A PIECE OF COAL. 
189 
Mississippi, to make a layer over the dead plants. 
You will easily understand that this mud would have 
many pieces of dead trees and plants in it, which were 
stifled and died as it covered them over; and thus the 
remains would be preserved like those which we find 
now in the roof of the coal-galleries. 
But still there are the thick sandstones in the coal- 
mine to be explained. How did they come there? 
To explain them, we must suppose that the ground 
went on sinking till the sea covered the whole place 
where once the swamp had been, and then sea-sand 
would be thrown down over the clay and gradually 
pressed down by the weight of new sand above, till it 
formed solid sandstone and our coal-bed became buried 
deeper and deeper in the earth. 
At last, after long ages, when the thick mass of 
sandstones above the bed b (Fig. 48, p. 177) had been 
laid down, the sinking must have stopped and the land 
have risen a little, so that the sea was driven back; 
and then the rivers would bring down earth again and 
make another clay-bed. Then a new forest would 
spring up, the ferns, Calamites, Lepidodendrons, and 
Sigillarias would gradually form another jungle, and 
many hundreds of feet above the buried coal-bed b, a 
second bed of peat and vegetable matter would begin 
to accumulate to form the coal-bed a. 
Such is the history of how the coal which we now 
dig out of the depths of the earth once grew as beauti- 
ful plants on the surface. We cannot tell exactly all 
the ground over which these forests grew, because 
some of the coal they made has been carried away 
