The Douglas Seam occurs within the Newcastle Member of the Upper Cretaceous Pender Formation (Nanaimo Group) approximately 18 metres above the Newcastle Seam. The seam area extends from Newcastle Island to just south of the Nanaimo River in a north-northwest trending zone. The Douglas Seam has been mined extensively from a workable area of 15.3 kilometres by 2.8 kilometres. The most important mine was the No. 1 mine (092GSW041) which was in operation for 55 years (1883 to 1938) and produced approximately 16,329,000 tonnes. Along strike from the No. 1 mine, and the Douglas Slope and shaft in the north, are the New Douglas Slope, New Douglas mine, 1911 (New Douglas Slope), Southfield No. 1, No. 2, and No. 4 Slopes, Southfield No. 3 and No. 5 mines, Reserve mine (092GSW037), Fiddick and Richardson Slopes (092GSW034) and the Morden mine (092GSW032). To the south of these are the Alexandria (092GSW025) and Granby mines (092GSW051).
The seam is high volatile bituminous in rank and has similar chemical characteristics to the Wellington seam. Other similarities include the rapid and frequent thickness variations and the structural features. Seam thickness averages 1.5 to 1.8 metres and is up to 9.1 metres. Variations in thickness are commonly caused by undulations in the floor which is predominantly shale. The seam is overlain by conglomerate to shale and sandy shale. Rock partings within the seam are common. Structures include pinches, swells, small faults, shears and rolls.
The Douglas seam strikes northwest and generally dips shallowly northeast. Northwest trending faults are common, bounding the area to the west for example, and an east-west to east-northeast set of faults also cut the coal bearing strata. The seam tends to be strongly sheared with abundant slickensides.