44 PRINCIPLES OF PALAZZONTOLOGY. 
Of these primary rock divisions, the Laurentian, Cambrian, 
Silurian, Devonian, Carboniferous, and Permian are collec- 
tively grouped together under the name of the Avzmary or 
Paleozoic 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 Secondary or Afesozoic formations (Gr. mesos, inter- 
mediate ; oe, life); the organic remains of this ‘“‘ Middle-Life ” 
period being, on the whole, intermediate in their characters 
between those of the palzozoic epoch and those of more 
modern strata. Lastly, the Eocene, Miocene, and Pliocene 
formations are grouped together as the Zértiary or Kaznozoic 
rocks (Gr. ainos, 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-Tertzary deposits are placed with the Kainozoic, or 
may be considered as forming a separate Quaternary system. 
CHAPTER <IV. 
THE BREAKS 1N THE GEOLOGICAL Ane 
PALZONTOLOGICAL FECOKD: 
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 
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 
