44 PRINCIPLES OF PAL.^ONTOLOGY. 



Of these primary rock divisions, the Laurentian, Cambrian, 

 Silurian, Devonian, Carboniferous, and Permian are collec- 

 tively grouped together under the name of the Primary or 

 PalcBOzoic rocks (Gr. palaios, ancient ; zoe, life). Not only do 

 they constitute the oldest stratified accumulations, but from 

 the extreme divergence between their animals and plants and 

 those now in existence, 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 to- 

 gether as the Seco7idary or Mesozoic formations (Gr. niesos^ inter- 

 mediate ; 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. kaifios, 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 Fost-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 

 PAL^ONTOLOGICAL 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. ^Ve must, 

 however, guard ourselves against too literal an interpretation 

 of the word "contemporaneous," and we must bear in mind 

 the enormously - prolonged periods of time with which the 

 geologist has to deal. When we say that two groups of strata 



