28 ANCIENT PLANTS 



show their concretionary structure so clearly, but some- 

 times it can be seen that they are made with concentric 

 bands or markings like those characteristic of ordinary 

 mineral concretions (see fig. 15). Concretions are formed 

 by the crystallization of minerals round some centre, and 

 it must have happened that in the coal seams in which 

 the coal-ball concretions are found that this process took 

 place in the soft plant mass before it hardened. Recent 

 research has found that there is good evidence that those 



ALB 



Fig. 16. — Mass ot Coal with Coal Balls, a and B both enclosing part of the same stem L 



seams^ resulted from the slow accumulation of plant 

 debris under the salt or brackish water in whose swamps 

 the plants were growing, and that as they were collect- 

 ing the ground slowly sank till they were quite below 

 the level of the sea and were covered by marine silt. 

 At the same time some of the minerals present in the 

 sea water, which must have saturated the mass, crystal- 

 lized partly and deposited themselves round centres in 

 the plant tissues, and by enclosing them and penetrating 



' This refers only lo the " coal-ball "-bearing seams; there are many other coals 

 which have certainly collected in other ways. See Stopes & Watson, Appendix, 

 p. 187. 



