260 HISTORICAL GEOLOGY. 



zoic, or sometimes called transition system of rocks ; 

 3. Mesozoic,* in the Secondary system ; 4. Cenozoic,f in 

 the Tertiary and Quaternary systems ; and, 5. Psycho- 

 zoic,]; in the present system of sediments. These five are 

 separated in the diagram (Fig. 163) by the heavy lines, 

 and their names are given on the right. 



How separated. These primary divisions (unless we 

 except the last) are separated by a universal or almost 

 universal unconformity, indicating wide-spread changes 

 in physical geography at these times ; and by sweeping 

 changes in organic forms, involving not only species, but 

 genera, families, and orders. The changes between the 

 last two were not so great either in the rocks or in the 

 organisms, but the introduction of man, and the sweep- 

 ing changes going on now by his agency, are deemed of 

 sufficient importance to make this a primary division. 



Ag-es. The whole history of the earth is divided, on 

 a different principle, into seven ages, characterized each 

 by a dominant class. In some cases these correspond to, 

 and in some are subdivisions of, the eras. These ages, 

 and their corresponding rocks, when they are subdivisions, 

 are separated usually by unconformity, but not so uni- 

 versal ; and by changes of organisms, but not so sweeping. 

 They are 1. Archaean or Eozoic age, corresponding to 

 the Eozoic era. 2. Age of Mollusks, or age of Inverte- 

 brates, corresponding to the Silurian system of rocks. 



3. The age of Fishes, corresponding to the Devonian. 



4. The age of Acrogens, or age of Amphibians, corre- 

 sponding to carboniferous strata. 5. The age of Eeptiles, 

 corresponding to the Mesozoic era and Secondary rocks. 

 6. The age of Mammals, corresponding to the Oenozoic 

 era and the Tertiary and Quaternary rocks. 7. The age 

 of Man, corresponding to the Psychozoic era and the 

 present sediments. 



* Mesozoic = middle life, f Cenozoic = recent life. $ Psychozoic = 

 rational life. 



