P. J. McCabe and J. A. Breyer write: In their paper on organic sediments of the Mississippi Delta, Kosters et al. 1987 begin by stating that, although the mires of the delta have long been considered prime examples of modern coal-forming environments, recent workers (notably McCabe 1984) have suggested otherwise. Their paper concludes that the ‘Mississippi Delta peats provide a good analogue for some coals of the geological record.' Although the Mississippi peats may be an analogue for some coal beds, we feel that it is important to stress that these peats do not provide an analogue for coals suitable for exploitation. The raw data presented by Kosters (1983), Kosters & Bailey (1983), and Kosters et al. 1987 clearly show that if the organic-rich sediments of the Mississippi Delta were preserved in the geological record, they would form carbonaceous shales with thin coaly stringers, rather than economic coal deposits. In this discussion, we shall review two factors which are critical in determining whether a coal bed is economic or not; seam thickness and ash content. Other factors which influence the mineability or marketability of coal, including lateral continuity of seams, and S and Na contents, may also be influenced by the depositional setting of the peat.

The United States Geological Survey bases its reserve calculations for bituminous and anthracitic coal on seams at least 0.7 m thick and its reserve calculations for sub-bituminous coals and lignites on seams at least 1.5 m thick (Wood et al. 1983). Most coals mined

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