226 Proceedings of the Royal Irish Academy. 



composite mass even down to the present sea-level, onr knowledge of 

 the banded gneisses of Boylagh might have been limited to those which 

 arose upon the side-walls of the caldron.^ As it is, from specimens a 

 few centimetres square up to moorlands that are not to be traversed in 

 a day, we may see throughout Boylagh what attacks are made from 

 below upon the materials of a rising mountain-chain. The final out- 

 come is a consolidation of the anticlinal ridges, by the intimate pene- 

 tration of igneous material, which crystallizes within their cores ; and 

 ribs are added to the Earth's crust, like those of Donegal, which 

 successfully resist later systems of folding, and still hold their own 

 among the rugged highlands of the world. 



Fig. 1 . Ideal section to illustrate the structure of the granite mass 

 south of the Gweebarra. — A great group of sediments has become 

 folded into a complex anticlinal mass, with production of schistose 

 features in most of the rocks. Granite has intruded during the 

 formation of this compound arch, and especially into the anticlinals, 

 where pressure is relieved ; it has found its way most easily along the 

 planes of bedding or foliation, as the case may be, in the overlying 

 mass. Parts of the latter mass are absorbed ; but flakes remain, pro- 

 ducing a composite rock, and imparting a gneissic structure to the 

 granite. Denudation, acting continuously during these changes, has 

 now worn down the rocks to the surface indicated by the line XZ. 

 Above the point A, the parallel intrusions suggest on this surface 

 that we are on the edge of an uptilted laccolite. At this point it 

 would be very difficult to determine how far the metamorphism of the 

 schists was previous to, and how far due to, the intrusion of the 

 granite. Above B, we have a granite moorland with occasional 

 gneissic structure. As we approach C, the origin of this structure 

 becomes again traceable ; and ultimately composite " leptynolites " 

 and " granitised schists " are seen to pass into the ordinary schists on 

 the right-hand side of the complex anticlinal. 



VI. — Conclusion. 



The references above made to the work of others show that the 

 explanation now put forward for the gneissic structures in Boylagh 

 is one that has raised a certain amount of controversy in the case of 

 other European areas. It is not to be expected that all gneisses 



^ Compare " Metam. Rocks in E. Tyrone and S. Donegal," Trans. Eoy. 

 Irisli Acad., vol. xxxi., pp. 468 and 469. 



