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 stone in the photograph shows 'the top with triangular holes for receiving the cross for working the stone - probably backwards and forwards'. The purpose of the stone in the Dartmoor tin mining industry is unknown.