Calcareous Nodules (‘ Coal Balls’) and Their Significance. 269 
differentiation was a general phenomenon where calcareous 
nodules occur. 
It will be seen, however, from the above table, that the 
two floras in Lancashire are by no means mutually exclusive. 
The differences are proportional rather than absolute. In 
Yorkshire and on some Continental horizons the ‘ roof * as 
well as the * seam/ nodules contain swamp plants. 1 Other 
considerations must also be taken into account. For example, 
the Cordaitales are rarely recorded from the ‘ seam ’ nodules 
of Lancashire, yet Cordaitean wood (with the characteristic 
multiseriate pitting) is often seen in the fusainised portions 
of Palaeozoic coals, and has been recognised in the vitrainised 
constituents. Again, the differences between fresh, brackish, 
and salt water forms are by no means sharply marked off (see 
P- 7 )- 
Dr. Stopes and Professor Watson have admirably described 
the mental picture of the series of events which led to the 
formation of the Lancashire calcareous nodules : ‘ Groves of 
large trees with smaller herbs and ferns finding place between 
and around their stems grew in the flat swampy levels between 
the higher ground and the sea. The water round their roots 
was brackish or salt, as is the water in the Mangrove swamps 
to-day, and into its quiet pools and shallows twigs and 
branches, stems, leaves and fruits fell or were blown. These 
fragments sank into the mass of debris already saturated and 
were shut out from the atmosphere and preserved by the salt 
water in which they lay immersed. Parts of the plants 
decayed and thus liberated the organic carbon, which began 
its slow task of reducing the sulphates and depositing them as 
insoluble carbonates. This process continued long without 
the entry of impurities or the deposition of anything but plant 
remains, and the rootlets of the living plants wandered among 
the dead ones, finding their way even through the heart of 
their stems or seeds. 
‘ At the same time the land was slowly sinking, and when 
several feet of debris had accumulated the level sank more 
abruptly till the plants were well submerged and the place 
where the forest trees had lived was covered by the waters of 
an arm of the sea. Over them was deposited fine mud, with 
the shells of Goniatites and Aviculopecten which lived and died 
in the waters. The plant masses below were continually 
withdrawing the sulphates of lime and magnesium from the 
sea water and depositing them as carbonates round the many 
centres started among the fragments of plants. The supply 
1 Crookall, R., in ‘ The Country around Huddersfield and Halifax,’ 
Mem. Geol. Surv., 1930, p. 166. The writer intends shortly to publish 
an account of the stratigraphical and geographical distribution of British 
Carboniferous Petrifactions. 
1933 Dec. 1 
