IV] 



COAL-BALLS. 



85 



In the Coal-Measures of England, especially in the neigh- 

 bourhood of Halifax in Yorkshire, and in South Lancashire, the 

 seams of coal occasionally contain calcareous nodules varying in 

 size from a nut to a man's head, and consisting of about 70 % of 

 carbonate of calcium and magnesium, and 30 "j^ of oxide of iron, 

 sulphide of iron, &c.i The nodules, often spoken of by English 

 writers as ' coal-balls,' contain numerous fragments of plants in 

 which the minute cellular structure is preserved with remark- 

 able perfection. It should be noted that the term coal-ball 

 is also applied to rounded or subangular pieces of coal which 

 are occasionally met with in coal seams, and especially in 



rv IV n IX 



nivra 



, lyxm IV vn 



Fig. 17. A thin section of a calcareous nodule from the Coal-Measures. Biuney 

 collection, Woodwardian Museum, Cambridge. Very slightly reduced. 



certain French coal iields. To avoid confusion it is better to 



speak of the plant-containing nodules as calcareous nodules, 



restricting the term coal-ball to true coal pebbles. A section 



1 Cash and Hick (78). 



