Geological Features of the North-east of Ireland. 133 



It should be added, that in the county of Londonderry water- 

 worn fragments of basalt really occur in considerable quantity, 

 particularly near the valley of the Roe. 



It is impossible to dismiss the subject of denudation without re- 

 marking, that appearances analogous to those so forcibly described 

 by Saussure as occurring in the Jura mountains, (where scattered,^ 

 fragments of the alpine rocks attest the subsequent excavation of 

 the great valley now separating the two chains) are also presented, 

 though on a much smaller scale, among the hills we have just ex- 

 amined, where rolled fragments of primitive rocks are often found 

 on the summit of high basaltic ridges, which are at present cut off 

 from all communication with the primitive districts by numerous 

 intervening valleys. (See Sampson's Statistical Account of the 

 County of Londonderry.) 



