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all the beds of any coal-basin is well known to be so great and rapid, that, in the different parts of the same basin, it is often difficult, and sometimes impossible, to establish any correlation, while in adjacent basins, such as those of Wales and Bristol, or of Hainaut and Liège, such attempts have, with few exceptions, hitherto utterly failed. There are, however, general features which serve to show some relationship.
The great central mass of from 2,000 to 3000 ft. of rock called Pennant exists in both the Welsh and Bristol coal-field; and the total thickness of Coal Measures is not very different, being, say, 10,500 ft. in the one, and 8,500 ft. in the other, with workable seams of coal, 76 in Wales, and 55 in Somerset. In the Hainaut (or Mons and Charleroi) basin, the measures are 9,400 ft. thick, with 110 seams of coal; in the Liège basin 7,600 ft., with 85 seams; and in Westphalia, 7,200 ft., with 117 seams. On the other hand, none of our central or northern coal basins, with the exception of the Lancashire field, exceed half these dimensions, and more generally are nearer one-fourth. Further, the difference which exists between the north country coals and those of Wales and Somerset, the preponderance of caking-coals in the former, and of anthracite, steam, and smiths' coal in the latter, equally exists between our north country coals and those of Belgium, which latter show, on the other hand, close affinities with those of Wales and Bristol. I am informed by two experienced Belgian coal-mining engineers and good geologists, who have twice visited our coal districts, that the only coals they found like those of Belgium were the coals of South Wales and Radstock; there was the same form of cleavage, the same character of measures, and the same fitness for like economical purposes. Organic remains afford us little help; and not sufficient is yet known of their relative distribution. The plants, are, as usual, the same; so also are shells of the genus Anthracosia, and a number of small Entomostraca ; and the marine forms are scarcer than in some of our central and northern fields.
That, therefore, which best indicates the relation between the coal-fields of the south-west of England and those of the north of France and of Belgium, is the similarity of mass and structure, uniformity of subjection to like physical causes, and identity of relation to the underlying older and to the overlying newer formations.
These physical features are of much importance and interest. Of the underground prolongation of the axis of the Ardennes through the south of England there can be little doubt; nor can there be much doubt that the same great contortions of the strata which in Belgium folded alike the Coal Measures, the Mountain Limestone, Devonian, and Silurian series, and were the cause of similar folds in the same rocks of Somerset and Wales, were continued along the whole line of disturbance, and that the preservation of portions of the same great supplementary coal trough is to be looked for underground in the intermediate area, just as they exist above ground in the proved area. The intermediate subordinate barriers dividing the coalbasins can, I conceive, in no way permanently affect the great major disturbance, by which the presence of the coal measures is ruled.
Admitting, however, that the coal measures were originally present, the question has been mooted whether they have been removed by subsequent denudation.
It has been urged that the Coal Measures become unproductive, and thin out under the Chalk, as they range from Valenciennes towards Calais, and, therefore, that the coal-trough or basin ends there. It is perfectly true that the Coal Measures do thin out between Béthune and Calais, but not in the sense of their dying out owing to their deposition near the edge of a basir. In that case, each seam, each stratum would gradually become thinner and disappear, but such is not the fact. None of the beds of the Belgian coal-field are thick ; the average does not exceed 2 ft. At Valenciennes it is the same; whereas M. Burat states that the mean thickness of the beds actually increases westward of Béthune to more than 2 ft. With respect, also, to the extreme end of this basin, the lower beds there brought up correspond with the bottom beds of the Hainaut basin, where the lower 650 feet consist altogether of unproductive measures. The thinning-out is, in fact, due to denundation, just as the Bristol coal-field thins out at Cromhall to resume in the Forest of Dean, or the coal-field of Liège thins out at Nameche to resume at Namur in the great fields of Charleroi and Mons.
The deterioration of the coal in the small coal-field of Hardinghen, near Boulogne, has also been adduced against the occurrence of workable coal in south-eastern England; but Mr. Godwin-Austen has shown that this Hardinghen coal-field is one of those small local developments of coal-bearing strata intercalated in the Mountain Limestone, and is of older date than the great Belgian coal-field. It has, therefore, no bearing on this part of the question.
Another objection, to which much weight has been attached, is, that as the coal-field of Bath and Bristol forms an independent basin, cut off both on the east and on the west by ridges of Millstone Grit and Mountain Limestone, we have there reached the eastern boundary of the coal measures. It is probable that such a bounding ridge does exist, though, as the edge of the basin is there covered by secondary rocks, there is some uncertainty about the disposition of the palæozoic rocks under them. Admitting, however, the basin to be complete and isolated, that is no proof that the older Palæozoic rocks prevail exclusively to the east; for the coal measures of the Somerset basin maintain their full development to the edge of the basin, and are there cut off by denudation, and not brought to an end by thinning out. They form part of a more extended mass, of which we have there one fragment, while on the west another portion exists in the Welsh basin, and another in the newly-discovered small basin of the Severn valley; and there is no reason why on the east the same disposition should not prevail.
The Severn Valley basin is entirely covered by the New Red Sandstone; and as the Welsh basin is bounded on the east and the Bristol basin on the west by Mountain Limestone, the same objective argument might have been used in either case to show the impossibility of Coal Measures occurring in this intermediate area, or of their extending beyond the boundaries of either
But the fact is, it is the very nature of the great line of disturbance to have minor rolls and flexures of the strata at, or nearly at, right angles to it, and so causing breaks in the coaltrough, which would otherwise flank it without interruption; thus the Aix-la-Chapelle coal-field is separated by older rocks from that of Liège, which is again separated by a ridge of Mountain Limestone from that of Hainaut. So, in the case of south-western England, we have the separate basins of South Wales, Severn Valley, and Bristol—the extremes of the intervening belts of older rocks being two miles at Nameche and eighteen miles in Wales. These barriers are clearly only local; and the division of the coal measures into separate basins appears to be their ordinary condition along this great line of disturbance. The length of the two known portions of the axis included between Pembrokeshire and Frome, and between Calais and Westphalia, is 472 miles; and in this distance we find eight separate and distinct coal-fields. The combined length of these eight coal-fields is about 350 miles, leaving about 122 miles occupied by intervening tracts of older rocks; so that nearly three quarters of the whole length is occupied by coal strata. I consider that a structure which is constant above ground, so far as the axis of disturbance can be traced, is, in all probability, continued underground in connection with the range of the same line of disturbance; and I see no reason why the coalstrata should not occupy as great a proportional length and breadth in the underground and unknown as in the above ground and explored area. It is certain the basin-shaped form of the Somerset coal field is no reason why other coal basins,
fragments of the same great original trough, should not exist underground between Somerset and the north of France and Belgium.
With respect to the possibility of denudation having removed the intervening Coal Measures, enormous as the extent of denudation must have been previously to the deposition of the Permian strata, we cannot admit its exceptional action in this
Denudation has removed from the crest of the Mendips a mass of strata possibly equal to two miles or more in height, and from that of the Ardennes as much as three or four miles; and it has also worn extensive channels between many of our coal-fields ; so that the power of such an agent cannot be denied. (See Sect. Plate LXXXV.). But it is a power of planing down exposed surfaces rather than of excavating very deep troughs. Notwithstanding the extent of its action on the Mendips and Ardennes, deep troughs of coal measures are left flanking their northern slopes. These troughs descend to more than a mile beneath the level of the sea; and I do not think it probable that the intermediate underground portions of the trough through South Eastern England, where the axis lies lower, have suffered more than those on the higher levels, except to the extent caused by the later denudation which preceded the Cretaceous period. But this would not affect the main bulk of the coal measures. The Belgian coal-field, which was exposed to the action of both these denudations, still retains vast proportions.
At the same time the pre-cretaceous denudation was very irregular in its action, giving rise to hills and plains. At one place near Mons the chalk and tertiary strata are above 900 ft. thick; whilst at another, on about the same level, and only a few miles distant, they are not 100 ft. thick—an old underground hill of highly inclined coal measures rising in the midst of the unconformable newer strata, and giving rise to this difference. This shows that in the English chalk area we may possibly find irregular old surfaces of this kind, so that the coal measures may exist at places nearer the surface than we have estimated.
We have alluded before to the great length and small width of the Belgian coal-fields. That of Liège is forty-five miles long, with a mean width of less than four miles, whilst that of Hainaut and Valenciennes, with a width scarcely greater, is 119 miles long. The presence of lower carboniferous rocks so far north as Harwich, and the extent of north range of the Bristol coal-field, render it possible that the coal trough in the intermediate area may have a greater expansion than in Belgium ; but we have nothing else to guide us, unless it be that the lateral pressure in the intermediate ground was less than in the Ardennes and the Mendips, where it has exercised its maximum elevatory force. In that case the coal trough in this intermediate area would be less compressed and more expanded, and we might consequently look to find larger coalbasins than those of either Somerset or Liège.
The strata on the south side of the Liège coal-field rise abruptly against highly inclined and faulted Devonian rocks, and on the north side they rise, at a less angle, beneath Cretaceous or Tertiary strata ; and to the westward the great palæozoic axis of the Ardennes, consisting of Silurian and Devonian rocks, Mountain Limestone, and Coal Measures, passes westward under the Chalk of the north of France, and has been followed underground as far as Calais, where it lies at a a depth of 1,032 ft. ; while in the direction of Boulogne the old rocks keep nearer the surface, crop out from beneath the chalk downs surrounding the Boulonnais, and disappear near the channel under an unconformable series of Jurassic and Wealden strata.
We may, I think, look for a prolongation of this old palæozoic surface of highly inclined, contorted, and faulted rocks at no great depth under the same Cretaceous and Tertiary area of the south-east of England. For, although the old palæozoic surface descends rapidly from 200 ft. above the sealevel in the Boulonnais to 1,032 ft. below it at Calais, it rises at Ostend 47 ft. higher than at Calais, and, crossing the Channel, it is found at Harwich within a few feet of the same depth as at Calais, from which it is eighty miles distant in a northerly direction. Passing westward, we find the palæozoic rocks under London, 105 miles distant from, and 102 feet higher than under Calais, and 106 feet higher than at Harwich. Allowing for irregularities of the old surface as evinced by the well at Crossness, near Plumstead, which was still in the Gault at a depth of 944 feet, or some 14 feet below the level of the palæozoic rocks at Kentish Town, we may still consider that in the area between these three points, and other parts on the same range of the south-east of England, the palæozoic rocks will probably be found not to be more than from 1,000 to 1,200 ft. beneath the sea-level.
Projecting the line another 100 miles westward, we reach the neighbourhood of Bath and Frome, where the coal measures are (as before mentioned) lost, at a depth of about 500 feet, beneath Liassic and Jurassic strata. In the intermediate area between that place and London no trial-pits and no wells have been carried to a depth of anything like 1,000 feet beneath the sealevel. The deepest well with which I am acquainted is one near Chobham, in Surrey, through tertiary strata and chalk to a depth of about 800 feet, or 600 feet beneath the sea-level.