24 Proceedings of the Royal Irish AcaJemy. 



Geology. — The lake lies in a volcanic area on basalt of Miocene age ; and 

 this basalt rises from the lake on three sides until it reaches altitudes of about 

 1,500 feet. On the south of the lake, and fonning a very low marshy ground 

 here, is a thick deposit of clays and sands, with lignite and clay ironstone. 

 The banks are formed of drift, etc., which covers the basalt and also the clay 

 above mentioned. This great southern clay deposit is of considerable thick- 

 ness, lying, according to the British Association Report of 1874, under 50 feet 

 of drift, and consisting of more than 260 feet of stratified grey and blue clays 

 and sand. This may be taken as the former delta of either the Upper Bann 

 or the river Blackwater. The opinion of Edward Hardman, of the Geological 

 Survey, is that the lake was formed at the close of the Pliocene period before 

 glacial times, and that after the basaltic flow had ceased, subsidences took 

 place over a large area, producing a depression. In all probability the area of 

 the lake was formerly very considerably greater than it is now. 



The drainage channels into the lake and the outflowing livers must have 

 varied during Glacial times ; but the question of this must be left to the 

 geologist. We should like to emphasize at this point the necessity for the 

 geologists to say whether Lough Xeagh has ever been an arm of the sea or 

 been occupied by salt water. As will be seen later on, there are features, such 

 as the presence of Mysis relicta in the waters of the lake, which have given 

 rise to the theory that Lough iS!"eagh was once an arm of the sea, and that it 

 was occupied by salt water. This question will be discussed later ; but the 

 presence of Mysis relicta supplies no evidence for the history of the lake. This 

 must be left to the geologist. 



At the present time the water of Lough Xeagh is derived chiefly from the 

 inflowing rivers, the Upper Bann, Blackwater, Ballinderry, Moyola, and Main 

 Water. There are in addition several smaller rivers, one of which, the 

 Six-mile Water, enters the lake at Antrim. It was from the mouth of this 

 river that the collecting expeditions set out. It will be rather important to 

 know from what ground this drainage-water comes. The Upper Bann flows 

 over Silurian rocks and Miocene basalt. Near the lake it flows over clay and 

 alluvium. In aU the details which follow it must be remembered that the 

 formations given are covered to a certain extent with boulder clay, allmium, 

 or bog ; and the amount of actual rock-cutting that is carried out by the 

 rivers is therefore not indicated. As a general rule, in the upper courses of 

 the rivers the rocks are exposed. The Blackwater flows over Carboniferous 

 limestone and Tiiassie rocks, the latter predominating, and further over bog 

 and alluvium. The Ballinderiy flows over Tiiassie rocks and Miocene basalt ; 

 the river Moyola cliiefly over Miocene basalt ; the Main Water flows over 

 basalt, and the same applies to the Six-mile Water. 



