156 



CARBONIFEROUS LIMESTONE OF CAERMARTHENSHIRE. 



rocks to which this work is more peculiarly devoted. (See Map.) The natural faces of 

 the rocks forming the northern and north-western escarpments of this coal-basin, are 

 indeed of great value in explaining the regular order and succession of the strata between 

 the carboniferous and Silurian Systems, for these sections exhibit a full and unbroken 

 sequence of all the intermediate strata, nothing being left to hypothesis or the imagina- 

 tion. (See PL 31. fig. 1.) 



The carboniferous limestone of the South Welsh coal-field, as seen in Monmouth, 

 Brecon, and Caermarthen, is on the whole a strong regularly bedded mass, partly light 

 coloured, rarely oolitic, and occasionally of so dark a tint as to be termed " black 

 marble 1 ." It is overlaid by the conglomerates and sandstones of the millstone grit or 

 base of the coal-field, and rests upon another conglomerate, very similar to the mill- 

 stone grit, but generally of a redder colour, which constitutes the uppermost stratum of 

 the Old Red Sandstone. In its course through these counties the limestone occupies the 

 form of a tortuous girdle, conforming to the shattered outline of the coal measures 2 . 



It is of inconsiderable thickness when compared with the formation of the same age 

 in Yorkshire, Denbighshire, or even in the adjacent parts of Monmouthshire and Glou- 

 cestershire, rarely if ever expanding to more than five hundred feet. In that portion 

 of the escarpment which has been the least dislocated, as to the south of Abergavenny, 

 and Crickhowell, the average inclination of the beds is very slight, not exceeding 7° or 

 8°, and the dip is towards the centre of the coal-basin. 



With this feeble development of the chief mass of limestone, the upper and lower lime- 

 stone shale are also of small dimensions when compared with the same rocks in the 

 northern counties, or in the environs of Bristol, and in the east of Monmouthshire, where 

 we shall soon describe them. Even, however, in the South Welsh coal-basin, the lower 

 limestone shale may be traced as a continuous band along the escarpment, and when 

 hidden from view by herbage or detritus, its place is usually marked by boggy ground, 

 springs, or rivulets, the waters of which, having been absorbed by the porous strata of 

 the overlying coal grits and limestones, are thrown out by this argillaceous band, and 

 fall in rills over the scarped and lofty edges of the Old Red Sandstone. It is not in my 

 power to describe all the varieties in lithological structure, and all the organic remains 

 of the formation in this region ; but a few remarks upon an interesting accidental feature 

 are worthy of notice. My attention was called to this circumstance by the Rev. H. Lloyd, 

 who had found detached fragments of this peculiar limestone. The surfaces of the stone 

 present a glazed or varnished appearance, which at first sight I thought might be owing 



1 The black limestone or marble is largely developed in Caermarthenshire (Llangyndeyrn, &c). The Earl of 

 Cawdor's new mansion at Golden Grove, near Llandeilo, is bnilt of it. " 



3 Some of the most extensive of the dislocations producing this outline are seen to the north-east and north- 

 west of Merthyr Tidvil, where they will be specially described by Mr. Conybeare. The great dislocations in the 

 neighbourhood of the Caermarthenshire Fans, or highest points of South Wales, will be specially explained 

 at the end of this chapter. 



