THE DIVISIOXS OF THE GEOLOGICAL TIME-SCALE. 4/ 



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 

 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 < 



g MiocEJii 1 / Rocky Mountain 

 Revolutions 



Paleozoici 



QUATERNAHv"^ Glaciul revolutiou 



Palisade revolution 



Appalachian 



Acadian 



Taconic 



? Pre-Cambrian •* 



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 



