THEORIES OF THE CAUSES OF FAULTING 195 



may, in instances, be the accompaniment of stresses 

 between horizontally drifting segments. 



KINDS OF CRUSTAL MOVEMENTS 

 Briefly the different kinds of crustal movements 

 may be placed in three groups: 



1. Up and down (flotational) movements pro- 

 duced by transfers of load from one place to another 

 through processes of erosion and transportation. 



2. Horizontal-drifting movements of segments of 

 the earth's crust produced by stresses of revolution, 

 sub-crustal undertow and other unknown causes. 



3. Compressional movements of the kinds previous- 

 ly mentioned. 



Faulting, folding and volcanism are more or less 

 associated with the above mentioned kinds of move- 

 ments. 



Thus these three great kinds of directional move- 

 ment of segments — compressional, drifting and flota- 

 tional — have each been factors in producing our faults 

 and folds and the resultant highland and lowland physi- 

 ography. Of the three, the up and down flotation has 

 produced the most conspicuous effects upon our 

 physiography, as seen in the great master rifts and 

 fault-block highlands and valleys. Each and all, in 

 my opinion, are superb illustrations of the work of 

 isostasy and of the near truthfulness of the theories 

 of our modern school of physiographers and geo- 

 physicists. 



THE UP AND DOWN MOVEMENTS 

 Up and down movements take place slowly through- 

 out long intervals of time and have produced the chief 

 aspects of our magnificent physiography, including 



