Analysis of 104 samples from the sediment of the Santa Clara River drainage basin in Southern California provides evidence that the silt fraction is more representative of total sediment composition than is either the sand or clay fraction. The mineralogy of the coarse silt (4-6 phi ) is similar to that of the sand fraction, whereas that of the fine silt (6-8 phi ) is closer to the clay fraction. Silt-size particles have resulted mainly from disintegration of igneous and metamorphic rocks. Fractured quartz and feldspars produced the light (density < 2.89) silt-size fraction; broken and cleaved mafic minerals and accessory minerals produced the heavy (density > 2.89) silt-size. Two suites of silt-size, heavy minerals occur in the shelf samples. The mineral assemblage of one of the suites suggests that the basin's rock and soil are its source, that of the other suite that the source or sources are other than the basin. Certain components of the basin suite can be recognized by the soil-derived minerals (leucoxene and anatase), those derived from the Sespe Formation from South Mountains (hyacinth), and minerals from other sedimentary source rocks and soils (hematite-altered biotite, abraded zircon and apatite, weathered clinozoisite, and geothite), or from metamorphic source rocks (blue-green hornblende, actinolite, and sphene).