DEFORM A riON OF ROCKS I 8 1 



angles to the less horizontal pressure. It may be that the 

 tangential forces in either direction constitute a vertical couple, 

 and in such cases monoclinal folds may be produced.^ 



While for a given district it may be assumed for definite areas 

 that the average direction of the maximum and mean forces 

 remain the same for considerable intervals of time, it does not 

 follow that in the adjacent districts, or in another part of the 

 same district, the relative values of these may not be reversed, 

 and thus the direction of the closer folds for one part of the dis- 

 district be that of more open folds of another part of the district. 



Moreover, as a result of the action of the formations as 

 transmitters of forces, plications may be formed intermediate 

 between the two prevalent directions. Minor plications deviat- 

 ing from the general directions of folding are particularly likely 

 to be seen when the two tangential forces are about equal, thus 

 forming domes, and at places where a strong formation plunges 

 below the surface as a result of the cross folding. In such cases 

 the strata, especially the weaker strata, may be in a set of radial 

 minor plications, mantling around the dome or mantling around 

 the plunging mass of strong material. At each point the axes 

 of the minor plications represent the dip of the slopes of the 

 dome or of the stronger bed. Such plications result from the 

 necessary readjustment between the beds. To illustrate — when 

 a piece of leather is placed upon a hemisphere, it will fit closely 

 at the top, but cannot be made to fit along the sides of the 

 hemisphere unless portions be cut out. In nature the rocks can- 

 not be cut out, and as a consequence we have the radial flutings, 

 which in the case of anticlines plunge away from the crest, and 

 in the case of synclines plunge toward the trough. These flut- 

 ings, and the consequent radiating or converging character of 

 the minor axial planes, are frequently of great assistance in 

 determining whether a given mountain mass is an anticlinorium 

 or a synclinorium. However, it appears that the arrangement of 

 these flutings is no exception to the general law, for there is 

 always a marked tendency for strong beds to decompose the 



' Loc. cit., (A), pp. 626-627; (B), pp. 343-345. 



