82 WILLIAM H. HOBBS 



hence resistant masses lying outside;^ a view which has been 

 rather generally accepted, it would seem, and which is followed, 

 among others, by Arldt.^ 



To accepting this conception there is an insuperable objection 

 from the standpoint of mechanics. The necessary reduction in 

 area of the strata through duplication by folding and "over- 

 thrusting" is certainly great, and if this duplicated and often 

 reduplicated expanse of strata has come from within the area 

 inclosed by the arc, one of two consequences must have followed. 

 Either a hiatus must have developed near the center of the area, 



C^-^^^^^^^ ^:^^ -.^g^^^.S'^^^^:^ 



Fig. 7. — Diagrams to contrast the conceptions of centrifugally distributed 

 (A) and centripetally arranged (5) forces in the production of mountain arcs. The 

 lower figures are enlarged sections along the dotted lines, and the arrows in aU cases 

 give the directions of the (assumed) active forces. 



or else the strata must have become greatly attenuated, an effect 

 which should be increasingly apparent in the upper and later folds 

 of the series and throughout in the upper limbs more than in the 

 lower (Fig. 7, Aaa'). If, upon the other hand, the arcs are to be 

 explained as a result of centripetally distributed forces coming 

 from outside the area inclosed by the arc, these difficulties are not 

 encountered for the reason that contraction of surface of large 

 portions of the earth's outer shell may be assumed to supply the 



■ Ed. Suess, Antlitz der Erde, III, 1-2. 



- Th. Arldt, Die Entwicklung der Kontinente und Hirer Lebewelt, Leipzig, 1907. 



