This photograph is dated 14 April 1888. It shows evidence of the tin mining industry which operated on Dartmoor until around 1939.
Blowing houses (sometimes called blowing mills) contained small blast furnaces into which a forced draught of air was blown using waterwheel-powered bellows. Cassiterite (tin oxide) ore was smelted in the blast furnace and cast into ingots using a mould stone.
Burnard comments that the upper surface of the stone is smooth from grinding and suggests that it was the lower stone for a mill. Milling between two stones in crazing mills was part of the Dartmoor tin mining industry. However the known examples of crazing mill stones are circular and so the purpose of the stone in the photograph is not known.