58 CONCRETIONS IN THE LOWER COAL-MEASURES. _ [| Feb. 1902, 
Conclusion. 
We may summarize the foregoing particulars shortly as follows: — 
Coal was formed in stagnant sea-water by anaérobic decay brought 
about by bacteria; the calcium-sulphate in the sea-water was reduced 
and ultimately converted into carbonate, which first separated in 
the cells of water-logged vegetable matter and then around it, 
forming a concretion which grew in the bacterial jelly and hence 
acquired its rounded shape. At the same time sulphide of iron 
was formed from the iron in the fine mud and became part of 
the concretion thus formed, being subsequently converted into iron- 
pyrites by pressure or some other agency. 
