THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 53 



ifcrovs period (Mississippian or Subcarbonifcrous), Mcso- 

 carboniferoiis period (coal measures), and Eocarbonifcrous 

 (Permian). 



In the Triassic and Jurassic eras no divisions have been 

 defined which can be recognized in other continents than 

 where described ; hence the periods are equivalent to the eras, 

 one period for each. 



The division of the Cretaceous into Eocrctaceous and 

 Neocretaccoiis periods is fairly well recognized in several con- 

 tinents. 



In the Tertiary era Eocene is the first period, and the 

 Neocene period includes the Miocene and the Pliocene. 

 And finally the Recent period may be regarded as geologically 

 the time of the living of the fauna associated with man. 



Uce of the Term Epoch in the Time-Scale. — The term Epoch 

 may be appropriately applied as an expression for the time- 

 duration of each local formation : thus we may speak of the 

 epoch of the Iberg limestone of the Hartz; of the Psammites 

 of Condroz in France; of the Marwood beds of England; of 

 the Dominik slates of Russia ; of the Chemung of eastern 

 North America; of the Lime Creek beds of Iowa. These 

 are each of them well-defined formations in separate regions, 

 each having a distinct geological structure, thickness, and 

 relative stratigraphic position, and the period of each is neo- 

 devonian ; but the faunas, although distinctive and constitut- 

 ing the means of determining the geological age, are not alike; 

 and in time-values it is not possible to say that one is or is 

 not the exact equivalent of the other. An epoch, upon this 

 basis, would be a definite division of a period, distinguishable 

 in the history of the organisms of a restricted region, but not 

 of universal application. 



With the present means of correlation it is impossible to 

 attain a greater degree of precision, in comparing the fossil 

 fauna-floras of widely separate regions, than to distinguish 

 the periodsby their characteristic species. 



A Comparative Time-scale for the Study of the History of Organ- 

 isms. — The tabulation of these facts and nomenclatures pro- 

 duces a standard geological time-scale for use in discussing 

 the historv of org-anisms. 



