182 Scientific Proceedings, Royal Dublin Society. 



country margining Mount Leinster, Jukes suggests that they "are mere tongues of 

 mica-schist and gneiss, let into the granite while that was yet fluid, portions of that 

 molten matter being, at the same time, squeezed and injected between the beds of the 

 aqueous rocks for some distance, all this taking place of course while the two rocks Mere 

 buried deeply under many thousand feet of superincumbent rock." (g.s.m.) The grain 

 so characteristic of intrusive granite is probably introduced by thrusting. 



[Note in Press. — Since this was in type, a report on the N.W. 

 Highlands of Scotland by the officers of the Geological Survey of 

 Scotland has been published in the Quarterly Journal of the Geol. 

 Soc. Lond., vol. xliv., No. 175, pp. 378, &c. In this Paper the 

 effect of thrust in producing foliation is very exhaustively ex- 

 plained, and such thrust would explain the "grain" in the Leinster 

 granite. As, however, the different systems of thrusts that have 

 from time to time affected the rocks of this territory have still to 

 be worked out, we cannot here refer further to the subject.] 



The Leinster range granite, as pointed out by Jukes, seems always to be margined by 

 rocks of Ordovieian age, and never by the older Cambrians. The Ordovicians to the 

 eastward of the great granite intrude were altered by metapepsis, or regional metamor- 

 pbism ; those to the westward of the range by paroptesis, or contact metamorphism. 

 Towards the western margin, especially, but also in smaller tracts elsewhere, there is 

 growan. Portions of this growan are evidently decomposed Leinster Type granite; but 

 some of it, especially in the Co. Dublin, seems to be composed of " Aughrim Type 

 granite." This latter granite is evidently much newer than the former, and it is sup- 

 posed to be of either Devonian or Carboniferous age, perhaps even later. Generally 

 the Leinster Type granite is more or less even-grained, but some of it is porphyritic. 



Associated with this granite are intrudes of Quartz-Rock, as at the White Rock, 

 northward of Tinabely. 



The Elvans and the Felstones, the adjuncts of the Leinster Type granite, cannot 

 be satisfactorily traced, being so much obscured by drift ; tbey occur in the associated 

 schistose rocks. 



Some of the whinstones in the schists are also probably adjuncts of the Leinster 

 Type granite ; this, bowever, can only be suggested, as it has not as yet been proved. 



Aughowle Type Granite, or Pegmatyte. — This forms a considerable tract in 

 the parish of Aughowle, at the junction of Carlow and Wicklow. The rock is a peg- 

 matyte, made up of large crystalline pieces of quartz and felspar (often microcline), with 

 flakes of mica. The rock is not recognized in the country as a granite, but is called 

 " some sort of a bastard rock." In aspect it is somewhat similar to the pegmatyte of 

 Ontario and Pennsylvania, in which are situated the " mica mines ; " but, unfortunately 

 for the home country, the veins of mica in the Aughowle pegmatj-te are of very small 

 dimensions. In the mass of the rock the mica is usually on the faces of the felspar. 

 The Plumose granite of Jukes is a variety of this pegmatyte, in which the mica is 

 arranged like plumes of feathers. This pegmatyte is similar to the endogenous granite 

 of Sterry Hunt, which he considers is due to the deposition of minerals from solution 

 in open veins or spaces. If this was the origin of the Aughowle pegmatyte, there must 

 have been in the granite mass a large cistern full of water, highly charged with mineral 



