Samples from clay veins found in various Norwegian mines, tunnels, etc. have been investigated, mainly by X-ray diffraction techniques. The clays investigated are of varying mineral composition. Montmorillonite is the commonest clay mineral; less common are illite, vermiculite and dickite. Chlorite occurs in most of the veins. More occasionally calcite, apophyllite, pyrite, and oxides and hydroxides of Fe and Mn occur. The clay veins have been formed by low-temperature hydrothermal solutions which have mostly followed minor faults.