244 NATURAL SCIENCE [October 



the case of the Seven Churches, Co. Wicklow : " All the ruins are 

 on a bank of drift, stretching across the main valley, and formed 

 partly of the detritus from that valley, but chiefly perhaps from the 

 other steeper and narrower valley, which must at one time have 

 emptied its drainage into the old harbour just above this point, 

 and brought down the detritus, of which the tidal currents formed 

 the bar." 



There are, I have no doubt, drift formations in other places, 

 which may have originated after the manner of the ' harbour-bar.' 

 But we must bear in mind the well-marked difference in form of 

 these accumulations and the esker proper. 



" Others, however," continues Jukes, " especially those numerous 

 ones which run in various directions all over the great central plain 

 of Ireland, can only have been formed in the open sea by the action 

 of different currents, as that sea became shallow in consequence of 

 the elevation of its bed." Now, as it appears to me, this statement 

 would go further if we could suppose the bottom of this shallow sea 

 already thickly strewn with the prepared materials — the sand, 

 gravel, clay, and rock fragments. As the waters would become 

 shallow, these deposits would more and more come within reach of 

 the surface currents, and, doubtless, would in many places become 

 heaped up in mounds or ridges of some kind. But I am not sure 

 that this is exactly what Jukes intends to lay down. 



The ' counter-current ' theory has been worked out with boldness 

 and clearness by Mr G. H. Kinahan, in his interesting " Manual of 

 the Geology of Ireland," chapter xiv. In this work there is no un- 

 certainty like that just mentioned. The author includes the eskers 

 among " the moraine drifts that are undoubtedly post-glacial." These 

 drifts lie divides into periods corresponding with alleged pauses in 

 sea depression "at or about the 300 feet contour line," " at or about 

 the 100 feet contour line," etc. 



The margin of the esker sea, he explains, " will be found in 

 places on the hills sometimes as a shelf cut in the sides, at other 

 times as a beach accumulation." But, as Geikie has shown in the 

 case of the Parallel Roads of Glenroy, near Fort William, in 

 Scotland, the notches and old beach lines may have resulted from 

 fresh water dammed up in the mountain valleys by the obstruction 

 of great glaciers in the plains adjoining. At one time Geikie held 

 the same opinion as Kinahan regarding the marine origin of the eskers. 

 ' But," he writes, " having since seen reason for believing that the 

 sea has had no share in the formation of the Scotch gravel ridges, 

 which are in the same category as the Irish eskers, I now look upon 

 the latter as having been heaped up principally by the action of 

 sub-glacial waters during the final melting of the confluent glaciers." 

 This goes a long way towards reducing the esker sea to a myth. 



