G. S. Kinahan — Water Basin of Lough Derg. 493 



with drift. At the present time, the deepest spot in Lough Derg is 

 only eleven feet below the low-water of ordinary spring-tides at 

 Limerick ; but as the level of the lake is 108 feet higher, there would 

 even now be an underground drainage if any of these holes are con- 

 nected with subterranean passages. During the Glacial Period, if 

 the land was higher than at present, the water would find vents 

 through such passages, denuding away their walls and enlarging the 

 deeps from which it flowed. After the Glacial Period water would 

 still flow through them, until eventually the land sank so low that 

 they were incapable of acting ; or even previous to this, they might 

 be choked by successive depositions of mud and marl. Even now 

 there are reasons for supposing that some such passages may exist 

 out of Lough Derg, as in the summer less water leaves the lake at 

 Killaloe than flows into it by the main river at Portumna and from 

 the numerous other smaller feeders ; part of this deficit is certainly 

 due to evaporation, but the whole of it could scarcely be thus 

 accounted for. 



Many of the Irish lake basins, especially those in the Carbon- 

 iferous Limestone, which now have a surface drainage, may once have 

 had subterranean vents. This appears probable from the various 

 systems of lakes connected with subterranean rivers that still exist 

 in different places in the island, and we vs^ould call especial 

 attention to those connected with the Eiver Fergus, Co. Clare, 

 since a system of lakes somewhat similar to this may have for- 

 merly existed in the basin of Lough Derg. These lakes are of 

 considerable size and are irregularly situated, and the river is 

 thus described by the late Mr. F. J. Foot, " The Eiver Fergus rises 

 in Lough Fergus, between Corrofin and Ennistimon, at an elevation 

 of about 350 feet above the sea, flows eastward and northward for 

 two miles and a half, when it receives the Clooneen river ; thence it 

 takes an easterly course for a mile and a half, when, on entering the 

 limestone groimd, it suddenly disappears in a swallow-hole or vertical 

 cavit}^ in the rock. Half a mile to the east, it again emerges to the 

 light, from a cavern called Poulnaboe, from which it flows down 

 into InchiLjuin Lough and thence into Lough Atedaun. No visible 

 river flows out of this Lough, but the Fergus is supposed to have 

 a subterranean course in a direction of about E.S.E. to Dromore 

 Lough, whence it flows southwai'd, now above and now below 

 ground, to Ennis, and thence to the Shannon." In the basin of 

 Lough Derg, if the deeps below the 50-feet line of soundings were 

 mapped, they would make a system very similar to those connected 

 vp'ith the Eiver Fergus, and the similarity woidd be more complete, 

 if each deep was connected with its fellows by subterranean 

 passages ; while the great deep (119 feet) in the long east and west 

 reach had a passage from it to the sea somewhere in the neighbour- 

 hood of Limerick. 



I have now offered suggestions as to the mode of formation of 

 the basin of Lough Derg ; the}'' are antagonistic to the views enter- 

 tained by many persons ; still they appear to me more reasonable 

 than any others; and the physical agencies appealed to are all 



