xxi. 3 DIVISIONS OF THE TERTIARY 571 



that can be correlated with the previously known Eocene marine beds 

 in other parts of the world, at which level there is a still wider variety 

 of placentals. 



It is clear, therefore, that there was a period of time of unknown 

 duration, following the disappearance of the dinosaurs, during which 

 the placentals were becoming differentiated. The old belief in the 

 sudden appearance of various types in the Eocene was, therefore, 

 perhaps chiefly an artefact of the conditions of fossilization at the 

 time. 



3. Divisions and climates of the Tertiary Period 



The whole Tertiary or Cenozoic period is now divided as follows: 



The names were originally given by Lyell to indicate the percentages 

 of modern species of shells; for curiosity these latter are given (ap- 

 proximately) in the third column. During the Palaeocene, Eocene, 

 and Oligocene the mountains raised during the Laramide revolution 

 were eroded. The climate was cold in the palaearctic region early in 

 the period, but it later became warmer and damper and there were 

 probably extensive forests during the later Eocene and Oligocene. 

 Palms then grew over much of Europe and there were forests where 

 are now immense areas of steppe. The climate probably showed 

 marked seasonal changes, at least in many parts of the world, and 

 deposits of the time often show a lamination ('varving') that indicates 

 an alternation of wet and dry seasons. During this first part of the 

 Cenozoic there was some invasion of the land by water, but this was 

 on a much smaller scale than during the inundations of earlier periods ; 

 indeed, the main land-masses have remained approximately constant 

 throughout the Tertiary. 



During the early Miocene there was a time of intense crustal 

 disturbance, the Cascadian revolution. In this many of the earth's 

 main mountain chains, the Rockies, Andes, Alps, Himalayas, as well 



