Petrographic composition of coal seams is discussed in connection with the rate of subsidence in some Australian sedimentary basins. Cyclic variation of floor/dirt band --> vitrinite-rich coal --> vitrinite-poor coal --> dirt band/roof can be seen within a seam. Coal seams deposited under unstable conditions are composed of incomplete cycles which lack vitrinite-poor coal, and so are rich in vitrinite, while coal seams deposited under stable conditions are composed of a few complete cycles, and are relatively poor in vitrinite. Such a contrast between coal seams can be seen within a sedimentary basin such as the Sydney Basin in Australia. Coal seams in "shelf sediments" on the stable basement rocks are, for the same reason, much poorer in vitrinite than those deposited under more variable conditions.