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