196 CARBONIFEROUS. 



The Pennant Grit is essentially a sandy series. De la Beche 

 described a section at the Town Hill, Swansea, of 3246 feet of 

 strata belonging to the series, of which 2125 feet were sandstone. 

 It comprises some seams of workable coal (Hughes's vein, five 

 feet), underclay, and shale. Mr. E. Daniel divides the Pennant 

 Grit into three groups. 



One of the most remarkable beds in the South Wales coal-field 

 is that discovered by Logan in the Pennant sandstones, on either 

 side of the Tawe Valley, Swansea. It consists of a conglomerate 

 containing pebbles of coal, sometimes four inches in diameter, 

 pebbles of ironstone, and boulders of granite and mica-slate ; these 

 latter very rarely.^ The presence of this bed indicates that the 

 Lower Coal-measures must have been (at any rate locally) con- 

 solidated and upheaved prior to the accumulation of the Pennant 

 Grit ; and this disturbance might perhaps have had something 

 to do with the production of the anthracite. Pebbles of anthracite 

 have been noticed in the Pennant Grit and Upper Coal-measures 

 of the Bristol Coal-field. - 



The Pennant Grit forms a marked escarpment above the Lower 

 Coal-measures, from the Garth Hill (1650 feet high) near Cardiff, 

 by Llantrissant and Margam to Aberafon, Neath, and Swansea. 

 North of the coal-basin the Pennant Grit is conspicuous near 

 Aberdare and Merthyr Tydfil. 



The South Wales Coal-field thus forms a basin which is one of 

 the most uniform and well-marked in the country, although its 

 continuity is broken by the bays of Swansea, Caermarthen, and St. 

 Bride ; and the Coal-measures themselves stand up in bold hills, 

 for the most part above the southern edge of the basin. 



One advantage of the deep valleys is the beautiful scenery they 

 afford to ramblers in search of the picturesque ; ^ another advantage 

 lies in the fact that the coal crops out along their sides, and can 

 often be worked by adits and galleries driven into the hills, instead 

 of by shafts sunk from their summits. 



In the Upper Coal-measures are the beds of Penllergare north of 

 Swansea, and of Llanelly ; and they occur at JMynydd Drumau and 

 Mynydd March Howel. 



Remains of Anthrakcrpdon, an air-breathing Reptile, have been 

 found in the Coal-measures of Llantrissant, by Mr. J. E. Lee. 



15. DEVONSHIRE CULM-MEASURES. 



The Coal -measure rocks of Devonshire can only for the sake of 

 convenience be grouped with those of other parts, for they certainly 

 form a very unproductive series so far as coal is concerned. They 

 were grouped with the Carboniferous System in 1836 by Sedgwick 

 and Murchison. 



J T. G. S. (2), vi. 491. 



- Geol. Mag. 1S65, p. 134 ; see also H. K. Jordan, Q.J. xxxiii. 932. 

 ^ See papers by Dr. G. P. Bevan, Geologist, i. 49, 124; iii. 90 ; and G. Mag. 

 1865, p. 158. 



