KiLROE — -Silurian and Metamorphic Eocka. 



135 



The revision of the Castlebar and Ballina district on the lines 

 decided upon by the Director-General was completed by Mr. McHenry 

 in 1893/ at which stage of the survey work it was believed that in 

 the area now under description : 



1st. Archaean rocks existed in Eelniullet Peninsula, at Achill 

 Head, in the Ox Mountains, and south-east of Clifden in Galway, as 

 well as probably at Kylemore, near Leenane. 



2nd. The younger schist series extended away from these tracts 

 of Archaean, and had a visible base at two points at least, viz. ; near 

 Achill Head, and north-west of Castlebar. ^ 



My connexion with the problems involved dates back to 1892, 

 when I was instructed to gather what information I could, for future 

 use in mapping, first, in the region of Belmullet. Going northward 

 to Erris Head, I was somewhat puzzled to find that the coarsely- 

 crystalline gneiss alternated with thin bands of dark-gray mica-schist 

 in such a manner as to suggest the invasion of an originally sedi- 

 mentary series, by massive bands of coarse pegmatitic granites. I was 

 further perplexed to discover roundish -flattened pebbles (or fragments 

 which looked extremely like pebbles) in fine-grained gneissose rocks 

 near Erris Head, which had been taken for mylonized igneous masses, 

 originally like the coarse pegmatites. I then concurred, and do now, 

 in the original reading of these fine-grained gneissose rocks, in which 

 they were described and mapped in 1876 as quartzites ; and the same 

 remark applies to the continuation of this series across Eroad Haven, 

 where it forms the cliffs at Benwee Head, and eastward to Eelderg. 



Coarse pegmatitic gneiss forms the middle portion of the Mullet, 

 west and south of the town ; and there, during my examination of the 

 ground in 1892, I also found the gneiss to contain lenticular masses 

 of black slate. 



It appeared to me then, and there is now no doubt regarding 

 the conclusion, that the supposed Archaean gneisses of this region had 

 been intruded as pegmatitic granites into the Dalradian series ; that 



1 Summaries of Progress, 1892 and 1893. Annual Report of the Geo. Surv. and 

 Museum of Prac. Geology, Appendix E, 1892, p. 267 ; 1893, p. 270. 



- It may be noted that the debatable point whether Archaean rocks occur 

 amongst the Dalradian, or whether the latter should be regarded as Archaean, is 

 not here discussed, or even touched upon. The senses in which the terms are used 

 throughout the Paper are those accepted by the Geological Survey (say) in 1890. 

 The Archaean rocks were then regarded as probably Azoic, possibly the original 

 crust ; the Dalradian as consisting of an entirely newer series of sedimentary origin, 

 probably once fossil-bearing, though now for the most part highly metamorphosed. 



