STRATIGRAnilC CLASSIFICATION DIASTROPITIC CRITERIA 40' 



central and northern parts 

 of the continent. Something 

 like this is thought to have 

 occurred repeatedly, and 

 probably many times, dur- 

 ing past geological ages, and 

 the theory would serve very 

 well in explaining the alter- 

 nating north and south tilt- 

 ing of the surface of North 

 America that is so clearly 

 indicated by the distribution 

 of Ordovician and Silurian 

 faunas and sediments de- 

 sci'ibed on pages 367 to 371. 

 Tw,o groups of move- 

 ments — The movements de- 

 fined. — Diastrophic move- 

 ments are divisible into two 

 generalized and not sharply 

 differentiated groups, ( 1 ) 

 those that are active and 

 relatively impulsive in oper- 

 ation, and (2) those that 

 are relatively gentle in 

 operation and much more 

 gradual in the production 

 of results. The former 

 are chiefly responsible for 

 the wrinkles of the earth's 

 crust, and their function is 

 primarily orogenic. On the 

 continents these relatively 

 active movements originated 

 and repeatedly rebuilt the 

 mountain chains of the pres- 

 ent day; in the oceanic 

 basins the> occasioned the 

 great undulations of their 

 bottoms. Though the same 



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