THE GEANITIC AIS'D SCHISTOSE KOCKS OF ^^ORTHEEN DONEGAL. 221 



24. On the Geais^ttic and Schistose Eocks of Noetheen Donegal. 

 By C. Callaway, Esq., D.Sc, F.G.S. (Read March 11, 1885.) 



Contents. 



I. The Granite. 



1. State of previous Opinion. 



2. The Grranite Promontory. 



3. Kelations between the Granite and the adjacent Schists, 



Section in Barnesbeg Gap. 



Section on Lough Greenan. 



Section south of Creeslough. 



Section north of Dunlewy Church. 



Ground round Bunbeg. 



Granite of Dunaff Head and Malin Head. 



4. The Fohation in the Granite. 



Arguments for the Metamorphic Origin of the Granite. 

 Objections to the Metamorphic Hypothesis. 

 Possible Explanations of the Gneissic Structure. 



II. The Schistose Eocks. 



Section from Londonderry to Dunaff Head. 



Drumahoe Quarry, Londonderry, Cashel Hill, Shore north of 

 Fahan, Buncrana Bay to BalljTiarry Bay, Ballynarry Bay to 

 Dunaff Head. 

 Section from Malin to Malin Head. 

 Section between Croagh and Horn Head. 

 Section from Dunlewy to Bunbeg. 



III. Proof of Lateral Thrust. 



In my paper * on " The Age of the Newer Gneissic Eocks of the 

 ISTorthern Highlands," I suggested that a stndy of the old rocks of 

 Ireland would probably aid in connecting the Scottish types with 

 those of South Britain ; and, in the summer of 1884, I visited the 

 north of Ireland to ascertain how far my suspicion was justified by 

 the facts. As Professor Hull had announced the discovery of 

 Laurentian gneiss in the same area, I included the relations between 

 this group and the schists in my investigation. A continuance of 

 stormy weather prevented the completion of my plans ; but I ob- 

 tained satisfactory proof of the nature and relations of the chief 

 granitoid mass, and collected partial material for the study of the 

 schistose rocks. As the result of a month's work, I came to the 

 conclusion that the schistose rocks of jS^orthern Donegal were sepa- 

 rable into two groups, and that the so-called " Laurentian " was 

 merely a mass of granite intrusive in the older of them. 



The President (Professor Bonney) , with his usual kindness, has 

 examined fifty-five microscopic sections of Irish rocks, and he 

 permits me to use at my discretion the rough notes which he has 

 furnished. It is to be regretted that pressure of other work has 

 prevented him from undertaking a more elaborate investigation. 



* Quart. Journ. Geol. Soc. Aug. 1883, p. 355. 

 Q.J.G.S. No. 162. B 



