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Proceedings of the Royal Irish Acadeni f/. 



the sub-aerial hypothesis. The chief objection to this hypothesis 

 seems to be the length of time since the Miocene epoch — comparable, 

 say, to 15,000,000 years — which would have to be added to that 

 already calculated for the differential lowering of the surface. A 

 diagram of the Irish mountain groups, and the plain passing through 

 the chief summits, is shown to illustrate the remarkable conditions just 

 explained (Plate IV., fig. 3). 



On referring to the diagram it will be noticed that the general 

 level of the hill-tops stands somewhat higher in the south of Ireland 

 than in the north, which would be all the more remarkable if the 

 hypothetical plane were originally approximately parallel to the 

 present datum plane ; for the mountain summits of the south are 

 chiefly Old Ked Sandstone, and probably more easily denuded than 

 the granites and quartzites of the northern summits. It is likely, 

 therefore, that a slight tilting upward has occurred in the south, sinc(; 

 the flow of the Shannon commenced — a tilting which, in an angular 

 measurement, might be reckoned in minutes rather than degrees. It 

 could not, for any prolonged period, have been greater than 1 in 

 2,500,1 gjgg lY^Q waters of Lough Allen and Lough Ree would 

 permanently flow off by the Erne valley. An uptilt in the east of 

 300 feet in 70 miles would have sent those waters into Galway Bay, 

 and one in the west of 261 feet in 60 miles would have sent them into 

 the Barrow basin ; while an almost imperceptible sag in the Shannon 

 basin itself would convert it into an arm of the sea. The existing con- 

 ditions, therefore, seem remarkably stable, and probably entitle us to 

 infer a high degree of rigidity for the crust in this western part of 

 the British region, throughout later Tertiary and recent times — 

 though it be fully recognised that oscillations of level in regard to the 

 whole island have occurred. 



The conditions above described, as well as the regularity of 

 geological boundaries at the entrances to the Shannon gorge, north 

 of Killaloe, afford disproof of any local crust-movements, such as 

 convulsive rents, &c., which might be supposed to have formed 

 the gorge ; and the stability and prolonged continuance of those 

 conditions warrant our reference to the drainage systems of Wales, 

 the neighbouring portion of the British region. These are discussed 

 in an elaborate paper read before the Geological Society by Mr. Aubrey 

 Strahan, m.a., f.k.s., in May, 1902,^ the facts and conclusions of 



1 202 feet, in say 90 miles, is the present slope from the Woodford River to 

 the north coast-line. 



- Quart. Journ. Geol. Soc, vol. Iviii., part ii., p. 213. 



