282 GENERAL PRINCIPLES. 



principles that geologists have established the divisions of time and 

 the corresponding divisions of strata. 



The whole history of the earth is divided into five eras, with corre- 

 sponding rock-systems. These are : 1. Archman or Eozoic * era, em- 

 bodied in the Laurentian system ; 2. Palceozoic f era, embodied in the 

 Palaeozoic or Primary system ; 3. Mesozoic J era, recorded in the Sec- 

 ondary system ; 4. Cenozoic* recorded in the Tertiary and Quater- 

 nary systems ; and, 5. The Psychozoic era, or era of Mind, recorded in 

 the recent system. 



These grand divisions, with the exception of the last, are founded 

 on an almost universal unconformity of the rock-system, and a very 

 great and apparently sudden change in the life-system, a change affect- 

 ing not only species but also genera, families, and even orders. Be- 

 tween the last and the preceding, it is true, neither the unconformity 

 of the rock-system nor the change in the life-system is so great as in 

 the others ; but the introduction of man upon the scene and the sweep- 

 ing changes which are now going on through his agency are deemed 

 sufficient to make this one of the grand divisions of time. 



We have already seen (p. 179) that unconformity is the result of 

 deposit of strata on old eroded land-surfaces, and that it therefore 

 always indicates an oscillation of the crust, and an emergence and sub- 

 mergence of land. In every such case, as already explained, a portion 

 of the record is lost, which may or may not be recovered elsewhere. 

 It is certain that if the lost leaves could be all recovered, and the rec- 

 ord made complete, the suddenness of the break in the life-system 

 would disappear. Nevertheless, it is also certain that these general 

 unconformities indicate times of great change in physical geography, 

 and therefore of climate, and therefore of rapid changes of organic 

 forms ; and therefore, also, they mark the natural boundaries of the 

 great divisions of time. 



Ages. — Again, the whole history of the earth is otherwise divided 

 into seven ages, founded, with perhaps the exception of the first, on the 

 culmination of certain great classes of organisms. These are : 1. The 

 Arclman or Eozoic Age, represented by the Laurentian system of 

 rocks ; 2. The Age of Molluslcs, or Age of Invertebrates, represented 

 by the Silurian system of rocks ; 3. The Age of Fishes, represented by 

 the Devonian rocks ; 4. The Age of Acrogens, or sometimes called the 

 Age of Amphibians, represented by the Carboniferous rocks ; 5. The 

 Age of Reptiles, represented by the Secondary rocks ; 6. The Age of 

 Mammals, by the Tertiary and Quaternary ; and, 7. The Age of 

 Man, by the recent rocks. 



In the accompanying diagram (Fig. 254), vertical height represents 



Dawn of animal life. f Old life. % Middle life. # Recent life. 



