vi MOUNTAINS 217 



those in the interior, and consequently they 

 could not lie flat between A'" and B"', but 

 must be thrown into folds, commencing along 

 any line of least resistance. Sometimes in- 

 deed the strata are completely inverted, as 

 in Fig. 19, and in other cases they have 

 been squeezed for miles out of their original 

 position. This explanation was first, I be- 

 lieve, suggested by Steno. It has been re- 

 cently developed by Ball arid Suess, and espe- 

 cially by Heim. In this manner it is probable 

 that most mountain chains originated. 1 



The structure of mountain districts confirms 

 this theoretical explanation. It is obvious 

 of course that when strata are thrown into 

 folds, they will, if strained too much, give 

 way at the summit of the fold. Before doing 

 so, however, they are stretched and conse- 

 quently loosened, while on the other hand the 

 strata at the bottom of the fold are compressed : 

 the former, therefore, are rendered more sus- 

 ceptible of disintegration, the latter on the 

 contrary acquire greater powers of resistance. 



1 See especially Helm's great work, Unt. u. d. Mechanismus 

 der Gebirgsbildung. 



