278 Reports and Proceedings — 



Geological Society of London. 



I— 'March 11, 1885.— Prof. T. G. Bonney, D.Sc, LL.D., F.E.S., 

 President, in the Chair. — The following communications were read : 



1. " The Granitic and Schistose Eocks of Donegal and some other 

 Parts of Ireland." By C. Callaway, D.Sc, F.G.S.^ 



The author first recalled attention to the current theories on the 

 nature of the Donegal granitic rock, one which described it as a 

 highly metamorphosed portion of a sedimentary series, another which 

 regarded it as a mass of Laurentian gneiss. In his view, however, 

 it was a true igneous granite, posterior in age to the associated 

 schists. In six districts examined it was intrusive and sent out 

 veins. The aj)parent interstratification with bedded rocks was 

 explained as a series of comparatively regular intrusions. Where the 

 granite was seen in contact with limestone, the latter contained 

 garnets and other accessory minerals. No gradation could be dis- 

 covered between the granite and any other rock, the junctions (even 

 in the case of small fragments of schist immersed in granite) being 

 well marked. 



The granite was distinctly foliated. In some localities there was 

 merely a linear arrangement of the mica ; but near the western 

 margin of the granite promontory there was a striping of light and 

 dark bands, the colour of the latter being due to the abundance of 

 black mica. The gneissic structure was attributed to lateral pres- 

 sure, the existence of which in the associated strata was seen in the 

 conversion of grits into schist-like rocks, in the production of 

 cleavage in beds of coarse materials, in the crushed condition of 

 some masses, in the overthrow of folds, and in the production of 

 planes of thrust. The direction of the pressure was perpendicular 

 to the planes of foliation in the granite. 



The schistose rocks of the region were divided into two groups. 

 The Lough Foyle series consisted of quartzites, quartzose grits with 

 a mineralized matrix, slaty-looking schists, fine-grained satiny 

 schists, black phyllites, and crystalline limestones and dolomites. 

 The semicrystalline condition of most of these rocks was character- 

 istic. This series was well seen at Londonderry and on Lough 

 Foyle, and formed a broad band striking to the south-west. These 

 rocks were compared with similar types in the Hill of Howth (north 

 of Dublin), near Aughrim (co. Wicklow), and South of Wexford. 

 The Leinster semicrystalline masses were quite unlike the Wicklow 

 Cambrians, and bore a strong resemblance to the slaty series of 

 Anglesey. They were lithologically intermediate between the Donegal 

 and Anglesey groups, and from a comparison of all these areas the 

 author referred the Lough-Foyle Series, with some confidence, to the 

 Pebidian system. The prolongation of the Lough-Foyle rocks into 

 the Grampian region was well known, and Ireland thus served to 

 connect some parts of the Scottish highlands with South Britain. 



1 This abstract was uufortuately omitted from the last number. — Edit. Geol. Mag. 

 ^ See paper by the same author, p. 258. 



