526 Geological Society. 



Mr. Sharpe points out, that up to the moment of his taking up the 

 subject no one of the authors quoted had expressed a doubt of the 

 existence of a great thickness of fossiliferous beds below the Caradoc 

 sandstone and Llandeilo flags, although it was admitted that these 

 supposed beds could not be distinguished by their fossils from the 

 Lower Silurian ; and he states that the object of his communication 

 is to show the error of this view as relates to the Bala rocks, 

 which he proposes to prove to be the equivalent of the Lower 

 Silurian beds described by Mr. Murchison, and not part of an older 

 series ; and he infers from analogy that the same will be found to be 

 the case in other parts of North Wales which he has not visited, where 

 he conjectures that all the rocks containing shells of Lower Silurian 

 species will also prove equivalents of the Lower Silurian beds. In- 

 stead of continuing the Silurian system downwards through a vast 

 thickness of slate rocks, Mr. Sharpe proposes to strike out one of its 

 original members, regarding the Caradoc sandstone and Llandeilo 

 flags as one and the same formation which has received different 

 names according to its mineral character ; he observes, in confirma- 

 tion of this view, that both formations are never equally developed 

 in the same district, and that the fossils found throughout are too 

 nearly the same to warrant the separation of the lower beds under a 

 separate name. Still Mr. Sharpe believes that there are in Wales, 

 as in Westmoreland and Cumberland, vast accumulations of slatv 

 rocks below the Silurian system, in which no fossils have been found, 

 and which must retain the appropriate name of Cambrian rocks. 



Mr. Sharpe did not map the district in detail, but he traced two 

 sections to show the position of the Bala beds with regard to the 

 Berwyns, as he considered the question to turn upon the accuracy 

 or error of the statement of Mr. Murchison, p. 308, " that the Bala 

 limestone dips under the chief mass of the Berwyns." 



The first section begins westward, at the igneous chain of Arenig 

 Mawr, the natural boundary to the district ; it crosses the town of 

 Bala, and ends eastward at the Calettwr, where a dark slate, the 

 upper bed of the Bala series, abuts unconformably against the clay- 

 slate of Moel-halog, which is referred to the Cambrian system. This 

 section places the Bala beds in a detached trough, and shows that 

 they do not dip under the Berwyns : but their succession is not well 

 shown, owing to the disturbed state of the surface. 



The other section is in two parts ; from the head of the lake of 

 Bala up the Twrch to Bwlch y Groes, and across the Dyfi by Dinas 

 Mowddy and Mallwyd, which line the author recommends to 

 those who wish to study this series, as the rocks are well exposed in 

 the upper part of the valleys of the Twrch and Dyfi : on the west it 

 begins at the northern prolongation of the igneous chain of Arran 

 Mowddy, and continues eastward through a conformable succession 

 of beds up to the Upper Silurian ; each section shows the whole of 

 the Bala series, the upper bed of blue slate, which on the Calettwr 

 rests unconformably against the Cambrian clay-slate, being the same 

 which is overlaid conformably beyond Mallwyd by an Upper Silurian 

 series of soft blue or liver-coloured shales alternating with hard, grey 

 grits, without cleavage or fossils, dipping east-south-east, which Mr. 



