CHAPTER XXXI 

 CENOZOIC ERA — TERTIARY PERIOD 



The history of the Cenozoic era brings us by gradual steps to 

 the present order of things. Of no part of geological history have 

 such full and diversified records been preserved as of the Ceno- 

 zoic, and yet this very fulness is a source of difficulty and embar- 

 rassment when we attempt to arrange the various phenomena in 

 their chronological order. 



The sedimentary rocks of the Cenozoic era are, for the most 

 part, quite loose and uncompacted ; it is relatively rare to find 

 hard rocks, such as so generally characterize the older formations. 

 They are also most frequently undisturbed, retaining nearly their 

 original horizontal positions, except when they have been upturned 

 in the formation of great mountain chains. Another characteristic 

 feature of Cenozoic strata is their locally restricted range ; only 

 in the oldest parts of the group do we find such widely extended 

 formations as are common in the Palaeozoic and Mesozoic groups, 

 and the later Cenozoic strata become more and more local in their 

 character. 



The climate of the era underwent some very remarkable and 

 inexplicable changes. At the beginning it resembled that of the 

 Cretaceous in its generally mild and equable character, a luxuriant 

 vegetation flourishing far within the Arctic Circle ; but by very 

 slow gradations the climate grew colder, culminating in the Clacial 

 Age, when much of the land in the Northern Hemisphere was 

 covered with sheets of ice and snow and reduced to the condition 

 of modern Greenland. 



The life of the Cenozoic era is very clearly demarcated from 

 that of the Mesozoic, though many modern characteristics began 

 in the Cretaceous or even earlier. The peculiar Mesozoic Am- 



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