Charlesworth — Glacial Geology of North- West of Ireland. 227 



with their extvaordioary profusion, and with the amount of degradation of 

 tlie granite hills they represent. 



The great frequency of the Eskdale and Galloway granites in the 

 Lancashire drifts would seem to have similarly impressed T. M. Eeade, who 

 made some very apposite remarks, which may here be quoted :— 



'■ It seems curious that such little patches of granite should have 

 yielded such a harvest of blocks . . . Probably the reason why the 

 granites, syenites, and other igneous rocks occur in larger proportion in 

 the drift than would seem to^ be due to the area they cover in situ, is 

 that they naturally break out in larger blocks, and moreover, they are 

 generally found at a high level."' 



The outcrop of the Bamesmore granite, covering an area of some 

 18 sq. miles, has given rise to boulders which dot an area of over 1,200 sq. 

 miles. The reason suggested by T. M. Eeade is without doubt applicable 

 here. The granite is traversed by a regular system of joints which in certain 

 parts of its outcrop impart to it an almost platy structure. 



It must also be remembered in this connexion that the fan is only the 

 smaller part of the area of dispersal of the granite erratics. Large numbers, 

 though what proportion it is impossible to- say, must have been carried out 

 to sea. The occurrence of the submerged part of the fan makes it also 

 impossible to estimate even roughly the thickness of granite removed from 

 the Barnesmore Hills and the extent of the glacial degradation. 



The distribution of these granite boulders, spreading out fan-shaped from 

 Barnesmore, together with their increase in size and numbers as traced to 

 these hills,- points unmistakably to the latter as their parent source. The 

 question of their origin has, however, been the subject of some controversy, 

 and rocks of various areas, near and remote, have been assigned as their point 

 of departure. 



The earliest of these identifications occurs in '■ Frost and Fire " (1865), 

 where the boulders were regarded as having travelled from Aberdeen. This 

 requires no combating to-day. F.E. Harte, in his eminently sane paper " On the 

 Post-Tertiary Geology of the County of Donegal, etc.,"^ very strongly advocates 

 the local or Donegal origin of the granites, contending against the idea of 



' On the Drift Beds of the North-west of England and North Wales. Quart. Journ. 

 Geol. Soc, vol. xxxix (1893), p. 120. 



■ E.xcept in the strip immediately s\UTOunding the Bamesmore granite margin both 

 to north and south. This was noticed by the Geological Survey, wlio explained 

 (Londonderry Memoir, p. 30) this anomalous behaviour of the erratics, by supposing 

 that the granitic detritus, collected during the maximum phase of the glaciation, was 

 transported as super- or en-glaoial material to a considerable distance. 

 * Journ. Roy. Geol. Soc. Ireland, vol. ii (1871), pp. 30-67. 



