1 88 THE FAIRY-LAND OF SCIENCE. 
the earth is sifted out and left behind. In this way 
the spongy mass of dead plants remains free from 
earthy grains, while the water and the shade of the 
thick forest of trees prevent the leaves, stems, etc., 
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 grows thicker and thicker, while tall cedar trees 
and evergreens live and die in these vast, swampy for- 
ests, 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 Dis- 
mal Swamp, and would keep out all earthy matter, so 
that year after year the plants would die and form 
a thick bed of peat, afterward 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 
