KiLROE — The Shannon : its Course and Geological History. 81 



annual rainfall than the present ; or that the area presented strata 

 much more easily denuded than limestone. With regard to this latter 

 alternative, there must have been a period during the formation of 

 the basin when the area affected was formed of non-calcareous strata 

 — those of the Pendleside, Millstone Grit, and Coal-measures series — 

 and therefore less easily removable than limestone. The denudation 

 of these members, however, may have preceded the Cretaceous 

 period ; and the hollows may have been since filled with more soluble 

 and friable strata. Even this supposition, however, does not help to 

 diminish materially the period necessary for erosion, on the assump- 

 tion of uniform operation ; for the Thames drains an area consisting 

 almost entirely of Secondary strata, and some Tertiary — just such as 

 might have covered Ireland while the Shannon basin was to some 

 extent being formed ; yet the present rate of waste of the Thames 

 area is comparable to that of the Shannon area. The calculations, 

 therefore, lead us to regard the forces producing denudation as 

 variable, or as having acted much more vigorously at some periods 

 than at others. This agrees entirely with the conclusion arrived at 

 by students of subterranean as well as superficial erosion.^ If we 

 take even half the time calculated and set down — 15,000,000 years — 

 as the time expended in the denudation of Ireland, including the 

 sculpturing of the present physical features, and the severance of the 

 island from Great Britain, it seems a very long period when we think 

 of the small proportion it must hold to the seons necessary for the 

 filling up of the entire geological record. 



We have an irrefutable argument presented in at least two Irish 

 regions, for the post-Eocene age of the present surface features of 

 Ireland. One is the existence of the Mourne group of hills, which 

 consist in large part, and from summits almost to base, of Tertiary 

 granite^ ; the other is the occurrence of a dyke of Tertiary basalt 

 which may be traced across the top of Errigal (2466 feet). Botli 

 of these igneous intrusions must have invaded strata at least on a 



^ See Martel's data in Spelunea," vol. vi., 1906. Later on in the present 

 paper a probable cause is suggested by which the presumed vast duration of the 

 processes of waste may have been considerably curtailed. 



- For the Tertiary age of the Mourne granites, see " Ancient Volcanoes of 

 Great Britain," by Sir A. Geikie, d.c.l., f.r.s., vol. ii., p. 421. They probably 

 belong to the same general epoch as the Antrim basalts — shown to be Eocene by 

 Mr. J. Starkie Gardner, f.l.s., f.g.s., &c., Quart. Journ. Geol. Soc, vol. xii. 

 (1885), p. 82. The dyke of basalt across Errigal was traced by the present 

 writer. 



