AND THE FOEMATION OF VALLEYS. 97 



is also the lateral contraction to be taken into 

 account, as some of the strata are seen to shrink more 

 than others, and consequently to add to the width of 

 the valleys. 



In many places there are long, narrow valleys or 

 gorges, that appear to be in a great measure due to 

 the contraction of the rocks along lines of fault or 

 breaks ; these often form marked features. In the 

 mountainous portions of Scotland and Ireland some 

 of these lines of breaks and faults appear to be post- 

 glacial, as the rocks forming their sides are not ice- 

 dressed ; besides, no part of the ravines formed by 

 them is occupied by moraine or boulder-clay drifts, 

 and if their line crosses an accumulation of drift, 

 there is either a marked hollow or a displacement of 

 the drift, the latter often being higher on one side 

 than the other. In Ireland this is well exemplified 

 in the counties of Tyrone and Londonderry, where in 

 numerous places the original sheet of boulder-clay 

 drift, or Till, as it is called in Ulster, has been broken 

 up by recent breaks, generally faults ; the original 

 fissures, however, have been subsequently modified, 

 in some cases by water, probably during the " Esker 

 sea " period, but in others they seem only to have 

 been affected by meteoric action. 



In "West Galway and Mayo, narrow fissures run- 

 ning along lines of faults and breaks form conspi- 

 cuous features in places not only among the hills, 



G 



