130 Reviews — Geology of South Wales Coalfield. 



IT. — Memoirs of the Geological Survey of England and Wales. 

 The Geology of the South Wales Coalfield. Part V : The 

 Country round Merthyr Tydfil, being an account of the 

 region comprised in Sheet 231 of the map. By Aubrey 

 Strahan, M.A., F.E.S., F.G.S., Walcot Gibson, B.Sc, F.G.S., 

 and T. C. Cantrill, B.Sc. pp. viii and 132, with 2 plates and 

 12 figures in the text. (Wyman & Sons, 1904. Price Is. 6d.) 



THIS is the fifth part of the Memoir on the Geology of the South 

 Wales Coalfield, and is descriptive of Sheet 231 of the New- 

 Series one-inch map. This sheet includes a small tract of the 

 Pennant plateau and that portion of the north crop of the coalfield 

 which extends from Dowlais to Swansea Vale. The northern half 

 of the map is occupied by the scarps of the Lower Carboniferous 

 rocks and by the long dip-slopes of Old Red Sandstone, which form 

 the Fforest Fawr and lead up to the Brecknock Fans. 



The Old Red Sandstone includes an upper and impersistent 

 subdivision of grey grits and conglomerates, which is developed 

 round parts of the Carboniferous tract. The grits pass up 

 into the Carboniferous Limestone, and are known from fossil 

 evidence, obtained further to the east, to be of Upper Old Red 

 Sandstone age. The great mass of ' Brownstones ' lying below the 

 grey strata has yielded no fossils in this area, and is doubtfully 

 referred to the Lower Old Red Sandstone. The lowest strata 

 exposed are some green and red sandstones and marls which have 

 been named after the Senni Valley, where they were first dis- 

 tinguished. One of the finest exhibitions of the characteristic 

 scenery of the 'Brownstones' (which appear to be the backbone of 

 the Old Red in this area) may be seen from the northern edge of 

 Bryniau Gleision. On the far side of the deep Glyn Collwng 

 a plateau, formed by the hard uppermost rocks, slopes gently 

 upwards towards the north to a height of nearly 3,000 feet. 

 Though once continuous, this long slope has been deeply trenched 

 by stream-action into separate hill masses, each with a flat top 

 bordei-ed by precipitous escarpments. This is the structure of the 

 well-known Beacons of Brecon (Pen y Fan, 2,906 feet, and Corn Du, 

 2,863 feet), each of which supports a small but characteristic outlier 

 of the plateau beds consisting of well-bedded flaggy red sandstone, 

 in which occur a few scattered quartz pebbles. 



The Carboniferous Limestone Series includes the Lower Limestone 

 Shales below the main mass of limestone, and a thin impersistent 

 set of shales with thin dark limestones above that mass. These 

 dark limestones towards the top of the series present the somewhat 

 exceptional feature of weathering at the outcrop into rottenstone, 

 which has been exploited here and there to a considerable extent, 

 the best quality fetching £10 a ton. Analyses have been made with 

 a view of ascertaining what has occurred in the conversion of the 

 black limestone into rottenstone. A sample of the limestone 

 contained about 16 per cent, of silica. "A few very small cubes of 

 pyrites and flakes of colourless mica are the only visible impurities : 

 there is no detrital quartz, and the silica shown in the analysis must 



