TO QUESTION PERIODIC DIASTROPHISM 613 



If after some deformation a mountain zone becomes strong enough 

 to resist further deformation, the parts of the crust which were 

 sliding toward it, will start moving toward another zone of greater 

 weakness, while deformation in the first zone may cease for a long 

 period of time. Is it not just this change of the location of the site 

 of deformation, rather than the cessation of movement, which has 

 lead many to beHeve in the hypothesis of periodic diastrophism ? 



SUMMARY 



Periodic diastrophism is taken to mean world-wide periodicity 

 of orogenic movements and not local periodicity. This h3^othesis 

 appears to have three fundamental weaknesses. First, geological 

 evidence shows a series of diastrophic movements which are almost 

 continuous from the Ordovician to the present. Second, the belief 

 in an excess of diastrophism at some epochs of geological time over 

 those of other epochs is due either to the common assumption that 

 diastrophic movements come at the end of a period, when the evi- 

 dence is not sufficient to justify such conclusions; to the records of 

 diastrophic movements having been erased at some times more than 

 at others; to the unrecorded evidence of sub-oceanic diastrophism 

 which probably alternates in intensity with continental diastro- 

 phism as is shown by the record of epi-continental seas; or to the 

 shortening of the crust by minor flexures which leave no record of 

 their time, but probably are important in amounts. Thirdly, theo- 

 retical considerations show that the idea of periodic diastrophism is 

 contrary to what would be expected of a shrinking globe, which as 

 geological evidence suggests has a crust that is rather easily dis- 

 turbed by differences of weighting and therefore could not be expected 

 to accumulate large strains. Further, the crust would have to sup- 

 port a large part of its own weight in order not to fit itself constantly 

 to a shrinking interior and it is believed that it can support only a 

 small fraction of its own weight. 



Although admitting the possibiKty that future accurate studies 

 may demonstrate that diastrophism has been periodic, it seems clear 

 to the writer that in view of our present knowledge of the subject 

 each of the three weaknesses cited form a serious barrier in the 

 way of the acceptance of the hypothesis and taking them altogether 

 there is little reason to believe in periodic diastrophism. 



