Montmorillonite is not uncommon in the china clay of Devon and Cornwall but occurs in such small proportions that few commercial samples will give X-ray diagrams showing a line at 15-18 Å, in the oriented aggregate technique of Nagelschmidt (1940) and MacEwan (1946). Particle size fractionation is an essential starting point for the study of these clays, montmorillonite concentrating at the fine end. The sensitivity of the X-ray method may be greatly increased by a preliminary concentration in this manner. However, there are many clays with properties which suggest that a trace of montmorillonite is present, but in which it cannot even be identified by X-rays. If the fractionated clays are converted to the H-clay and the cation exchange capacity of the fractions measured by titration with alkali the presence of montmorillonite is shown by the much higher exchange capacity of the finest fractions. In this way montmorillonite may be detected, and roughly estimated, when the proportion present is as low as 0.2 per cent. of the total clay.