SOUTH-WEST IRELAND. 193 



(srah or haugfi), and eventually a plain bounded by 

 cliffs ; but in this case neither the flat nor the cliffs 

 are due to the river action, but to wind combined 

 with the water of the lake. True, river-flats may be 

 formed by rivers gradually gating into a hill on one 

 side, while the debris on the other side is levelled 

 during floods ; such flats, however, are of small 

 extent, and in Ireland all the extensive river-flats are 

 differently formed. When the rivers flow more or 

 less near the centre of their flats, they add yearly to 

 the alluvium, quantities of silt and such like sub- 

 stances that are suspended in the water during floods, 

 but never gravel and sand ; except immediately below 

 rapids, where the coarse materials can be driven on to 

 and scattered over the plain. Irish rivers, therefore, 

 may be related to the flats through which they flow in 

 four different ways : 1st, They may in some places 

 form their own flats and the marginal cliffs ; 2d, They 

 may fill a lake-basin with gravel, and form a flat, but 

 not the marginal cliffs ; 3d, They may occupy old 

 valleys and raise their floors by annual deposits of silt ; 

 and 4th, They may flow through old estuary-flats, 

 to which the marginal cliffs were formed previously. 

 In the first case, rivers would take ages to form a 

 flat of any extent, and to lower their beds subse- 

 quently as the land rose ; during which operation they 

 would probably leave behind upper-river gravels in 

 the valleys. In the second case, rivers might possibly 



N 



