T9IO. HiNCH. — Rosapenna Confaencc. — Geology. 169 



itself to the westward b}^ means of a river at its centre. 

 At Lackagh bridge very felspathic types of granite were 

 observed, and on the road to Carrigart the "muUion struc- 

 ture " of the micaceous quartzite was noted in a number of 

 openings ; this '' muUion structure " being the result of a 

 shearing movement across the bedding which produced a 

 peculiar fluting like that of a Gothic column. 



The visit to Glenveigh gave an opportunity of seeing the 

 northern end of the Gweebarra depression, a rift valle}^ which, 

 commencing near Glen, in the north of the county, runs in a 

 practically straight line for thirty miles to Gweebarra Ba5\ 

 This ver}^ remarkable depression has its origin in a series of 

 faults, and in the foundered bottom of the rift lie Glen Though, 

 Lough Veagh, and Lough Barra. The western side of Glen- 

 veigh is bounded b)^ enormous rock-precipices rising almost 

 verticall}^ from the waters of the lake ; the granite of which they 

 are formed being often foliated, and traversed by a large number 

 of basaltic dykes. As the road to Calabber Bridge was being 

 traversed Muckish and Errigal emerged to the westward, and 

 there could be no more striking example of the effect pro- 

 duced by denudation upon different rock-material than that 

 afforded by Errigal and the Derryveagh range. The latter 

 being composed of granite showed the usual rounded outlines, 

 so well known to those who have studied the Leinster massif, 

 but Errigal is formed of quartzite, and takes the shape of an 

 almost perfect cone, and there are few more striking scenes 

 than the beautifully white pyramid of Errigal rising from its 

 sombre foreground of diorite, schists, and limestone. On the 

 journey to Tory Island the massive quartzites of Horn Head 

 were seen rising to a height of 600 feet above the water, and 

 in a number of instances dykes of diorite could be traced 

 intruded between the bedding, and sometimes even .striking 

 across it horizontally. 



Tory Isi^and. 



Tory Island is composed to a great extent of granite, which 

 varies much in texture .; the grey type with large and in man)^ 

 cases almost perfect crystals of orthoclase being most pre- 

 valent on the portions of the island visited. The eastern end of 

 the island — the Doon peninsula — is formed of compact tabular 

 white quartzite carved by the sea into bays, headlands, and sea- 

 stacks (Plate 9), which looked most fantastic seen through the 

 blanket of fog which lay over the island the whole da3^ The 

 solid geology of Tory has been studied exhaustively by the 



