[UNDER-1 5 b.] A bed of clay beneath a stratum, now spec. under a seam of coal.
1661. J. Childrey, Brit. Baconica, 58. The rains that fall, wash by degrees the uppermost mould down into the Valleys,… but leaves the underclay behind.
1840. [see UNDERCLIFT].
1845. Lyell, Trav. N. Amer., I. 84. I was curious to know whether the Stigmariæ would be found here in the underclays.
1867. Smythe, Coal, 25. The floor, thill, or seat … of the coal is an underclay, generally good for fire-brick.
