Cole — The Intrusive Gneiss of Tirerrill and Drumahair. 369 



whicli consist of amphibolite, rather poor in garnet. l!^eitlicr in the 

 Survey collection nor my own have I found an eclogite from this 

 area ; yet the handsome occurrences of eclogite in Stonepark make it 

 probable that some such rock has furnished the green pyroxene to the 

 gneiss of Glennagoolagh. The marked banding of the gneiss is again 

 clearly associated with an abundance of inclusions ; and we have now 

 sufficient evidence from various parts of Tiremll and Drumahair to 

 show that this is a nonnal characteristic. 



The gneiss, then, of the ridge on the south side of Lough Gill 

 repeats the features of the granite floor of Donegal, and was probably 

 formed during the same epoch of intrusion. It has certainly absorbed 

 a Dalradian series on its margins ; and one is tempted to regard it as 

 of the same age as the Caledonian earth-movements. Yet we must 

 remember that the Gotlandian (Upper Silurian) conglomerates of 

 Lough I^afooey, in County Galway, contain pebbles of granite, 

 associated with quartzite, and prove that an earlier intrusion of 

 granite had taken place in these western highlands. It is always 

 possible that the composite rocks formed in Ai'chsean times may have 

 been brought to the siu'face at a far later epoch, and that they 

 then underwent a certain amount of mechanical deformation, "While 

 I do not think that such deformation is a prominent feature in 

 Tirerrill and Drumahair, it has been sufficient in other cases to lead to 

 a misapprehension as to the origin of the banding and flow-structure 

 throughout the gneissic mass.^ 



In conclusion, now that the composite origin of banded gneiss is 

 becoming a matter of general acceptance by geologists, it is well to 

 refer back to the views of M. Michel Levy, summarised by him in 

 1887.- Sixteen years ago M. Levy emphasised the similarity between, 

 more recent ribboned gneisses, formed by parallel intrusions of granite 

 into metamorphosed sediments, and the ancient yet complex masses, 

 which were commonly regarded as the primitive crust. Sederholm 

 in Finland, working on the earlier masses, and Duparc and Mrazee, 

 dealing with far more modern intrusions on Mont Blanc, may be 

 cited among those who have verified the master's generalisations. 

 Similar views have even found their way into the text-books; 

 and now that Mr. A. Harker^ has given us a convincing study of a 



' Compare op. cit., Proc. R.I. Acad., vol. xxiv, sect. B., pp. 220 and 221. 

 "■^^ "Sur Torigine des terrains cristallins primitifs," Bull. Soc. geol. de France, 

 3me. ser., t. xvi. (1887-8), pp. 102-113. 



•' " The Overthrust Ton-idonian Rocks of the Isle of Rum, and the Associated 

 Gneisses," Quart. Journ. Geol. Soc. London, vol. lis. (1903), pp. 207-215. 



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