THE TERTIARY ERA 55 



into four great eras. The first is the Eozoic, or the age 

 of the Archaean, often called Pre-Cambrian rocks ; 

 rocks largely volcanic, or greatly altered since their 

 formation, showing only obscure traces of the life which 

 no doubt existed. Then follow the Primary era, or, as 

 it is generally called, the Palaeozoic ; the Secondary or 

 Mesozoic ; and the Tertiary or Kainozoic. Palaeozoic is 

 used rather than Primary, as this word is ambiguous, 

 being also used for the crystalline rocks first formed by 

 the solidification of the molten surface of the earth. 

 But Secondary and Tertiary are still in constant use. 

 These long ages, or eras, were of very unequal duration ; 

 yet they mark such changes in the life of animal and 

 plant upon the earth that they form natural divisions. 

 The Palaeozoic was an immense period during which life 

 abounded in the seas, numberless species of mollusca, 

 crustaceans, corals, fish are found, and there were great 

 forests, which have formed the coal measures, on land, 

 forests of strange primeval vegetation, but in which 

 beautiful ferns, large and small, flourished in great 

 numbers. The Secondary Era may be called the age of 

 reptiles. To this era all the rocks we have so far studied 

 belong. Now we come to the last era, the Tertiary, the 

 age of the mammals. Instead of reptiles on land, in sea 

 and air, we find a complete change. The earth is occupied 

 by the mammalia ; the air belongs to the birds such 

 as we see to-day. The strange birds of the Oolitic 

 and Cretaceous have passed away. Birds have taken 

 their modern form. In some parts of the world 

 strata are found transitional between the Secondary and 

 Tertiary. 



The Tertiary is divided into four divisions, the Eocene, 

 the Oligocene (once called Upper Eocene), the Miocene, 

 and the Pliocene ; which words signify, Pliocene the 

 more recent period, Miocene the less recent, Eocene the 

 dawn of the recent. 



