254 Calcareous Nodules (‘ Coal Balls’) and Their Significance. 
usually invested with a coaly layer so that, apart from their 
hardness, are not easily recognised. On washing with dilute 
hydrochloric acid or on being allowed to ‘ weather,’ they 
become more conspicuous and are greyish-white or brownish- 
grey. Washed or ‘ weathered ’ nodules clearly show the 
direction of the bedding and often bear a median flange. 
Roof ’ nodules are found in the shaly roof of the seam. 
The term ‘ coal ball ’ is used to include nodules from all 
three situations, as is also the word ‘ bullion.’ * Roof ’ 
nodules are sometimes distinguished as ‘ baum-pots,’ ‘ posts,’ 
or ‘ goniatite nodules.’ 
Chemical Composition. — The term ‘ coal ball ’ is unfor- 
tunate, as rounded pebbles consisting of coal — i.e. true coal 
balls — occur occasionally in the sandstones of the Coal 
Measures. The ‘ coal balls ’ of miners, on the other hand, 
consist mainly of the carbonates of lime and magnesia. They 
are calcareous nodules. Their composition is very variable. 
Adjacent nodules may contain very different proportions of 
the two carbonates, as well as of the remaining constituents. 
This, however, has little or no effect on the excellence of the 
preservation of the enclosed plant tissues. 
In the case of ‘ seam ’ nodules, the remaining constituents, 
about io per cent., consist of the carbonates of manganese and 
iron, oxide of iron, pyrites, clay and carbon. Nodules 
occurring in the Bradford and Huddersfield districts are often 
so pyritous as to obscure the plant structures contained. 
The amount of carbon contained in the nodules varies 
greatly, depending on the degree of coalihcation attained 
before the petrifaction of the tissues set in. Stopes and 
Watson have estimated that about one-third of the organic 
matter of the cells is preserved, usually forming brown ulmic 
matter, but being locally carbonised and black. Dutch and 
German nodules have been described as differing from English 
‘ coal balls ’ in being pitch black. Many English nodules 
are, however, more or less black, and it is owing to the wealth 
of the material available in Britain that such nodules were 
discarded and have not usually come under the observation of 
Continental workers. 
While * seam ’ nodules contain only 0*2 per cent, of 
silicates and 92 per cent, of carbonates, * floor ’ nodules 
contain 8 per cent, of silicates and 84 per cent, of carbonates, 
and ‘ roof ’ nodules 6 per cent, of silicates and 86 per cent, 
of carbonates. These amounts are of significance in view of 
the origin of the ash in coal, most of which was usually 
derived from non-vegetable sources. It is evident that the 
silicates occurring in ‘ floor’ and ‘roof’ nodules represent sand. 
Formation, — ‘ Floor ’ nodules are clearly found in the 
place where they were formed and did not drift into position. 
The Naturalist 
