THE ROMANCE OF COAL AND OIL 
the roof is the shale, sandstone, or limestone bed formed 
from the mud, sand, and lime contents carried by the 
waters which inundated and covered the swamp. The 
under clay usually contains innumerable roots; the roof 
frequently bears impressions of leaves, stems, and seeds 
from the vegetation of the swamp; the coal itself, if 
microscopically examined, shows woody fibers, plant 
membranes, and spores. 
As a rule, the coal is very much metamorphosed vege¬ 
table matter which has been subjected to enormous pres¬ 
sure and heat. Many feet of peat and living plants were 
reduced to a very few feet of coal. During the process 
the structure of the plant materials was greatly changed 
and the details obliterated. But occasionally a mineral 
spring was active in the swamp, or the invading waters 
contained a high percentage of silica or lime. Lumps 
of vegetable matter were rapidly saturated with the silica 
or lime and a perfect preservation of the plant tissues 
instead of an amorphic, or formless, carbonization took 
place. Such lumps with their perfectly preserved plant 
structures are found, as we observed in an earlier chap¬ 
ter, sticking out of the coal like raisins from a cake. 
These coal balls, in which we can see under the micro¬ 
scope the anatomic structure of leaves, stems, reproduc¬ 
tive organs, and the smallest components of the plant 
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