18 Scientific Proceedings, Royal Dublin Society. 



from the others by the narrow tract of the Later Period Mocks that 

 extends from Mulroy Bay, south-west, to beyond Glen Swilly. To 

 the north-west they are distinctly covered unconformably by the 

 Later Period EocJcs ; but to the eastward the latter appear to be 

 upthrust on to them. To the southward, in the barony of Eaphoe, 

 the junction between the two is obscured by deep drift and bog ; 

 but in some places the newer formation appears to be upthrust on 

 to the older, while in others there is probably an ordinary un- 

 conformability. 



The gneiss and foliated granite series, with groups a and b of 

 the Grartan schist series, are cut off to the north-eastward (south 

 of Glen) by the Later Period Pocks, while the rest of the group 

 [e, d, e, and/) are cut off both to the north-east and south-west; 

 the groups d to /occurring in a tract only a few miles long. 



Series A consists principally of gneiss and foliated granite; 

 but with them there are subordinate limestones and schists, with, in 

 places, a variety of gneiss so quartzose that it may be called 

 gneissose, or micaceous quartzite. The last is of considerable 

 importance, as the Geological Surveyors have not been able to dis- 

 tinguish it from the later quartzites, and have mapped both as if 

 portions of the one group, this, as already mentioned, being a 

 similar mistake to that formerly made by the American geologists 

 in the neighbourhood of Lake Superior. Attention should also be 

 called to the pink, felsitio veins of segregation, as fragments of 

 them are often conspicuous in the Boulder-beds of the Later Period 

 Pocks. These fragments, however, may be derived from the thin, 

 irregular veins of a similar felsitic granite in the Gregory Hill 

 series. 



To the south-west, in the baronies of Boylagh and Bannagh, 

 the rocks of these different series appear to be severally represented, 

 they again coming out from under the Later Period Pocks. But 

 in Fan ad, between Mulroy Bay and Lough Swilly,^ but especially 



consist principally of slates, some veins teing roofing slates, and finely parallel obliquely 

 laminated sandstones. Many of the latter have the lamination so regular that the rock 

 has the appearance as of a gigantic book ^yhile another variety is full of pebbles 

 [MuUaghsawnites, Egan), generally the size of shot and peas, but sometimes larger. 

 The enclosures are generally quartz, but in some places they consist largely of pink, 

 felspathic fragments, the enclosures evidently having been derived from the veins in 

 the older granitic rocks and schists. 



