GEOLOG Y. 12T 
Cretaceous period we leave the Secondary Age, and pass 
on to the Tertiary. 
AGE OF PALMS, EXOGENS, AND MAMMALS. 
The Tertiary Age is subdivided into the Eocene, Miocene, 
and Pliocene periods. These names were chosen to express 
the result of a comparison made between the shells found 
in the rocks of the Tertiary formation and those living at 
the present day, the object in view being to determine 
whether many living shells are found petrified in the 
Tertiary rocks. Thus, in Sicily, of one hundred petrified 
shells, from seventy to ninety are found in existing seas; 
hence the name Pliocene, or most recent, was given to 
rocks containing such a large proportion of living shells. 
Those parts of the Tertiary formation known as Miocene, 
or less recent, have from forty to fifty per cent., while only 
the dawn of recent shells is expressed by the term Eocene. 
The subdivision of the Tertiary Age into these three periods, 
originally based on the proportion between the fossil and 
living shells, was afterward applied to Tertiary plants and 
animals generally, it being supposed that a proportion 
similar to that of fossil and living shells existed between 
Tertiary plants and animals and those of the present day. 
These. periods often, however, pass so gradually into one 
another, the lines of demarkation not being very well 
defined, that this classification is not always applicable. 
The Tertiary Age, notwithstanding the minor differences 
of its periods, is essentially an age of Mammals, Palms, 
and Exogens. There is no necessity of describing the 
details of its animal and vegetal life, since Asia and Africa, 
with their Hippopotami, Rhinoceroses, Elephants, Lions, 
and Tigers, living amidst the characteristic tropical plants, 
give one an excellent idea of what America, Great Britain, 
etc. were during their Tertiary Ages. To the Evolutionist the 
