CHRONOLOGICAL SUCCESSION. 45 



Of these primary rock divisions, the Laurentian, Cambrian, 

 Silurian, Devonian, Carboniferous, and Permian are collectively 

 grouped together under the name of the Primary or Pal&ozoic 

 rocks (G. palaios, ancient; zoe, life). Not only do they constitute 

 the oldest stratified accumulations, but from the extreme diver- 

 gence between their animals and plants and those now in exist- 

 ence, they may appropriately be considered as belonging to an 

 " Old-Life " period of the world's history. The Triassic, 

 Jurassic, and Cretaceous systems are grouped together as the 

 Secondary or Mesozoic formations (Gr. mesos, intermediate; 

 zoe, life); the organic remains of this "Middle-Life" period 

 being, on the whole, intermediate in their characters between 

 those of the palaeozoic epoch and those of more modern strata. 

 Lastly, the Eocene, Miocene, and Pliocene formations are 

 grouped together as the Tertiary or Kainozoic rocks (Gr. kainos, 

 new; zoe, life) ; because they constitute a " New-Life" period, in 

 which the organic remains approximate in character to those now 

 existing upon the globe. The so-called Post-Tertiary deposits are 

 placed with the Kainozoic, or may be considered as forming a 

 separate Quaternary system. 



CHAPTER IV. 



THE BREAKS IN THE GEOLOGICAL AND 

 PALMONTOLOGICAL RECORD. 



The term " contemporaneous " is usually applied by geolo- 

 gists to groups of strata in different regions which contain the 

 same fossils, or an assemblage of fossils in which many iden- 

 tical forms are present. That is to say, beds which contain 

 identical, or nearly identical, fossils, however widely separated 

 they may be from one another in point of actual distance, are 

 ordinarily believed to have been deposited during the same 

 period of the earth's history. This belief, indeed, constitutes 

 the keystone of the entire system of determining the age of 

 strata by their fossil contents; and if we take the word "con- 

 temporaneous " in a general and strictly geological sense, this 

 belief can be accepted as proved beyond denial. We must, 

 however, guard ourselves against too literal an interpretation 



