14 Schuchert and Barrett — Revised Geologic 



true, a basis for the natural division of geologic events, these 

 movements being in themselves and in their immediate con- 

 sequences the basis of such division. 



" The major movements of the earth's surface have con- 

 sisted of the sinking of the ocean bottoms and the withdrawal 

 of additional waters into the basins whose capacities were 

 thereby increased." Then, too, the master factor in the great 

 crustal readjustment has been the progressively greater " radial 

 shrinkage of the ocean bottoms" surpassing the "radial shrinkage 

 of the continental platforms to the average amount of some 

 10,000 or 12,000 feet." Besides these periodic crustal readjust- 

 ments resulting from internal causes, there are external read- 

 justments of long duration, the quiescent periods, working u to 

 precisely opposite ends, the degradation of the land and the 

 tilling of the basins." 



Terminology and definition. — The local warpings as a rule 

 bring about the development of formations and disconfor- 

 mities. These are grouped together into the periods by the 

 " minor diastrophic movements," or the " epicycles " of Willis 

 (1913), when local mountain ranges are developed. As there 

 are many of these movements and as they are not of the first 

 order of magnitude, we propose to call them disturbances, to 

 distinguish them from the rarer but far greater events, the 

 " major diastrophic movements," or revolutions of Dana, which 

 group the periods together into eras. 



It seems probable that the periods were all separated by dis- 

 turbances, events occurring now in this and now in that con- 

 tinent, but in each case enlarging some oceanic basin and so 

 reacting on the strand-line the world over. On the other hand, 

 during the revolutions, all of the oceans were enlarged by the 

 sinking or broadening of their basins, and all the continents 

 were more or less reelevated. These latter are the " critical 

 periods" in the history of the earth and are marked by the 

 following features (somewhat modified from LeConte, 1900): 



(1) By widespread deformation of the earth's crust, trans- 

 mitted from place to place. This leads to the elevation of 

 many and widely separated mountain ranges, followed by long 

 intervals of erosion and mountain removal, and therefore by 

 almost universal unconformities. Because of the long-enduring 

 intervals of lost record, the subsequent faunas are not only 

 very different, but appear as if suddenly or at least quickly 

 evolved. Each revolution or critical period is named after one 

 of the prominent mountain ranges formed at the time desig- 

 nated, for example, Laramide and Appalachian revolutions. 

 The subsequent interval is the transition period from one era 

 to the next. 



