GENERAL PRINCIPLES. 



283 



time, the strong horizontal lines divide the whole into eras, while the 

 lighter lines, where necessary, separate the ages. The shaded spaces 

 represent the origin, the increase and decrease, in the course of time, of 



Animals. 



Plants. 



Age of Man. 



Age of 

 Mammals. 



Age of 

 Keptiles. 



Age of 

 Acrogens. 



Age of 



Fishes. 



Age of In- 

 vertebrates. 



Archaean 



Age. 



I ;TJiMltttl"] 



Psychozoic. 



Cenozoic. 



1 Mesozoic. 



RBONIFERDUS 



Mill 



(LfC 



D E WON IAN 





/ CTJ R TAN 



LAURENTIAN 



SYSTEM 



S YS TE N\ 



Palaeozoic. 



Archaean, 

 or Eozoic. 



Fig. 254. 



the great dominant classes of animals and plants. To illustrate : The 

 class of reptiles commenced in the uppermost Carboniferous increased 

 to a maximum in the Secondary, and again decreased to the present 

 time. It will be seen that the ages correspond with the eras, except in 

 the case of the Palaeozoic era. This long and diversified era is clearly 

 divisible into three ages. 



Subdivisions. — The subdivisions of eras and ages into periods and 

 epochs are founded, as already explained (p. 201), on less general un- 

 conformity in the rock-system, and less conspicuous changes in the life- 

 system. The names of periods are often, and of epochs are nearly 

 always, local, and therefore different in different countries. We will, 

 of course, use those appropriate to American geology. The table on 

 page 200 represents, as far as periods, the classification used in this 

 country. We have added epochs only in the uppermost part, viz., in 

 the Tertiary and Quaternary. 



We give, also (Fig. 255), an ideal diagram of the principal groups 

 of strata which we shall notice, in the order of their 

 superposition, indicating also the principal places 

 of general unconformity. 



The terms used for the divisions of time, and 

 corresponding divisions of rocks, are shown in the 

 accompanying schedule : 



Order of Discussion. — Many geologists take up 



Time. 



Rocks. 



Eras ) 



Ages j" ' * 

 Periods . . 

 Epochs . . 



Systems. 



Series. 

 Groups. 



