TO QUESTION PERIODIC DIASTROPHISM 605 



the fossils are concerned because of wrong identifications and faulty- 

 interpretation of the restriction of certain species. For example, 

 the Laramie, after wliich many of the Rocky Mountain deformations 

 are supposed to have occurred, has proven to be a fictitious horizon 

 so that it is difficult to say whether the different ranges were con- 

 temporaneous or not. The extermination of a fauna seems often 

 to have been due to diastrophism. If diastrophism came at a some- 

 what different time in one part of the world than another, the faunas 

 might change at correspondingly different times. This also might 

 make diastrophism appear to have been contemporaneous, when it 

 actually was not. 



UNRECORDED DIASTROPHISM 



Even if we consider that the evidence for diastrophism shows a 

 greater amount of deformation at certain times during the geological 

 history, there is good ground for doubting the validity of such evi- 

 dence so far as it goes to prove that diastrophism was periodically 

 intensified. In some parts of the geological column we find almost 

 no traces of Hfe, in others the remains of life are most abundant. 

 The conclusion drawn from this evidence is not that life became 

 almost extinct at times, but that conditions for its preservation were 

 not as good. The same thing has probably been true of mountain 

 building. The rapid oscillation of marine incursions have in some 

 cases preserved the diastrophic record with more accuracy than when 

 there were long intervals on either side of the diastrophism without 

 marine invasions. 



Various other ways in which diastrophism is likely to have 

 occurred without leaving a record can be suggested. A few of these 

 will be discussed. 



I. Records removed by erosion or deposition. — If great erosion 

 follows the development of a mountain range, it is always possible 

 for the fossil ferous beds which were involved in the folding to be 

 removed and for pre-Cambrian metamorphics or intrusive cores 

 to be the only remnant of a folded system. If erosion continues in 

 the Colorado Rockies along the sides of the Front range, the 

 upturned sedimentary rocks, which give evidence as to the time of 

 the disturbance, may be removed leaving only a metamorphic core. 



