SummaryX‐ray and chemical studies of silt separates (2–20μ) from basaltic soils were made to determine the origin of the rather high cation‐exchange capacities (C.E.C.) found for these separates. The silts as isolated by the usual method of mechanical analysis consisted in the main of quartz and feldspars with variable amounts of clay minerals which occurred as aggregates. These aggregates were of two types:(a) Composed of clay‐size particles, i.e.<2μ cemented or bound into silt‐size aggregates by ‘free’ sesquioxides of which the basaltic soils have a high content. The aggregates were disrupted by reduction and the clay particles subsequently released.(b) ‘Pseudo‐aggregates’ which were shown to be individual particles of silt‐size dimensions and having a flaky appearance. These particles were either vermiculitic (under good drainage conditions) or montmorillonitic (under poor drainage conditions), and it is suggested that they form intermediate products in the weathering of primary‐rock minerals to clay minerals.Whilst the major contributor to the C.E.C. of the silts was undoubtedly the ‘pseudo‐aggregates’ and to a lesser extent the true clay aggregates, most of the silt separates had a much higher C.E.C. than implied by