568 CENOZOIC TIME. 



GENERAL OBSERVATIONS ON THE CENOZOIC. 



1. Time-ratios. — Using the same kind of data as on pp. 386 and 

 493 for determining the relative lengths of the ages and periods, we 

 have for the Tertiary period the maximum thickness of the Eocene 

 beds of Europe about 3000 feet, part of which is limestone, and for 

 the Miocene and Pliocene 7000 to 8000 (in the Molasse of Switzer- 

 land). It is therefore probable that the Tertiary period was about 

 half as long as the whole Mesozoic (p. 493). 



The data for the Post-tertiary are too uncertain for a satisfactory 

 estimate. The lapse of time during the period is more marked in 

 the extent of the valleys made than in the thickness of the rock 

 deposits. The latter are small, because apparently the earth's sur- 

 face was undergoing much smaller oscillations in this last end of 

 its history than in earlier times. From the extent of valleys over 

 the world, both fiords and land gorges, whose excavation was accom- 

 plished in the Post-tertiary, it is safe to infer that this period was 

 at least half as long as the Tertiary. 



Adopting these conclusions, the ratios for the Palaeozoic, Meso- 

 zoic, and the two periods of the Cenozoic will be 14 : 4 : 2 : 1. If 

 D'Orbigny's statements of the thickness of the European Cretaceous 

 (p. 493) are right, these ratios for the Palaeozoic, Mesozoic, and Ceno- 

 zoic are nearly 4:2:1. 



2. Geography. — The geographical progress of the Tertiary and 

 Post-tertiary periods took place in different directions. 



A. Tertiary period. — In the Tertiary, there was (1) the finishing of 

 the rocky substratum of the continents ; (2) the expansion of the 

 continental areas to their full limits, or their permanent recovery 

 from the waters of the ocean ; (3) the elevation of many of the 

 great mountains of the globe, or considerable portions of them, 

 through a large part of their height, as the Alps, Pyrenees, Apen- 

 nines, Himalayas, Andes, Rocky Mountains, the loftiest chains 

 of the globe, — a result not finally completed until the latter part of 

 the Tertiary. 



In North America there occurred a small extension of the con- 

 tinent on the Atlantic and Gulf borders ; a vast increase west of 

 the Mississippi ; a small rising of the land on the east and south, 

 an elevation of 6000 to 7000 feet in the Rocky Mountains (nearly 

 the whole height of the mass) and 2000 feet or more on the Pacific 

 border. 



The system of progress during the Tertiary was in each respect 

 a continuation of that which began with the Azoic. In North 

 America it was enlargement and elevation especially to the south- 



