166 GORGES AND RAVINES. 



afterwards carried out of them by the water. The 

 power of water to keep clear a river-bed having an 

 underground vent, is exemplified in the Beagh River, 

 County Galway, the stream that flows from the north 

 end of Lough Cooter. This river, after an open course 

 of about two miles, becomes subterranean. At and 

 near its disappearance its bed is enclosed by high banks 

 of boulder-clay drift, which contain blocks of lime- 

 stone and sandstone. These banks are yearly more 

 or less denuded, and the blocks in the drift are nume- 

 rous ; yet after a flood few of them remain in the bed 

 of the river, it being for the most part occupied by 

 a flinty, angular gravel lying on the denuded lime- 

 stone rock. During low water no open vent seems 

 to exist through which the blocks could be carried, 

 the water flowing into a pool with a muddy bottom, 

 at the base of the cliff. The limestone blocks might 

 be in part dissolved and carried away in solution : 

 this, however, could not happen with the sandstone. 

 But a close examination explains the process. The 

 vent, as viewed during low water, is a filter of stones, 

 with a greater or less coat of slime ; when, however, 

 a flood comes down, the mud and stones are disturbed 

 and a passage opened, which increases more and more 

 till a whirlpool is formed by the rush of the water 

 that rolls the stones over one another, and thus grinds 

 them up. Some may even find their way into the 

 subterranean passage; as lower down, where the stream 



