A PIECE OF COAL. 185 
&c., from being decomposed by the air and sun. 
And so year after year as the plants die they leave 
their remains for other plants to take root in, and the 
peaty mass gro^s thicker and thicker, while tall 
cedar trees and evergreens live and die in these vast, 
swampy forests, and being in loose ground are easily 
blown down by the wind, and leave their trunks to be 
covered up by the growing moss and weeds. 
Now we know that there were plenty of ferns and 
of large Calamites growing thickly together in the coal- 
forests, for we find their remains everywhere in the 
clay, so we can easily picture to ourselves how the 
dense jungle formed by these plants would fringe the 
coal-swamp, as the present plants do the Great 
Dismal Swamp, and would keep out all earthy matter, 
so that year after year the plants would die and form 
a thick bed of peat, afterwards to become coal. 
The next thing we have to account for is the bed of 
shale or hardened clay covering over the coal. Now 
we know that from time to time land has gone slowly 
up and down on our globe so as in some places to 
carry the dry ground under the sea, and in others to 
raise the sea-bed above the water. Let us suppose, 
then, that the great Dismal Swamp was gradually to 
sink down so that the sea washed over it and killed 
the reeds and shrubs. Then the streams from the 
west would not be sifted any longer but would bring 
down mud, and leave it, as in the delta of the Nile or 
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 
