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 algae 

 or animalculae. In order to test this question a quantity 

 of a common pond alga (Microcystis flos-aquae) 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 



82 



