Minllyn is a slate and slab quarry (nprn 310049) begun in the C19, the most productive phase of which began after the Carlyle Slate & Slab Company was formed in 1872. Work ceased in 1924. The L-shaped engine house, now roofless, oriented WSW-ENE with a return to the SSE at the NNE end. The external sides measure 10m by 9.5m, respectively, and the building is of mortared rubble stone with dressed quoins, the walls standing up to 2.5m high.
Inside, at the WSW end, is a former firebox above a brick-lined pit 2m N-S x 1.4m. Within it are stacked firebricks, all stamped 'LAMBERT / SHRIGLEY / MACCLESFIELD'. In the N wall is a large iron damper plate that was operated by a pulley, which opened to the flue leading to the chimney stack (nprn 527467). Further E along this wall is an iron pipe at ground level, presumably for feeding the boiler. There is no pit or engine mountings that give a clue as to an engine installed here, but it was probably horizontally mounted.
The building is attached to the NNW side of the mill (NPRN527460). Communication between the two was by stone steps 1.1m wide, but they are now mainly covered over.
Recorded as part of RCAHMW Uplands Initiative Project, W B Horton, H&H, 14/12/2012.