l»arl L' 1 VKIJDMFEllUl'S MMESTONK OF S. WALKS CO \ f,FI KLD. 1 liH 



12') feut at Kisca. is indisjjutaltlc. In tlu* Main Liiuestont', accoiiiil 

 L-au <tnly be talu-n of the sequence, Z to S^, as a whole, because an 

 accurate delimitation of (', and C^ is only possible in the nei^h- 

 lK)urhood ot Miskin : elsewhere west of the Taff, and in the almost 

 i)anvn dolomitie series east of the Taif, the C, C. b(»undarv can 

 only be sketched. The thickness of the series. Z to S^, estimateil 

 at iX'iO feet near Miskin. probably increases south-west of that 

 |»lace. Eastwards from Miskin it increases to at least 1250 feet in 

 tlie Taff valley, owinij to expansion of the C,^-|-S, beds. Kast of 

 the Taff, however, where the outcrop of the strata is tren<linii^ north- 

 eastwards, it diminishes to S.jO feet on Cefn-On, and to between 

 (350 and 700 feet near Machen. 



Overstep of the M illstone U rit. — Overstep is very gradual 

 aloni,' most of the tiutcro}>, but it becomes raj)id at those i)oints 

 where tlie zones of the Carboniferous Limestone swing north-east- 

 wards,* namely: (I) in the ground west of the Ely, (2j immediately 

 east of the TafF, and (H) immediately cast of the Khymney. 



In the area west of tlie Creigiau Fault, the junction of Carbon- 

 iferous Limestone and Millstone Grit is everywhere concealed, 

 either by Keuper or by Glacial deposits. But the rate of overstep 

 is undoubtedly rapid in the ground west of the Ely, and slow 

 ca.st of that river. At Ruthin, the JJ/bun(}j>//^//ifm-Ucds have a 

 thickness of at least GOO feet ; l)ut near lirynsaddler, 4 miles away 

 to the east, the position of the Millstone-Grit boundary, deduced 

 from Mr. S. Vivian's section of the Trecastle iron-mine,- indicates 

 that oidy about 100 feet of the Dibunophi/Uum Zone survives. 

 ( )n the other hand, overstej) does not progress much below the basr 

 of 1) in the distance of (5 miles between Hrvnsaddler and the 

 TafF valley. 



East of the Taff valley the base of the Millstone (Jrit can. as 

 a rule, be traced with considerable accuracy. No actual junction 

 with the Carboniferous Jjimestone is, however, exposed."^ Rapid 

 overstep cuts o»it at least liOO feet of the S^ beds in a distance of 

 little more than a mile north-east of the Taff valley, and another 

 sharp tmnsgression takes place immediately east of thq Khymney. 

 This latter transgression, which carries the Millstone Grit across 

 the base of S^ and far down into the underlying beds, pi*obably 

 into C,, coincides with the development of a thick lenticle of 

 ('luartz-conglomerate at the base of the Millstone Grit. It leads 

 to the maximum overstej) of the Millstone Grit within 

 the district here dealt with, ^ta i)oint midway l>etween the Rhymney 



' The Hignificant treinl is. of courKo, that of the base and zonen of the 

 CarbonifiTotis LiinoHtoni,'. Inasunurh as ovurntop is cauHiiiK' nipiil attenu- 

 ation of the oiiteroj) of the Carboniferous LimeHtone at these very points, the 

 trend of tliat outcrop as a whole has no siguificanee in this e«)iuiexion. 



- Trails. S. Wiilfs Inst. Kng. vol. xiv, No. 3 (1885) pi. xx\\. Roprinluced in 

 the Hridgrnd Menioir. fig. 11. p. 109. 



•* The junction formerly exposed in a railway-cutting on the western side 

 of the Ebbw valley (Xewi)ort Memoir, 2nd ed. p. 21) iu now concealed. 



Q. J.G.S. No. 21 M) L 



