176 THE RIVER VALLEYS OF 



especially as their continuity is broken up by various 

 systems of master-joints. 



Now, to proceed to the third and fourth proposi- 

 tions if they are found to be incorrect, it follows that 

 the fifth must be incorrect also. If the land-level 

 is stationary, or if it is gradually rising, the natural 

 tendency of the sea's action would be to form a 

 plain of denudation ; but if the land-level is sinking, 

 the result will be different, for although the sea 

 may tend to form a plain, it will not be one solely 

 of denudation, as the marine action fills up all the 

 low places, but does not excavate them. It would 

 appear that the relative level of Ireland has been 

 lowered since the historical period, as we know the 

 time when the sea flowed into Lough Swilly, and 

 other low places that are now bays ; while we have 

 traditions that the sea has encroached in other 

 places, as for instance in Galway Bay, the ancient 

 name of which is Lough Lurgan. Archaeological 

 discovery would lead us to believe that such tradi- 

 tions are correct, as in late years on the Aran 

 Islands have been found human habitations and 

 structures, which can be traced under the sands 

 below low- water mark ; while in various other places 

 on the coast of Ireland, bogs containing trees and 

 plants similar to those of the inland counties, 

 are found at low-water mark of spring-tides, they 

 being in places buried from twelve to twenty feet 



