THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 



and known present but a very imperfect representation of the 

 species that were living ; but of those preserved in one forma- 

 tion there are generally found in the succeeding formations 1 

 representatives of the same or closely allied genera ; so that, 

 for the kinds of organisms whose remains are best preserved, 

 the record is fairly continuous for the grander rock-systems < 

 in terms of the generic, and in some cases of the specific' 

 characters. 



Cenozoic 



Mesozoic < 



Paleozoic* 



Glacial revolution 



Rocky Mountain 

 Revolutions 



Palisade revolution 



Appalachian 

 Acadian " 



Taconic 



? Pre-Cambia'an * 



? Archaean 



FIG. 2. Diagram representing the order of succession from below upward of the formation of the 

 geological systems in North America and the approximate time at which the grander revolu- 

 tions eroded and disturbed the already made deposits. 



Time-ratios, or the Relative Time-value of the Several Sys- 

 tems. While the conditions of deposition for a particular 

 region remained relatively constant and uniform, the strata 

 were accumulated in successive beds one upon another; and 

 thus the thickness of the deposits of the same kind, with pro- 

 portionate thickness for deposits of different kinds, constitutes 

 a scale of definite time-value ; a foot of deposit representing 

 a period of time, and the relative time-separation of two 

 faunas is represented by the thickness of the strata between 

 them. It was on this principle that the time-ratios of Dana 

 were estimated. The maximum thickness of the known 



