The invention relates to vertical retorts for the distillation of shale and other bituminous substances, or for the calcining of ironstone etc. The retorts are circular in cross-section and have tapering lower ends. For the purpose of effecting a continuous or intermittent movement of the substance under treatment, there is mounted at the base of each retort a supporting disk or table, above which is an arm or scraper capable of being revolved by means of an upright spindle actuated by a ratchet and pawl or other means. The form of the table and scraper may be modified. The discharging hoppers of the two rows of retorts have their openings so arranged as to discharge into a single truck, which runs along a central passage underneath the retorts. Or they may discharge into a trough containing a worm, screw, or endless-band conveyer.