11. The best known callistophyte was documented from Late Pennsylvanian coal ball petrifactions in North America. 12. Thin sectioning was an early procedure used to analyse fossilised material contained in coal balls . 13. After applying acetone, a piece of cellulose acetate is placed on the coal ball . 14. It was not until 1922 that coal balls were discovered and identified in North America. 15. Coalification was thus prevented, and the peat was preserved and eventually became a coal ball . 16. North American coal balls are more widespread, both stratigraphically and geologically, than those in Europe. 17. Coal balls are not made of coal; they are non-flammable and useless for fuel.18. Chitaley's revisions begin after grinding the surface of the coal ball to a smooth finish. 19. David Watson analysed coal ball samples and decided that coal balls formed " in situ ". 20. David Watson analysed coal ball samples and decided that coal balls formed " in situ ".
