G. S. Kinahan — Water Basin of Lough Berg. 491 



during floods its waters are muddy, wliile at other times there is a 

 great evaporation from its large expanse of surface ; consequently 

 large depositions of mud and marl are contiuually taking place : 

 therefore the nature of the original bottom of the lake-basin cannot 

 be learned from the soundings ; but we do learn that all the deeps 

 are in the lines of the different longitudinal or transverse breaks, or 

 at the junction or crossing of two or more ; while most, if not all, of 

 the islands, rocks, shoals, or shallows, are on the upthrow sides of 

 the different faults. The latter fact is very apparent, except in the 

 long reach between Scarriff and Youghal Bays, as the existence of 

 the faults to the south of the Scarrilf valley have not as yet been 

 proved. If, however, as is highly probable, the previously-men- 

 tioned fault in the plain on the west of that valley extends through 

 it and into the long reach, all the deeps in that part of the lake- 

 basin are on the downthrow side of this fault. In concluding this 

 portion of the paper it may be observed, that if faults or breaks 

 have no connexion with the formation of lake-basins, it appears 

 remarkable that in this basin all the changes in the bearings of 

 the different lines of deeps should be connected with the strikes of 

 the different faults in Slieve Bernagh and Slieve Aughta, and that 

 the islands, rocks, and shallows should be on the upthrow sides of 

 these lines of faults. 



Supposed Fokmation of the Lake-Basin, 



We have now only to offer suggestions as to how the " rock 

 basin" of Lough Derg was excavated; for although its water is now 

 in most places margined with drift or alluvium, yet the probability 

 is that it was originally almost, if not entirely, surrounded by rocks. 



The faults and breaks were probably developed at such times as the 

 land was rising from under water, in which case the sea must have 

 been th« first excavator, marking out along those faults and breaks 

 valleys and ravines, which were afterwards occupied by rivers and • 

 ice ; for when the sea retired, these other denudants must have accom- 

 plished more or less work. It has been pointed out by Prof. J. S. 

 Newberry, M.D.,^ in relation to some of the large North American 

 lakes, that the original surface outlets from them have been ap- 

 parently obliterated by large accumulations of drift, and thereby the 

 waters have been forced to find other outlets ; and this probably may 

 also have been the case in regard to the basin of Lough Derg, as the 

 valley extending from Youghal Bay round the hills called Slieve 

 Arra. and down the Kilmastulla Eiver to the Shannon, a few miles 

 south of Killaloe, is now occupied by a deep accumulation of drift. 

 This drift, which is principally gravel and sand, apparently of 

 the "Esker sea period" may be of sufficient depth to fill a valley 

 whose original trough corresponded with the height of the rocky 

 barrier that forms the rapids and falls of Doonass, in the neigh- 

 bourhood of Castleconnel ; and if such a valley ever existed, either 

 marine action or meteoric abrasion would have been capable of ex- 

 cavating the basin of Lough Derg to a depth of about sixty feet 



1 " On the Surface-Geology of the Basin of the Great Lakes, and the Valley of the 

 Mississippi," by Prof. Newberry, M.D., U.S. Geologist. Ann. Jyyccum Nat. Hist., 

 New York, 1869, vol. ix. See Geol. Mag., 1870, Vol. VII. p. 227. 



