194 THE RIVER VALLEYS OF SOUTH-WEST IRELAND. 



form upper gravels; for as the land rose and the 

 embouchures of the lake-basins wore down, the rivers 

 would partly re-excavate the gravels they had pre- 

 viously deposited in the lake-basins. In the third 

 case, the flats may or may not be solely due to the 

 rivers. When sections are opened across them, it is 

 often found that at the first they were a bog or 

 morass, and that vegetable growth and decay formed 

 the beginning of the flat, while subsequently river 

 silt was deposited over the peat. In other cases will 

 be found a substratum of gravel or shingle, probably 

 an estuary formation, on which the river alluvium 

 has been deposited. Such rivers, however, might 

 easily lower their beds if the land rose, and the bar- 

 rier across the lower end was capable of being easily 

 denuded. But in the fourth case, rivers have little or 

 no work to do ; for prior to and during the rising of 

 the land from under the sea, the flats and terraces, 

 with the upper and lower gravels, would &e formed. 



