136 Proceedings of the Royal Irish Academy. 



the member invaded was chiefly the black and dark-gray slate or schist 

 underlying the quartzite ; and that the pebbly or conglomeratic beds 

 that I noticed near Erris Head represent the boulder -deposit which 

 forms so distinct a zone between dark-gray schist below and 

 quartzite above, throughout Donegal. 



Early in 1893, I undertook the examination of the metamorphic 

 region north of Castlebar ; and here, it seemed to me, upon exami- 

 nation of the supposed base of the Dalradian series, that it was in 

 reality the sole of a tlirust plane, which leaves the rocks on each side 

 practically of the same age ; and that the rounded pieces of rock, 

 taken for water-worn pebbles embedded in younger strata, are but 

 detached lumps rounded by movement beneath the over-thrust mass. 

 I could not distinguish the rocks, described as Archaean (sheared) 

 granulites north of Castlebar, from ordinary mica-schist and sheared 

 grits, and found in these rocks, in proximity to the Lough. Conn granite, 

 the following section, which proves the granite to be distinctively 

 intrusive, rather than an unsheared band of Archaean rock from which 

 the adjoining supposed granulites had been formed by intense 

 shearing : — 



Fig. 1. — Sectiona] View, nearly two miles N. of Castlebar, showing micaceous 

 and felspathic quartzite (U) and epi-diorite (cross-hatched), invaded by 

 granite (G). 



I also found in this series an unquestionable quartzite band, indicative 

 of the existence in the series of undoubted sedimentary strata. 



These observations, therefore, in the Castlebar as in the Eelmullet 

 tracts, convinced myself that Arch[Ean rocks, according to the original 

 acceptation of the term, do not exist there. The coarse, pegmatitic 

 gneisses of the Mullet have invaded the Dalradian series ; and the 

 metamorphosed sedimentary rocks north of Castlebar, invaded by the 

 granites and gneisses of the Ox Mountains, equally belong to the 



