182 Proceedings of the Royal Irish Academy. 



consist of thin-bedded, green or dark-coloured slates, flaggy sandstones, and 

 coarse conglomerates. 



Dotted over this area of Tyrone occur a few small patches of granite, which 

 have been described by Professor Grenville A. J. Cole, f.e.s.' 



Such in brief is the geographical distribution of the various types of rock in 

 the north-west of Ireland; the coimtless complications, not necessary for our 

 immediate pui-pose, have for the sake of clarity been omitted, though it will 

 be readily understood that these could not be ignored in the actual field 

 research. Thus the frequent occurrence of schist bands in the quartzite series 

 of Donegal and the perhaps still more frequent association of quartzites with 

 the true schist belts have exercised considerable influence upon the distribution 

 of the erratics of these two rock-types. 



rv. — Signs of Glaciation axd Gexeeai Chaeactek of the 

 Drift Deposits. 



Evidence of the severe glaciation of the north-west of Ireland is every- 

 where apparent ; the beautifully rounded slopes, the dome-like hUls, the 

 moulded ridges and outcrops, and the abounding roclies riioutmitiees alike 

 attest this. The almost entire absence of aretes, and the occurrence of 

 erratics and striated surfaces on the summits of the highest hills, suggest that 

 the whole country was completely buried beneath a sheet of ice, and that at 

 the period of maximum glaciation not even the highest peaks projected as 

 nunataks above the ice-surface. 



The record of the march of the glaciers is chiefly presented, as in other 

 areas, by striae, roches inoutonn^es, and enatics. The correlative phenomena 

 of sub-glacial erosion and deposition agree in theii- indications of directions of 

 ice-flow. 



As in other centres of radiation located in mountainous regions, excess of 

 erosion charactemes the central hiUy area, excess of deposition the peripheral 

 belt — e.g., the low-lying coimtry of schist west of the Eiver Foyle, the low- 

 lands of County Tyrone and of Gweedore, and the low strip of carboniferous 

 country which sweeps round Donegal Bay on its northern and eastern shores. 

 • Drift is also thickly laid down in the hays of the western coast, where it 

 probably furnished the material for the building of the magnificent sand-dunes 

 which in so many cases fringe this coast and swing across its larger indenta- 

 tions. Much was doubtless carried beyond the present coasts, so that the 



' The Greology of Slieve Gallion, in the County of Londonderry, Trans. Roy. 

 Dublin Soc, vol. vi (Ser. ii) (1897), p. 213 ; On ^Metamorphic Rocks in Eastern Tyrone 

 and Southern Donegal, Trans. Roy. Irish Acad., vol. xxxi (1900), p. 431 ; see also Mem. 

 Geol. Surv., Sheet 34, pp. 14:-17. 



