Summary: Two unusual cavities at a Northumberland colliery are described. Although apparently of tectonic origin they do not seem to be related to any major fault. Sideronatrite, not previously recorded in this country, is thought to result from the alteration of the marcasite and pyrite lining the cavities by underground saline waters. These waters are the source of the baryte and gypsum that are also found in the cavities and of the salt stalactites in the disused roadways of the colliery.