Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/36

14 The thickness of this series of sandstones is very great; it has been estimated by Mr. Hull at 1500 feet; and possibly it reaches that thickness; but it is difficult to define the uppermost limit of the group, as it appears to graduate into the overlying Bunter, as may be seen by following the ravine Shellbrook upwards towards the S.E.

These sandstones have recently been bored through near the village of Whittington, two miles N.E. of Oswestry, and six miles south of the Dee-side section. The boring, after leaving the drift, passed through bright red soft sandstone belonging to the "New Red," and entered these dark sandstones, which proved to be 620 feet thick. They there consist of coarse- and fine-grained dark red and brown sandstones, occasionally streaked with white; and near their base there is a bed of white rock 2 feet thick. With the exception of a thin bed containing a few fragments of darker rock, there are no conglomerates or brecciated beds; at this point these upper sandstones rest immediately upon the lower portion of the red marls of group 3, the Ifton Coal-measures and the upper portion of the red marls being absent. At Croeswylan, two miles south of this boring, these upper sandstones have thinned out to about 70 feet in thickness, and they rest upon the red marls. At Llynclys, two miles to the south of Croeswylan, they rest immediately upon the Carboniferous Limestone and Millstone Grit. At Alberbury, seven miles S.E. of Llynclys, they cover the calcareous conglomerate for which that place is celebrated.

I have been careful to trace these upper sandstones in their progress southwards, because of their bearing upon the inferences to be drawn presently as to their true stratigraphical position.

In the year 1859 I described the coal-seams at the top of group 3, section 11, as ordinary Coal-measures, and I argued from them for an extension of the Coal-measures eastward; for I should here observe that in the maps of the Geological Survey the boundary between the Coal-measures and the Permian crosses the horizontal section at point A (Pl. I. B). In 1869 Mr. Hull, in speaking of the red marls, group 3, as seen at Newbridge and at Hafod-y-bwch Colliery, describes them as Upper Coal-measures, and he begins the Permian at the base of the dark sandstone of the Dee, group 4, section 11.

Subsequently to the publication of my paper in 1859 I observed from time to time the unconformability of groups 2 and 3 to the underlying Coal-measures—an unconformability amounting sometimes to between 700 and 800 feet; I noticed also the identity of the red marls of group 3 with those mapped as Permian at Plassau, N.E. of Ruabon, and elsewhere. On reading the description by the late Sir R.I. Murchison and Prof. Harkness of the Permian strata of the N.W. of England, I was struck with the similarity of the plant-remains of the shales in group 3 of their section No. 3, to those found at Ifton, section 11 in the same group, as well as with the