818 SoLLAS — A Map to show the Distribution of Eskers in Ireland. 



the hypothesis o£ a melting ice-sheet. It might readily result from subsequent 

 denudation. Surprise has sometimes been expressed that the eskers should have 

 survived the turbulent movement of the streams discharged from the disappearing 

 ice. Perhaps in these great gaps we have a measure of the breaching efficacy of 

 the liberated floods. Equally possible, however, is the suggestion that the gaps 

 are original, marking stretches of the glacial rivers where sediment was never 

 deposited as might readily happen for reasons so many and obvious that it were 

 waste of time to consider them. 



The eskers of Ireland are usually confined to comparatively low ground ; they 

 are consequently mostly distributed over drift-covered country. The ground on 

 which they stand seldom rises more than 350 feet above the sea-level, though some 

 large isolated mounds are found at a height of 400 feet. Their height above the 

 surrounding country is rarely over 70 feet, not often over 60 feet. 



Their side slopes are subject to great variation, but are often steep, some- 

 times making an angle of 35° with the horizon; both sides may be almost 

 equally inclined, but more commonly one side has a steep, and the other a gentle 

 slope. 



When both sides are fairly steep, it is probable that the esker was deposited in 

 a narrow channel and was supported by the ice on both sides. When one side is 

 gently sloping it is possible that the channel was wide, and that the esker rested 

 against the ice only on one side. 



The run of an esker system, as we may term the direction towards which its 

 several chains and branches converge, may be with the slope of the ground, or 

 against it. The system of Ballyhaunis is an examjile of the former case ; the sub- 

 system of Portumna of the latter. A system which runs with the slope of the ground 

 in one part of its course may, however, run against it in another ; as, for instance, 

 the system of the Midlands, which runs with the ground on its western side, as in 

 the case of the Portumna sub-system just cited, and against it on its eastern side, 

 thus descending to the Shannon in the one case, and rising from it in the other. 

 The explanation of this may fairly be postjjoned until we have taken into account 

 the direction of the movement of the ice. 



The structure of the Irish eskers, like that of those known in other parts of the 

 world, differs greatly in different eskers, and even in different parts of the same 

 esker. In some places, as pointed out in the earliest Memoirs of the Geological 

 Survey, it is confused, consisting of a rubble of blocks of stone only slightly 

 waterworn, but without glacial scratches, piled together in disorder ; in other places 

 it is beautifully stratified, coarse and fine gravel, sand, and occasionally clay, 

 alternating in rapid succession. False bedding is common, and in many cases a 

 rude tendency to so-called anticlinal structure is seen, i.e. the beds of stratified 

 material conform more or less closely to the slopes of the esker. I should prefer 



