214 FRANCIS PARKER SHEPARD 



lateral support of the range, it is not inconceivable that the condi- 

 tions would change again sufi&ciently to renew the support. A 

 recurrence of vulcanism beneath the range would produce a new 

 uplift by the expansion of the underlying material. 



A CHALLENGE OF THE BASIS OF PERIODIC DIASTROPHISM 



If the history of the earth actually shows periodic diastrophism 

 and this periodicity is due to a long accumulation of strains followed 

 by readjustment, then isostasy has not always been operative. 

 On the other hand, if the apparent periodicity of diastrophism is 

 due to periodic shrinkage of the interior, the isostatic condition 

 may not be affected. However, there is perhaps some reason to 

 question the whole idea of periodicity. 



Continuity of diastrophism in the past does not mean that there 

 was diastrophism of all parts of the crust at all times, but that 

 the earth's crust was taking up shrinkage of the earth's interior 

 by being constantly folded or faulted in one or several places 

 according to the concentration of the shortening. Therefore, 

 periodic folding in one region does not prove that diastrophism 

 was universally periodic. Folding very likely occurred in the weak- 

 est zones of the crust. After a certain amount of shortening a 

 zone may become more rigid and allow the folding to be transferred 

 elsewhere. 



Wide marine invasions of the lower parts of the continents are 

 thought to indicate periods of quiet unaccompanied by diastrophism. 

 If the shortening of the crust is temporarily transferred from the 

 continents to the ocean basins, the folding under these basins 

 will raise the sea-level and cause an inundation of the lower parts of 

 the continents which will produce the epi-continental seas without 

 a cessation of diastrophic movements. 



If diastrophism has been continuous in the past, would its 

 records of times of disturbance be equally continuous ? We have 

 a vast store of illustrations of past diastrophic movements, but in 

 very few instances are we able to give the exact time at which the 

 orogenic movementwas initiated and the time at which it terminated. 

 Much too often a clue as to the time of origin of one small section 

 of a great mountain system has to be taken as the time of origin 



