Kinahan — Formation of Lough Corrib. 491 



ought to have acted more on the soft limestones than on the hard 

 metamorphic rocks. This seems to be contrary to facts, as most of 

 the portion over the latter is deep, while nearly all the shallow part 

 of the lake is cut out of the limestone. That the waters have scarcely 

 affected the metamorphic rocks since the glacial period seems proved 

 by those rocks — so far as can be seen under the water when the lake 

 is at its lowest summer level — ^being ice scratched, rounded, and in 

 places polished. 



If the water of the lake did not corrode away the rocks, could 

 they have been worn away by running water and the other subaerial 

 agencies ? This at first seems to be impossible, as the barrier of 

 metamorphic rocks, across the lake's outlet at Galway, is even now 

 higher than most of the bottom of the lake.^ However, if water 

 and the other sub^rial agencies have the power of wearing away 

 rock, it might not have been impossible here ; for from the " Rock- 

 basin," part of it being in a limestone country, there may have been 

 subterranean passages through which the drainage could escape. 

 There was one at Castlegar that carried off part of the water, and 

 why might there not have been others ? In the Annals of the Four 

 Masters, a.d. 1178, and in O'Flaherty's History of H-iar Connaught,^ 

 we find four times during the historic period, namely a.d. 1178, 1190, 

 1647, and 1683, " that the river at Galway suddenly went dry. and 

 remained so for a few hours during very low tides." Might not this 

 point to old subterranean passages ? If these did exist, they would 

 have been outlets from the lake, while the land was higher than 

 now, but would cease to act when the land sank, and the surface of 

 the lake was only a few feet above the level of the sea. Moreover, 

 they would be liable to be choked up by thousand, etc., swept into 

 them by the waves, as the water could not flow through them 

 except when the tide was very low, and it may have been during 

 four of these very low tides that the phenomena, recorded by the 

 Four Masters and O'Flaherty, occurred. For if the entrance to one 

 of these joassages was left dry for a few hours, or with only a small 

 depth of water over it, the weight of the water in the lake might 

 force out the sand, etc., that filled the passage, and so find a vent 

 through which to flow instead of down the open river at Galway, 

 until the returning tide dammed up the water, and as the tide did not 

 for years recede so far, the passage would again be choked up. 



Let what seems in favour of subterranean passages now be con- 

 sidered. On examining the chart of the lake it will be observed 

 that north of a N.E. and S. W. line, drawn from Oughterard (k on Map, 

 PL XVin.) to the south point of Inchiquin (n on Map, PI. XIX.) the 

 lake is generally deep, while, southward of it, it is shallow, and that 

 none of the latter, excepting the narrow passage at the Ferry of Knock, 

 is over 29 feet deep ; ^ also that S.B. of this line it is shallowest toward 



^ This barrier was lowered considerably during the navigation and drainage works 

 previously mentioned. 



2 History of H-iar Connaught, by Eoderic O'Flaherty, Esq. Edited by James 

 Hardiman, M.R.T.A. Pages 28 and 29. 



2 The depths given are those on the chart, which was made before the drainage and 

 navigation works j now the lake is a little shallower. 



