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11 nearly half of the valuable mineral deposits in the world occur in the rocks of this period. 15 coal deposits form
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Geological Timescale
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10 the presence of stromatolites indicates that shallow seas cover much of Earth during intervals in this period.
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30 colonies of tiny invertebrates call graptolites flourished and vertebrates appear, fish did not have jaws or teeth and their bodies were covered with bony plates
