228 Proceedings of the Royal Irish Academy. 



their importation from some north-eastern sonice. His suggeslioii, liowcver, 

 of their derivation from the main Donegal granite is clearly incompatible 

 with their distribution as set out above and represented on the maps (fig. 1 

 and PI. VII). The petrological and strnctural differences between the two 

 rocks — the one, the Donegal granite, almost invariably exhibiting planes of 

 foliation, the other lacking such features — are equally conclusively against 

 this view. " 



It is interesting to note that Portlock, in his " Pieport " (p. 640), concluded 

 that the large granite boulders of County iJerry most probably came from 

 Donegal, 



J. E. Kilroe, in his paper presented to the Belfast Nat. Field Club in 1913, 

 referring to the granite boulders " of striking resemblance to Barnesmore 

 granite" found in the drifts of Xorth Derry, says : — 



"I prefer to think that these erratics have come either from Scotland 

 or from a submerged source to the north of Ireland" (p. 648). 



Yet it is strange that these erratics only should have been so favoured 

 during this Scottish glaciation. Moreover, granites are totally absent from 

 the only indisputably undisturbed Scottish drifts, those of Eglinton and the 

 neighbourhood, riirthermore, these granite boulders, often resting, as we 

 have seen, on the surface, are the product of the last glaciation, from 

 whichever direction the ice of that glaciation may have come, and their 

 occurrence should in consequence be reconcilable with the phenomena of the 

 retreat in the district, i.e. with the morainic accumulations and the marginal 

 drainage. These, however, as will be demonstrated in the sequel, unmistak- 

 ably show open and ice-free country to the north and north-east, and ice 

 pressure from the south and south-west — a state of affairs patently irreconcil- 

 able with a simultaneous transport of erratics from the north-east. 



Elsewhere, J. It. Kilroe, referring to the Scottish glaciation, remarks'^ : — 



" Confirmatory evidence for the westward movement is to be found 

 iu the absence of granitic blocks from the lower boulder-clay of Glen 

 Swilly, and from the boulder-clay which rests on the granite at the 

 north entrance of Barnesmore Gap." 



It may be observed that granite boulders were encountered during the 

 progress of this present investigation in the latter area, so tliat this objection 



' Q. J. G. S. (1888), p. 831. In the Geol. Sury. Mem. of tliis district (Sheets 3, 4, 

 etc., p. 107) the same error has arisen. It is there stated : "Along tlio headwaters of tlie 

 Rivers Finn and Swilly" the boulder-clay contains "ice-scored boulders, blocks, and 

 fragments of rock, which have unquestionably been derived from the country to the cast 

 and north-east." I have been quite unable to discover any evidence in justification of 

 this statement. 



