Coal-mining. (north. dial.) Also swally, swolly. [? Local variant of SWALLOW sb.2 Cf. SWILLY sb.] A depression in coal strata; a local thickening in a seam of coal.
1849. Greenwell, Coal-trade Terms Northumb. & Durh. (1851), 54. Swelly, or Swally, a gradual depression or dish in the strata.
1863. Warrington Smith, Addr. Brit. Assoc., Swellies, or narrow depressions in the Low Main coal.
1883. Gresley, Gloss. Coal-mining, Swelly, also Swally, also Swilly.
