Summary: From field and petrographic evidence, some scoriaceous rock from the upper ignimbrite horizon of the Mount Rivers Volcanic Member of the Carboniferous Dyrring Formation in the Mirannie-Mount-Rivers District, New South Wales, Australia, is believed to have a recent origin.

By heating unaltered dacitic ignimbrite in a furnace it was possible to produce in the temperature range 600–1230 °C changes in the rock similar to those observed in specimens of scoria collected in the field. Thus a temperature range for the alterations was established. Two possible sources of heat are suggested as being responsible for the production of the scoriaceous rock, namely a bush fire and a lightning strike, with a preference for the former.