128 



HISTORICAL GEOLOGY. 



the transitions from one set of Tertiary or Cretaceous strata to 

 another were, in the two, cotemporaneous. The point should be 

 proved, not assumed. We add, therefore, — 



Sixthly. It is an important object in geology to ascertain as nearly 

 as possible the parallelism between the periods and epochs marked 

 off on each continent, and study out the precise equivalents of the 

 rocks, each for each, that all the special histories may read as parts 

 of one general history, and thus contribute to the perfection of one 

 geological system. 



Progress of life as the basis of the subdivision into geo- 

 logical ages. — The general principles in the progress of life upon 

 which the ages are based are shown in the annexed table.* 



Fig. 133. 



ANIMALS. 



PLANTS. 



Age of Man. 

 Age of Mammals 

 Age of Reptiles. 



Carboniferous Age 



Age of Fishes, or 

 Devonian. 



Age of Mollusks, 

 or Silurian. 





Azoic. 



The horizontal bands represent the ages, in succession ; the ver- 

 tical correspond to different groups of animals and plants. 



The Eadiates begin with the Lower Silurian, and continue till 

 now, rather increasing throughout the ages. 



The Mollusks have their beginning at the same time, and continue 

 increasing to the age of Eeptiles ; they then pass their maximum 

 (as indicated in the figure) and decline. 



* The system of ages is essentially the same with that proposed by Professor 

 Agassiz, — the only difference consisting in calling the Silurian the age of Mol- 

 lusks, instead of considering both the Silurian and Devonian the age of Fishes. 



