246 NATURAL SCIENCE [October 



Southward of this current there would be a bay, somewhat similarly 

 circumstanced to Wexford Bay, oft' which banks would form between 

 Galway Bay and Dublin, that is in the line of country occupied by 

 the principal eskers. 



That the author was himself sensible of the difficulty already 

 alluded to, is clear from the words above quoted : " The typical 

 eskers are very unlike shoals." By the conflict, or the separation, 

 of flow-tide currents you can have shoals and flats and mud- 

 banks, but can you have an esker like one of those already described ? 

 Let anyone put the question to himself as he stands beside one of 

 these ridges, or walks its road-like top, for only in such circum- 

 stances can the difficulty be fully realized. 



Kinahan, however, goes further, and explains how subsequent 

 denudation may have played a part in the modelling process. 



Although it cannot be affirmed, yet it appears possible (he says) 

 that as the sea shallowed, and the shoals and banks became ' awash,' 

 the current should have the power of changing the massive banks 

 into narrow ridges, for at the half-tide or ' awash,' portions of banks, 

 or in the shallow places where two currents collide, there are esker- 

 like ridges as St Patrick's Bridge between Kilmore and the Saltees 

 Co. Wexford. 



But with all this before me I am unable to account to my own 

 satisfaction for the form of the esker proper. It is not easy to 

 conceive how by any process of marine or aerial denudation great 

 massive banks could be attenuated to the slender fitmre of an esker 

 running for miles like an artificial earthwork. One may 

 ask how the denuding forces could waste all but the back-bone, and 

 yet spare the latter which was, after all, no more likely to resist 

 erosion than the rest of the drift matter. 



In the first edition of his Manual, Mr Kinahan attached particu- 

 lar importance to the ' Head of tide ' origin of banks, etc. But at 

 a later period he abandoned this in favour of what I have sketched 

 as the ' Cross-current theory.' He believes that, allowing the marine 

 origin of eskers, the various details and complications of the drift 

 formations of the central plain of Ireland could be explained by the 

 i colliding ' or meeting of the flow-tide currents branching from the 

 main, with these coming through the straits — now valleys — in the 

 surrounding hills, as also the different eskers to the north of the 

 main current. 



It is indeed remarkable that the great eskers of the central 

 valley are opposite some great gap or valley which, on the hypo- 

 thesis of an esker sea, was at one time a strait or channel. For 

 example, the Parsonstown esker is opposite the great gap of Eoscrea 

 between the Slieve Bloom mountains and the mountains of North 

 Tipperary. The East Galway groups are in relation with the open- 



