HOW IS COAL FORMED! 
57 
tlie stumps which are found in coal-beds argue to the 
contrary, for trees evidently grew where the stumps are 
found. Then, too, only impure coal could have been 
thus formed, for at the mouths of rivers the vegeta¬ 
tion must have been mixed with great quantities of 
mud brought down with the trees by the freshets. 
The quality of coal depends upon the amount of car¬ 
bon it contains. The very best 
hard coal does not contain over 
ninety per cent of carbon; most 
of that which we use in our stoves 
and furnaces consists of scarcely 
more than half carbon. 
We may get some idea of the 
abundance of vegetation which 
flourished during this age from the 
number of fossils. They sum up 
eight thousand six hundred and 
sixty different kinds of plants. 
These plants consisted of cone¬ 
bearing trees something like our sandstone, 
pines, great ferns (mostly tree- 
ferns) ; one kind, the lepidodendron, has the bark 
peculiarly marked in five-sided figures. 
It will be interesting to gather some facts con¬ 
cerning the time it must have taken to form the coal¬ 
beds. Geologists have computed, from the decay 
going on in forests at the present time, that it would 
take one hundred years to form a layer one-eighth of 
an inch thick. To produce the aggregate depth of 
two hundred and fifty feet it wmuld require at least 
Fig. 33.—Cast of lepi¬ 
dodendron bark in 
