FERNS, FOSSILS AND FUEL 
these swamps were destroyed by water and by sand and 
mud. This process repeated itself many times, and each 
time accumulated vegetation was covered up and buried 
in thick mud and sand. This compressed and buried 
swamp vegetation turned into coal seams. 
There are as many coal seams in Illinois as there 
were subsidences of these bays to a point below water 
level. Each inundation of the swamp left shale, sand¬ 
stone, or limestone beds above the coal. The plants of 
the swamp left their leaves, stems, reproductive organs, 
and seeds in the shales and sandstones that covered the 
swamps, and their roots are frequently found in the old 
subsoil of the swamp which became the clay bed on which 
the coal seam rests. The result is that we have a large 
number of plants preserved from the Pennsylvanian 
period, but they are all remnants of swamp flora and do 
not represent the upland vegetation of that period. We 
know practically nothing about the latter except what 
we may infer from some of the fossil woods and seeds 
that we occasionally find in sandstones or which have 
been washed out of them and can be picked up in the 
ravines between the Pennsylvanian rocks. 
The shore swamps of the Pennsylvanian period were 
probably intersected by rivers, and the deltas of these 
old river beds are now the best fossil plant deposits. 
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