THE STRENGTH OF THE EARTH'S CRUST 657 



degree of isostasy, there is but little such adjustment within areas 

 200 to 300 km. in diameter, or of limited differential relief. Indi- 

 vidual mountains and mountain ranges may stand by virtue of the 

 rigidity of the crust. Even under the level plains equally great 

 loads are permanently borne, loads produced by widespread irregu- 

 larities of density not in accord with the topography above. 

 Isostasy, then, is nearly perfect, or is very imperfect, or even non- 

 existent, according to the size and relief of the area considered. 



The third division, the mode of maintenance of isostasy and its 

 bearings on problems of the crust, remains to be considered. This 

 condition of isostatic equilibrium exists at present in spite of the 

 leveling surface actions and compressive crustal movements of all 

 past geologic time. There must be, consequently, some internal 

 mode of restoring more or less perfectly an isostatic condition, 

 either by frequent small movements, or by more infrequent and 

 larger ones. 



Erosion and sedimentation result in a lateral transfer of matter, 

 and to maintain isostasy there must be some lateral counter- 

 movement in the earth below, but in regard to how or where or 

 when this is done, and as to what are its effects, there has been no 

 unanimity of opinion, nor convincing demonstration. 



In considering the problems of crustal dynamics some authors 

 have regarded earth shrinkage and consequent tangentially com- 

 pressive forces as controlling the nature of diastrophism, including 

 movements of both orogenic and epeirogenic character; others, the 

 advocates of extreme isostasy, have thought to see even in folding 

 only the secondary effects of movements maintaining isostatic 

 equilibrium. The first point of view emphasizes the strength and 

 elasticity of the crust, with long-deferred periodic discharge of 

 stress. The second point of view calls for an interpretation based 

 on the weakness and plasticity of the crust, with resulting nearly 

 continuous small movements restoring the delicate vertical balance 

 destroyed by gradational actions. To what degree are the two 

 points of view compatible and within what limits is each dominant ? 

 The problem of this chapter involves, therefore, not only the mode 

 but the limits and effects of the movements which more or less 

 completely maintain or restore isostasy. 



