KiLROE — The Shannon : its Course and Geological History. 8-5 



and 14, granite; 9 and 10 Silurian grits; and the last four, of Old 

 lied Sandstone strata. It cannot be regarded as a mere coincidence 

 that hill-summits formed of rocks of such different natures could all 

 range upon or approximate to a plane ; and it is therefore probable 

 that in the summits of the mountain groups we have the vestiges of a 

 i^reat plain of denudation, such as was postulated in 1862 by Professor 

 Jukes. That authority, as already noted, believed the plain to have 

 been one due to marine erosion ; and Professor Hull concurred in the 

 belief, which he extended so far as to attribute the existence of 

 plains found at lower levels in the interior to the same origin. It 

 is not easy to see why this hypothesis should not be accepted, at least 

 for the original plain upon which the Shannon commenced to run in 

 Tertiary times ; but I did not think the hypothesis could apply in the 

 case of the lower planes connecting certain hill-tops in the interior, 

 since it would have to be supposed that the causes which produced 

 them — the erosion of ocean-waves and currents — left the outworks, 

 the higher grounds around the island, unaffected ; and this throughout 

 the long period necessary for the formation of the plains within. 



The work of Mr. W. M. Davis, in America, tends to veto the 

 above hypothesis in so far as marine erosion is supposed to be the 

 agent which produced the plain. He shows that sub-aerial waste, 

 operating upon an uplifted area, retained at a certain level, can do 

 precisely the work with which marine erosion has been credited.^ 

 This hypothesis, moreover, in the case of Ireland, would account for 

 the existence of plains at lower levels — produced, let us say, where 

 similarity of conditions prevailed. And, as against the previous 

 hypothesis, it may be mentioned that it is likely the denudation 

 which formed the great Irish plain commenced when the land showed 

 itself above the Miocene sea, rather than when it was sinking. The 

 great Cardigan bulge, which initiated the drainage directions in 

 Wales, took place when the land was emerging ; and this was probably 

 concurrent with the intrusion of the Mourne granophyres, accom- 

 panied by bulging of the invaded Silurian strata, which must have 

 been cleared away to expose the summit of Slieve Donard, prior, as 

 we have seen, to the initiation of the Shannon. If, according to this 

 reasoning, the ancient great plain of Ireland was formed during 

 emergence, this would have been an unfavourable condition for the 

 formation of the plain by marine erosion, and would tell in favour of 



^ " Physical Geography," by William Morris Davis, 1899. The plains in 

 (question the author terras " peneplains," p. 152. 



