180 Proceedings of the Royal Irish Academy > 



surface above that covering the lim of hills and to cause the reversion of the 

 ice-flow. 



III. — Geological Steuctdee axd Phtsical Featdees. 



The map (PL IX)_ attached to this paper shows the leading physical 

 features of the r^on. The reader is refeiTed to the Greologieal Map of the 

 British Isles (scale 25 miles = 1 inch) for the general geological structure. 



The Donegal granite' fomis a twin-range of mountains trending N.E.-S.W., 

 and sepai-ated by the deep and narrow " Gweeban-a Eift." To the south lie 

 the Glendowan Mountains, to the north, the DeiTyA-eagh HiHs. falling steeply 

 to the gi-anite plain of the Bosses. These gi^nite ranges are flanked by 

 parallel ridges, composed chiefly of quartzite, the northern, the Muckish 

 -Errigal Eidge, falling almost pi'ecipitously to the schist country of the north- 

 west, the southern, extending from near the town of Glenties, by Aghla 

 Mountain, Sci-aigs, Knock Salt (1646),* and continuing as the Knoekalla ridge 

 of Fanad and the Eaghtin More r-ange of Inishowen. A still more southerly 

 ridge is found in Inishowen, culminating in Slieve Snaght. 



The greater part of the remainder of Donegal is composed of schist. This 

 rock generally underlies the lai-ger depressions and valleys of the country, 

 though to the east of Donegal Bay it fonns part of the main watei-shed of 

 north-wKt Ireland, and further east the Sperrin Mountains and the hills 

 extending south-west from Omagh. Associated with the schist are occasional 

 bands of limestone, intruded sUls of diorite and epidiorite, and, in certain 

 areas, light-coloured acid dykes.' 



^ This gTanite is red or white in colour and distinctly foliated. Detailed acconnts of 

 its petrology are given in an appendix to the Geological Survey Memoir, descriptive of 

 sheets 3, 4, 5, 9, 10, 11, 15, 16. 



- The figures in brackets throughout this paper indicate the altitude of the summits 

 of hills above sea-leveL 



For the convenience of the reader unacquainted with the detailed geography of the 

 country, the use of little-knowu place-names has been avoided, as far as consistency with 

 the value of the paper for future workers permitted. The most important of the names 

 employed are inserted on the accompanying maps and sketches ; and throughout the 

 paper, names to be found on the 6-inch maps, but not engraved on the 1-inch maps of 

 the Ordnance Survey, have not been used. 



^ Aa the terms " felstone " and " felstone p>orphyry " have been used for these dykes 

 in the Memoirs of the Geological Surrey of this region, it is here deemed advisable to 

 retain this nomenclature in order to avoid confusion, though under these terms are 

 included rocks which, though of similar external appearance, are of very different 

 petrological composition. They possess ustially porphyritic crystals of grey and pink 

 felspar, biotite and black prisms of hornblende and less commonly blebs of quartz, 

 aU embedded in a crypto- or microcrystalline groundmass. Though occurring singly in 

 other areas, they are more particularly characteristic of the Dnnfanaghy, Horn Head, 

 Rosguill, I n ishfree Bay, and Gweedore regions. 



