21 



evidences of the extensive denudation of the secondary and 

 trappean rocks, and the markings of the former sea lines. 



This part of the paper was illustrated by a large section of 

 the district, taken between Divis Mountain and Castlereagh, 

 passing across the valleys of the Blackstaff and Lagan rivers, 

 and shewing the succession of strata from the trap in Divis 

 downwards through the Chalk, Greensand, Lias, Keuper marls, 

 and New Red Sandstone, to the Silurian rocks of the County 

 Down, and the enormous amount of material that must 

 have been torn away to give the present contour of the 

 country. 



The various agencies that are known to be capable of wearing 

 down rock were then discussed, and the opinion of eminent 

 geologists on this point were cited, and the conclusion was 

 expressed strongly, that such denudation as there is evidence of 

 in this district could only have been accomplished by icebergs 

 on the gigantic scale of those observed at the present days 

 around the coast of Greenland ; the whole country being sab- 

 merged 2,000 feet or more, and strong ocean currents sweeping 

 over it from north to south. 



The various levels at which the land had paused on its 

 subsequent emergence from the ocean was then referred to, and 

 among other evidence of one of the more ancient sea levels 

 was adduced the fact of the caves in the Knockagh, Carnmoney 

 Hill, and first cave of Cave Hill, being about the same general 

 elevation of 600 feet above present sea level, and being 

 characteristic of sea- worn caves at present being hollowed out 

 on the Antrim Coast. The action of glaciers in wearing off 

 the edges of the earlier beaches was mentioned, in explanation 

 of the difficulty of identifying the higher ones, except under 

 very peculiar conditions. This difficulty does not exist in 

 respect of the latest sea margin, which can be readily traced 

 around the entire district, and, indeed, around the whole coast 

 of Ireland and Scotland, at a height of about twenty feet above 

 present high water mark. 



The evidence of the Roman wall, at the Firth of Clyde, not 

 having changed its relative level to the Firth since its erection, 

 was given, in connexion with the observed rate of wearing of sea- 



