122 ME. F. DIXET XyD PROr. T. Y. SIBLY OX THE [vol. IxXlii^ 



mudstoiie, which ranges from some level in C^ up into S^, The 

 Lower Limestone Shales of the Ebbw valley, on the other hand, 

 are miaffeeted by contemporaneous dolomitization, and present a 

 normal, fossiliferous character. 



Between the Ewenu}^ valley and the Ebbw valley, therefore, the 

 D i bu uop J/i/ 1 1 II iii^ Beds and the Main SeminuJa Zone are cut out by 

 overstep of the Millstone Grrit; while the surviving members of the 

 Main Limestone suffer much attenuation, and change from a mainly 

 non-dolomitic series of standard limestones into an almost un- 

 broken succession of crystalline dolomites and dolomite-mudstones. 

 The Lower Limestone Shales, though also suffering attenuation,, 

 maintain their character substantially unaltered. 



In the western part of the district, a well-detined series of zonal 

 divisions in the Main Limestone can be traced as fai- east as the 

 neighbourhood of the Creigiau Fault. But contempomneous dolo- 

 mitization increases rapidly in its vertical extent as we proceed 

 eastwards, and obliterates part of this zonal succession in the out- 

 crop between the Creigiau Fault and the river Taft', In the Taff 

 valley, contem.poraneous dolomitization has affected almost the 

 whole of the Main Limestone up to, and including, the basal beds- 

 of S., : but the bulk of S.^ has escaped alteration, except for localized 

 vein-dolomitization. East of the Taff valley the greater part 

 of S., is soon cut out by overstep. The surviving members of the 

 Main Limestone remain almost w^holly dolomitic ; but in C.,, 

 and again at the base of S.„ the formation of dolomite has been 

 partly due to vein-dolomitization, Dolomitization and the develop- 

 ment of J/of//o7r^ -phase deposits obliterate the faunal succession so 

 largely that only two zonal horizons can be traced in the Main 

 Limestone east of the Taff : these are (1) Z^, Avhich can be recog- 

 nized at a few localities, and (2) the base of S.,, a well-marked 

 horizon which can be traced as far as the Rhymney valle}^, where 

 it is overstepped by the Millstone Grit. The il/o^/Zo/rc -phase 

 deposits, essentially dolomite-mudstones with subordinate calcite- 

 mudstones. become conspicuous in the upper part of the Main 

 Limestone a little east of the Taff. and increase in a north-easterly 

 direction until, in the ground between the Rhynmey and the Ebbw, 

 they form nearly half of the Main Limestone as there developed.^. 



Attenuation of the strata. — Marked attenuation in a 

 north-easterly direction from the Taff valley is shown by tlie 

 Lower Limestone Shales and those divisions of the Main Limestone 

 which persist. As regards the Lower Limestone Shales, estimates 

 of thickness west of the Taff are of little value, owing to the 

 prevalence of drift ; but a great reduction of thickness north-east- 

 wards from the Taff valley, from about 260 feet at Tongwynlais to 



^ These constitute an argillaceous group which, as stated in the Newport 

 Memoir .(2nd ed. p. 20) ' appears atEisca and runs thence south-westwards . , , .' 

 It is in consequence of unconformable retrogression of the Millstone Grit, as 

 the Carboniferous Limestone is traced from the north-east, that the beds in 

 question make their appearance in the Ebbw valley at Risca, 



