110 THE LAKE-BASINS OF IARCQNNAUGHT. 



Drift-basins may be situated in hollows in drift 

 accumulations, but most of those remarked in larcori- 

 naught are partly formed of rock and partly of drift ; 

 the drift occurring as a mass or bar across a hollow 

 in the rocks, and thereby pounding-in the water. 

 Some drift-basins situated in low valleys, such as 

 the basin of Kylemore Lake, appear due to bars and 

 mounds of sand and gravel that extend across, or 

 accumulate in, a valley; the ridge and mounds appa- 

 rently being ancient sea- work. The formation of such 

 gra.vel accumulation will hereafter be more minutely 

 described. Another kind of true drift-basin is in- 

 stanced by that of Bealawaum Lake in the corry 

 north-east of the summit of Mweelrea; the hollow 

 apparently having been formed, in a great measure, at 

 the same time as the drift, as it is probable that such 

 irregular accumulations of drift are the debris con- 

 tained in the ice when it finally disappeared. 



For the formation of basins partly rock and partly 

 drift three agents may be suggested, viz., marine, 

 glacial, and meteoric action. 



Marine action may build up a bar across the end 

 of a narrow bay or estuary, into which a stream 

 flows ; and such a mound now exists at the east end 

 of Cleggan Bay (Map, fig. 16, PL III.) This bar 

 is evidently due to the meeting of the fresh and 

 salt water; having begun as a shoal that gradually 

 grew higher and higher by the addition of debris 



