coal seam

The nature of the pavement of the coal seam, particularly as to hardness and softness; and if soft, to what depth it may be so.

A coal seam with a soft pavement and a hard roof is the most subject to a “creep.”

A stratum of fire clay commonly underlies a coal seam, and there occur also beds of iron ore.

The mining engineer, Monsieur Arnaud, was of opinion that on sinking deeper into the coal-seam a better quality would be met with.

That's how it is you never find anything but shale or slate (which is mud-rock) or sandstone above a coal seam.