86 



Proceedings of the Royal Irish Academy. 



aggregate thickness of these rocks may imply a great extension west- 

 ward of the Secondary formations. If we bear in mind the conditions 

 which are believed to have existed in Wales, all this western part of 

 the British area may have borne a garment of Secondary rocks, if not 

 also some of Tertiary date. The gaps in our Irish records, however — 

 the absence of great groups, Oolitic and Lower Cretaceous, for 

 example — suggests the necessity for caution, in conjecturing a general 

 extension and substantial thickness of these rocks over Ireland ; indeed 

 whether the Secondary formations were ever represented in force in 

 the Shannon area may always remain an open question. No trace of 

 them has been reported, even in the glacial drifts of this region. ^ If 

 they did exist, they may have been cleared away while the plain of 

 denudation was being formed, prior to the initiation of the Shannon 

 basin. The depth at which Wales must have lain to admit of the 

 country being almost covered with Cretaceous strata, as Mr. Strahan 

 states, and the proximity of Wales to present Irish land, may imply 

 that Cretaceous strata also covered a large part of our area. We 

 cannot say that any of it was covered with ocean water in the 

 Eocene period ; but the existence of land during that period in the 

 present north-east corner can scarcely be used as an argument to 

 the contrary, for this tract may have been exceptional, and have 

 subsided subsequently to the volcanic activity which, for the time, 

 prevailed in the region. 



The ground, then, upon which the present drainage originated was 

 probably formed for the most part of Palceozoic strata, including con- 

 siderable areas of the Upper Carboniferous strata, partly of Secondary, 

 and possibly in part of Tertiary. The crust-movements later than 

 the Eocene epoch have not affected the directions of any of the Irish 

 rivers, those at least in the middle of the country. The direction of 

 the Erne was probably determined by a jST. N. W. line of weakness, of 

 Charnian direction and date — to use Professor Lapworth's term for 

 the system of dislocations and fissures of Tertiary age. The W. 

 trend of the valley is that of a great Tertiary basalt dyke, some 100 

 yards in width, which I traced in 1883 along the eastern side of Upper 

 Lough Erne. 2 An instance of pre-Carboniferous erosion with the 

 formation of a gap filled, subsequently, with limestone occurs in 



1 A chalk-flint pebble wliieh I picked up from the Shannon alluvial deposits, 

 south of Castleconnell, may testify to the wide distribution of northern drifts 

 rather than to the recent existence of chalk in situ in the Shannon basin. 



- Explanation of Sheet 57 of the Geological Survey Maps, p. 16. 



