AMERICA THE OLD WORLD. 
15 
may enable us to determine the age of any Plu- 
tonic rock by its mode of crystallization ; so that 
the mineralogist will as readily tell you by its 
crystals whether a bit of stone of igneous origin 
belongs to this or that period of the world’s his- 
tory, as the palaeontologist will tell you by its 
fossils whether a piece of rock of aqueous origin 
belongs to the Silurian or Devonian or Carbon- 
iferous deposits. 
Although subsequent investigations have mul- 
tiplied so extensively not only the number of geo- 
logical periods, but also the successive creations 
that have characterized them, yet the first gen- 
eral division into three great eras was neverthe- 
less founded upon a broad and true generaliza- 
tion. In the first stratified rocks in which any 
organic remains are found, the highest animals 
are fishes, and the highest plants are crypto- 
gams ; in the middle periods reptiles come in, 
accompanied by fern and moss forests ; in later 
times quadrupeds are introduced, with a dicoty- 
ledonous vegetation. So closely does the march 
of animal and vegetable life keep pace with the 
material progress of the world, that we may well 
consider these three divisions, included under 
the first general classification of its physical his- 
tory, as the three Ages of Nature ; the more im- 
portant epochs which subdivide them may be 
compared to so many great dynasties, while the 
