A band of impure, siliceous, crinoidal limestone of the Upper Pennsylvanian to Lower Permian Mount Mark Formation (Buttle Lake Formation limestone), extends discontinuously north-northeast for 4 kilometres. The Nanaimo River limestone crosses the Nanaimo River 2 kilometres below the outlet of Fourth Lake and is up to 500 metres in width. The limestone is in contact with sandstone, chert and lime- stone of the Mississippian to Pennsylvannian Fourth Lake Formation, Buttle Lake Group, to the east. Bedding at one point strikes 048 degrees and dips 25 degrees northwest. This sequence is hornfelsed by a granitic intrusion outcropping to the west.