786 SoLLAS — A Map to show the Distribution of EsJcers in Ireland. 



loose material have been deposited from water, as the structure of this one clearly 

 proves it to have been, and yet have acquired such steeply sloping sides ? The 

 only answer that could be given was that, during deposition, boundary walls must 

 have existed by which it was held up. What, then, were these walls ? Might 

 they not have been a lateral extension of the esker itself, a lateral extension so 

 wide as to constitute an alluvial plateau ? An answer in the affirmative has been 

 given by at least one Irish geologist, Mr. M. Harrison ;* but such a solution has 

 been rejected by others, including the late Professor Jukes, and as it seems to me 

 on very sufficient grounds ; for, in the first place, the existing drainage system of 

 the country is not related to the eskers in a manner which would suggest such an 

 explanation ; and, in the next place, the structure of the eskers themselves is defi- 

 nitely opposed to it, since the stratified material of which they consist is frequently 

 arranged so as to dip parallel to the sides of the ridge, and, in addition, the 

 separate ridges sometimes unite so as to inclose deep hollows between them, 

 which could not, by any possibility, have been excavated by a stream. f 



It is impossible to explain an esker as the denuded remains of a plateau ; and 

 ao-ain we inquire what were the retaining walls which held it up during its 

 deposition ? Whatever they were, they no longer exist, and have utterly vanished 

 from the face of the land. The only substance at all likely so to behave is ice. 

 But we have independent evidence that the whole of Ireland was at one time 

 buried under a system of confluent glaciers, and it is known that streams of water 

 flow over, under, and through such ice-sheets. Let the channel of some such 

 stream become choked up by debris, and a cast of it will be formed in sand, 

 D-ravel, and pebbles, which, on the disappearance of the ice, might form a ridge 

 in all respects similar to an esker. 



If such an explanation were on the whole true, it would follow that an indi- 

 vidual esker should resemble, in its wanderings over the country, the course of a 

 river, it should branch and meander ; and similarly a system of eskers, if such 

 should exist, as might naturally be expected, should often repeat the characters of 

 a river system. To see whether this was the case, I prepared from the published 

 maps of the Geological Sm-vey the maps which this Paper is written to illustrate, 

 and on reference to it the resemblance, which we have deduced, will be manifest 

 at a glance. 



It is now clear why we did not succeed in finding marine shells at Maryborough; 

 their absence is just what our present explanation would lead us to expect. 



The next step was to acquire a knowledge of the literature of eskers ; and 

 from this it appears that a similar explanation to that we have just attained 



*M. Harrison: On the Origin of Eshers. — Proo. Belfast Field Club, ser. 2, vol. i., p. 100, 1875. In 

 Sweden, this view had already been proposed by Tornebobm and others. 

 ■f Jukes' Memoirs of the Geological Survey, Nos. 98, 99, 108, 109, p. 8. 



