478 Proceedings of the Royal Irish Academy. 



five miles as they extend. Through one of these valleys the infant 

 Shannon meanders southwards, and issues out of a cavern at the hase 

 of Cuilca known as the "Shannon pot." 



I have traversed all these mountains and examined their strata, 

 escarpments, and intervening valleys. (The latter reveal the lime- 

 stone surface). On the face of some of these escarpments thin seams 

 of coal crop out, and in the escarpment in the opposite side of 

 the valley there are corresponding seams, and similar strata as found 

 on the other side, and on both sides horizontal. Some of these valleys 

 have been scooped out to a depth of 1600 feet. Notwithstanding the 

 presence of these deep valleys, all the geological phenomena found in 

 the locality bear evidence in favour of the hypothesis that all these 

 mountain ridges which rest on a portion of the great limestone 

 plain which covers the centre of Ireland, and are principally com- 

 posed of sandstone, at one period formed one continuous undulat- 

 ing plateau covering the limestone formation in this part of the 

 country ; but the greater portion of it has been removed by denuding 

 forces, leaving these ridges behind — presenting now the appearance of 

 streaks and patches of snow, which sometimes remain on higher sum- 

 mits after a thaw, when the sheet which covered the country has 

 passed away. 



The rain water which courses down the sides of these mountain 

 ridges converges into little rivers which traverse the surface until they 

 reach the limestone formation at a lower level, when they generally 

 penetrate its strata, and form subterranean passages which often result 

 in deep ravines. I have examined a great many of these "swallow 

 holes" which form the entrance to these underground water ducts, 

 and found rounded sandstones amongst the debris in their bottoms, of 

 considerable size, which were transported hither by the current from 

 higher levels, where the sandstone thins out on the surface of the 

 limestone. This description of the geological features of the loca- 

 lity, confused as it is, may help us to understand more about the denu- 

 dation of the sandstone formation, which was continuous from this 

 locality over Knockninny hill before it became detached by denuding 

 forces from the main formation in the locality. 



Standing on the east shoulder of Cuilca you observe a valley which 

 passes from its base (to which I have already referred) eastward, and 

 broadens and deepens as it. extends, and passes at almost right angles 

 through the mountain ridge which bounds the valley on the south- 

 west side of Knockninny, then broadens into the valley of Lough Erne 

 and encompasses Knockninny at its extremities ; and supposing a 

 current of water filled this valley from the base of Cuilca down to 

 Lough Erne, a distance of five miles, Knockninny, which is situated 

 at its lower end, would appear an island, and before the valley which 

 surrounds it was sculptured out by subaerial agents, the waters which 

 passed down the slopes of Cuilca passed over the top of EJiockninny 

 into Lough Erne. 



The evidence supporting this hypothesis I have found on the sur-- 



