124 ME. F. DIXEY AXD PEOF. T. F. SIBLY OX THE [vol. Ixxiii, 



and the Ebbw rivers. Noi'thwards from that point, towards Kisca, 

 the quartz-conglomerate dies out, and simultaneously the base of 

 the Millstone Grit retrogresses, uncovering the C^ + S^ beds on the 

 western side of the Ebbw valley, but not recrossing the base of S.^. 

 The retrogression is, however, merely a local incident. Immediately 

 north of the Ebbw river, just outside the limit of the area here 

 described, the Millstone Grrit resumes its overstep on to lower beds 

 in the Main Limestone. 



V. Detailed Descriptiox of the Successiox. 



Note in regard to localities. — Important localities 

 throughout the district have been indicated upon our copies of the 

 6-inch Ordnance Survey maps by symbols or locality-numbers, 

 as, for example, A 1, A 2. The complete descriptive symbol for any 

 locality therefore consists of the number of the appropriate 6-inch 

 sheet (Grlamorgan or Monmouth), followed by the locality-number ; 

 for example : Grlam. 36 SWA 4 designates locality 4 in the 6-inch 

 sheet Glamorgan 36 SW. These descriptive symbols are quoted in 

 this section of the paper, and a majority of them are marked upon 

 the maps (Pis. XV & XVI) which illustrate the paper. 



(A) Lower Limestone Shales. 



As compared with members of the Main Limestone, the beds 

 constituting the Lower Limestone Shales suffer little change of 

 character wdien traced through the district here dealt with. The 

 only notable variation is an increasing development of shale in 

 proportion to limestone in the lower beds, when followed from east 

 to west. But the strata undergo a stead}^ expansion in thickness 

 south-westwards from the Ebbw valle}" to the Taff valley, and the 

 expansion is apparently maintained along their outcrop west of the 

 Taff. Between Kisca and Tongwynlais, the thickness of the Lower 

 Limestone Shales increases from about 125 to 260 feet ; near 

 Miskin it probably exceeds 300 feet, although a partial masking 

 of the beds by Glacial gravel renders this estimate uncertain ; 

 farther west, the drift-cover prevents any determination of 

 thickness. 



No section in the district here described exposes the junction of 

 the Lower Limestone Shales with the Old Red Sandstone, although* 

 a band of calcareous grit which is the lowest bed exposed in a 

 sti'eam- section below Coed-y-Moch^ai, m the Ebbw valley, marks 

 the approximate, if not the precise, base of the Carboniferous 

 (fig. 5, p. 127). But there is no reason to doubt the conformable 

 relation of the two formatiq^is. 



East of the Taff, the characters of the Lower Limestone Shales 

 are well displa3^ed in fairW numerous sections ; west of the Taff, 

 the beds are extensively covered by drift, and exposures are less 

 satisfactory. The development east of the Taff is, therefore, 

 described first. 



