88 Proceedings of the Royal Irish Academ ij. 



O'Briensbridge^ (Plate Y.), along an alternative course, possibly one 

 used by a branch of the Shannon.^ There is no evidence of back- 

 cutting upward from Limerick, such as might be expected along this 

 important water-way ; and the points and directions in which the 

 tributaries from the high grounds on each side here join it are not in 

 harmony with the circumstances which might be looked for in a long- 

 established river-system. Thus, under existing conditions the ]S"ewport 

 and Annagh Eivers form a system with a pronounced valley, indepen- 

 dently of the Shannon, instead of being directly tributary. They 

 turn sharply southward a few miles to the east of the main stream, 

 and are tributary only after joining the Mulkear, which empties into 

 the Shannon somewhat against its current. Again, the latest glacia- 

 tion was effected by an ice-sheet which moved over the Cratloe 

 liills, probably from the high grounds in west Clare, and fanned 

 out eastward at Castleconnell to south-eastward at Limerick 

 without being in the least affected in direction by the Shannon 

 valley. In consideration of these unwonted circumstances, and 

 having discovered what seems to me a reasonable means of account- 

 ing for an intermission of the Shannon erosion in this locality, I 

 cannot hesitate to see in the new appearance of its course a strong 

 suggestion of the river not having occupied it for a time at least ; 

 that, in fact, the river is, in a sense, a recent visitant there — a 

 monarch returned, after a long absence, to a part of his dominions 

 which by force of circumstances he had abandoned. 



There is little doubt that the gorge above Killaloe owed its origin 

 to river-erosion, until a late stage in its history. This is the view 

 maintained throughout the present paper.^ With regard, however, to 

 the latest stages, I would here submit : — 



1st. That, prior to these latest stages, the river-erosion of the gap 

 at length failed to keep pace with the comparatively rapid lowering 

 of the ground to the north, where solution of the limestone allowed 

 of the outspreading of Lough Derg, until the water found an easier 

 alternative line of discharge through the Scarriff valley, by which it 

 again reached the estuary, and kept the Kilrush gap open. 



1 And not very different from the geneial southerly slope of the ground. 



2 Mr. Geo. W. Lamplugh, f.r.s., &c., who first noted the singularly new 

 appearance of the Shannon course below Killaloe, considered that it might 

 originally have taken the line indicated above, that is by Clonlara, rather than 

 by Castleconnell. 



3 Notwithstanding comparisons with valley -formation by ice-erosion referred 

 to later on. 



