_Cave-hole._ A depression at the surface, caused by a fall of roof in the mine. _Chain pillars._ Heavy pillars of coal, lining one or both sides of the gangway, and left for the protection of that passage. _Chamber._ See Breast. _Chestnut coal._ One of the regular sizes of prepared anthracite. _Choice damp._ See After-damp. _Cleavage._ The property of splitting on a certain plane. _Collar._ The upper horizontal crosspiece uniting the legs in the timbering of a drift, tunnel, slope, or gangway. _Colliery._ All the workings of one mine, both underground and at the surface. _Conglomerate._ The rock strata lying next beneath the coal measures. _Counter-gangway._ A gangway which is tributary to the main gangway, and from which a new section of coal is worked. _Cracker boss._ The officer in charge of the screen room in a breaker. _Creep._ A crush in which the pillars are forced down into the floor or up into the roof of the mine. _Cribbing._ The timber lining of a shaft, extending usually from the surface to bed-rock. _Crop-fall._ A caving in of the surface at the outcrop. _Cross-heading._ A narrow opening for ventilation, driven through a wall of coal separating two passages or breasts. _Crush._ A settling downward of the strata overlying a portion of an excavated coal seam. _Culm._ All coal refuse finer than buckwheat size. _Dip._ The angle which any inclined stratum makes with a horizontal line. _Door boy._ A boy who opens and shuts the door placed across any passageway in the mines to control the direction of the ventilating current. _Double entry._ One of the systems by which openings into the bituminous coal mines are made. _Downcast._ The passage or way through which air is drawn into a mine. _Drift._ A water-level entrance to a mine, driven in from the surface on the coal. _Drill._ Any tool used for boring holes in the rock or coal. _Driving._ Excavating any horizontal passage in or into the mines. _Drum._ A revolving cylinder, at the head of any hoisting-way, on which the winding rope is coiled. _Egg coal._ One of the regular sizes of prepared anthracite. _Entrance._ See Cross-heading. _Entry._ The main entrance and traveling road in bituminous mines. _Face._ The end wall at the inner or working extremity of any excavation in or into the mines. In bituminous mines the vertical plane of cleavage at right angles to the butt cleavage. _Fan._ A machine used to force a ventilating current of air through a mine. _Fault._ A displacement of strata in which the measures on one side of a fissure are pushed up above the corresponding measures on the other side. _Fire-board._ A blackboard, fixed near the main entrance of a mine, on which the fire boss indicates each morning the amount and location of dangerous gases. _Fire boss._ An official whose duty it is to examine the workings for accumulations of dangerous gases. _Fire clay._ The geological formation which is usually found immediately underlying a coal bed.