A PIECE OF COAL. 1 87 
of Northumberland and Durham, where all is now black 
with coal-dust, and grimy with the smoke of furnaces ; 
and where the noise of hammers and steam-engines, 
and of carts and trucks hurrying to and fro, makes the 
country re-echo with the sound of labour ; there ages 
ago in the silent swamp shaded with monster trees, one 
thin layer of plants after another was formed, year after 
year, to become the coal we now value so much. In 
Lancashire, busy Lancashire, the same thing was 
happening, and even in the middle of Yorkshire and 
Derbyshire the sea must have come up and washed a 
silent shore where a vast forest spread out over at least 
700 or 800 square miles. In Staffordshire, too, which 
is now almost the middle of England, another small 
coal-field tells the same story, while in South Wales 
the deep coal-mines and number of coal-seams re- 
mind us how for centuries and centuries forests must 
have flourished and have disappeared over and over 
again under the sand of the sea. 
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. 
