20 ANCIENT PLANTS 



though in some specimens there is a considerable 

 quantity of iron sulphide, and in all there is at least 

 5 per cent of various impurities and some quantity of 

 carbon. 



The important mineral compounds, CaCOg and 

 MgCOj, are mixed in very different quantities, and even 



Fig. 1 1. — Pliotograpli of Section across Stem of Sphenofhyllum from a Lancashire 



"coal ball", showing perfect preservation of woody tissue 



w, wood ; c, cortex. 



in coal balls lying quite close to each other there is 

 often much dissimilarity in this respect. In whatever 

 proportion these minerals are combined, it seems to 

 make but little difference to their preservative power, 

 and in good " coal balls " they may completely replace 

 and petrify each individual cell of the plants in them. 

 Fig. 1 1 shows a section across the wood of a stem 

 preserved in a "coal ball", and illustrates a degree of 

 perfection which is not uncommon. In the course of 



