178 THE BOOK OF NATURE STUDY 



composed and their relation to one another ; only they are usually 

 traversed by a single river belonging to the principal system. 

 Thus a miniature set of features may be locally developed along 

 the lines occupied mainly by the broad features of the greater 

 system of folds and drainage. 



At times both the major system of folds and the minor system 

 of wrinkles may be broken by faults, which will destroy the 

 symmetry of the sides of a fold ; and introduce in parts of the 

 area unmatched beds of resistant rocks. These bands will be 

 in part the repetition of those found in normal position, but they 

 will react on denudation as if they were independent beds. Thus 

 there may arise two or more parallel scarps, each one resulting 

 from the repetition of the same rock by faulting. Again, faults or 

 folds crossing the strike of the rocks may cause this strike to 

 bend out of its normal course, and thus make the scarp feature 

 bend in a similar fashion. Facts like these do much to relieve any 

 monotony in landscape which might result from the development 

 of a too simple tectonic system, and too great consistency in the 

 composition of the rocks. 



The fissures, joints, and cleavage which traverse rocks often 

 have a marked influence on the details of landscape. Thus a 

 joint system running in definite directions will indicate the 

 direction of easiest erosion to the streams. These will tend to 

 settle along the joint planes, and either to cut trenches parallel 

 to them, or to wind from one system to another. This is often 

 very noticeable in limestone country, where, as will shortly be 

 seen, another influence comes into play. The outlines of buttresses, 

 gullies, and ridges of rock mountains are markedly influenced by 

 joint faces, as may be seen in limestone, grit, or granite countries. 

 The production of cleavage is often responsible for very irregular 

 and very sharp ridges, like some of those on Snowdon and its 

 neighbours. 



Crystalline rocks, especially the intrusive ones, being much 

 less regular than strata in their direction, and in the rocks with 

 which they come into contact, being devoid of bedding, and tra- 

 versed by peculiar types of joints, give rise to very strongly- 

 marked scenery, but of a type which rarely falls into line with 

 that produced by the denudation of strata. Most of these rocks 



