TO QUESTION THE THEORY OF PERIODIC 

 DIASTROPHISM 



FRANCIS PARKER SHEPARD 



University of Illinois, Urbana, Illinois 



Periodic diastrophism as it is understood by T. C. Chamberlin, 

 R. T. Chamberlin, Schuchert, Willis, and most other writers, does 

 not mean that diastrophism has only been locally periodic, but that 

 periodicity has been world wide. The writer has no desire to attack 

 the well-established idea of peneplanation and relative quiescence 

 locally over long periods, but rather to challenge such statements as: 

 "the earth has passed through periods of diastrophic activity alter- 

 nating with periods of relative quiescence,"^ and to replace this idea 

 with the suggestion, that diastrophism has been continuous and that 

 the crust has been constantly shortening to fit itself to a shrinking 

 interior. Among American geologists the theory of periodic dias- 

 trophism is generally accepted, so that it is no small undertaking to 

 attempt its overthrow. However, it seems to be of vital importance 

 to a knowledge of the causes and modes of deformation of the earth's 

 crust to know whether such deformation is continuous or intermit- 

 tent. Also attempts to correlate geological time by diastrophism 

 have been made. At least the subject should be critically examined. 



In the present article periodic diastrophism will be considered 

 from three points of view. First, a brief examination of the times 

 of diastrophism will be made to see where the gaps, if there are any 

 such, occur in the diastrophic record. Secondly, the apparent con- 

 centration of diastrophism at a few times during the geological 

 history will be considered. Thirdly, the idea of accumulation of 

 strains within the earth over long periods before yielding will be 

 questioned. 



THE GEOLOGICAL RECORD OF DIASTROPHISM 



Since the conception of periodic diastrophism is based primarily 

 on field observations, some consideration of the evidence offered by 

 these will be made. The accompanying table has been compiled 



' R. T. Chamberlin, "Periodicity of Paleozoic Diastrophism," Jour, of GeoL, 

 Vol. XXII, p. 315. 



599 



