238 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



The very question arises from our knowledge that geological 

 evolution has not been uniform. On the contrary, as is commonly 

 understood and admitted, orogeny, volcanism, degradation, and 

 aggradation, and all other constructive and destructive processes 

 show great differences in their intensity from period to period, and 

 these variations are cyclic and rhythmic. The measure of the 

 rhythm is indicated by the formation of the mountain chains. 



Since the beginning of the Cambrian period, the evolution of the 

 earth has passed through three geologic cycles of higher order and 

 commenced a fourth. The transition from one to another is marked 

 by the unconformities which in the stratigraphical series follow the 

 Caledonian, Hercynian, and Alpine diastrophism. The great cycles 

 are: 



The first part of each cycle comprises an anorogenic phase of 

 long duration. The cycles enter then in a first orogenic phase, a 

 forerunner of the main mountain-making movements. After this, 

 relatively anorogenic periods follow in the second part of each cycle. 

 From this point, however, disturbances become more frequent, and 

 at the end of the cycle, the main mountain folding takes place, to 

 continue here and there, with decreasing strength, in the beginning 

 of the following cycle. 



The above tabulation in cycles of the geological periods is in 

 no manner meant to assert that these were of equal duration, nor 

 that the corresponding phases from different cycles represent quite 

 similar stages in geological evolution. It is of course possible to 

 dispose the periods in cycles in many other ways. But however we 

 arrange them, we shall find that the rhythm of orogeny has regulated 

 the general course of the histoiy of the earth. 



Thus volcanic activity has always been not only geographically 

 combined with the zones of folding and faulting, but also chrono- 

 logically connected with the formation of mountain chains, being 

 strongest during, and immediately after, orogenic times, feeblest 

 in anorogenic periods. 



The great transgressions and regressions of the sea follow the 

 same rhythm. In the periods of orogeny and immediately after, 

 there is considerable general regression. The land areas were ex- 

 tensive then, as they are at present. The greatest transgressions, 

 again, appeared in the anorogenic intervals. 



