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 subaërial 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 subærial agencies have the power of wearing away rock, it might not have been impossible here; for from the "Rockbasin," 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,2 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 the sand, 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 passages 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 considered. 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. XVIII.) to the south point of Inchiquin (N on Map, Pl. 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.E. of this line it is shallowest toward 1 This barrier was lowered considerably during the navigation and drainage works previously mentioned. 2 History of H-iar Connaught, by Roderic O'Flaherty, Esq. Edited by James Hardiman, M.R.T.A. Pages 28 and 29. 3 The depths given are those on the chart, which was made before the drainage and navigation works; now the lake is a little shallower. |