KiLKOE — The Shannon: its Course and Geological History. 95 



evident that we must take into account a considerable amount of 

 glacial erosion throughout the country as a whole, in the lowering of 

 the central plain, to its present level. Evidences for a great thickness 

 of ice are to be met with in many places, perhaps as great as 2,000 to 

 3,000 feet, if not more ; and when we consider the work done, 

 according to Hess, by glaciers of 300 or 500 metres in thickness, 

 operating for 50,000 to 70,000 years, we might be tempted to 

 dispense altogether with the agencies of sub-aerial denudation in 

 carrying off 2,500 feet of solid strata from the surface of Ireland. 

 The true estimate of time, probably, lies between 50,000 years — if 

 thick ice could be supposed to have accomplished such work — and 

 30,000,000, or the still less estimate of 15,000,000 years, for reduction 

 by sub-aerial forces alone. 



The differential lowering of the surface, resulting in the present 

 surface features, seems more consistent with the mild reduction due 

 to sub-aerial waste than with the drastic mechanical force of an over- 

 whelming moving ice-sheet. Thus, in the south of Ireland, the most 

 soluble rock, limestone, invariably occupies the lowest ground ; the 

 calcareous Silurian slate, and fine grits, occupy the next level ; and 

 the coarse non-calcareous Old Red Sandstone and conglomerates 

 form the highest ground — excluding the mid-Ireland granites of 

 Dublin and Wicklow. The conditions are well exhibited in the 

 Slieve-na-man, Galtymore, and Comeragh tracts. 



Another circumstance may be mentioned which also tells strongly 

 against the predominance of glacial over sub- aerial waste, including 

 river-erosion, namely, the nature of the boulder-clays. They are, to 

 •a large extent, practically impervious to water ; yet in the very region 

 with which we are at present chiefly concerned — amongst the valleys^ 

 of the Keeper Hill group — I noticed, some years ago, that while it 

 was almost impossible to find, in some of those deposits, a scrap of 

 limestone, small pieces of chert could be picked up in abundance. 

 These indicate that the boulder- clays had been carried from off the 

 limestone tract ; but that being no longer, when found, surrounded 

 by or attached to fragments of their original limestone matrix, the 

 latter must have been completely dissolved away before the clay 

 containing the cherts was picked up and borne along by the ice to be 

 deposited where such are now to be seen. This would seem to hav(> 

 been the case with much of the boulder-clay of Ireland. Instances, 

 however, frequently occur — for example, the very gravelly so-called 

 boulder-clays — in which fragments of limestone are quite plentiful — 

 constituting, in fact, a large percentage of the mass — and angular. In 



