HARDWICKE'S SCIENCE-GOSSIP. 



i73 



Logan detected among the hard siliceous beds of 

 this series, specimens of Wavellite (hydrous phosphate 

 of alumina), a mineral found at the base of the Coal- 

 measures between Barnstaple and South Molton.* 



While De la Beche hesitated whether to regard the 

 beds as the equivalent of the Millstone Grit, or of 

 the upper part of the Carboniferous Limestone, John 

 Phillips placed them (doubtfully) in the Yoredale 

 series.f Somewhat analogous beds are developed in 

 Gloucestershire.^ 



Millstone Grit —The Millstone Grit, or "Fare- 

 well Rock," is of uncertain thickness and extension 

 in the district. On the northern margin of the coal- 

 basin it forms a continuous band and generally a 

 marked feature, from near Haverfordwest to Ponty- 

 pool, having at Merthyr Tydfil a thickness of 330 feet. 



At Mynydd Garreg the lower portions consist of 

 sandstone and conglomerate, the upper of arenaceous 

 shales and flaggy sandstone ; these indeed are its 

 prevalent characters, the conglomerate being com- 

 posed of quartz pebbles, the sandstone sometimes 

 passing into quartzite. Large blocks of these rocks 

 are generally scattered about on the line of outcrop. 

 The shales and occasional coal-seams render the 

 boundary line with the Coal-measures above very 

 vague. 



The Millstone Grit forms a thin and inconspicuous 

 band north of Cardiff, but farther west, at Cefn 

 Hirgoed and Cefn Crubwr, the beds stand out in bold 

 ridges, and here the broad outcrop is probably dupli- 

 cated by a fault. To the north-east of Pyle the beds 

 are much concealed by Drift. 



As before mentioned the Millstone Grit is not defi- 

 nitely identified in the Gower peninsula, and it 

 remains to be proved whether the Gower shales 

 should be correlated with it, or whether it be repre- 

 sented by certain sandstones above them and now in- 

 cluded with the Coal-measures. 



At Cwm Afon, near Aberafon, a seam of coal two 

 feet in thickness, called the Crow's-foot vein, is worked 

 in the Millstone Grit. 



Coal-medsures. — Coming now to the Coal- 

 measures, we find them to be divided, as in the coal- 

 fields of Bristol and Somerset, into two productive 

 series separated by a comparatively unprofitable group 

 of sandstones called the Pennant Grit. 



The Lower Coal-measures comprise shales with 

 beds of sandstone and seams of coal. They are 

 known also as the "Ironstone series," being rich in 

 clay-ironstone, an impure carbonate of iron which 

 yields from 50 to 80 per cent, of ore. This renders 

 the Lower Coal-measures of particular value, and 

 fortunately the strata are rolled into two or more 

 anticlinal axes, one of which running through the 

 neighbourhood of Maesteg, brings the Lower Coal- 



* See De la Beche, op. cit. pp. 133, 134, 143, 144. 

 t Phillips, " Manual of Geology," pp. 169, 170. 

 J Buckland and Conybeare, Trans. Geol. Soc. 2nd series. 

 Vol. i. pp. 223, 240, 248 ; Stoddart, Geol. Mag. yol. ii. p. 83. 



measures near the surface, and thus renders them 

 of easy access over a much larger area than would 

 otherwise have been the case. 



The total thickness of the division is about 850 

 feet ; but it varies from 423 feet at Pontypool to 

 812 feet at Merthyr Tydfil. 



The coal-seams are ordinarily from two to six feet 

 in thickness, and rarely as much as nine feet. The 

 junction of the Lower Coal-measures and Pennant 

 Grit above, is marked near Maesteg, east of Neath, 

 by two or three beds of quartz rock, known as the 

 " Cockshoot Rock," and this is of local value in 

 fixing the position of the seams. 



A remarkable feature in connection with the coal, 

 is that while bituminous in the neighbourhood of 

 Swansea, and to the east of Neath, it becomes 

 anthracitic westward, and northward. The changes, 

 which appear to be gradual, are chiefly due to a loss 

 of oxygen and hydrogen, and it is considered clear 

 that similar changes must be going on where carbonic 

 acid gas (choke clamp) and carburetted-hydrogen (fire 

 damp) are being given off. That the change from 

 ordinary, or bituminous coal, to anthracite or stone 

 coal,* might be connected with igneous eruption, has 

 been suggested from the fact that in Pembrokeshire, 

 eruptive rocks occur in proximity to the Coal- 

 measures ; but Mr. L. C. Miall has shown that 

 coal altered by contact with igneous rocks does not 

 form anthracite, but "cinder coal" or "soot coal." 

 According to his experiments, coal loses its volatile 

 constituents at ordinary temperatures, and this is 

 facilitated by disturbance of the strata.f By the 

 changes undergone, fuel of great variety, and suitable 

 for numerous purposes, has been furnished. 



The Pennant Grit is essentially a sandy series. 

 At the Town Hill, Swansea, De la Beche described a 

 section of 3246 feet of strata belonging to this series, 

 of which 2125 feet were sandstone. It comprises 

 besides, seams of workable coal, under-clay, and 

 shale. Hughes's vein, one of the seams worked near 

 Swansea, is about five feet in thickness. At Llanharry 

 the thickness of the Pennant Grit is 2700 feet, 

 while at Pontypool it becomes reduced to 1474 feet. 

 Mr. E. Daniel divides it into three groups, in the 

 sections he prepared for the Royal Coal Commis- 

 sion, under the direction of Mr. H. H. Vivian, M.P. 

 And these sections, which were published by the 

 Geological Survey, may be consulted for full details of 

 the Coal-measures in the neighbourhood of Swansea.^ 



One of the most remarkable beds in the South 

 Wales coal-field is that discovered by Logan in the 

 Pennant sandstones of the Town Hill, and of Cilfay 

 Hill, on either side of the Tawe Valley, Swansea. 



* The term Culm is applied to the inferior kinds of anthra- 

 cite, and sometimes to the small fragments of the better kinds. 



f Proc. Geol. and Polytechnic Soc. W. Riding of Yorks. 

 New series, part i. p. 22. 



% They are numbered 53, 57, 58, and 59. See also vertical 

 sections, Nos. 1, 2, 3, 4, and 6; and horizontal sections, Nos. 7. 

 8, and 9, previously published by the Geological Survey. 



