246 NATURAL SCIENCE [October 
Southward of this current there would be a bay, somewhat similarly 
circumstanced to Wexford Bay, off 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 aérial denudation great 
massive banks could be attenuated to the slender figure 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- 
jar importance to the ‘ Head of tide’ origin of banks, ete. 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 
‘ 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 Roscrea 
between the Slieve Bloom mountains and the mountains of North 
‘Tipperary. .The East Galway groups are in relation with the open- 
