472 Revieivs — The Work of the Geological Survey. 



Each geologist contributes an account of the area surveyed by 

 himself, and the whole memoir has been edited by Mr. Strahan. 



Two subdivisions of the Coal-measures, namely, the Pennant 

 series and the Lower Coal series, occupy nearly the whole area. 

 In the latter occurs a group of seams which yield the well-known 

 smokeless steam-coal. These seams are illustrated in the present 

 memoir by a folding plate giving six vertical sections ; various 

 other sections are also given in the text, while sheets of vertical 

 sections, Nos. 83-85, are published separately. The seams of the 

 Lower Coal series vary greatly in thickness ; thus, in the Aberaman 

 Colliery in a section prepared by Mr. E. M. Hann (given in detail 

 on pp. 12-14 of this memoir), in a depth of 283 yds. 1 ft., or 

 850 feet, no fewer than 28 seams are passed through having an 

 aggregate thickness of 15 yds. 2 ft. 8 ins. ; but of this amount only 

 4 seams are more than a yard in thickness, 6 are over 2 feet 

 thick, 11 exceed one foot in thickness, and 8 are less than a foot; 

 so that it is probable that not more than 9 out of the 15 yards 

 in thickness of coal, if even so much, are won at this colliery. 

 It may be of interest to notice that, though the seams can be 

 recognized for long distances in an east and west direction, yet 

 both these and the measures associated with them change so rapidly 

 southwards that only a general correlation by groups is possible. In 

 this part of the coalfield some massive sandstones, much resembling 

 Pennant, develop in the upper part of the Lower Coal series. 



The Pennant series shows a no less remarkable expansion both 

 in a southward and westward direction. At the same time its upper 

 part becomes split up by shales and coal-seams, which have yielded 

 the bulk of the coal near Neath, Swansea, and Llanelly. Thus both 

 the top and the bottom of the Pennant is less distinct than in 

 Monmouthshire, where the subdivision consisted almost exclusively 

 of sandstone. 



The Upper or Llantwit Coal-measures occur in two small outliers 

 only in the area comprised in Sheet 248. 



In the South Crop, where the high dip usual in the southern 

 margin of the coalfield prevails, the Millstone Grit and what may 

 be the top of the Carboniferous Limestone come into view, but they 

 are partly ovei'spread by Triassic strata, which have been laid down 

 unconformably upon their truncated edges. The Coal-measures 

 themselves are partly thus covered, and it is worthy of note that 

 the Triassic conglomei'ates consist, not of Coal-measure rocks, but 

 of fragments of Carboniferous Limestone. 



The Secondary rocks, which include Keuper, EhEetic, and Lias, 

 are described in a separate chapter. They enter the map under 

 description along its southern margin only. 



The faults and disturbances are grouped into (1) the east and 

 west folds and the Moel Gilan Fault, and (2) the north-north- 

 westerly faults. They are discussed in detail in chapter viii. 



Chapter ix, after giving a general account of the glacial deposits, 

 deals with their occurrence in each valley. In chapter x the 

 principal economic products of the region are enumerated. 



