90 THE RELATIONS BETWEEN FAULTS, OPEN JOINTS, 



however, would be indirectly due to cracks, as with- 

 out breaks, through which the matter could reach 

 the surface, we would have no eruptions. 



In some mountain groups, such as that of Dublin 

 and Wicklow, some of the valleys have an appearance 

 that at first might lead an observer to imagine that 

 they had been denuded gradually bit by bit ; but 

 among other hills, such as those forming the highlands 

 of Scotland, or the mountainous parts of Galway or 

 Mayo, it is generally apparent that the valleys are 

 connected with breaks in the subjacent rocks. 



A few valleys seem to have no connection with 

 lateral breaks ; such valleys, however, are only minor 

 features, and scarcely deserve the name when com- 

 pared with the others. 



In most straight, or nearly straight, valleys, or in 

 those formed of a series of straight portions, the con- 

 nection with the associated breaks is easily traced 

 out; but in many tortuous valleys it is not so 

 apparent ; nevertheless, after a little examination, it is 

 seldom that proof cannot be found that all the different 

 lines, no matter how irregular, have connections with 

 breaks, either faults or joints. A ravine which 

 illustrates this fact occurs on the east slopes of Slieve 

 Gallion, Co. Londonderry. This glen is most irregular, 

 and after a casual examination its excavations might 

 be supposed to be due solely to the influence of rain 

 and rivers. This, however, is not the case, as the rocks 



