Charlesworth — Glacial Gcolojjj of North-West of Ireland. 185 



many are situated just where the ice passed off the land on to the sea. As 

 examples may be cited the granites outcropping at the extremities of the 

 northern peninsulas, e.g. the deep red hornblende granite of Eosquill, a rock 

 recognized in the field with the greatest facility, and the granites of Fanad, 

 Dunaff Head, Malin, and Ardmalin.i Equally useless from this point of 

 view are the hornblendic gneissic rocks of the islands of Torbeg, Tormore, and 

 Inislitrahull, which differ in appearance and character from the rocks of the 

 mainland. The chocolate-coloured sandstones and conglomerates of O.ll.S. 

 age, which lie at the northern foot of Knockalla in Fanad, are also virtually 

 valueless for this purpose. 



From these considerations it follows that rocks whose erratics are easily 

 recognisable and traceable to their source are relatively few ; notable 

 exceptions are tlie granites of Donegal, Barnesmore, and Tyrone, whose fans 

 of dispersal will be fully described in the sequel. 



If, for the reasons just briefly outlined, erratics do not, with certain notable 

 exceptions, very often afford much indication of the direction of ice-flow, the 

 rocks fortunately are generally of a nature to retain ice-stria tions. In many 

 districts, more especially in the central area of erosion, striae are remarkably 

 common ; of those observed only a small minority are inserted on tlie map 

 (PI. IX). 



The various kinds of rock have behaved differently in this respect. Coarse 

 diorite, though frequently wioitto^me'c, is seldom grooved or striated; especially 

 is this true of the coarser varieties, which have weathered badly, and of those 

 which are highly jointed. Schist, which is soft and easily decomposed, is not 

 frequently striated, for, though to a certain extent receptive of such scratches, 

 these are readily removed by its rapid disintegration. Thus in the Sperrin 

 Mountains, where bare schist covers a not inconsiderable area, not a single 

 striated surface was observed, either by myself or by the officers of the 

 Geological Survey. Yet where striated schist is encountered, the scratches 

 and grooves are extremely distinct. Granite is usually too coarsely crystal- 

 line and too readily decomposed to admit of striae preservation. Hence in 

 the Eosses, striations are relatively rare, though bare granite covers virtually 

 the whole region.^ 



Quartzites have commonly preserved the finest ice-polish and the most 

 delicate ice-markings. 



' The granites of Ardraaliu and Malin are red and coarser than the grey Dunatt' Head 

 granite, and, unlike the latter, show clear evidence of metamorphism. The Fanad 

 granite is also red, and in places foliated. 



■On the map (PI. IX) all the striae observed on the granite outcrops are inserted. 



