AND THE FORMATION OF VALLEYS. 91 



are traversed by systems of joints: one system (a, a, 

 fig. 22. PL IV.), parallel to the strike of the beds, 

 while the other (b, b) is slightly oblique to it. Of 

 these systems of joint lines the stream (c, d) has 

 taken advantage, at one time excavating along the 

 lines of one system, at another time along the lines 

 of the other ; thus a ravine has been cut apparently 

 most irregularly, but really systematically. In all 

 such valleys or ravines the protruding angles will be 

 more or less modified, or even carried away by 

 meteoric abrasion, as in many of the Devonshire 

 valleys, where the relation between the old joints, 

 openings, and the present valleys, is often very much 

 obliterated and obscured. 



Valleys in such accumulations as glacial-drift, on 

 account of its irregular composition, are not as uni- 

 form as rock-valleys ; nevertheless, in most tracts of 

 country they are inclined to occur in more or less 

 regular systems, as will be seen by examining the 

 geological maps of the central plain of Ireland, 

 essentially a drift country, where, in the different 

 areas, the long narrow bogs, the stream-courses and 

 minor valley-systems, usually have more or less par- 

 allel bearings. 



In most countries it would seem as if the main 

 valleys were formed in the rocks prior to the drift 

 period, during which they were wholly or partially 

 filled, while subsequently they have been more or 



