Reviews — Geology of South Wales Coalfield. 13! 



csxist as a thin framework of chalcedonic character between the 

 <;alcite grains." What has happened is that the carbonates have 

 been removed, leaving behind siliceous matter of the above peculiar" 

 ■character. As much as 80 per cent, of this siliceous material i^ 

 found in the best samples of rottenstone, whilst alumina and ferric' 

 oxide are in less amount than is the case in inferior samples. 



In the Millstone Grit three subdivisions are made, the most 

 •conspicuous member being a massive quartz-conglomerate at the 

 base of the formation. The fossils yielded by some shaly bands 

 show that marine conditions prevailed during the deposition of 

 strata of Coal-measure aspect. Upwards the Millstone Grit graduates 

 into the Coal-measures, the upper and lower limits of the Farewell 

 Kock being ill-defined. As regards fossils from the shales of the 

 Millstone Grit, as many as 17 localities are given, some belonging 

 to another sheet, whilst the species quoted include Spirorbis pusillus, 

 Lingula mytiloides, OrtMs Michelini, Productus semireticulatus, 

 Spirifera glabra, two species of Carbonicola, Pterinopecten papyraceus, 

 Gastrioceras carbonarium, GlipTiioceras reticulatum, etc. Mr. Strahan 

 observes that, as a general rule, Carbonicola occurs in the upper 

 part of the shale, while purely marine forms such as crinoids, 

 brachiopods, and cephalopods are confined to the lower part. 



As regards the Coal-measures, the same author observes that the 

 part of the North Crop included within the area under description 

 extends from the anthracitic region of Ystradgynlais to the steam- 

 coal region of Dowlais. While not always recognizable individually, 

 the lower coals as a group can readily be identified in the two 

 regions. The nine-foot seam of Dowlais and Aberdare corresponds 

 closely in position with the nine-foot of the anthracitic region. 

 The anthracitic region is regarded as commencing at the Vale of 

 Neath, but there appears to be no definite boundary between it and 

 the steam-coal area. Moreover, the anthracitisation is far from 

 complete in the Yale of Neath, and continues to increase westwards 

 beyond the limits of the map under description. Though all the 

 seams are subject to the loss of bituminous matter in a west, or 

 rather north-westward, direction, the lower seams are the first to 

 show it. Thus, in any one vertical sinking the lower coals are 

 on the whole less bituminous than the upper. The base of the 

 Pennant is drawn arbitrarily at the Ehondda, No. 2 seam, which 

 is known by another name west of the Neath. The measures in 

 which the principal seams lie are illustrated in vertical sections — 

 Sheet 83, Nos. 1, 2, 12. and 13; Sheet 84, Nos. 12, 13, and 19; 

 and Sheet 85, Nos. 12-16. 



The stratigraphy of the region is fully described, and further 

 illustrated by a section from the Old Red Sandstone scarp to the 

 Pennant scarp at Carn Mosyn (plate i). In addition to a large 

 number of normal faults running in the N.N. W. direction — a system 

 which prevails throughout the entire coalfield — there are two 

 great belts of disturbances, known as the Cribarth and Neath 

 disturbances, which traverse this region from E.N.E. to W.S.W. 

 They form the forerunners of a long series of similar disturbances, 



