256 Calcareous Nodules {‘Coal Balls') and Their Significance. 
being derived from the sea water. This view is supported by 
the xerophytic structure of the petrified leaves, by the presence 
of magnesium in the petrifactions and other features. It 
accounts for the rarity of seams containing * coal balls * and 
for the fact that seams bearing ‘ coal balls ’ always have a 
roof containing marine shells. 
Professor P. F. Kendall has pointed out that, ‘ It is 
probable that other coal seams have accumulated under 
similar conditions . . . without, however, giving rise to the 
formation of coal balls. Two such, though not in the Upper 
Carboniferous, may be mentioned. A small coal seam at 
Skateraw, near Dunbar, rests on a highly coralliferous lime- 
stone, into which Stigmarian rhizomes can be seen to ramify. 
The other example is seen in the well-known fragmentary coal 
seam at Meals Bank Quarry, Ingleton. The rootlets from this 
seam descend through the underclay of the coal and are 
traceable to a depth of several feet in the underlying compact 
limestone which, it is inferred, must have been in the condition 
of ooze when the plants were growing.’ 
Although the salinity of the sea was less during the 
Carboniferous Period than at present, the plants which went to 
form coal (chiefly Pteridophytes) could not live in definitely 
salt water. Where a coal has been laid down under truly 
maritime conditions, the vegetable debris must, therefore, 
have accumulated by drift. 
Botanical Contents. — To determine their contents, ‘ coal 
balls ’ are sectioned at right angles to the bedding planes. 
‘ Seam ’ nodules consist of petrified plant remains, while 
‘ floor ’ and (some) ‘ roof ’ nodules contain petrified plants. 
The ‘ roof ’ nodules found in Holland contain no plant remains. 
In the case of ‘ seam ’ nodules, the petrifactions clearly 
represent part of the plants which formed the adjacent coal. 
As has been observed above, occasional stems or roots may be 
seen which are continuous from the ‘ coal ball ’ into the 
surrounding coal. Had these organs not been preserved by 
impregnation with mineral matter in solution, they would 
have been coalified and formed a normal part of the seam. 
When sectioned, ‘ floor ’ nodules are seen to be ramified 
by roots and rootlets, their only contents. 
Of ‘seam’ nodules 80 to 90 per cent, consist of the club- 
moss rhizome known as Stigmaria (especially in Yorkshire) 
or of the Seed-fern stem known as Lyginopteris (especially in 
Lancashire ) . 
‘ Roof ’ nodules usually mainly contain tissues which were 
resistant to decay (i.e. woody and corky tissues), the softer 
parts (which are generally well preserved in 4 seam and ‘ floor ’ 
nodules) having decayed. 
{To be continued). 
The Naturalist 
