ON THE "LITTLE LIMESTONE" AND ITS ACCOMPANYING COAL IN SOUTH NORTHUMBERLAND.
BY G. A. LEBOUR, F.G.S. LONDON AND BELGIUM, F.R.G.S., &c.
THE coals of the Carboniferous Limestone series in the North of England must eventually acquire a much greater commercial importance than they have hitherto had. As the Coal-measure seams become exhausted, the thinner and inferior, but by no means always bad, coals of the West of Durham and Northumberland will be sought after and worked ; and the writer ventures to say, that it will be found that they are more numerous, thicker, and of better quality than they are usually supposed to be. It is as a small contribution to a more extended and accurate knowledge of. these lower carboniferous seams that this paper is brought before the Institute. It is based entirely on personal observation and on authoritative documents, and so far as it goes, may, it is hoped, be found useful in future investigations..
The coal seams in the Carboniferous Limestone are too numerous to be all mentioned here, and many of them are too unimportant for detailed notice ; they can, however, be conveniently divided into five groups (as far as the southern half of the county is concerned), which for convenience are named as follows in ascending order
1. The Lewisburn Coal Group.
2. The Plashetts Do.
3. The Redesdale Do.
4. The Shilbottle Do.
5. The Acomb Do.
It is with the most important seam of the last, or Acomb group, that the writer is now engaged. In a former paper,* he gave a brief account of the third group.
The position of each seam in the series of shales, sandstones, and limestones of every thickness, which constitute the Carboniferous Limestone series in the North of England, is best determined by ascertaining its relation to some well-known or easily-traced bed of limestone, either above or below it. By coal miners in the west this is so well understood that in many cases the seam takes the name of its "guiding" limestone. Thus the coal seam about to be described is often known as the " Little Limestone coal." This "Little Limestone" is a very constant stratum, recognizable over a large extent of country, from the Alston mining district to Coquetdale, and no doubt still further north, Plate XIV. In Westgarth Forster's section ( "A Treatise on a Section of the Strata," etc., by Westgarth Forster, 1821, p. 165) it is described as being the second limestone of his " Lead Measures," the first being the " Fell-top Limestone," and the third the "Great Limestone." This arrangement holds good for a great portion of the south-westerly extent of these beds, but on reaching the Tyne a great change is discoverable ; not, it is true, in their relative positions, which are tolerably constant throughout their known course, but in the appearance, between the " Fell-top " and the " Little " limestones, of three other calcareous beds, quite as important in thickness and in quality as the best of our Northumbrian limestones. The writer `does not know these interpolated beds south of Corbridge, but from that place to as far north as the Wansbeck at least, they form conspicuous characteristics in the geological features of the country-more especially is this the case with the two upper beds, which appear to be more constant than the third and lowest one. The latter varies exceedingly in thickness, from twenty-five to five or six feet in three or four miles in some places. - This putting in of additional limestones in a well-known series has been, probably, the cause of much of the misapprehension which has often prevailed with regard to the identity of the beds below them. The three limestones in question are well seen in the neighbourhood of Belsay, where the upper two are, or have been, largely quarried-the highest in the South Park, and the next in the North Park ; the third and lowest is very thin here, but can be seen cropping out in the Belsay Burn, west of the park, just south of the Military Road ; however, near Matfen Moor, this bed is fairly thick, and has been quarried. Before leaving the subject of these, so to speak, extra. limestones, the occurrence of two thin seams of coal, in connexion with them, should be mentioned, with the continuity of which the writer is not acquainted; both have been worked, however, on a small _scale-the upper one, about twenty inches thick, and lying immediately below the second of these limestones, at the Gallow Hill Quarry, near Bolam, and the lower, about eighteen inches thick, and about forty feet below the third limestone, near the spot mentioned above by the Belsay Burn.
A considerable but varying thickness of strata occurs between the lowest of the intercalated limestones, and the " Little Limestone," not less than 1,250 feet in the Matfen and Inghoe district. The thickness between the Fell-top and the Little, would here be about 1,450 feet. In the Alston region this great mass is reduced to little more than 330 feet. This important thickening of this set of beds, together with the addition of the calcareous elements above described to the north-west, does not seem to have been pointed out before ; but it agrees thoroughly with the gradual change of the entire lower carboniferous series from south to north. Accompanying this general thickening of the mass of strata above the °1 Little Limestone," there is a very obvious disproportion between the increase in thickness of the shales and that of the sandstones, of which this mass consists :-in the south-west, the total thickness of the sandstones is nearly equal to that of the shales, the latter predominating slightly, while on the other hand to the north-west, the shales form scarcely one-fourth of the entire mass. The sandstones moreover grow more and more gritty to the north-west, and are in places almost conglomerates. Thus at Inghoe Crags, Shafthoe Crags, and Rothley Crags, quartz pebbles are commonly found in the coarse grit larger than pigeons' eggs. Practically, indeed, from the Tyne to the Wansbeck the beds overlying the "Little Limestone" may be considered as one great deposit of sandstone, varying in texture from fine flagstones to the coarsest grit, and divided by bands of shale of no great thickness. One seam of coal at least there is in this series of grits, which in the Alston district is about four feet thick, and lies about half-way down between the " Fell-top" and the "Little Limestone," and which the writer is inclined to identify with the Oakwood Coal of St. John Lee, and Fallowfield, with some little doubt however. At the latter places the depth from the Oakwood coal to the " Little Limestone" is about 260 feet.
In the district under consideration, which may be roughly defined as extending from the South Tyne about Haltwhistle, to the Wansbeck about Wallington, the grit series can be easily studied, dipping at a. low angle to east from the Wansbeck to Matfen, thence bending round towards Stagshaw Bank, and (except in faulted areas for short distances) keeping a westerly strike with a somewhat higher and southerly dip to beyond Blenkinsopp Castle on the borders of Cumberland. The isolated Warden Hill, and the. long slope from the Roman Wall to the Tyne extending from Acoanh to Corbridge, are formed of them, as are also (with a northerly
strike) the broad areas between Matfen and Stamfordham, and between Capheaton and Belsay. The whole range is marked by step-like lines of scarps and crags, the escarpments of course all facing the west. Between Matfen and Stamfordham the craggy nature of the series is least shown, owing to the old valley of the Pont being filled up by a very considerable thickness of drift clay and sand.
The "Little Limestone" is usually separated by a bed of shale from the last-mentioned grits, the thickness of which shale varies from 8 to occasionally over 30 feet. The limestone itself is very constant in its thickness, being 9 feet at Alston, 9 to 13 feet at Blanchland on the Derwent, 16 feet at Acomb, 17 feet at Matfen, and 16 feet at Inghoe. Considering the distance, these variations are very insignificant, but even with regard to this bed, the rule seems to hold of new beds wedging in towards the north for, as will be seen in the sections in Plate XV., a thin parting of shale makes its appearance in the Stagshaw Bank section, which gradually increases in the Matfen section, and in the two given of the Fenwick borings. The character of the limestone is that of the average fairly good calcareous beds of the district ; it is seldom burned for agricultural purposes, but is frequently quarried for road metal. It is of a blueish grey colour when newly fractured, and weathers a reddish brown, but not so red as many of the other limestones. In the Alston district, it has been found very productive of lead, but the author is not aware that north of the Tyne this has ever been in any degree its character, unless it be so to a very limited extent at Fallowfield, where it reaches its greatest measured thickness, 18 feet. The line of outcrop of this limestone is so intimately connected with that of its accompanying coal, that the range of both will be considered together after describing the characteristics of the latter. Immediately below the " Little Limestone " in the Alston district, comes a shale some 20 feet thick which is, however, anything but constant; in the Derwent district it dwindles to 2 or 3 feet, it is absent altogether at Bardon Mills, is represented by about 25 feet of "grey beds" or arenaceous shale at Acomb, thinning to 12 feet at Fallowfield main shaft, and to scarcely a foot of shale a mile further ; at Stagshaw Bank Colliery it is replaced by a sandstone 8 feet, with a shale below it 7 feet thick ; at Matfen it has again disappeared altogether, but it reappears a little to the north-west as a slowly increasing band of shale in the Fenwick sections ; at Inghoe it is 45 feet thick.
Below this shale in the Alston section there is the tipper seam of the "Little Limestone Coal," about 3 feet thick ; at Shieldon, Bardon Mills, and Matfen, it lies directly below the limestone ; in the other sections given it lies below the shale as at Alston or below its representative, the distance between the limestone and the coal varying from nothing to 30 feet. This top seam is, it is believed, invariably present in the south-western sections, and as far north and east as Bardon Mills at least. It is absent at Acomb as a separate bed, and likewise at the westernmost Fallowfield section ; thence it is again found separate at the second section near that place, at Stagshaw Bank and at Matfen; at Fenwick, Inghoe, and as far north as the writer knows it, it is absent.
Except at Matfen, where a shale bed underlies it, the top seam is succeeded either by sandstone or by that arenaceous kind of laminated rock known by sinkers as " Grey beds ;" the arrangement at each place will be seen by referring to the sections in Plate XV. The thickness of these intermediate beds varies from nothing (as at Acomb, Fallowfield shaft, and Fenwick), to about 25 feet (as at Stagshaw Bank).
The lower seam, where it is separated from the upper, is in some cases (as in the Alston district) thinner than it, but it thickens a little to the north-east, when it is generally about double the thickness of the top coal, as at Bardon Mills, Stagshaw Bank, and Matfen. Where the top coal is absent as such the thickness of the lower seam is much increased as at Acomb and Fallowfield ; west of. the latter place, however, where the top coal is present as a separate seam, the bottom seam is divided by five or six feet of shale into two parts, the lowest of which is the thinnest. This is the only place the writer knows at which three seams are found on this horizon, though such an arrangement may very probably obtain elsewhere.
There are two ways of accounting for the various relative positions and thicknesses of the seams : the one being that the top coal is an independent bed frequently thinning out altogether for a space and reappearing. Although this opinion may be held by some, yet it is submitted that a glance at the sections placed in juxtaposition, as in the diagram, will show that the real explanation is that the "Little Limestone Coal" as a whole, is split into two seams very usually, and into three seams sometimes by the intercalation of beds of shale and sandstone of no great thickness. That this is so is supported by the fact that where the seams are separate the sum of their thickness is pretty nearly equal to the thickness of the seam where it is single. Unfortunately no sections are known which show any of the actual points of junction of the upper and lower portions of the seam.
It does not come within the purposes of this paper to describe in detail the beds lying below the "Little Limestone Coal," between it and the well-known "Great Limestone," but it should be noted that the thickness of these beds is quite as variable as that of those above, perhaps more so. Among these beds shale greatly predominates in almost every locality known to the writer, the chief member being a bed of black shale which immediately overlies the "Great Limestone," and which is very well developed and exposed in the large Fourstones quarries. Above this shalebed, between it and the coal is, in the Acomb and Haydon,Bridge districts at least, a thin bed of limestone which, where it is known, may serve as a guide to the coal. This little stratum was pointed out to the writer for the first time by Mr. Benson, of Allerwash, and might be very easily overlooked, as it is too thin to form a feature on the. surface, and very few pit sections or borings range to below the coal. The only places where the writer has seen this limestone are the bed of the South Tyne, below Allerwash, and that of Silly Burn, on the north side of the main river east of Haydon Bridge.
The run of the outcrop of the "Little Limestone" and its coal is not difficult to trace in the district to which these remarks refer. From Blenkinsopp to Beamwham (faults of no very great throw being here unnoticed) the strike is nearly east and west, near the latter place a large fault, having a very considerable downthrow to the west, throws these beds up to the south side of the South Tyne, whence the dip and the shape of the valley bring them once more on the north or left bank, just at the east end of Haydon Bridge ; thence to a little below Allerwash the outcrop keeps clear notwithstanding large accumulations of clay and gravel. At the latter place the beds are exposed by the river, which they cross, and with a gently-curved line on the south side come once more, for the last time, to the north bank at Fourstones. From Fourstones, skirting the northern flank of Warden Hill, the crop wends its way, crossing the North Tyne close to Wall Mill, following the base of Wall Crags, and reaching the Military Road at Planetree. Just north of St. Oswald's chapel our line is stopped by a dyke of basalt, which, being a filled up fault, throws the beds up to the east some 50 feet; this brings the " Little Limestone" on to the road again, where it forms a great spread running to the south (the ground being here a dip-slope) between the whim-dyke and Hill Head ; at the latter place a small fault running at right angles to the dyke again throws the beds a little up to the east, that is still more south of the road, between Coldlaw and Greenfield ; the strike now begins slightly to bend to the north-east, and the "Little Limestone" is again brought on to the Roman road, where it forms another spread along the western side of the Great Fallowfield vein, which is here nearly at an end both as to length and throw.
(This crossing and recrossing of the site of the Roman way by the "Little Limestone" has puzzled former observers who were unable to make an extended examination of the country to the north and south. Thus the late lair. George Tate, in his " Geology of the Roman Wall," appended to Dr. Bruce's magnificent work, has mistaken the reappearances of this limestone for the outcrops of different successive beds, and he has drawn them as such in the map which accompanies his memoir).
This vein or fault throws down to the east, and the outcrop of both coal and limestone now runs a little south of cast (but with a S.S.E. dip) between Grottington and Stagshaw Bank. A little to the east of the toll-bar a fault again throws up the limestone to the south site of the road at Halton Shields, where it is largely quarried, and half a mile further yet another fault, with a throw the reverse of the last, throws the beds down to the east due south of Great Whittington. Here the dip is south-easterly, and as it very soon changes to almost due east, the strike from this point may be said to be due north as far as the Wansbeck. The outcrop through this district is, as has been noticed previously, very well marked, lying as it does at the foot of the bold escarpment of the overlying grits, which are rarely interrupted in that distance. No fault of importance crosses its path, and one basaltic whir-dyke only slightly throws it down to the north-west, about a mile south of Capheaton. The amount of dip between the Pont and the Wansbeck is on an average about four degrees or less, seldom rising to five degrees, and not unfrequently being not more than three degrees.
The "Little Limestone" and its coal are both of them known in that portion of South Northumberland which lies to the south of the Tyne, but the writer has purposely abstained from entering into that part of the subject, as he would have been unable to speak authoritatively with regard to it, his acquaintance with that much-faulted and very difficult part of the country being but limited.
He has thought it unnecessary to give the range of the strata described in a map as this will be found delineated with great care in the published maps of the Geological Survey. However, such diagrammatic horizontal sections have been given as may help to a clearer view of the stratigraphical relations of these higher beds of the Lower Carboniferous series as they appear in a large portion of Northumberland.
Besides the local details which, however dry and uninteresting, will it is hoped be found valuable by mining men, the chief points which the writer has endeavoured to bring forward in this paper are :-
1. The exact position of the " Little Limestone" and its coal with regard to the "Fell-top Limestone" on the one hand and the "Great Limestone" on the other.
2. The great thickening of the mass of beds lying above the "Little Limestone" which takes place from south to north and is accompanied by the interpolation of at least three new beds of limestone which are unknown in the Alston district, and by an increase in the coarseness of the grits themselves.
3. The division of the "Little Limestone Coal" commonly into two and occasionally into three seams.
The CHAIRMAN said, he would be glad to hear any remarks on that very interesting paper.
Mr. T. J. BEWICK said, it must be satisfactory to the members of the Institute to find a gentleman in Mr. Lebour's position, coming forward to give information, and aid them in their investigations into the geological features of the district. Mr. Lebour, he believed, up to a recent date, had opportunities which few of them enjoyed, and had been at liberty to go where he chose and leisurely scan the ground. It was rarely the lot of any of them, as mining engineers, to have that opportunity, and, therefore, Mr. Lebour was what might be called a link between one professional man and another; and, personally, he was extremely glad that Mr. Lebour had come forward with that paper. Parts of the district and the coal seam to which Mr. Lebour referred, were well known to him, and with most of what Mr. Lebour had said, he entirely agreed. It was not a remarkably uniform bed of coal; for, as Mr. Lebour had mentioned, it was occasionally divided, and not unfrequently was very puzzling to those who had to develop it. The same thing occurred with reference to the other strata to which Mr. Lebour had alluded, which were found, in one part, thickening, and, in other cases, thinning and dividing, and until a series was obtained, or perhaps one or two beds of different rocks which could be well identified, it was most difficult to recognise them. IIe had, at that moment, in his own experience, a case in that very field, in which he could not say he was quite satisfied that he knew in what bed of limestone he was working. The investigations of Mr. Lebour were probably extended to fields beyond his (Mr. Bewick's) observation, and, therefore, perhaps he would be able to define it better than he (Mr. Bewick) could do. So far as he had been able to make out these several rocks, even the " Great Limestone" itself, which was one of our most uniform beds, he knew from actual experience, were divided in the northern part of the field. In Alston Moor, in Allendale and Weardale, in Teesdale, and into Yorkshire, it was one uniform bed, seldom less than nine fathoms, and rarely, if ever, more than twelve fathoms thick, and, therefore, a bed, which of all others, could be traced, and it had a peculiarity in it which few of the others possessed, and that was, that at about 20 or 24 feet from its top, there was what was called the "black bed," a stratum of shale. This applied with remarkable exactness over the field he mentioned; but in Tynedale, the same peculiarity does not occur ; and his impression at that moment was that this black bed, which was rarely more than two feet in the lead-mining districts proper, increased in thickness north from Allendale. At Whitfield Hall, for instance, it was, he believed, about 20 or 24 feet; perhaps Mr. Lebour could correct him if he was wrong, but certainly it was more than 18 feet. Farther north, the. thickness increased; and he should not be surprised to find in Tynedale, that, in addition to the shale, there might be sandstone, or what, in mining phraseology, may be called a grey bed, which is an arenaceous shale. He thought they would find in Tynedale, all round this very district which Mr. Lebour had described and illustrated, that the great limestone of the lead-mining districts is divided, and does not maintain the same characteristics which it has in Alston, Allendale, Weardale, and Teesdale. He had had some opportunity of judging of these same mountain limestone beds in the north, towards Little Mill, and in that neighbourhood, and also in some of the intervening places, but he had not traced them right through; yet he might say that he was unable to recognise the beds of Little Mill and those in Tynedale. Another point, he might mention, occurred with reference to those same limestones further south. In the southern part of Yorkshire, Wharfdale, and towards Skipton, they are found just exactly the,.-everse of what occurs here. Here, in the north, the shales and sandstones predominate; in the south, at the places he had mentioned; the limestone does so. He believed he was correct in saying that in the southern part of Wharfdale, in the neighbourhood of Coniston and Kettlewell, there is a thickness of about 2,000 feet of limestone-almost continuous-with mere films or thin divisions of shale. There was no doubt these were the same limestones which are here, and which pass through all the mining districts the mountain or carboniferous limestone. There was a gradual change in passing from one part of the island to another, although, geologically, they were the same formation.
Professor PAGE said, that this gradual thickening of the carboniferous limestone to the south and south-east, and its thinning and breaking up into several beds to the north and north-west, had been long ago noticed, as Mr. Bewick was aware, by Mr. Hull, who attributed the fact to the deep sea water being to the south and south-east, while towards the north and north-west, the shoal or shallow water was approached. Another reason which Mr. Hull did not sufficiently allude to was, that in Northumberland, and particularly in the Scotch coal-field, there were a great many limestones occurring, interpolated and broken through by the trap-rocks increasing to the north-west and north, showing that volcanic action had been going on, interrupting the continuity of the calcareous beds, while no change was taking place towards the south and south-east to interrupt that continuity. Hence the sudden transition from a thick limestone to a thin one, and from that to these thick shales, which made it extremely puzzling to identify the beds, as he dared say both Mr. Lebour and Mr. Bewick were well aware. In these various beds, however, whether five, eight, or ten, there were certain fossil forms peculiar to each, and if these were carefully examined, they would be found occurring in certain beds, dying out, and from time to time re-appearing in other beds; and these formed much better tests for distinguishing the limestones than any purely lithological one. This was a very important fact, and he thought if Mr. Lebour or Mr. Bewick were to devote close attention to the recurrence of these special fossils in the various beds, there would be little difficulty in identifying them, even at thirty or forty miles distance. There was another point: he should have liked very much if Mr. Lebour had given some information respecting the argillaceous nature of these limestones. They were aware that Great Britain was now becoming a great. cement-making country. Cement is made from chalk and clay, an artificial mixture, while it is very well known that in Scotland and Northumberland, there were many beds of argillaceous limestone which required only to be burnt in peculiar kilns and converted at once into hydraulic limestone. Within the last year, Stuart and Co., of Edinburgh, had tried seven different varieties of the argillaceous limestones of Scotland, and found them answer their purpose equally well with the artificial cement, and they are now making a selenite mortar from the Scotch limestones, and not buying a single ounce of Portland cement for their manufacture. It would be worth while, economically, to direct attention to the argillaceous nature of these limestones-for many were highly argillaceous-to see how far they would be found to be adapted for hydraulic cement instead of falling back on artificial mixtures such as is the practice on the banks of the Tyne.
Mr. LEBOUR said, with regard to the power of these limestones to be used for cement, he might say that some of the upper limestones had been tried ; he did not know whether this Little Limestone had been, but the Great Limestone had been tried and found wanting altogether without admixture. But when the lower series are reached many of the limestones are very well adapted for cement making, especially those of the Calciferous Sandstone or Tuedian series. There are, doubtless, a large number of these argillaceous limestones, which no doubt snake very good cement, but the limestone which was the special subject of this paper had never, he thought, been tried in that respect, nor did he think its argillaceous character was sufficiently marked to make it yield good cement.
Mr. BEWICK might mention, that he believed Mr. Benson, of Fourstones, was at this moment erecting cement works with a view to utilize some part of the products which he had at that place, and which consisted of limestone, sandstone, and shale. What Mr. Benson's plans were, he did not know ; but lie was erecting a very large manufactory at Fourstones, and he believed with this object. If even these limestones exist in a state in which they can be converted into cement, they would not be commercially valuable, unless near a railway.
The CHAIRMAN asked if any other member had any remark to make ? If not, he hoped they would pass a vote of thanks to Mr. Lebour for his paper, and then adjourn the discussion until the nest meeting.
The vote of thanks was then put and carried unanimously