the plains of Switzerland. Venetz, who had collected evidence of the oscillations of the Swiss glaciers in historic times, was the first publicly to maintain the same doctrine in Switzerland. Warmly supported by De Charpentier, this doctrine was extended by Agassiz, as an explanation of the erratic phenomena to other regions in which it is less applicable -Britain, the north of Europe and America; and since his residence in America, he still applies it to the wide-spread erratic deposits of that country. On his hypothesis the whole northern hemisphere, if not the whole world, was covered by a vast cere