Alberbury, and form a narrow belt extending to Haughmond Hil, near Shrewsbury. The rising ground to the west of the coal-fields marks the position of the Millstone Grit, which forms one of the meridianal bands of elevated and broken ground already spoken of. The thickness of this series varies from 1000 to 1500 feet, decreasing from north to south; it contains one or two seams of workable coal. Beyond its westerly margin is a parallel band formed of Carboniferous Limestone, this formation frequently presents in strong relief those terraces which distinguish it in its outward aspects from the neighbouring strata. The beds of partially bare limestone may be observed rising from beneath each other in succession, and ending off in abrupt faces of cliff. In the heights above Llangollen this feature is particularly striking, and the author has a vivid recollection of the effect produced on his own mind, when for the first time he caught sight of those mighty walls of grey rock ranging for several miles from north to south, tier above tier, appearing like colossal breastworks thrown up by some race of Titans to defend the plains of England from the warriors who peopled the mountains of Wales. In truth, however, the work is too stupendous for any race of gods or men. The architects were the Polypes of the deep, and those walls were shaped and chiselled by the waves of old Atlantis, which once chafed and flung themselves against their bases. The Carboniferous Limestone is of special interest to the mineralogist, as being the repository of several systems of mineral veins. The tract of country from the sea-coast near Rhyl to the north of Llangollen has for many centuries been the site of mines from which argentiferous galena, calamine, and blende have been extracted. From the discovery of stone-headed hammers in some of the old works of this part of the country, it is probable these operations were commenced in the time of the ancient Britons; that they were worked in the Roman times is certain, and the present yield, according to the returns collected by Mr. R. Hunt, for 1860, are as follows:— The lodes in this district, as laid down on the Maps of the Geological Survey by Mr. Warington Smyth, appear in greatest number at the junction of the Millstone Grit and Carboniferous Limestone, including strata of both formations. Many of the lodes are faults, and they range generally across the strike of the beds-that is to say, approximately from east to west. In the direction of Holywell, there are great beds of chert intercalated with the limestone, and the lodes in this position are generally very productive. One of the most remarkable lodes in this district is the "Great Minera Vein," the most productive in North Wales. It is a fault ranging northwest, and having a down-throw to the north-east of about 350 yards. It has been worked from very ancient times, and in 1860, yielded upwards of 4000 tons of lead ore, and 1125 tons of zinc ore (blende). This great fracture enters the Denbighshire coal-fields, and probably produces important changes in the strata; its effect, however, has not yet been ascertained, as the strata are not yet explored in the direction of its prolongation. Besides the lodes already referred to, there are several remarkable longitudinal fissures traversing the limestone in the direction of the strike from north to south. They cut down through the strata to a great but unknown depth, and are filled with rubbish; they are not generally metalliferous. One of these, called the "Catshole cross course, has been traced for a distance of twelve miles from Holywell southwards, and at its northern extremity it becomes a lode.* We now proceed to the more immediate subject of this articlethe description of the coal-fields themselves; having laid a foundation for this superstructure by the above somewhat desultory account of the rocks which immediately underlie the coal-formation. The Flintshire and Denbighshire coal-fields were originally continuous with each other, as they each present a similar succession of strata, and the coal-series of each is almost identical. Of the two fields, the Denbighshire is by far the most economically important, not only from its greater area, but on account of the arrangement of the strata. While the coal-seams in Flintshire are repeatedly dislocated and thrown out by faults, which almost invariably prevent their attaining great depths,-those of Denbighshire, for the most part, dip steadily eastward, and are covered over by conformable strata, under which they may eventually be followed to the limiting depth of coal-mining. From this it arises, that while the one is rapidly approaching exhaustion, the other is practically inexhaus tible. FLINTSHIRE COAL-FIELD. Northward, the boundary of the Flintshire coal-field is the estuary of the Dee-westward and southward, the Millstone Grit. From the Point of Air to Bagillt, near Flint, there occurs a narrow hand, sometimes interrupted, of Coal-measures, in which there are a few collieries, one of which has an air-shaft about a mile out from the margin of high water mark. We are not certain whether it is at present in use. From Bagillt, southwards and south-eastwards, extends the main area of the field. A large portion of this, extending from the shore inwards towards Northop, is composed of Lower Coal-measures with only thin seams. The most important tract is that which stretches from Mold to Hawarden and the Dee, and southwards to Treiddyn. Nevertheless, such is the dislocated state of the beds, that nearly all the thick coals are thrown out three or four times in succession, after having "set in" along certain bands of country stretching north and south. The depth over these bands being nowhere great, much of the coal has already been raised, and there is little prospect of the discovery of large tracts of new ground, unless by sinking over the wide tract of alluvium which borders the Dee. The general succession of the coals, and their intervening strata over the central part of the coal-field, is very well illustrated by the following section taken at Rhyd-y-Craliad Colliery,† near Mold. * See Map of the Geological Survey, 79 S.E.; also Horizontal Section, sheet 43. + From the "Explanation" of Horizontal Section of the Geological Survey, sheet 43. The section above given is abridged from the original. COAL-SERIES NEAR MOLD, FLINTSHIRE. Yds. Ft. In. 6 Four-feet Coal, with a band of cannel at bottom.. 0 Strata, principally black shale Strong black shale Light blue shale Black-band Ironstone.. 1 1 0 1 2 0 5 0 6 4 2 4 2 0 10 6 6 6 0 0 2 1 0 The Main Coal is everywhere a most valuable seam. Though only seven feet in thickness at Mold, it becomes as much as eleven or twelve feet in the direction of Hawarden, and is even thicker at the new colliery recently commenced there. At Bryn-ffymon, a four feet coal has been worked, which is supposed to lie sixty-five yards below the Main seam. The Cannel seam, recently opened up at Leeswood, but which, I am assured by Mr. P. Higson, jun., who is well acquainted with the district, has been worked many years since, and was known to the miners as a sort of black bass, having a marvellous power of blazing when ignited, lies about 100 yards below the Main Coal. It is probably identical with the Four-feet seam mentioned above, as there is good evidence for believing that it passes into a seam of ordinary coal in other districts. At Leeswood, however, it occurs as one of the most valuable seams of cannel in the kingdom, yielding, in its lower part it is said, 1500 or 2000 cubic feet of gas per ton more than the celebrated cannel of Wigan. It is probably rich also in mineral oil. The following is the section at Leeswood: SECTION THROUGH THE CANNEL SEAM AT LEESWOOD. It may prove worth the while to proprietors, who possess property over this field in which the Main Coal has been worked, to ascertain, by boring, whether a seam, calculated to bring so large a profit as this, may not exist under their estate. Of its great value for the production of gas there can be no doubt. At the base of all these coal-seams there occurs a considerable thickness of strata, principally shales, belonging to the Lower Coalmeasures, and resting upon the Millstone Grit. The seams they contain are thin, and rarely worked. The area of the Flintshire Coal-field is about 35 square miles. More than half the original quantity of coal has either been raised or destroyed, leaving about twenty millions of tons for future supply. In 1860, there were 590,500 tons of coal raised, so that, at the present rate of production, this supply would only last for about 35 years. The number of collieries in work in the same year was 40. THE DENBIGHSHIRE COAL-FIELD. The northern extremity of this coal-field terminates along the great fault already described, about one mile south of Hope. Leaving this line, and gradually receding from it, the base of the Coalmeasures bend round southward by Minera, and thence takes a due southerly course, crossing the Dee at Pont Cysyllian, where the strata are traversed by a large fault, and extending thence to Trefonen, south of Oswestry. Its length is about eighteen miles, and its breadth, at Wrexham, about four and a-half miles. Towards its southern extremity, it contracts considerably-probably from two causes: first, the overlapping of the New Red Sandstone; and secondly, from the gradual thinning of the strata in that direction. Throughout its whole length the dip is easterly, and the inclination generally lessens towards its eastern margin. Nowhere, except in the immediate neighbourhood of faults, is the dip very steep-the average being about 10 degrees. Along the valley of the Alyn, near Gresford, the dip of the Upper Coal-measures varies from 5° to 10°. This is also the dip in and around Ruabon, Wynnstay, and on the south bank of the Dee; and, as most of the seams are at accessible depths under these tracts, they are calculated to become of extreme value for future supply, although still unbroached. The general structure of this coal-field may be expressed by the accompanying section drawn across the noble escarpment of Cefn-y-Fedw, and the town of Ruabon. The general series of the main portion of the Denbighshire Field is as follows: Permian Strata.-Probably attaining a thickness of 2,000 feet, consisting of interstratified red, purple, and brown sandstones (sometimes calcareous) and marls. These beds are well shown in the River Clywedog near Wrexham, and in the Valley of the Dee between Plas-yn-coed and Eyton Hall. Upper Coal-measures.-About 1,000 feet in thickness. They consist of greyish and yellow sandstones, with coal-plants, interstratified with red and grey clays and shales. There are a few very thin coal-seams, but never of workable thickness. This series is well shown along the banks of the Alyn, from Gresford westward, also around Ruabon, Wynnstay, along the Dee, near its junction with the river Ceiriog. Middle Coal-measures.-About 800 feet in thickness. Consisting of grey and yellow sandstones, clays, shales, with bands of valuable ironstone and beds of coal. Over these strata the principal collieries are situated. Lower Coal-measures.-About 1,000 feet in thickness. Consisting of shales and sandstones, with a few thin seams of coal. |