SILURIAN DISTRICT OF RHYMNEY AND PEN-Y-LAN. 339 





By W. J. SOLLAS, M.A., F.G.S. 



[abstract.] 



THE paper commences with a history of previous observations 

 on the district ; a description of the geographical distribu- 

 tion, geological structure, and vertical succession of the Silurian 

 rocks is next given. They comprise beds belonging to the 

 Wenlock and Ludlow groups, and pass conformably upwards into 

 the Old Red Sandstone. The district affords a good base for a 

 measurement of the thickness of the Old Red Sandstone on the 

 south of the South-Wales Coalfield. This was found to be a little 

 over 4,000 feet. The thinning out of the Old Red Sandstone and 

 Silurian strata, together with the marked change which takes place 

 correspondingly in the Hthological characters of the latter formation 

 on passing from the north to the south side of the coalfield were 

 taken to indicate an approach to a shore-hne. This shore-line 

 belonged to land which, as shown by the great thickness of the 

 Devonian beds, could not have extended far south. It corresponded 

 to Mr. Etheridge's barrier between the Old Red Sandstone and 



