20 FORMATION OF THE EARTH 



place among living beings as a distinct period in the earth's 

 history. 



Each of the eras just defined has been divided into many 

 periods corresponding alike to the formation of certain parts of 

 the great folds briefly described above, to a certain phase in the 

 evolution of life, or to specific characteristics of the deposits 

 then formed. We shall confine ourselves to enumerating these 

 in the order of their formation, beginning with the oldest. 

 Their names will be so many landmarks to which we can relate 

 the various developments to be recorded in connexion with the 

 evolution of life on our planet. 



The oldest known deposits have been completely trans- 

 formed into crystalline rocks or mica schists in which traces 

 only of fossils have been discovered. They belong to a pre- 

 Cambrian era, in which two periods are recognized : the 

 Archaean and the Algonkian. It is followed by the primary 

 epoch, comprising five periods : i, the Cambrian whose deposits 

 contain the earliest well-characterized remains of living beings ; 

 2, the Silurian ; 3, the Devonian ; 4, the Carboniferous, whose 

 rich vegetation produced the most important coal deposits of 

 our country ; 5, the Permian, which immediately precedes the 

 secondary period. 



The secondary period, in its turn, is divided into three great 

 periods : 1, the Triassic, the period of transition ; 2, the 

 Jurassic, during which enormous coral reefs such as those 

 encountered to-day in tropical regions were formed along our 

 coasts ; 3, the Cretaceous, in which the oceans were deepened 

 and a fine calcareous ooze formed on their floor which later 

 became chalk. 



Finally, the Tertiary era, which witnessed the appearance 

 and multiplication of animals more and more similar to those 

 of our own times, has been subdivided into two great periods 

 according to the proportion of animals with representatives 

 still existing encountered in their fauna : the Eogene or 

 Nummulitic, during which the sea was full of very simple 

 organisms, which formed disc-shaped shells — nummulites — and 

 the Neogene period, rich in animals of our present fauna. 

 These periods have been again divided into two subdivisions : 

 the Eogene, into the Eocene and Oligocene, and the Neogene 

 into the Miocene and Pliocene. Sometimes another, the 

 Pleistocene, corresponding to the quaternary, is added. 



