542 



GEOLOGY. 



Tlie Great Periodic Movements. 



Mountain-forming movements. — Along certain tracts, usually near 

 the borders of the continents, and at certain times, usually separated 

 by long intervals, the crust was folded into gigantic wrinkles, and these 

 constitute the chief type of mountains, though not the only type. The 



P^..'"A 



Fig. 449. — The great Eurasian mountain tract. Jones Relief Globe. (Photo, by 



R. T. Chamberlin.) 



characteristic force in this folding was lateral thrust. The strata were not 

 only arched, but often closely folded, and sometimes intensely crumpled. 

 In extreme cases, like the Alps, the folds flared out above, giving over- 

 turn dips and reversed strata, as illustrated in the chapter on Structural 

 Geology, pp. 501-511. In these cases there was an upward as well 

 as a horizontal movement, for the folds themselves were lifted; but the 

 horizontal thrust so much preponderated, and was so much the more 



