Humic acid drilling-mud "thinners" derived from oxidized coal have been found to contain from 600,000 to 4,000,000 pollen and spores per gram of coal. Various brands of quebracho thinners contain, in addition to the primary constituent, quebracho extract, varying amounts of oxidized coal as a filler. Hence, such quebracho thinners are also a source of coal-derived pollen and spores. Palynologists, working with cuttings from which the drilling mud cannot be completely eliminated, must reckon with the built-in flora of the mud thinners.

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