Subsidence and Elevation of Land.

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rocks as the softer slates, the chalk, and the oolites, would be borne from different parts of the coast, and drifting about in the melting ice, would be dropped, as we find them in the boulderclay, in some cases side by side, in large unmixed masses, in others, in detached fragments, dispersed through the mud.

Regarding the boulder-clay as a littoral deposit, the sinking of the land, during its formation, will explain the greater altitudes at which it occurs in the interior than in the coast. As the subsidence continued, the previous coast line, broken by islands and promontories, would be converted into an open sea, in which boulders and heaps of detritus from remote quarters would still continue to be occasionally dropped. The most recent portions of the upper erratics would therefore differ from the lower, in part from being formed in a more open sea, and in part from the initigated rigour of the climate at the close of the glacial period. When the amount of depression reached to about two thousand feet, only the summits of our higher mountains would remain. above the sea as small scattered islands, which, on the commencement of the movement of elevation, would be left as masses of frost-shaken rock, surrounded by heaps of their own angular ruins, the very conditions which our mountain peaks now exhibit. As the elevation continued, large portions of the sand and gravel of the upper erratics would be removed from higher to lower levels, forming terraces of rolled materials, in the situations where we find them, bordering the great lines of drainage at various heights. Large areas of the lower erratics would be exposed by the denuding process, and would be partially covered by outlying patches of the upper sand and gravel; while along the great lines of drainage, the two would be almost entirely removed, and the subjacent rocks exposed. Out of these combined operations would arise all that irregularity of distribution which appears to be one of the prevailing characteristics of the erratic deposits. Our higher mountains would be clothed with glaciers both during the subsidence and elevation. Those of the latter period, as suggested by Mr. Darwin, may have assisted in clearing the great valleys of the marine deposits, and may have left those deep grooves on the rocky surface, which conform to the course of the valleys.

The contorted state of Cromer Cliffs have long presented difficulties to which the narratives of the polar voyagers appear to offer a satisfactory solution. The conditions to be explained are remarkable, and preclude the idea of their having resulted from a force acting from below. The contorted strata consist of laminated beds of the upper erratics occupying hollows in the boulder-clay. Beneath them are strata of crag and chalk, in their original horizontal position. There are also

many serious objections to the contortions having been caused by the ploughing action of grounding icebergs. Sir Edward Parry found, however, a stratum of compact blue ice fixed ten feet below the water for several miles along the coast of Melville Island, the remains, as he supposed, of floes driven on shore by the heavy pressure of large ice fields, and remaining fixed in the viscous mud. To ice thus fixed and covered with marine deposits, afterwards laid dry by elevation, he attributed the phenomenon of underground ice in cold countries. Laminated beds. of the upper erratics, accumulated over such masses, would subside, as the ice melted, on the return of a milder climate, into the cavity which it had previously occupied, and the walls of clay bounding the cavity being squeezed together, under the pressure of three or four hundred feet of strata, acting laterally in the vicinity of the cavities or weak places, like the creep in coal mines, would produce, with the aid of irregularities in the upper and under surface of the ice, every variety of contorted strata which the cliffs of the Norfolk coast exhibit.

If the gradual submergence of the land proceeded from north to south, the erratic deposits would be the deepest in the northern parts of the island, and at some southern point, where the climatal conditions precluded the formation of boulder-clay as a littoral deposit, the upper erratics would overlap or extend beyond the lower. Here, again, the theory is the expression of facts, for the boulder-clay does not extend beyond the northern confines of the valley of the Thames; but the chalk and eocene tertiaries, south of that river, are covered with thin beds of a peculiar subangular gravel, which may be considered as a modification of the upper erratics of the northern district, though some geologists, in the absence of marine remains to define the age of this gravel, refer it to an older portion of the tertiary era. The history of the pleistocene epoch does not terminate with the elevation of the erratic tertiaries; neither are their accumulation and denudation the only operations which have modified the influence of the rock formations on the soil. In every district to which our observations have extended, tabular hills, up to an elevation of somewhat less than one thousand feet, are covered with a thin film of soil, of different composition from the bed on which it rests, whether that bed belong to the erratic, tertiaric, or to older strata exposed by denudation. On clays it is more sandy, on sands more clayey than the subjacent bed. It deepens, from less than six inches, on summits and at the heads of valleys, to four or five feet and more near the coast. It is composed, in some cases, of finely divided matter, forming a deep loam. It contains, in others, alternating seams and collections of fragments derived from different neighbouring forma

tions which imply aqueous transport. In others it is wholly replaced by angular or very slightly worn debris. In some cases it abounds with large angular fragments, which must have required for their transport forces of greater intensity than ordinary atmospheric action. It is not a mere talus, for it is spread over table lands, as well as at the base of cliffs. It is the principal cause of those variations of soil within small areas, and without corresponding changes in the mineral character of the rock on which they rest, which are found, whenever reduced to order by mapping them, to be dependent on contours. The geologist who first drew attention to this deposit called it the warp of the drift," or "erratic warp," from a belief that it was formed by the last wash of the erratic sea on the emerging land, and from its resemblance, in low situations, to the deposit left by muddy waters in the process of warping land. Phenomena, however, were discovered by more extended observation, which indicated the lapse of a considerable interval of time between the desiccation of the bed of the erratic sea and the formation of this warp; and during the interval the country appears to have been repeopled by many of the large pachyderms which had flourished there before the submergence of the erratic block period. Estuary and freshwater beds, formed on the denuding surface of the erratic deposits, and containing exclusively living species of shells, associated with bones and teeth of the elephant, rhinoceros, horse, and a species of bos, are covered in West Norfolk by this deposit, which fills pipes, furrows, and cavities excavated in the freshwater beds. It extends, of variable depth, to the summit level of the district, filling similar excavations, in whatever beds it is in contact with, whether they belong to the erratic or to older formations. Similar deposits cover the mammalian gravel of the ancient wide-spread alluvium of the Thames, and are found in the district south of it, holding the same relative position to the subangular flint gravel of that district, which they hold to the undoubted upper erratics north of the Thames. It has recently been described in some parts of the southern district by Mr. Austin, under the name of "head," as a subaerial accumulation, the result of atmospheric agency of a peculiar kind. If we understand his paper rightly, this was the melting of snow. It has also been described by Sir R. Murchison, in a paper recently read before the Geological Society, but not yet published, as it exists in other portions of the same district, under the name of "angular flint breccia of the southern counties," and attributed to anomalous marine action-the bursting over the land of waves analogous to those of earthquakes. His descriptions apply to the upper part of the bone bed of Dr. Mantel, at Brighton, which is based on an ancient beach con

taining marine remains, with rolled pebbles of granite, and other crystalline rocks. Similar pebbles have been found beneath the mammalian deposits of the valley of the Thames. This alone would be an indication that they accumulated after the desiccation and denudation of the bed of the glacial sea; for whatever proofs the red and mammalian crag may afford of the commencement of an arctic fauna, the true erratic phenomena of the transport of granitic fragments do not begin till after that period. There is, however, independent evidence, first pointed out by Dr. Buckland, that the valley of the Thames, unlike those of Norfolk and Wales, was excavated after the transport of the gravel of the Midland counties, which we regard as belonging to the latter portion of the erratic period.

The attention of geologists is now strongly fixed on these questions, and we may hope that the superficial deposits will receive more careful investigation than has yet been bestowed on them. With regard to that deposit, which one eminent geologist attributes to anomalous atmospheric action, another to anomalous marine action, and which others assert to have resulted from the decomposition in situ of the rock on which it rests by ordinary atmospheric action, all we contend for is, that it is an aqueous deposit of some kind or other on which the distribution of soils in a great measure depends. We do not attempt to define the nature of the agencies which produced it, beyond this, that if marine, they differed from ordinary marine action-if atmospheric, they differed from ordinary atmospheric action; that they produced phenomena different from those which, in the lower erratics, we have traced to the action of shore ice, and different from those which, in the upper erratics, we have traced to the action of ice floating in open seas.

The practical questions in the Geology of Agriculture, connected with the pleistocene deposits, are independent of questions in theoretical geology respecting their origin. In whatever manner they may have been formed, the extensive areas which they cover, the great elevation to which they extend, their frequently great depth, and the mixture which they contain of matter derived from a great variety of rocks, are unquestionable facts, which render them of the utmost importance in an agricultural point of view. The state of our knowledge respecting the distribution of soils is disgraceful to an age when so much is doing in the application of science to agriculture. Having lately endeavoured to compile, from agricultural authorities, a map of the soils of England, for comparison with maps of the strata, we were compelled to abandon the attempt from want of materials. The only resources are the maps, on a very small scale, attached to the Reports of the Board of Agriculture,

and they resemble fractions, most of which require reduction to a common denominator, while some are incommensurable, from want of a uniform nomenclature. Foxbench, rammel, woodland, clum, clunch, cledge, keale, pinnock, hassock, stone shatter, malm rock, hazel, black hover, and bears' muck, with red land, black land, and white land, are some of the wild flowers which we have culled from the fields of agricultural nomenclature. Each of the last three terms is applied to the soils of more than one geological formation, and the white land of one county is often geologically the black land of another. We found Arthur Young, the Secretary to the Board, complaining, as well as other reporters, of the loose and indefinite manner in which the common terms, sand, loam, and clay are used in different districts, and different parts of the same district, everything stronger than the average quality of a sandy district being called clay, though it may scarcely, with reference to composition, deserve the name of sandy loam. Whether the agricultural districts described accord with geological areas, or are independent of them, we are invariably told of the numerous and intricate variations contained in every area assigned to any given description of soil. Districts of clay, sand, and loam are laid down, with the reservation, that though those are the prevailing characters in each, the cases of exception are nearly as numerous as those of the rule; and in most counties a "district of various soils" is formed, comprising not unfrequently the larger portion, in which the variations are said to be so numerous and intricate, as to defy classification.

The oolites and the chalk are the strongholds of those who derive soils exclusively from the rock below by ordinary atmospheric decomposition, in situ, and who found on that assumption another, that the composition of the soil and the rock are identical. If, however, we trace these strata through the maps and reports of the Board of Agriculture, or the more recent essays in the Journal of the Royal Agricultural Society of England, it will be found that, though soils on the chalk ought by hypothesis to be white and calcareous, non-calcareous sands, loams, and clays alone are described, and those of any colour but white, from Flamborough Head till the counties of Cambridge, Beds, and Herts are reached. Even there these chalky soils are stated to be confined to steep escarpments and lofty summits, so that they occupy but a small portion of the area allotted to chalk on geological maps. In Buckinghamshire, the variations of soil on the chalk are described as so numerous that scarcely a single parish can be characterized as consisting of one description. It is the same in the western counties, and on the North and South Downs of Kent and Sussex. Even in Hampshire and Wiltshire,