Shales and other bituminous minerals are distilled in horizontal retorts arranged side by side and with furnaces beneath their front ends. The furnace gases pass, preferably through a brickwork grating, into spaces between the retorts and beneath a horizontal partition towards the back ends. They return above the partition to the front of the retorts, and finally enter a horizontal flue leading to a chimney. The front end of each retort is fitted with a hopper for charging and with a door for discharging. The products of distillation pass through perforated partitions inside the retorts and are conveyed away by pipes at the back.