FERNS, FOSSILS AND FUEL 
bodies, show exactly the material out of which the coal 
was formed. The structures thus revealed convince us 
that basic biologic processes existed in Pennsylvanian times 
as they do now, and that plants lived and reproduced 
themselves in the same way then as at the present time. 
There are differences in form but not in principle. 
It has been established that the qualities of our coal 
deposits depend to a large extent on the material from 
which they were made. A coal that was formed from 
wood is different from a coal that contains a large num¬ 
ber of spore remains. The latter is oily, has a larger 
percentage of volatile substances, and is better for oil 
production by low temperature distillation, while the 
former is more suitable for making coke. Such differences 
as these can be determined beforehand by a knowledge 
of the fossil forms that are found in the coal. 
The theory is now pretty generally accepted that most 
coal seams have been formed in situ . In other words, the 
plant material from which they are built up actually 
grew on the spot where the coal seam is. This is called 
autochthonous formation of coal. The other theory, now 
held by only a few geologists, holds that the plant material 
of the coal bed was carried from the original swamps 
by water currents into lake beds and did not grow on 
the spot. This theory is called the allochthonous forma- 
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