212 ON THE SILURIAN GRITS OF CORWEN, NORTH WALES. 



conformity 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 un- 

 derlying 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 Den- 

 bighshire. 



Mr. Hopkinson 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. Hughes, in reply, pointed out that the Graptolites found 

 in the slates of Penyglog were not those of the Graptolitic Mud- 

 stones 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 Sed- 

 bergh district. He had not himself succeeded in finding Pentamerus 

 oblong us in the Corwen beds ; but Mr. Salter recorded it from Cyrny- 

 brain. 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 President insisted strongly on the necessity of studying 

 both the Palaeontology 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. 



