MOUNTAIN GROWTH 



241 



Throughout his discussion of diastrophism Kober emphasizes the part 

 played by compressive forces^ which probably have their origin in con- 

 tinued contraction of the earth body. He even conceives that horsts 

 and graben in a region of block-faulting are bounded typically by reverse 

 rather than normal faults (figure 8). The Ehine Yalley and the 

 African Rifts are explained b}' failure at the crests of broad anticlinal 

 arches^ and so are essentially features due to compression. Unlike many 

 exponents of the contraction theory, however, he is alive to the import- 



FiGURE 9. — Schema cines OeMrges mit Massendefekt 



(Dotted line represents theoretical surface of norinal gravity.) 



Kober's figure 21, page 97. 



ance of isostatic control. Relief features are largely compensated,, 

 mountain ranges standing high by virtue of deficiency in density (figure 

 9). When an orogenetic zone is undergoing folding, more material is 

 forced into the depths than above the general surface. There is con- 

 tinual conflict between tangential and vertical forces. If excessive mat- 

 ter is crowded into a zone, sinking will eventually ensue, although a 

 certain degree of overload may be borne for a time. Thus uplift and 

 subsidence may occur alternately in essentially the same zone through 

 several geologic periods. 



