Cole — The Intrusive Gneiss of TirerriU and Drumahair. 363 



Both authors speak of the "bedding" of the gneiss; and the general 

 variations in its structure are well described. Mr. Kilroe^ notes its 

 tendency to pass into quartzite, by the disappearance of felspar and 

 mica; and these quartzose areas are shown upon the map. The same 

 author lays proper stress on the hornblendic inclusions observed by Mr. 

 Hardman, and gives excellent figures of them. He declines, however, 

 to regard the rock as a conglomerate, and makes the important obser- 

 vation that " thin streaks of hornblendic schist and gneiss also occur 

 in the same place which bifurcate, and thus become lost in the con- 

 taining rock." Sir A. Geikie is quoted in a foot-note as considering 

 the basic masses as " geodes — segregations of hornblende rock in the 

 gneiss." At that date this was the common way out of all such diffi- 

 culties, and Professor Sollas^ was probably one of the first British 

 geologists to enter a protest against the assumption of local segregation 

 as opposed to igneous absorption and inclusion. 



From experience gained in southern and central Donegal,^ I was 

 led to conclude that these interesting rounded masses of amphibolite, 

 and the conspicuous banding of the gneiss throughout the ridge, were 

 phenomena of igneous intrusion, i.e., that a granite magma had 

 penetrated an earlier series of rocks along the axis of the Ox Moun- 

 tains. Nothing could be better, from this point of view, than Mr. 

 Kilroe's descriptions and figures of the phenomena near Ballydawley 

 Lough ; and it is noteworthy that this author refrained from drawing 

 any conclusion on his own account. I am fortunately able to add 

 details of similar features from other portions of the baronies of TirerriU 

 and Drumahair, which will, I think, materially assist in a correct 

 appreciation of the ground. 



Taken as a whole, the gneiss of the area may be regarded as a 

 granitoid rock, consisting of quartz and potash-felspar, fairly free 

 from mica, but occasionally containing biotite. Tlie micaceous por- 

 tions are arranged in strings and bands, and sometimes impart a 

 superbly gnarled and striped character to the mass. In the town- 

 lands of Dromore, Crossboy, and Sillery, east of the Correagh or 

 Slishwood valley, white quartz-veins have penetrated the rock along 



1 Memou- to sheet 55, p. 15. 



- " Eelalion of granite to gabbio of Bainavave," Trans. E. I. Acad., vol. 

 XXX. (1894), p. 502. 



* " On metamorphic rocks in Eastern T}'rone and Soutberu Donegal," Trans. 

 R. I. Acad., vol. xxxi. (1900), pp. 453 and 464. 



" On composite gneisses in Boj'lagb," Proc. R. I. Acad., vol. xxiv., section B 

 (1902), p. 203. 



