SummaryResults are given of the exchangeable calcium, magnesium, sodium, and potassium contents of the sand, silt, and clay fractions separated by purely physicaI methods from sixteen horizons, mainly subsoils from basaltic soil profiles in Northern Ireland. Relatively high exchangeable‐calcium values are recorded, ranging from 6.9 to 20.9 m.e. per cent. for sands, from g to 28 m.e. per cent. for ‘silts, and from 8.3 to 35 m.e. per cent. for clay separates.Exchangeable‐magnesium values are also high, particularly for separates from subsoils with imperfect drainage, and values as high as 17.2, 27.3, and 32–2 m.e. per cent. are recorded for sand, silt, and clay separates, respectively. The exchangeable‐magnesium contents usually increase with depth in the subsoil and in many cases the exchangeable‐magnesium values for silts are as high as or higher than values for corresponding clay separates. Comparisons of the values for sands, ‘ilts’ and for corresponding clays suggest that the high values exhibited by the silts (and often by the sands) cannot be attributed, to any considerable extent, to incomplete dispersion, e.g. to the presence of normally dispersible clay aggregates in the silts or sand separates. It is suggested that these high values and the associated high cation‐exchange capacities of these silts (and coarser fractions) may be due to the presence of partially weathered rock minerals which may be broken down, e.g. by acid treatments and dispersion procedures prior to deta