The Fern Alliance 
The twigs, leaves, and fruits fell into the quiet pools 
and shallows, and were swallowed up and preserved in 
the salty or brackish mud. Rootlets of living plants 
wandered amongst and sometimes through these buried 
stems and fruits. Then the whole mass was submerged 
by an arm of the sea, and concretions of carbonate were 
formed out of the remains by the withdrawal of calcium 
and magnesium sulphates. 
The rivers and currents in the estuary brought frag- 
ments of plants from the higher lands, which were laid 
down and pressed flat by the silt which accumulated 
above the older forest, and these water-borne twigs now 
occur in the shales formed by the silt. 
One can see to-day in the Solway silt, round, blackish 
concretions which are due to peat fragments of a much 
older date that have drifted in the water and are now 
decaying into a carbonaceous mass. 
Many petroleum deposits seem to be really estuarine 
or lake-muds similarly submerged after their formation, 
for salts and marine fishes or shells occur in them. 
The oil was probably formed originally by minute algz 
or animalculz. In order to test this question a quantity 
of a common pond alga (Microcystis flos-aquz) was 
chemically tested and petroleum was really obtained 
from it. 
As regards many details of life in the coal-measure 
forests we are very well informed, thanks to the skill 
and ingenuity of geological botanists. 
It is known, for instance, that a root fungus (Mycor- 
hiza) assisted some of the coal-measure trees to do their 
work. 
Some of the fern-like plants were probably, even in 
those days, attacked by rust fungi.* Dr. Scott has 
figured germinating spores of some of the coal-measure 
plants. Mr. Gordon has even figured the prothallium 
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