268 Calcareous Nodules (‘ Coal Balls') and Their Significance. 
The three types of * coal ball/ as found in Lancashire, 
their contents and characteristics, are compared in the 
accompanying table, together with the conclusions to be 
drawn . 
Observation. 
‘ Floor ’ Nodules contain only roots 
and rootlets (Stigmaria), with mud, 
but no marine shells ( goniatites , 
Aviculopecten , etc.). 
' Seam ’ Nodules represent a mass 
of numerous plant remains penetrated 
by roots, but which are free from mud 
or shells. 
Inference. 
They were formed in the original soil 
of the coal forest. 
‘ Seam ’ nodules were formed in the 
coalifying vegetable debris and were 
not transported by water. Decaying 
plants formed a peaty mass on which 
their descendants lived. 
The surface of the nodule is not 
smooth, as has been maintained. 
Stems which occur in a nodule may 
extend into the surrounding coal. 
Even soft plant tissues are often not 
torn or crushed. 
The nodules, as such, were not 
carried into the seam by the agency 
of water, but formed in situ on the 
swamp in an estuary. 
The roots contain exceptionally large 
air spaces, adapted to facilitate 
breathing in the muddy soil. 
Leaves which are preserved in the 
nodules have sunken pores to restrict 
the amount of water lost. 
‘ Roof ' Nodules may contain only 
one plant fragment. This is not 
penetrated by roots. The mud is 
fine and evenly bedded ; marine 
shells are present ; quartz grains 
are absent. 
The plant tissues preserved are 
mainly those resistant to decay ( e.g . 
wood) ; they are sometimes torn or 
crushed . 
The air spaces in the roots (and other 
features) are not abnormally de- 
veloped. The leaves do not bear 
sunken stomata. 
The plants found in ' floor ’ and 
‘ seam ' nodules must have grown in a 
swamp . 
Though water was abundant, the 
plants must have lived in a moist 
soil containing salts or organic acids. 
The single plant -fragment was carried 
by water to its present position. 
The material composing the nodule, 
however, did not accumulate on an 
area which was then an estuary or 
lagoon or on a shore, but some distance 
from land, on the bed of the sea. 
Only resistant tissues could with- 
stand transport and even these may 
have been damaged. 
The included remains are those of 
land plants which lived under normal 
climatic conditions. 
There has been some tendency to exaggerate this differentia- 
tion of the floras found in ‘ seam — and ‘ roof ’ — nodules in 
Lancashire, the ‘ seam ’ nodules being predominantly swamp 
vegetation and the ‘ roof ’ nodules containing certain land- 
forms. In addition, some writers have thought that such 
The Naturalist 
