Cii. XIII] 



SECONDARY, AND PRIMARY EPOCHS 



301 



temp 

 migrations 



an accelerating rather than a retarding influence on the usual 



rate of fluctuation. 



We must go back as far as the older Miocene formations 

 of our table at p. 139, in order to arrive at a period when the 

 marine shells differ as a whole from those now existing. The 

 antecedent Tertiary strata from the beginning of the Eocene 

 to the close of the Lower Miocene comprise two great revo- 

 lutions in organic life, as measured by the change of testacea 

 each of them equal in magnitude to that which has happened 

 in the interval which separates the Lower Miocene period 



So that we should thus obtain three periods, 



times 



terms 



Miocene 



5 and Pliocene in our table. A fourth of equal dura- 

 tions indicated by the change in organic life which occurred 

 between the end of the Cretaceous and the beginning of the 

 Eocene epoch, a great gap, to which as yet few geological 



records refer. 



million 



above 



claim 

 r reasons 

 complete 



revolution in species. The antecedent Cretaceous, Jurassic, 



mor 



lmpor 



inner 



may 



Permian 

 Trias. 



which separates it 



om 



the 



To these eight periods we may add, continuing 



more 



ous, Devonian, 



Cambrian; so that we should 



have twelve in all, without reckoning the antecedent^ Lau- 

 rentian formations, which are older than the Cambrian or 



primordial 



If each, therefore, of the 



twelve periods represents twenty million of years on principles 

 above explained, we should have a total of two hundred and 



millions 



Another 



since the beginning of the Cambrian period. 



Supposed variations in the temperature of space.-— 

 astronomical hypothesis respecting the possible cause of 

 secular variations in climate, has been proposed by a dis- 



