ON A HILLSIDE IN DONEGAL 349 



Wicklow, above the romantic water-slide in Glenmacnass. He 

 has already noticed the great curving joints of the granite 

 bending down from the highland towards the foothills, which 

 he now approaches, as if parallel with a surface of cooling, 

 parallel, that is, with the margin of the intrusive mass. He 

 now sees, running in much the same general direction through 

 the granite, and parallel with the rock-surface that forms the 

 water-slide, bands of dark micaceous material, which prepare 

 him for encountering a contact with the schists. The granite 

 is locally modified in structure and, clearly, in composition also. 

 Looking farther down the valley, the student sees the walls 

 formed of mica schist, the result of the contact-alteration of 

 Silurian shales by the granite core of the Leinster Chain. The 

 strike of the foliation of these schists, which is undoubtedly that 

 of the original uptilted stratification, is parallel with the dark 

 bands in the granite. Is, then, the student to conclude that the 

 pressure that upraised the Silurian strata produced micaceous 

 sheets in the granite parallel with the junction, and subsequent 

 to the partial or complete consolidation of the mass ; or that 

 the dark bands are relics of the schists, and that the whole 

 phenomena, the metamorphism of the Silurian beds and the 

 modification of the granite into a handsome streaky gneiss, 

 are manifestations of continuous action, and alike due to the 

 conditions of intrusion ? 



J. B. Jukes has written truly of Co. Wicklow, " In the great 

 majority of instances, the folia of the mica-schist, whether 

 straight or puckered, are certainly parallel to the grit bands, 

 and therefore to the original lamination and stratification of the 

 rock." l But the principle of lit par lit injection was not present 

 to his mind ; and the foliation in the granites of Donegal and 

 Connemara 2 only served to connect them, in his view, with 

 the sediments, as representing an extreme of metamorphism. 

 Unpopular as this view has since become, the student finds it 

 replaced by one presenting equal difficulties. As we have said, 

 he is asked to regard the marginal streakiness and darkening 

 of the granite as due to pressure acting on it. 3 The surfaces of 



1 Student's Manual of Geology, third edition, revised by Sir A. Geikie (1872), 

 p. 229. 



2 Ibid. p. 145. 



3 Sollas, " Contributions to a Knowledge of the Granites of Leinster," Set. Trans. 

 R, Dublin Soc, vol. xxix. (1890) pp. 496 and 501. 



