CHAPTEK YII. 
THE ERA OF ANCIENT LIFE.—Continued, 
THE CARBONIFEROUS AGE. 
Investigation of formation Number 4, the rocks 
that lie exposed on top of those formed in the Devo¬ 
nian age, has brought to light fossils of quite a different 
character. Among the strata are layers of black rock. 
In central Illinois this rock is not nearly so hard as 
limestone, and seems to be quite unlike it in structure. 
We call this rock coal. Alternating with the coal 
layers are layers of shale and limestone. The period 
during; which the coal was formed is known as the 
Carboniferous age; the lower layers are called the 
Subcarboniferous. 
The number of layers or coal-seams, as they are 
called, varies in different places from a dozen to one 
hundred and seventeen, the latter being in Europe, 
where the total thickness amounts to two hundred 
and seventy-four feet. In Illinois the total thickness 
of the coal-seams is seventy feet. The layers vary 
from a few inches to forty feet in thickness, but in 
few places are they over seven or eight feet thick. 
The coal-seams are by no means all the rock that 
was formed during this age; perhaps they are not 
over one-fiftieth part of it. 
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