490 Kinahan — Formation of Roch-basins. 



gar (t on Map, PI. XIX) .^ On all sides of the lake are rocks extending 

 under it, or, to use Professor Eamsay's term, it lies in a " Eock- 

 basin." What excavated this Eock-basin ? I propose in this paper 

 to consider. 



It does not seem to have been formed by faults or displacements 

 of any kind, as in different localities peculiar beds of rocks are either 

 traceable across it, or will be found opposite one another on each 

 side. One of these localities is half-way between Oughterard (k on 

 Map, PL XVIII.) and the Ferry of Knock (o on Map, PL XIX.), where 

 a remarkable set of cherty limestones can be traced in the islands 

 across it. Another is between the mouth of the Owenriflf or Owen- 

 fough (l on Map, PL XYIII.), the river that flows by Oughterard, and 

 Ballycurren (m on Map, PL XIX.), where dolomite beds extend across 

 the lake, and others are farther north, Avhere peculiar beds of rocks are 

 found ojDposite one another on each side. Neither does it seem to 

 have been made by marine denudation, for though I believe it was 

 that force which formed the main features of the neighbouring 

 mountains, and diuring that time the rush of tide out of the narrow 

 valley of Maum (a on Map, PL XVIII.) might have scooped a depres- 

 sion opposite the mouth of the valley ; yet it seems hardly possible 

 that marine denudation could have excavated out the entire " Eock- 

 basin" now occupied by Lough Corrib. 



K the "Eock-basin" were not caused by faults, or excavated by 

 marine denudation, could it have been formed by either running or 

 solid water {Ice), or corroded out by the waters of the lake ? The 

 facts in connection with the corroding powers of the waters of the 

 lake will first be considered. 



The works carried on, about fifteen years ago, to improve the 

 drainage of the neighbouring country, and to make the lake navigable 

 from Galway upwards, have lowered the Lough about three feet, 

 and made patent the following facts, to which my attention was 

 first directed by Professor King, of the Queen's College, Galway. 

 These seem to prove, as will immediately be seen, Col. Greenwood's 

 remark in " Eain and Elvers," " that water by itself has very little 

 power on rocks." 



The limestones that are exposed to the dash of the water 

 during the winter gales, and at other times to wind and weather, 

 are rapidly corroding away ; next to them the limestones, which, 

 previous to the lowering of the lake, were exj)osed during the 

 summer months ; while the rocks that are now exposed during 

 the summer months are only beginning to weather, and the rocks 

 that are below the summer level and always covered with water, 

 seem scarcely, if at all, weathered. Moreover, if the water of the 

 lake corroded away the rocks and formed the " Eock-basin," it 



parts of the lake that are respectively below the 100 and 50 feet contour lines ; the 

 former being surrounded with a broken line, and the latter by dots. Not to crowd 

 the map, all the places that are referred to in this paper are marked by letters, while 

 other names are left out. 



1 The passage to the Castlegar outlet was closed about fifteen years ago by the 

 Board of Works, to facilitate the nayigation of the Lough. 



