Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/266

212 in the Corwen area between the Lower Grits and the underlying Bala beds in any of the sections he had examined; and he was inclined to believe that this area remained under water during nearly, if not quite, the whole period that the more central parts of Wales were above sea-level. No one had attempted to deny that there was marked unconformity in the Longmynd and some other districts between the May-Hill Sandstones and the underlying rocks; but the beds which could in any way be classed with the Llandovery rocks attained there only to a few hundred feet in thickness at the most, whilst thousands of feet represent that period in South Wales, and apparently also in parts of Denbighshire.

Mr. gave a list of Graptolites he had found in the flaggy slates, which included forms characteristic of beds at the summit of the Coniston Mudstones or the base of the Coniston Flags, and stated that a similar series occurred in equivalent beds in Scotland.

Prof., in reply, pointed out that the Graptolites found in the slates of Penyglog were not those of the Graptolitic Mudstones or Stockdale Shales, but agreed exactly with those of the Coniston Flags. So also near Austwick, on the borders of the Lake-district, the Graptolitic Mudstones had not yet been discovered, though they were well developed not far to the north in the Sedbergh district. He had not himself succeeded in finding Pentamerus oblongus in the Corwen beds; but Mr. Salter recorded it from Cyrnybrain. He thought the Corwen beds were on the horizon of the calcareous conglomerate of Austwick, and that the Penyglog grit was the equivalent of the Austwick grit, while the flaggy slates of Penyglog represented the flags between the Austwick conglomerate and Austwick grit.

The insisted strongly on the necessity of studying both the Palæontology and the Field-geology of any district, before attempting to come to any definite conclusion as to its geologica structure and the relative age of the deposits forming it.