CuAKLESWORTH — Glacial Geology of North-West of Ireland. 225 



noted at the head of the overllow chauiiel falling to Douglas Bridge, while 

 a second was seen to the north-east of Lisnafin Glebe (ca. three-quarters 

 of a mile N.-E. of tlie first). A few others were encountered in the great 

 amphitheatre N.-E. of ISTewtown Stewart, on the hillsides east of Sion Mills, and 

 . on the flanks of Meenasliesk. They become increasingly plentiful as we proceed 

 northward to Strabane, in which direction we are clearly passing towards tlie 

 main track. In the fine section in the stream flowing north from Mullaghclogha 

 (2,088), about one mile south of Craig and on the nofth side of the Sperrin 

 watershed, one gianite boulder was discovered at an altitude of roughly 800 

 feet, O.D. Another was seen south of Fairy Lodge,^ and one near Crabarkey." 

 Occasional boulders were observed in the Eoe valley below Unngiven, and an 

 odd erratic was found near Largyreagh and Bolea, both near the foot of Keady 

 Mountain, while sporadic boulders were observed at intervals along the Eoyle 

 estuary and the foot of the basalt escarpment to ]\'Iagilligan, e.g. near the 

 "Murder Hole" at an altitude of ca. 800 feet, O.D. 



The southern limit therefore crosses the valley of the Mourne River 

 obliquely and near its head, runs roughly coincident with the watershed 

 between the Derg and Finn, cuts across the Foyle in the neighbourhood of 

 Victoria Bridge, and skirting the northern slopes of the Koram and Sperrin 

 Mountains, passes just south of Eeeny and Dungiven and along the edge of 

 the basalt escarpment to the mouth of Lough Toyle. 



It is interesting to note that Portlock, remarking on the occurrence and 

 distribution of the granite boulders, wrote :— ■' 



" In the more southern portions of Derry, they become very rare, or 

 are reduced to fragments of small size in the gravel." 



Within the limits briefly sketched above and represented on the 

 accompanying maps (fig. 1 aird PI. IX) lies a fan-shaped area, over which the 

 boulders are scattered. They increase very perceptibly iir size'' and number 

 from the margins into the interior, and from Inishowen Head towards the 

 source of the granite in the Barnesmore Hills. 



The grairites at Inishowen Head have been transported ca. 52 miles from 

 the nearest possible point of the granite outcrop. Since they were tracked 



' Ca. two miles E.S.E. of Feeny. Portlock uientioiis "occasional granite" in the 

 pai'ish of Bauagher. (Report, p. 747). 



- Ca. two miles S. of Duugiven. This was the farthest south in the Roe valley. 



■' Report, p. 631. 



'• The largest erratic observed was the boulder named " Greystone," ca. half a mile N. 

 of Drumdoit, and two miles S. W. of Clady ; it is roughly 11 feet long, its other dimensions 

 being in proportion. Another large boulder, measuring 12 x 9 x 8 feet, was observed at 

 Cooladerry, just S. of Raphoe. 



