594 CENOZOIC ERA— AGE OF MAMMALS. 



and what took place in Glacial times, is the comparative universality 

 of the oscillations then, and especially their coincidence with certain 

 astronomical changes, which greatly increased their effect upon climate. 



Other and more direct methods of estimation, however, such as the 

 recession of waterfalls (p. 15) and of lake-shores and the extreme fresh- 

 ness of glacial scorings and polishings, seem to indicate a much shorter 

 time since the disappearance of the ice-sheet. This is again a strong 

 reason for believing that geographical changes were the main cause of 

 the climate. 



3. The Quaternary a Period of Revolution — a Transition between 

 the Cenozoic and the Modern Eras. — We have already seen (pp. 282 and 

 294) that between the great eras, and perhaps also at other times, there 

 have been periods of oscillation of the earth's crust, and therefore of 

 changes of physical geography, marked by unconformity of strata; and 

 changes of climate, marked by apparently abrupt changes of species. 

 These have been the critical periods of the earth's history — periods 

 of revolution and rapid change. But for that very reason they are 

 also periods of lost records. We have already spoken of the lost inter- 

 val at the end of the Archaean, evidently the greatest of all ; again, of a 

 lost interval at the end of the Palaeozoic, partly recovered in the Per- 

 mian, evidently the next greatest ; again, of a lost interval at the end 

 of the Cretaceous, in a large measure recovered in the Laramie beds. 

 There are doubtless many others of less extent. These periods are 

 always marked by unconformity of the strata and change in the life- 

 system. The old geologists regarded these changes as sudden and 

 cataclysmic. All geologists now regard the suddenness as largely ap- 

 parent, and the result of lost record. 



Now, the Quaternary is also a critical period. It corresponds 

 with one of the lost intervals ; only, in this case, on account of its near- 

 ness to us, the record has been recovered. By the study of this period, 

 therefore, we may hope to solve many problems which have heretofore 

 puzzled us. Here, for example, we have oscillations of the crust on a 

 grand scale, producing great changes of physical geography and cli- 

 mate, and therefore of fauna and flora. Here we have unconformity, 

 now being produced by sedimentation on old eroded land-surfaces in 

 all the region affected by the oscillations — marine sediments in fiords 

 and river sediments in old river-channels. But we observe that in this 

 case these effects have been produced slowly, and that the fauna and 

 flora have not been suddenly destroyed and suddenly recreated, but 

 have continued to live throughout, the species gradually changing. 

 But, what is still more interesting, much light is thrown also on the 

 hitherto insoluble problem of the mode and the cause of the compara- 

 tively rapid change of species in these critical periods. The attentive 

 study of the Quaternary shows that, in addition to the direct effect of 



