19 1 o. HiNCH. — Rosapoina Conference, — Geology. 167 



The geological axis of the North-western Highlands passes 

 from north-east to southwest through the great rift valley of 

 Glenveigh and Gweebarra. The granite in which the valle}^ 

 has been formed rises on the north-west side into the 

 Derryveagh Mountains and on the south-east into the Glen- 

 dowan hills. To the north west of the Derryveagh range rises 

 the Errigal and Muckish range, composed of metamorphic 

 rocks, while to the south-east of the Glendowan hills rise 

 the Lough Salt and Scraigs range, composed of the same 

 metamorphic types. 



This three-fold parallel series of mountains — a central 

 granite ridge flanked by a metamorphic series on the north- 

 west and south-east — had its origin at the close of the Silurian 

 Period as a result of the *' Caledonian " earth-movements, a 

 system of intense plications which, acting over the British 

 area from Connacht to the Highlands of Scotland, produced a 

 general system of folding, the longitudinal axis running in 

 general from north-east to south-west. Prolonged erosion has 

 no doubt modified to a considerable extent the original 

 contours of the land ; later earth-movements, and even the ice 

 sheets of the Glacial Period, played their parts, but the 

 general outline that was then impressed on the Donegal 

 Highlands remains the dominant tectonic feature of the 

 district. 



This general formation of the countr}' can be well seen on 

 the journey from Letterkenny to Creeslough. First the 

 traveller meets the rugged hills of the Lough Salt district, 

 then succeed the sweeping granite outlines of Glendowen 

 and Derryveagh, while away to the w^est appear first Muckish 

 and then Errigal, and as the train crosses the viaduct and 

 over the Owencarrow River the rift valley of Glenveigh opens 

 out gloriously to the south west. 



When the broad tectonic outlines of the district have been 

 pointed out, the extraordinary length of the lakes of Donegal 

 as compared with their width receives an explanation. There 

 can be no doubt that glacial erosion and glacial deposition, 

 .and even the solution of the limestone, have given rise to 

 certain features in the lakes ; but the '"' Caledonian " earth- 

 movements produced results which no later occurrences have 

 been able to obscure. 



