part 2] CAEEOXiFEEors limestone of s. wales coalfield, 161 



G-rit. as in the example figured in the Abergavenny Memoir ; and, 

 indeed, it was difficult to see how such an area could be invaded by 

 currents strong enough to distribute coarse gravel without some 

 washing-up of the sea-bottom. The matter was complicated by 

 the piping of the limestone, which had taken place on so great a 

 scale as to eventuate in wholesale subsidence of the Millstone Grit 

 and to cause difficult}^ in mapping its base. He was not satisfied 

 that there was evidence of an unconformity^ of a pronounced order, 

 such as would involve the supposition of upheaval and prolonged 

 denudation. 



All the Carboniferous subdivisions shared in the attenuation, 

 clearly indicating the existence of Carboniferous land at no great 

 distance to the north. The shore-line appeared to have run in a 

 direction genemlly parallel to the existing margin of the Coalfield. 

 Part of the margin of the Carboniferous Limestone could be 

 recognized in Pembrokeshire, and eastwards near the Welsh Border. 

 The different aspects under which the limestone presents itself at 

 various distances from the sho]*e-line afford an unrivalled oi^portunity 

 of studying the types developed under littoral conditions, such as 

 dolomites, oolites, pisolites, interbedded conglomerates and others. 

 He looked forward to the prosecution of researches on which so 

 promising a commencement had been made. 



Mr. E. E. L. Dixox said that he would refer to two points only. 

 The more important was the overstep of the Millstone G-rit across 

 successively lower horizons of the Carboniferous Limestone. This 

 overstep continues along the outcrop that bounds the eastern end 

 of the coalfield, north-east of the area described in the paper. For 

 some miles, however: that is, to beyond Pontypool, it results in little 

 further thinning of the Lower Avonian dolomites ; but, from Cwm 

 Afon north-eastwards, it rapidly increases and cuts out almost, if not 

 quite, the whole of the remaining dolomites. Xevei-theless the 

 underlying Lower Limestone Shales maintain their thickness un- 

 changed, and present the same rock-types and the same fauna, 

 indicative of the Cleistopora Zone, as elsewliere along the outcrop. 

 There can be no doubt that they do not represent tlie Main Lime- 

 stone + Lower Limestone Shales of other localities. 



This is the extreme amount of the overstep. The outcroj) here- 

 abouts swings westwards, and, in that direction, successivelv-higher 

 subzones of the Main Limestone emerge from beneath the Millstone 

 Grit — so i-apidly that in less than 6 miles appear several hundred 

 feet of limestone, in which all the zones, \\\) to the Upper Seiuinula 

 Subzone, S,, are recognizable. 



In this area, also, important confirmatory evidence of the uncon- 

 formable nature of the Main Limestone-Millstone Grit junction 

 has been obtained, of which details will be pubhshed later. 

 Although no discordance of dip is to be observed in actual exposures 

 of the junction (which fortunately are to be seen at several places 

 in this area), the latter is sharp and irregnlar, and in the largest 

 exposure, in the Llammarch dingle near Bryn-mawr, is sufficiently 

 transgressive to cut out 6 feet of limestone in a distance of 35 vards. 



