FERNS, FOSSILS AND FUEL 
reached the Pennsylvanian formations, for the Pennsyl¬ 
vanian in that region does not contain oil pools. A 
chunk of rock taken from the greatest depth of the drill 
hole, the so-called drill core, was shipped to a paleo- 
botanist. He was able to determine from the plant fos¬ 
sils in it that the rock belonged to a certain part of the 
Cretaceous period and not to the Pennsylvanian. So the 
drilling went on. 
The identification of different coal seams is frequently 
made in the same way on the basis of fossil plants, es¬ 
pecially in regions where the coal beds are very much dis¬ 
turbed and twisted, so that the drill core may run through 
the same coal seam several times. This often happens 
in the Ruhr district in Germany and in the Belgian coal 
fields. 
All the coal that we know was produced from swamps 
that had been buried in geologic times under an accumu¬ 
lation of sediments. Therefore, a knowledge of coal goes 
hand in hand with a knowledge of the fossil plants which 
have made it up. In coal geology we distinguish between 
the under clay on which the coal seam rests, the coal bed 
itself, and the roof which immediately overlays it. The 
under clay, as we have already noticed, is the subsoil on 
which the swamp grew; the coal bed is the carboniferous 
material formed from the vegetation of the swamp; and 
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