MECHANICS OF FORMA TION OF ARCUA TE MOUNTAINS 205 



to be utilized. Arcuate 

 structure representing a 

 reduplication of strata in 

 recumbent and ruptured 

 folds requires that the 

 duplicated material shall 

 have migrated centripet- 

 ally from outside the arc 

 and implies that the mass 

 within is relatively the 

 more rigid. 



The arcuate ranges of 

 Asia, regarded not indi- 

 vidually but as a system, 

 favor the conclusion which 

 we have reached of centri- 

 petally distributed thrusts 

 directed toward the center 

 of the system: (i) by their 

 plan of arrangement; (2) 

 by the position of the rela- 

 tively rigid area (Angara 

 Land); (3) by the greater 

 geological age of the cen- 

 tral, and the newer forma- 

 tion of the peripheral 

 arcuate ranges; and (4) 

 by the present locus of in- 

 tense volcanic and seismic 

 activity along the outer 



margm 



eoS'J'-^^ 



Fig. 37. — Examples of listric surfaces de- 

 veloped at the forward end of major slides: 

 a and b, Buffalo Mountain, southern Appalach- 

 ians (Keith); c, Rigihochfiuh, northern Alps 

 (Tobler and Buxtorf); d, Belgian coal-fields 

 from St. Eloi to St. Leon (Briart as reproduced 

 by Suess); e, Belgian coal-field at Fontaine- 

 I'Eveque (Briart and Suess); /, the same near 

 Denain and Anzin (Bertrand and Suess); g, 

 North-central Carpathians (Uhlig). For better 

 comparison all sections have been made to look 

 north, northeast, or east. 



Fig. 38. — One of the sections through Roan Mountain in the southern Appa- 

 lachians (after Keith). 



