Coals of the Early Miocene Maryville Coal Measures show well developed dulling-up trends, defined by a decrease in the proportion of bright coal, that culminate in sapropelic horizons. Field determination of lithotype is difficult but the use of polished surfaces enables classification by either the brightness or Stopes-Heerlen systems. During upwards dulling, gross petrographic composition remains essentially constant, at 80-90% huminite (vitrinite), 10-15% liptinite and 1-4% inertinite, except in sapropelic types, which are liptinite dominated. Dull bands are characterized petrographically by fragmentary macerals, especially detrogelinite, whereas bright bands are composed of intact plant fragments. Dulling up is related to the development of raised (ombrogenous) mires where growth and subsequent preservation of large, intact plant fragments is favoured during early stages of mire development and around the mire margins.