506 CENOZOIC TIME — MAMMALIAN AGE. 



introduced, it was preceded in the system of history by Primary 

 and Secondary. The first of these terms was thrown out when the 

 crystalline rocks so called were proved to belong to no particular 

 age, — though not without an ineffectual attempt to substitute it 

 for Palaeozoic ; and the second, after use for a while under a re- 

 stricted signification, has given way to Mesozoic. Tertiary holds 

 its place simply because of the convenience of continuing an ac- 

 cepted name. 



I. TERTIARY PERIOD (19). 



Epochs. — The Tertiary has not been satisfactorily divided into 

 epochs either in America or Europe. This arises from the fact that 

 in this later part of the Earth's history the changes in progress 

 were of a less comprehensive or more local character than in 

 earlier time. Consequently, the subdivisions often vary widely on 

 different parts even of the same continent. 



Lyell has divided the beds into three series, according to the 

 proportions of living species : — 



1. Eocene (from ?/wf, dawn, and ttaivog, recent, the same word from 

 which Cenozoic is derived) : species all extinct. (When the term 

 was introduced, 5 or 10 per cent, were supposed to be recent.) 



2. Miocene (from /xeiuv, less, etc.): 10 to 40 per cent, living 

 species. 



3. Pliocene (from irleiuv, more, etc.) : 50 to 90 per cent, living. 

 But these proportions are not capable of general application. It 



is possible that beds in America containing all extinct species may 

 be synchronous with those of Europe, in which there are ten or 

 fifteen species of recent shells. Moreover, not even the subdivisions 

 in different parts of Europe can be made to correspond to these 

 epochs : still, they are convenient terms for Lower, Middle, and Upper 

 Tertiary, and with proper caution may be used to the advantage of 

 the science. 



1. AMERICAN TERTIARY. 



Epochs : — 1. Claiborne, or that of the Tertiary beds of Claiborne* 

 Alabama; 2. Jackson, or that of the beds of Jackson, Mississippi; 

 3. Vicksburg, or that of the beds of Vicksburg', Miss. The species 

 of animals of these three epochs are all extinct, and the beds have 

 been referred to the Eocene. 4. Yorktown, or that of the beds of 

 Yorktown, Virginia, in which fifteen to thirty per cent, of the spe- 

 cies are living, — usually called Miocene. 5. Sumter, or that of the 

 beds of Sumter and Darlington districts of South Carolina, in which 



