Chakleswokth — Glacial Geology of North- West of Ireland. 229 



can no longer be sustainei.1, wliile Uieir aliseiice from Glen Swilly, it is 

 submittsd, is readily explained on the view of the mode of glaeiatioii, as set 

 forth in this eommiiiiication. 



An examination of the map (fig\ 1), indicating tlie limits of the boulder 

 dispersal from the Barnesniore and Donegal granite outcrops, shows that 

 there is, south of Milford, no overlap of the Barnesmore and Donegal granite 

 boulders, but that, on the contrary, there occurs a wedge-shaped strip of 

 country, tapering northwards to Milford, which is completely destitute of 

 granite erratics from either source, save perhaps for a rare and undetected 

 straggler. Their absence, therefore, is not suggestive of Scottish glaciation, 

 but must bo ascribed to the passage of the granite-laden streams to the east 

 and west of this strip and of Glen Swilly to which J. 11. Kilroe more particu- 

 larly referred, furnishing at the same time additional confirmation of the 

 Donegal centre of radiation. 



The Barnesmore origin of the erratics is supported by the lithology and 

 petrology of the boulders and of the Barnesmore granite in siiu. These 

 rocks are alike, distinctly red in coloui-, though frequently pink, and exhibit 

 a fairly medium texture; foliation, as already pointed out, is absent. 



Professor H. J. Seymour, from an examination of rock sections of the 

 boulders obtained daring the course of the survey of the Londonderry district, 

 and a comparison of these with sections of the Barnesmore granite, concluded 

 that the rocks were petrologically identical.' 'J'his external resemblance aud 

 petrological identity, when supported by the distribution of the boulders, by 

 the evidence furnished by the striae, the general glaciation, and by the conclu- 

 sions drawn from the mode of retreat as proved by marginal phenomena, 

 demonstrate conclusively the Barnesmore source of these countless granite 

 boulders.- 



Located in the country lying between the Swilly and Foyle and to the 

 north of the Finn valley, are a number of dykes and sills of lamprophyre 

 and " diorite,"^ and other rock types. Their boulder trains furnish additional 

 evidence of the lines of ice-flow. The conclusions based upon them confii'm 

 tliose drawn from the striae, aud from the distribution of the granites of 



' Loudondeiry Memoir, p. 30. 



= The above description of the disti-ibution of tlie erratics from the Donegal aud 

 Barnesmore granite outcrops respective!}', clearly shows that the boulders of granite 

 occurring in the country to the east of this, and referred hj the indefatigable workers 

 of the Belfast Nat. Field Club, with various degrees of probability, to these sources, uuist 

 have been derived, apart from the granites of Tyrone, from parent masses outside the 

 region under review. 



^ Petrographical descriptions of these are giveu in the Londonderry Memoir (p. 13), 

 and in the Geol. Survey Blemoir, Sheet 17 (pp- 33-38). 



