226 Proceedings of the Royal Irish Academy. 



to the south of the hills to St. John's Point, they have been traced across 

 country from sea to sea, a distance in a straight line of 77 miles. On the 

 south of the axis the boulders are strewn over an area exceeding 300 sq. miles, 

 on the north of the ice-shed over a fan exceeding 900 sq. miles ; these figures 

 take no account of the odd boulders which have wandered into the territory 

 of the Donegal granite erratics, nor of those which found their way into the 

 drainage basin of the Fairy Water, or of the Strule aboveA^'ictoria Bridge. 



This great dispersal of the granite boulders on the north of the axis was 

 due to the sweeping out of ever widening arcs as the ice proceeded outward 

 from its centre in the Barnesmore Hills, the glaciers streaming towards the 

 Swilly and Mulroy Bay on the west, and on the east down the valleys of 

 the Finn and Foyle towards Magilligau. This factor was doubtless aided by 

 a process of " nudging," due to variations in the relative pressures behind the 

 component streams. By this lateral swinging of the ice-sheet, a tract of 

 ground vacated by one stream would be usurped by an adjacent, for the time 

 being, more powerful stream, which would transport any boulders already 

 deposited in its path by the earlier occupant. The sporadic boulders 

 occurring at considerable intervals on tlie margin of the great granite boulder 

 country, e.g. those discovered near Victoiia Bridge, were most probably, 

 therefore, not transported directly by ice from the Barnesmore Hills, but 

 by glaciers proceeding from the adjacent non-gi-anitic part of the ice-shed. 

 These straggling boulders, passed out in this way to the margin of the fan, 

 some wandering out in the one direction into the country north of the 

 Sperrin Mountains, and in the other into Mulroy Bay, }]ave effectually 

 blurred the real boundaries of the granite-laden ice streaming from the 

 Barnesmore Hills. 



This seems to be the only possible explanation of the occurrence of a 

 boulder of this granite, just north of Dunteige, S. of Bessy Bell, and of a second 

 granite boulder, near the National School of Ballynatubbrit (ca. four miles 

 S.-E. of Newtown Stewart). Other granite boulders, referable to Barnesmore 

 and encountered in this area outside the fan of dispersal as delineated above, 

 include those found, among other places, at Eosnaniuck Bridge, N. of Omagh, 

 just E. of Deer's Leap, at Mossey's Hill, S. of Gortin (washed over from 

 " Lake OuUion "), at Mountjoy, and at Ballynatubbrit.' 



Traversing the granite boulder-country, especially in the main track of 

 these erratics, one is impressed with tlieir disproportion', both in respect to 

 size and to numbers, as compared with the other ingredients of the drift. 



1 None of these granites is included in the fan of dispersal as represented on the 

 map (%. 1). 



