TO QUESTION PERIODIC DIASTROPHISM 603 



active to a moderate degree, but very much concentrated at certain 

 times. While there seems to be very Httle evidence that such has 

 been the case in the Cenozoic, there is some evidence for it in the 

 Paleozoic and perhaps in the Mesozoic. Most authors of textbooks 

 and many other geologists speak of the end of the Paleozoic and the 

 end of the Cretaceous as being times of very great disturbances all 

 over the world. The end of the Ordovician, the end of Silurian, and 

 the late Devonian are other times when diastrophism is said to have 

 been more common than usual.^ 



In considering the probability of this type of periodicity, several 

 points have to be borne in mind. In the first place, to have real 

 evidence of periodicity, in each case there must be fossil-bearing 

 horizons both directly above the unconformity and directly below. 

 Such evidence is lacking in all but a few cases, as will be readily 

 seen if the evidence for the times of deformation for these so-called 

 epochs of great orogeny is examined. If the formation which is 

 folded belongs to the next period, and if the exact horizon is unknown 

 in each case, the deformation is generally referred with confidence 

 to the end of the first period. If there is Silurian strata involved in 

 folding and Devonian strata overlying the unconformity, provided 

 that nothing is known about the exact age in either case, the defor- 

 mation may have occurred well within the Silurian or even well 

 within the Devonian period. If on the average an unconformity 

 following mountain building represents a stratigraphic break of 

 three-fourths of a period, the plotting of this extent of time will show 

 that the faunas above and below the break represent different periods 

 except when the center of the time break occurs within one-eighth 

 of the center of the period. If the interval of the stratigraphic 

 break is larger than this (and it generally is), the chances become 

 still greater for discovering faunas of different periods above and 

 below. Therefore, even if there is abundant evidence that diastro- 

 phism in many parts of the world occurred at the break in the strati- 

 graphic record between periods, the diastrophism may have been 

 distributed widely over the two periods. 



Also, not infrequently, diastrophism is considered to have 

 occurred directly after the close of a period when the evidence merely 



' R. T. Chamberlin, Jour, of Geol., Vol. XXXI, pp. 318-32. 



