704 THE COLLIERY GUARDIAN. April 1, 1915. the two seams at Maes-llydan to be the same as that worked in Sylen Colliery, and the writer has been led to the same conclusion, because if the Maes-llydan seam were not the Brondini, it must be a seam lying at a considerably greater depth in the sequence than is com- patible with the estimated throw of the Llannon disturb- ance at this position. It is true that the disturbance which passes between the two positions is an upthrow to the south which would bring the deeper seams nearer to the surface, but in the Tack borehole, about to be noticed, no workable seam beneath the Brondini was intersected, although more than 100 yds. of lower strata were proved. As stated, the Llannon disturbance is an upthrow to the south, and this, coupled with the easterly dip that exists on both sides, is the reason that the seam crop- ping out at Maes-llydan on the south side of the fault is on the same level as it is at Sylen Colliery on the north side of the fault. A borehole was drilled a few years ago at a position 1,400 yds. E.N.E. of Sylen Colliery, and it is known as the Tack borehole. Its sectional details were courteously given to the writer by Messrs. Forster- Brown and Rees, and an abstract of it is given below. It, however, should be stated that the writer alone is responsible for the two correlations shown. o D .Z#£7. .Yarcfc. 6p 4 1 M- ! <£ follows that the dip is 148 yds. 2 ft. over the 1,400 yds. distance, or an average of *4 in. to the yd., but in the colliery the dip is about 4|in. The regularity of dip in this direction shows that the disturbance which the Survey authorities have marked as passing some 25 yds. to the north of the borehole passes to the south of it, and this view is supported by the fact that if the disturbance were on the north side, and so near, it would, with its hade to the south, have been inter- sected by the borehole, but no sign of it was detected, all the cores being regular. The exact position where the disturbance crosses the outcrop of the lower of the two Maes-llydan seams is at the point 450 yds. north of Maes-llydan House. At this position the old colliers lost the seam. The throw of Llannon disturbance north of Maes- llydan may be estimated as follows :—It has been stated that in Sylen Colliery the eastward dip is 4|in. to the yard, which for a distance of 633 yds. would amount to 79 yds., and the surface at this 633 yds. position on the north side of the disturbance is but a short distance from the severed outcrop of the Maes-llydan, or Bron- dini seam, previously noticed, on the south side of the disturbance; consequently its displacement here is about 80 yds., but it attains a much greater magnitude farther to the east. 56 4- 169 = 225yds. above the Brondini on the other, or down side of the fault, instead of 300; consequently its throw here will be 300 — 225 = 75 yds., as compared with 245 yds. at Dafen, nearly three miles to the south. It is probable that the fault dies out before reaching the outcrop of the Brondini seam on the north side of Mynydd-iSylen. This illustrates the fact to which attention is repeat- edly drawn in the Memoirs, that in going north from the synclines, the throws of the faults are greatly reduced, or die out. The above fault has diminished at the rate of about 60 yds. per mile in going north. (Fig/ 3.) Taking a general view of the Coalpit Hall inlier, it appears that the seams which dip to the south-east in Pant-yr-un Valley, and are probably fully 300 yds. deep near Coed-cyw Mill, are thrown up to the south by the Llannon disturbance and the accompanying fold so as to crop around the denuded crest of the Coalpit Hall anti- cline; consequently the disturbance must be of great magnitude near Coalpit Hall and farther to the east. From the Llannon disturbance near Maes-llydan the Brondini seam has a remarkably small dip to the south —its full dip being to the south-east—for in a distance of 1,000 yds. the difference in the level of its outcrop is 100 ft. only, as is shown on Geological Sheets LIV., N.E. and LIV., S.E., and it cannot be traced farther to the south owing to the surface accumulations, and to a large disturbance which will shortly be noticed. Before leaving the consideration of the Brondini seam in Mynydd-iSylen, a few remarks will be made upon its relationship to the “base of the Pennant.” In the Memoirs of the Geological Survey, the Brondini seam is stated to be the representative of the Garnswllt Vein of the Loughor Valley,, of the Ynysarwed seam of Neath district, and of No. 2 Rhondda, and for that reason it was taken by the Survey authorities to represent the base of the Pennant; but in Part II., pp.178-80, the writer showed the Garnswllt seam to be about 200 yds. too high in the series to be No. 2 Rhondda; therefore the Brondini must also be too high to be at the base, and on p. 210 of Part II. this point was further dis- cussed, and the horizon of No. 2 Rhondda north of Llannon was shown, on pp. 218-19, to be 182 yds. beneath the Brondini seam. The information which the writer has obtained since then, from the levellings in Great Mountain Colliery, shows that the base of the Pennant must extend considerably farther to the north than is shown on the Geological Sheet XLVII., S.E.; the base near Tumble is but a short distance above the Cockshot rock, which forms a prominent feature a little to the south of Tumble. This is an interesting circumstance, because in Mon- Fig. 2.—Horizontal Section. ■f' t J"'' a? ---------------- Section of Tack Borehole. Depth. Description. Thickness. Yds. ft. in. Yds. ft. m. 18 2 0 .. . Surface clay 18 2 0 Pennant rock 75 1 9 94 1 0 .. . Coal 0 0 3 Pennant rock 33 0 0 Shale.. 2 1 1 Coal. 0 0 11 132 1 0 .. Upper Brondini seam ■< Shale 1 2 10 . Coal. 0 1 2 Blue sandy shale 15 0 11 'Coal. 0 1 4 148 2 0 .. Lower Brondini seam ■< Shale 0 0 5 . Coal. 0 1 4 Sandy shale and rock... 16 1 6 165 1 0 .. Coal 0 0 6 Shale 5 0 7 170 2 0 Coal 0 0 5 Shale and fireclay 7 0 10 178 1 0 . .. Coal 0 1 2 Shale and rock 10 1 1 189 0 0 . .. Coal 0 0 11 Shale with rock bands 28 1 0 Pennant rock 17 0 2 Black shale 0 1 1 235 0 0 . .. Coal ..... 0 0 9 Shale with rock bands 12 2 8 . 248 0 0 . .. Coal 0 0 4 Grey sandy shale 1 2 2 Rock 1 0 10 251 0 0 It is practically certain that the two seams in the borehole section, which the writer calls “ Upper Brondini” and “Lower Brondini,” are correctly correlated, the latter, at 148 yds. 2 ft., being the seam that is now worked in Sylen Colliery; and as the surface at both positions is close to the 600 ft. contour line, it Coalpit Hall Inlier*—At a distance of 1,060 yds. east of Maes-llydan a seam of coal is shown to crop out with an easterly dip, until it is cut off by the “ supposed Bryn-Serfyll fault,” and on the east and down side of - this fault two other seams are shown to crop around an anticline. Consideration will now be given to the question : What positions in the sequence do these seams occupy? The Survey authorities considered the lower of the two Coalpit Haff seams to be the Brondini. The writer considers this correlation to be untenable, and regards the two Coalpit Hall seams as being distinct from the seam shown on the west side of the “ supposed Bryn-Serfyll fault,” each of these being distinct from the Maes-llydan, or Brondini seam, worked at Sylen Colliery still farther to the west. The writer shows that as no seams of coal have been found between the Pentre Vein and the Llwydcoed Veins in Llannon district, it is probable that the two Coalpit Hall coals are the Llwydcoed, or Pant-yr-un Veins; at all events, it is abundantly clear that they are seams considerably above the Brondini and Pentre . Veins. This correlation of the Coalpit Hall coals with tne Pant-yr-un seams enables the following estimate to be made of the throw of the Bryn-Selfyll fault at this posi- tion. The Pentre Vein is shown to be 31yds. deep on the west side of the fault, and the Brondini, 138 yds. beneath the Pentre, will be 169 yds. deep. On the east side of the fault the supposed Pant-yr-un seams crop out, and these are 300 yds. higher in the series than the Brondini, but they are denuded on the crest of the anti- cline at the line of section, at which position, if not denuded, they would be 56 yds. higher, and therefore * Spelt Cockpit Hall by the Survey. mouthshire and East Glamorgan a Cockshot rock occurs only a few yards beneath No. 2 Rhondda, or Tillery Vein, which lies at the base of the Pennant. In the important new sinking at Cwm, Llantwit-Fardre, this rock was intersected 12 yds. beneath No. 2, but it was only 5 ft. thick there. In the Ely Valley it occurs at Coed-Ely pits a few feet beneath No. 2, and in the Llanharan pits it also occurs, and on Mynydd-y-gaer, near Pencoed, it may be seen cropping out beneath the Malthouse, or No. 2 Rhondda. It disappears in some areas and comes in again; in fact, the Cockshot rocks are somewhat impersistent — they “ come in and die out ” in the same horizon over very large areas. Gwendraeth Valley to Llanelly. In the Gwendraeth Valley the dip at the outcrop posi- tion of the Rhas-fach seam is shown to be 19degs., or 12|in. to the yd., but in Pontyberem Colliery the dip for 1,400 yds. averages 14degs., or 9 in. to the yd, the prevailing line of full dip being S. 37-J- degs., E. A seam of coal which is shown at 90 yds. above the Big Vein, represents a seam that has recently been intersected in the pits of Blaen-hirwaun Colliery, 1,400 yds. N.E. of Tumble, at the stated distance over the Big Vein. It is 2 ft. 6 in. thick and of good quality. The writer, however, is not aware that this seam has yet been met with in Pontyberem district. Since the foregoing part of this paper was written, the writer has been enabled to make a fresh and accurate measurement of the thickness of the measures between the Carway-fawr seam and the Brondini at the Gwen- draeth Colliery, through the courtesy of Mr. Evan Jones, one of the owners. The true thickness between the Carway-fawr and the Brondini is 413 yds. at this position.