FERNS, FOSSILS AND FUEL 
other and to the plants of the present era. We can also 
make a purely chronological grouping of the individual 
floras which have covered the earth successively during 
the great epochs of the past. This sort of classification 
we call geologic or stratigraphic. 
Concerning the oldest age of the geological series, which 
is usually called the Archeozoic, we have only theories, 
but we are bound to conclude that life in its lowest forms 
must have existed even then. The limestone and silica 
deposits of that time must have originated from organic 
secretions, and there are also graphite deposits whose 
carbon was undoubtedly derived from organic remains 
of some sort. There are large sedimentary accumulations 
dating from this early epoch. The formations which can 
now be traced back to that era constitute the complex of 
Archaean rocks, whose age has been estimated at 
1,300,000,000 years. We also know that there was much 
mountain raising during Archeozoic times and that at 
their close the North American continent was uplifted. 
The next great era we call the Proterozoic. Great sedi¬ 
mentary deposits of conglomerate sandstones, shales, and 
limestones, containing a large quantity of iron, have come 
down from it. We find formations of this age around 
Lake Superior, in the Rocky Mountains, in Canada, in 
Brazil, in China, and in South Australia. There are no 
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