part 2] CAHUOMi Kifors li.mkstonk of s. wales coalfikm). 117 



Mr. K. K. L. Dixoii pointed out the eonteiiiporaneou.^ orij^in of 

 most of the dolomitizatioii in the outcrop comprised in the Nc\vjx)rt 

 miip, wliicli inckides tlic eastern part of our area, and we cannot 

 do better than quote his words : — 



• From the fact that the dolomite is comparable \vitli that of Gower, and 

 persists as such for upwai'ds of 15 miles, it may be inferred that much of the 

 dolomitizatioii was '•contemporaneous ' rather than the result of subsequent 

 alteration along- veins or faults. That the rocks were at first calcitic is shown 

 by their containiiif,'- crinoidal remains, and the expression *• contemporaneous "" 

 is intendetl as implying- dolomitizatioii while they were still under the influence 

 of the Carboniferous Limestone sea. One of the bands, however, in the 

 argillaceous g-roup was examined under the microscope, and found to have 

 been a dolomite-mud, such as might have been derived from the detrition of 

 dolomites.' ^ 



Vein-doh)niitization, due to a much hiter metasomatism, is 

 widespread, bein^ particularly well develojjcd, for example, in the 

 Seiiiiiiula Oolite of Creigiau and the Tafl" valley. Its great 

 development in the localities mentioned may be attributed to free 

 circulation of magnesian waters in highly-tissured limestones 

 adjacent to large faults, but elsewhere it appears to have resulted 

 chieHy from diffuse })ermeati()n. 



Inasnuich as contemjioraneous dolomitization was induced by 

 conditions which accompanied the deposition of the Carboniferous 

 Limestone, whereas the distribution of vein-dolomitization l>ears 

 no relation to those conditions, we have attempted a systematic 

 di.scriniination ; and the nature of the dolomitization as inferred 

 by us is expressed by means of conventional signs in the sections 

 illustrating this j)aper. Discrimination is often uncertiiin, how- 

 ever, and no uniform accuracy is claimed. ^Moreover, it is pro- 

 bable that many cases occur in which limestones partly alteretl by 

 contem))oraneous dolomitization wen- fuHluM- nlTt'cted by vein- 

 dolonutization at a later date. 



IV. (Ik.nkhai, Ac'corNT or ini; Si tckssion and iis 

 Later A I, N'aiha iion. 



The Lower Limestone Shales rest conformably U])on the Tppt r 

 C)ld lied Sandstone. The Main Limestone is succeeded uncon- 

 formably by the Millstone (irit, the base of the latter formation 

 overstepping from higher to lower horizons in the Main Limestone 

 when traced north-eastwards along tlu* outcrop. In genend, shales 

 form tlie lower niend)er of the Millstone (Jrit; but locally, in the 

 east of the district, a (piartz-eonglonierate is develoiu'il at the bast* 

 of that formation. 



The vertical sections i tiu". L'. p. II!)) illustrate the genenil lithology. 



' Newport Mi'inoir. 2nd ed. pji. !'.» 'Jo. Spct-imeu V.IVIW in the collection 

 of the Cteologieal Survey represents the baml mentioned in tiie last sentence 

 quoted. It is t«) rocks of tliis type, largely developeil in tlie secpience oast of 

 tile Taff. that the term dolomi te - ni nds t i>ne is iipplied in tliis j>aper. 



