Charleswoeth — Olacial Geology of North- West of Ireland. 209 



the complete oveiriding of tlie granite lulls by extraneous ice bearing these 

 erratics. This is donbtless the explanation of those occurrences at iJninifin 

 and Stragaddy on the borders of the granite, and also equally of the schist 

 boulders in the Tnshagh valley, more especially those in its eastern part. The 

 saine would also appear to be true of an oeca.sioual fragment of schist and 

 diorite at.Oweuearrow Bridge, though the greater part of these, as all those 

 found in the sections of G-lenveagh Bridge, were indubitably transported from 

 the band of metamorphic rocks which runs across, the head of Glenveagh in 

 the very heart of the granite country. This source is confirmed by their 

 association in the drift with erratics of basalt which have been derived froni 

 the outcrops of the dykes which cross the glen towards its head. 



. 5. The, Errigal-Mucliish Ridge. 

 Ice proceeding from the granite hills of Derryveagh to the south, passed 

 in a northerly direction over the whole length of the quartzite ridge of 

 Errigal and Muckish, with the vai'ying amounts of west in its motion 

 gradually increasing towards the south-west extremity of the range, though 

 even in the north, i.e. towards the Muckish end, there appears to have, been 

 still a westerly component in the direction of ice-flow. The lower layers of 

 this ice were constrained by the flanks of the hills, so that the ice-cm-rents 

 on the eastern slopes of Muckish were compelled to pass practically 

 south-north, or even 10 or 20° E. of N. Where the constraining influence 

 was no longer felt the ice immediately swerved round into the north-west, as 

 seen, for example, on the finely striated surfaces on the col just north of 

 Muckish, at an altitude of ca. 800 feet, O.D. Still further north, however, 

 the ice-flow conformed with the prevalent movement over this northerly 

 region, so that on Crockatee, to the north-east of Muckish, striae were 

 observed going S.-N., and 20° W. of N. 



This northerly ice-flow, oblique to the great depression extending from 

 Dunlewy to Creeslough, is confirmed by the composition of the drift, which is 

 singularly free from qnartzitic material, the boirlders being chiefly granite 

 from the south, together with the schist, quartz, and metamorphic limestone 

 of the district. The lines of flow from the granite hills to the lowlands 

 around Falcarragh were guided by the deep portals in the quartzite ridge, e.g. 

 the valleys containing Dunlewy and Alton Lough. Even the higher notches, 

 such as those containing the rock basins of L. Aluirg, L. Feenaue, and 

 L. Nabrachbraddy, exerted some influence. 



Granite boiilders occur on the slopes of Errigal. They were most 

 probably derived, as the trend-lines show, from the Poisoned Glen or the hills 

 imnrediately to the north and east of this depression. As the summits of 



