part L'^ I' AHIK)NlFKI{()r> MMKsTONK of s. walks cOAF-riEI.I). W-i 



Till' iiatiirc and siL^niitli-ancc of tin- liroad variations sliowii l)y 

 tlie ('arl)oniferous Liiin-stont' Scries in tlic extensive South Wales 

 outcrops liave been admirably defined by Dr. Stnihan in the follow- 

 iuLT word> : 



' Thu Ciirhoiiiferous Limestone Series presented a successton of stages in 

 tlevelojunent. The fullest ilevelupnieut was exhibited in the southernmost 

 occurrences, as reyanled both the sequence and the thickness of zones. 

 A second stajjre, showLnj,'- an incfjmplete secjuence an<l considerable attenuation, 

 with in'lications of near-shore ori{,'-in. was presented along i>arts of the margin 

 of the fi»alHeld. Lastly, in the nortiierninnst occurrences, near Abergavenny 

 and at Pen-cerig-ciilcli. the series dwindled away to 100 feet or less, while in 

 Pembrokeshire it was wholly overstepped. The inference followed that the 

 coast-line ran through Pembrokeshire, and not far north of Al^ergavennj'.' ^ 



Within the comparatively small extent of outeroj> now under 

 consideration the Carboniferous Limestone Series undergoes a 

 remarkable chauii^e. Here, as in most parts of Soutli Wales, two 

 litholo|irical <livisions are reeo«j;ni'/ed : namely, the Lower Limestone 

 Shales and the Main Limestone. Xot only does the whole series 

 suffer a notable <liminiition of thickness when traced north-i-ast- 

 wards aloni; the outcrop, but simultaneously the Main Limestone 

 ehan*,^'s in character from a formation com])osed chielly of ordinary 

 crinoidal limestones and oolites into an almost uninterrupted 

 succession of dolomites,- Tlie elucidation of this i-hanij^e has been 

 the (d»je«'t of our investiij^ations. 



Litth* zonal work had ])rece(b'd om- studies in the district. In 

 the east of the area Mr. K. E.L. Dixon, ^ who examined the out- 

 croj) for the ])urpo.se of eolleetini^ information for the second edition 

 of tlie NewjMtrt Memoir, reco^niized the eijuivalence of the Lowi'r 

 Limestone Shales to the Ciristnpnrn Zone, and noted the presence 

 of the Srmiiniltf Zone in the Taff valley. That ij^eoloij^ist further 

 attributed the ab.scnco of the DihiniophijUiiw Zone in the Taft 

 valley, and the apparent attenuation of the Sriii inula Zone farther 

 • •ast, to unconformable overstep by the Millstone (irit, a conclusion 

 which is conHrme<l by additional evidence adduced in this ])aper. 



In tlie west of the district, a visit to the neiiijhbourhood of 

 Miskin and Llanharry in IDOJ) enal)led Prof. (>. Deh'ijinc t(> 

 recognize the ZajthreutiH, Cauini/t, Srminuhi, and DihuuophyJhnu 

 Zones in the Main Limestone of that area.^ Our own <d)servations 

 accord with those of Prof. l)»dej)ine, exce])t as reupards his identiti- 

 cation of the Dihinioi>hi/llinn Zone : the exposures near Llanharry 

 which \\v assii^ns to the subzones D, and D.^ are refern'd by us to 

 the Srnilimlfi Zone, 'j'ln' subzoiir D, is rej)rescnt«*d in s^round 



' Q. J. (J. S. vol. Ixvii (llUl ) p. .'•(»7 (in discussiim of the paper by K. K. L. 

 l>ixon «fe A. Vaughan on 'The Carboniferous Succession in (tower'). 



' Newport Memoir, 2nd ed. p. U> ; and Bridgeml Memoir, p. «;. 



' Newjxirt Memoir. 'Jnd ed. ji. 20. 



■* ' Not*' on th<« Faunal Successi«m in the (.'arboniferous Limesti>ne (.\vonian) 

 near Llantrisant Station in the Hriilgend area. Smith Wales ' iJeol. Mag. dec. .'>, 

 vol. viii (1010) pp. r>7 70. 



