190 
THE FAIRY-LAND OF SCIENCE. 
since by rivers and cut down by the waves of the sea, 
but we can say that wherever there is coal now, there 
they must have been. 
But what is it that has changed these beds of dead 
plants into hard, stony coal? In the first place you 
must remember they have been pressed down under 
an enormous weight of rocks above them. We can 
learn something about this even from our common 
lead pencils. At one time the graphite or pure carbon, 
of which the blacklead (as we wrongly call it) of our 
pencils is made, was dug solid out of the earth. But 
so much has now been used that they are obliged 
to collect the graphite dust, and press it under a heavy 
weight, and this makes such solid pieces that they can 
cut them into leads for ordinary cedar pencils. 
Now the pressure which we can exert by machinery 
is absolutely nothing compared to the weight of all 
those hundreds of feet of solid rock which lie over the 
coal-beds, and which has pressed them down for thou- 
sands and perhaps millions of years; and besides this, 
we know that parts of the inside of the earth are very 
hot, and many of the rocks in which coal is found are 
altered by heat. So we can picture to ourselves that 
the coal was not only squeezed into a solid mass, but 
often much of the oil and gas which were in the leaves 
of the plants was driven out by heat, and the whole 
baked, as it were, into one substance. The difference 
between coal which flames and coal which burns only 
with a red heat, is chiefly that one has been baked 
and crushed more than the other. Coal which flames 
has still got in it the tar and the gas and the oils 
which the plant stored up in i{s leaves, and these when 
