Calcareous Nodules (‘ Coal Balls ’) and Their Significance. 273 
debris had been turned into coal and had hardened, and part of 
it denuded by water and re-deposited in the sand (which, 
nevertheless, become overlain by deposits including seams of 
coal). Again, uncrushed stems are occasionally found in coal. 
They show that pressure was not an all-important factor in 
coalification. Temperature, similarly, can have caused little 
differentiation between coals. 
Such observations point to factors operating prior to 
long-continued heat and pressure in the formation of coal. 
Further, there is abundant evidence of the presence of fungi 
and bacteria in the plant tissues preserved both in coal and 
in ‘ coal balls,’ and many other observations point to the 
conclusion that most (though not all) coals were formed in situ. 
The evidence adduced by Stopes and Watson, received above, 
must also be taken into account. 
It must here be said that the conditions under which 
various coals were laid down were by no means sharply marked 
off from each other. Accepting the conclusion of Stopes and 
Watson that coals which contain calcareous nodules were 
deposited in brackish water, there is reason to believe that 
most seams which do not contain such concretions were laid 
down in brackish rather than in fresh water. The aquatic 
worm Spirorbis is commonly found attached to the plant 
incrustations found in the roofs of seams from which * coal 
balls ’ are absent. Yet Spirorbis must be regarded as a 
typically marine genus, and its presence at least suggests 
brackish-water conditions. Even the so-called ‘ fresh-water 
molluscs ( Carbonicola , Anthracomya and Naiadites) found in 
the roofs of many seams which do not contain calcareous 
nodules are of doubtful habitat. Naiadites, in particular, 
appears to have favoured brackish water. Of the other forms 
some probably favoured fresh — and others brackish — waters. 
We must, therefore, conclude that most seams which 
contain f coal balls,’ like the majority of other seams, did not 
originate by drift ; that the vegetable debris in these, as in 
most other cases, accumulated in situ. 
SUMMARY. 
Calcareous nodules (‘ coal balls") consisting mainly of the carbonates 
of lime and magnesia, occur in association with a few seams of coal, chiefly 
in the Lower Coal Measures. They may be found in the fireclay floor of 
the seam (f floor ’ nodules), in the coal itself (‘ seam ’ nodules), and in 
the shaly roof of the seam (‘ roof ’ nodules). 
They represent concretions formed in and around plant remains, the 
vegetable tissues being petrified. The plants preserved in ‘ floor ’ and 
seam ' nodules usually grew ‘ in situ ’ ; those in ‘ roof ’ nodules drifted 
into position. The amount of drift, however, depended on physio graphical 
conditions , so that some ‘ roof ’ nodules may include certain land forms 
while others contain swamp -plants . 
Whereas the plants which formed most coals grew in fresh — or, more 
1933 Dec. 1 
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