Calcareous Nodules (‘ Coal Balls') and Their Significance. 271 
nodules above had drifted out to sea from other districts and 
bear in the character of their structures the impress of the 
different type of land on which they lived.’ 
Cell-structures seen in ‘ Coal Balls.’ — While the wood 
found in many coals of Tertiary age shows growth (so-called 
annual) rings, due to the alternation of the small thick -walled 
wood with the large thin -walled wood (the former usually 
being formed in the autumn and the latter in the spring), the 
wood found in Palaeozoic coals (mummified) and in ‘ coal 
balls ’ (petrified) rarely contains such structures. Palaeozoic 
wood is usually thin-walled and possesses large lumens. 
Thick -walled (‘ sclerotic ’) cells, in mass forming the tissue 
known as sclerenchyma, though common in Palaeozoic coals, 
are usually localised. They occur occasionally in pith or 
leaves (e.g. in Cordaites ) , but are more frequently found in 
the cortex of the stems (e.g. Lyginopteris and Medullosa among 
the Pteridosperms and Sigillaria and Lepidodendron among 
the Lycopods). The lumens of these cells are almost, or 
quite, closed by the thickening material. 
Again, while the wood of many Tertiary coals may contain 
large areas of resin-filled tissue, the resins found in Palseozoic 
‘ coal balls ’ and woods (e.g. Cordaites ) are more disseminated 
throughout the plant body. 
The Value of Fossil Plants in Stratigraphy. — It has 
been pointed out by Dr. D. A. Wray 1 * * that 4 Stopes and Watson 
have clearly shown that the flora of the roof shales may differ 
considerably from that of the underlying coal seam.’ From 
this Dr. Wray concludes that, ‘ The employment of plant 
remains as zonal indices is [further] complicated by ecological 
considerations.’ 
There can be no question that the stratigraphical use of 
fossil plants is complicated by ecological conditions. But, 
in my opinion, Dr. Wray’s argument passes from the particular 
to the general. 
In the first place, as has been observed above, not all 
‘ roof ’ nodules contain land plants. The presence of land 
plants in certain * roof ’ nodules is clearly related to the 
proximity of the locality to relatively high land. For 
example, where the material composing the roof of a seam was 
laid down near a low-lying tract of country, the plants included 
in the * roof ’ nodules, though more or less drifted into position, 
would be swamp plants which were not markedly dissimilar 
from the vegetation which went to form the seam itself (and 
the enclosed * seam ’ nodules). Where, on the other hand, 
the shore-line was steep, the remains of land plants (as well, 
1 Wray, D. A., ' The Major Subdivisions of the Coal Measures in 
Yorkshire,’ Summ. Prog, for 1931, Mem. Geol. Surv., 1932, Pt. II, p. 60. 
1933 Dec. 1 
