February 5, 1915. THE COLLIERY GUARDIAN. 279 The Geology of the South Wales Coalfield. THE PEMBROKESHIRE AREA. Part XI. of the memoir on the geology of the South Wales coalfield has been issued by the Geological Survey of England and North Wales. This volume deals with the country around Haverfordwest, and is an account of the region comprised .in sheet 228 of the map. The authors are Aubrey Strahan, M.A., Sc.D., F.R.S., F.G.S.; T. C. Cantrill, B.Sc., F.G.S.; E. E. L. Dixon, B.Sc., F.G.S.; H. H. Thomas, M.A., B.Sc., F.G.S.; and 0. T. Jones, M.A., D.Sc., F.G.S. In the preface to this memoir, Dr. Teall says that the country around Haverfordwest, which is described in this, the 11th, part of the memoir on the geology of the South Wales coalfield, is illustrated in sheet 228 of the new series 1 in. map. The re-survey has been made on the 6 in. scale by Dr. A. Strahan, Mr. T. C. Cantrill, Mr. E. E. L. Dixon, Mr. H. H. Thomas, and Dr. 0. T. Jones, under the superintendence of the first-named. On the original edition of the old series map the lower palaeozoic rocks were shown as undivided “ lower silurian,” but in the later edition some of the beds were differentiated by symbols. The present re-survey has shown that the “ lower silurian ” rocks are referable to the Cambrian, Ordovician, and Silurian systems. The subdivisions of each have been distinguished, and, as one of the results, their overstep by the old red sand- stone is conspicuously brought out. The overstep of the lower old red sandstone by the carboniferous rocks, which became noticeable in the district lying to the east, is completed in the region described in this volume, and the old red sandstone disappears west of the Eastern Cleddau. A little farther west the carboniferous lime- stone is overstepped by the millstone grit. The coal measures of this part of Pembrokeshire include repre- sentatives of the lower part only of the lower coal series of Carmarthenshire. The coals are all anthracitic, and of unusual value for special purposes, but owing to the highly disturbed condition of the strata they are not being extensively worked. Ironstone measures com- parable to those of the main coalfield were worked in former years. No triassic rocks have been found in situ, but evidence of their former presence over the plateau formed by the carboniferous limestone exists in places. Eocene beds are possibly represented by some sands with quartz pebbles, seen in one spot only. A series of extrusive and intrusive igneous rocks, lying between Benton and Johnston, has been proved to be of earlier age than the Upper Llandovery, and is believed to be pre-Cambrian. Another group of extrusive igneous rocks, in the neigh- bourhood of Wolfsdale, forms part odf the volcanic series developed at Trefgarn in sheet 210, and has been shown to be of Arenig ages. Structurally the district divides itself naturally into two parts. In the northern part lower palaeozoic strata have been thrown into shallow east-and-west folds, which are in the main of early date, and across the majority of which the old red sandstone and carboniferous rocks maintain an almost undisturbed course. Upon these folds powerful post-carboniferous movements in the same general direction have been superimposed. In the southern part the structure is dominated by post-carboniferous movements, which range a little north of west, and in which overthrusting from the' south is a conspicuous feature. These two systems, of the northern and southern parts respectively, are separated by a tract of undisturbed coal measures and millstone grit in the eastern part of the map, but coalesce westwards so as to involve the whole breadth of the coalfield. Observations on the character of the drift and the boulders show that the district under descrip- tion was crossed by ice moving in a general south- easterly or easterly direction. Some evidence of the existence of the raised beach of Glamorgan has been detected in the tidal parts of the River Cleddau, and the position of submerged forests in the foreshore has been recorded. Carboniferous Limestone Series. Classification and Correlation. — In the area under description this series has several outcrops, which differ considerably from one another in facies. In the outcrop north of the coalfield the development resembles that of Carmarthenshire, but is incomplete as a result of several causes in addition to the northerly thinning that affects carboniferous deposits in South Wales. Of the lowest zone shown in the table below, the lower part has pro- bably not been deposited; the two succeeding zones are wanting on account of an unconformity in the middle of the series; and the highest zone is overstepped by the millstone grit. Indeed, towards the west the attenuated series is probably transgressed completely. South of the coalfield, however, the series is almost complete. The lowest zone is fully developed; the unconformity in the middle is insignificant, or, in places, absent; and the hiatus at the junction with the over- lying millstone grit, if existant, can be but small. In one or other of the different outcrops the litho- logical or palaeontological subdivisions have been recognised (as shown in Table A). It may be added that the two facies, respectively characteristic of the outcrops to the north and to the south of the coalfield, are reproduced with great fidelity in corresponding positions in more eastern parts of South Wales. The result is that a closer comparison can be made in either case with a similarly situated outcrop to the east, than is possible between the northern and southern sides of the coalfield, though the'areas com- pared may be much farther apart than the opposite sides of the coalfield. These relationships can be best explained on the supposition that the area of deposition of. the carboniferous of South Wales shallowed north- ward towards a coast that had a general east-and-west trend. Table A. Litho- logical sub- division. Palaeontological zone or subzone. Thick- ness. — Main lime- stone Dibunophyllum zone, D : lower subzone, D1} alone known SpminulA zonp S f upper subzone, S2 bemmuia zone, b (iower subzone, S, Syringothyris zone, C: upper subzone, C3 (Unconformity in places.) Ft. 225-275 J 475-625 Upper Avonian Syringothyris zone C: lower subzone, Ci Zaphrentis zone (upper subzone, Z„ 1 (lower subzone, Z T (Horizon £) f upper subzone, Ka | lower subzone, Kly Cleistopora zone, K-( including a | Mo diol a - phase. U Km } About J 500 Lower lime- stone shales j>200-300 J Lower Avonian Millstone Grit Series. The millstone grit series has been mapped in three divisions as in the ground to the east, viz. :— Farewell Rock : Greenish-grey flaggy thick-bedded sand- stone, usually overlying highly current-bedded sand- stone. Middle Shales : Dark blue and grey .shales and mudstones, often concretionary, with numerous bands of fine- grained greenish flaggy sandstone and some “ mine ” (clay ironstone). Basal Grit : Grey-white quartzite with bands of dark blue and black shale usually yielding Goniatites and other marine fossils. The Farewell Rock makes its first appearance within the sheet at Telpyn Point, east of Amroth, where it con- sists of tough grey highly current-bedded quartzitic sand- stone in the lower part; and even-bedded sandstones in the upper part passing up insensibly into sandstone and sandy shales, which are followed by shales w’th some thin sandstone bands, on which rests a bed of fireclay and a thin coal seam. The upper part of the Farewell Rock is therefore intimately connected with the base of the coal measures, and any line drawn between them must be quite arbitrary. In practice it has been drawn where the lowest shales succeed the sandstones of the Farewell Rock. Its characters are maintained with hardly any change as far as the Western Cleddau, where it is noticed to contain some very coarse pebbly and con- glomeratic beds, which become more conspicuous to the west until ultimately the whole rock has taken on the pebbly form. Coal Measures : General Description. The eastern part only of the Pembrokeshire coalfield is included within the map under description. It has, broadly speaking, a synclinal form, in so much as the lowest measures, followed by the millstone grit and carboniferous limestone, come to the surface both to the north and south of it. Closer examination, however, shows that the various subdivisions lie in normal super- position in the northern side only of the trough, the central and southern parts being complicated by inverted folds and an almost infinite number of large and small overthrust faults. So completely do the superinduced structures dominate the region that it would be easy for a stranger to the carboniferous system to reverse the sequence of its subdivisions, for, the prevailing dip being southwards, the coal measures appear to dip under the millstone grit, and the grit under the lime- stone. The appearance is due partly to inversion, but chiefly to repeated overthrusting from the south. The following section of the measures has been proved in various shafts and drifts in the neighbourhood of Saundersfoot and Step aside. The records were made many years ago, and it is not known whether any cor- rection was made in them for dip. Probably none was made, and the thicknesses given will average about 8 per cent, more than the true thicknesses measured vertically to the bedding; but the strata have been so deformed by crushing and overthrusting that no precise measurement is possible :— Generalised Section of the Coal Measures of East Pembrokeshire . Tnhest be?ow ' Rock vein. Yd. ft. in. Yd. ft. in. Measures proved in the Moreton ’oxhole shaft. A upcast shaft; details unknown 'Measures with three thin seams... Rock Vein of the Timber Vein 58 62 0 12 0 0 4 0 0... 0... 6... 0 0... 13 3 0 0 0 series Measures 2 ft. to PH ^Low Vein 0 1 8... 13 1 8 Measures 17 1 4... 31 0 0 Timber Vein average 0 6 0... 33 0 0 ""Measures 78 2 0...111 2 0 ^<3 Rock Vein 0 1 0...112 0 0 Measures 28 1 0...140 1 0 .£3 02 Garland Vein 0 1 3...140 2 3 Measures 6 2 3...147 1 6 FiddleFs Vein 0 0 6...147 2 0 fl g Measures 7 2 1...155 1 1 ® § Under Garland Vein 0 1 4...155 2 5 ^Measures 15 0 7...171 0 0 02 . ' Lower Level Vein ,.ift. 8in. to 0 1 10...171 1 8 1 Measures 26 2 5...198 1 1 rfl fl S o i I Catshole Vein 0 0 8...198 1 9 flO ( .Measures 44 0 0...242 1 9 o Kilgetty Vein .1 ft. 6 in. to 0 1 9...243 0 3 Measures Lady^s Frolick Vein... 17 0 0...260 0 3 Measures Tin Pits Vein 63 0 o\.323 0 3 Measures Farewell Rock...... ... 10 0 0” .333 0 3 Though it is impossible to correlate with certainty the Pembrokeshire seams individually with those of Carmar- thenshire and Glamorganshire, we can say that the lower series only of those counties, and probably only the lower half of that series, is represented in the eastern part of this county. The existing measures appear to include a thickness of 363 yds. above the lowest work- able seam, and about 90 yds. more between that seam and the top of the millstone grit, that is, about 453 yds. in all. Measured up from the top of the Farewell Rock in the Gwendraeth Valley, this distance would take us well above the Big Vein, but would not include the Carway-fach and Carway-fawr veins. It may ■ be further suggested that the timber resembles the Stanllyd Vein in its association with soft shales, while the Rock Vein, with its rock roof, might correspond to the Big Vein. Gn this view the Lower Level Vein might corre- spond to the Gwendraeth Vein, and the Kilgetty Vein to the Pumpquart. It will be noticed that 120.yds. of measures are mentioned as existing above the Rock Vein of the Timber Vein series. They were proved in the upcast pit of the Moreton Colliery, of which no details have been preserved. It is known, however, that they contained no workable seam, and they may therefore correspond to a part of the barren measures that separate the Big from the Carway veins in the Gwendraeth Valley. The seams which have yielded the bulk of the coal are the Rock, Timber, Lower Level, and Kilgetty veins. The Timber Vein, though the thickest, suffered from a bad roof, and gained its name from the amount of support necessary. Formerly much mine (clay iron- stone) was worked on or near the coast between Amroth and Saundersfoot, and was swelted at Stepaside. The mine ground occurred at several horizons, but the best known courses lay below the Timber Vein (the Kilvelgy mine measures); above the Fiddler’s Vein as worked in the cliff east of Wiseman’s Bridge, 'in Lloyd’s or the Rooksnest Patch; above the Catshole Vein, worked in the same cliff in the Craigdam Patch; and above the Scad (supposed Kilgetty) Vein as worked in the Burrows Patch, a quarter of a mile west of Amroth. Mine was raised also in the cliff under He an Castle from measures lying above the Lower Level Vein. The Bridge Patch, which is the first to the east of Wiseman’s Bridge, is said to have been in the Fiddler’s mine ground, but the identity of the measures from Wiseman’s Bridge as far as Lloyd’s Patch is doubtful. Much coal was raised in the neighbourhood for use in the furnaces, but the works were abandoned about 50 years ago, and the coal now being worked is used for local domestic purposes, or for export chiefly as malting fuel or for horticultural pur- poses. In 1905 two collieries only were raising coal in the eastern part of the coalfield, namely, Bonville’s Court, where the Lower Level and Kilgetty veins are being worked, and the Jeffreston Colliery on the Timber Vein. All the coal of Pembrokeshire is anthracitic. Large coal is known commercially*as “ anthracite,” and small coal as “culm.” .Some analyses of Pembrokeshire anthracites are given elsewhere, and we need, here mention only that they show a percentage of carbon ranging from 93*26 to 95*68, and of hydrogen .ranging from 3*04 to 3*28, while the percentage of ash averages less than 1*00. The synclinal structure of the coalfield, to. which allusion has been made, is due to its having been involved on both its northern and southern sides in great belts of disturbance. On the south it is touched by the northern margin of the Armorican belt, which ranges through Belgium westwards to the south of Ireland, involving the north of France and the south of England. Part of that margin has already been recog- nised in the Vale of Glamorgan and in Gower, and the disturbances there observed would, if prolonged, strike the Pembrokeshire coast near the centre of the syncline, and would fall into line with the general trend of the folds and overthrusts which complicate the southern side of the coalfield. They commence in Pembroke- shire with a sharp fold which intersects the cliff at Amroth, and they range thence by Begelly towards Martletwy; north of them there is little or no disturb- ance of the strata for two or three miles, while south of them the folding and overthrusting increase rapidly in frequency and violence.. These disturbances all show evidence of overthrusting and overfolding from the south, while cleavage, parallel to the folding, sets in about 4 miles within the margin of the disturbed belt. To the north, on the other hand, the coalfield is bordered by a no less important belt of disturbances which affect the old red sandstone and Ordovician rocks, and which, ranging somewhat west of south, invade the coalfield near Martletwy. These link themselves on to the great west-south-west system, to which the folding and faulting of the Vale of Neath, Cribarth, Careg Gennen, and the Tawe Valley belong. They increase in frequency and intensity northwards, and generally show evidence of overfolding and overthrusting from the north. Here also cleavage, parallel to the folding, sets in about 4 miles north of the southern margin of the belt. It will be seen, therefore, that a part of the coalfield and of the rocks north of it occupies an interval between these two great belts of earth movement. In that interval there is comparatively little sign of disturbance, and within it for a breadth of about four miles the carboniferous rocks lie in a normal sequence with a steady dip of 15 degs. to 20 degs. to the south. The interval is narrower in Pembrokeshire than in Carmar- thenshire, the two belts tending to meet westwards, but becoming more and more nearly parallel as they proceed in that direction. The central belt of the coalfield ranging from Saundersfoot past Moreton and Jeffreston, includes the principal outcrops of the Timber Vein series, and con- tains the nearest approach to an axis in the much- deformed carboniferous syncline. The outcrops, with few exceptions, arrange themselves along a strip of country about one-half to one mile in width, and at first sight appear to run continuously for a distance of about 4| miles. Closer.examination shows this to be far from