page 111 note 1 Although lying outside the subject of the present paper, the history of these strata deserves a brief notice. They were first distinguished from the Triasin1865 by Maw, S. (GTEOL. MAG., Vol. II, pp. 380Google Scholar, 523, and Rep. Brit. Assoc, for 1865, Sections, p. 67), but erroneously described as Permian. in the same year Davies, D. C. suggested that they would probably prove to be Carboniferous (GEOL. MAG., Vol. II, 3Google Scholar. 476). During the resurvey of 1881 it was readily seen that the purple measures ay directly and conformably upon the limestone, but were unconformably overlapped by the Trias. They were known, moreover, to contain thin seams of coal, and to include strata indistinguishable in tint or character from Coal-measures. Their Carboniferous age, therefore, was no longer leftindoubt, but there remained the difficulty that they were quite unlike the rocks which lie next above the limestone in East Flintshire.
The East Flintshire Millstone Grit is of an abnormal type, for though in the southern part of the county it consists of quartz-grits and conglomerates of the usual character, towards the north it passes almost wholly into chert. With neither of these types had the purple measures anythingincommon, but after some little search I was able to match them exactly in some micaceous sandstones and shales which lie between the Millstone Grit and Middle Coal-measures near Mold, and which may be supposed to belong to the Lower Coal-measures of other regions. Subsequently, when the mapping showed that the Millstone Grit locally thinned out in Mid-Flintshire (“Geology of Flint”, etc., p. 53), leaving the supposed Lower Coal-measures in direct superposition to the limestone, I attached more importance to the lithologieal affinities of the purple strata, and concluded that they also were probably younger than the Millstone Grit (ibid., pp. 33, 69).
Mr. Morton, however, states that “the Purple Sandstone and Shale, coloured as Coal-measures on the Map, are really on the horizon of the Upper Black Limestone of Prestatyn and the Arenaceous Limestone of Mold, subdivisions which do not belong to the Coal-measures” (Proc. Liverpool Geol. Soc. 1897–1898, p. 280Google Scholar). This is true in the sense that they rest upon the “Upper Grey Limestone” of his classification, but it is not the case that they represent or pass horizontally into the black limestones. These latter are strictly localintheir development, and thicken and thin rapidly in East Flintshire.inSouth Flintshire they are represented by alternations of sandy limestones and sandstones, and in a corresponding position in the south end of the Vale of Clwyd strata of this character, though poorly developed, are seen dipping beneath the purple beds.
This poor development, the absence of the black limestone, and the local thinning out of the Millstone Grit, may be taken with the general attenuation of the limestone as indications of a Carboniferous shore-line at no great distance westwards, the only evidence we possess that Snowdonia had commenced existence as an elevated region in Carboniferous times.
