Charleswoktii — Glacial Geology of North -West of Ireland. 233 



prominence that one is forced to seek a cause for their presence. Such 

 au association can scarcely have originated in the south, a source of this 

 character being wanting there ; tliere is, however, a distinct multiplica- 

 tion of granite erratics, noticeahly as one proceeds southwaid hy Garvagh 

 and Kilrea, pointing to Slieve Gallion as the source. ... Of an extensive 

 movement from Donegal and Londonderry over tlie basalt plateau and 

 othei- intervening high grounds there seems no evidence whatever, and 

 no reasonable grounds for conjecture. We are, therefore, shut up to 

 tlie nortli or north-east, where the sea conceals the prolongation of the 

 Archaean Gneiss area." 



Like the erratics of granite, whose southern source Mr. Kilroe recognised, 

 these basic and metamorphic boulders are clearly leferable to the Slieve 

 Gallion and Co. 'J'yroue area, for petrologically these rocks are identical, 

 while, as Dr. A. E. Dwerryhonse has shown, the Bann was glaciated by ice 

 proceeiling northward. 



The sliell-be'aring deposits of the Eoe valley, on the one hand, and tlie 

 granites, schists, and epidiorites of the Bann, on the other, together con- 

 . stituting J. E. Kilroe's evidence of a southward-moving ice-mass, may be, 

 therefore, more satisfactorily regarded as products of the earlier and later 

 ice-sheets respectively, tlieir occurrence fully harmonising with and con- 

 firming ttie movements of the ice-sheets over this region as set out in 

 Dr. A. E. Dwerryhouse's paper and in this communication. 



At an earlier page it was shown that Lough Derg lay on the line of the 

 great ice-shed of Donegal. It is difficult in the field to trace the continuation 

 of this ice-shed to the south of this lake ; the hills are beautifullj^ rounded, 

 but the rocks have seldom retained any ice-scratches or other indubitable 

 evidence of the direction of ice-flow. 



In the earlier stages of the giaciation, the ice-shed doubtless ceased 

 somewhere in the neighbourhood of Lough Derg. Gradually a great mass 

 of ice would appear to have accumulated over the site of Lough Erne. The 

 great bastion of carboniferous rocks rising from its southern shores would 

 seem to have played a very important part in barring the progress of the ice 

 in this direction, causing it to pile up and finally to split on the northern 

 face, swerving to the west into Donegal Bay and eastwards over Enniskillen 

 into the Central Plain. In this way there was gradually formed an axis of 

 dispersion over Lough Erne, linking up the main ice-shed of Donegal with 

 that centred in the Sligo Hills. This ice prol)ably imparted a more easterly 

 motion to that moving over Tyrone, while the northerly movement of the ice 

 down the Bann ai;d over the Sperrin Mountains may be also in part ascribed 

 to its influence. 



