The ejected blocks of the Soufrière volcano consist of the mineral phases anorthite (An96-An89; average An93), olivine (Fo79-Fo67; most frequent interval Fo74-71), salite con taining 5–6 per cent Al2, O3, hastingsitic amphibole, and magnetite containing 6 per cent A12O3, 4 per cent MgO, 7 per cent TiO2. The minerals occur in various proportions and textures. They are virtually unzoned and represent material which has been ejected at, and quenched from, a high temperature. The interstitial scoria present among the mineral grains in the blocks has the composition of a saturated sub-alkaline aluminous basalt, and is believed to represent the liquid phase with which the minerals were in equilibrium at depth. The high-temperature mineralogy, the textures and structures of the rocks support the view that the blocks represent crystal cumulates which have crystallized under high water-vapor pressures from a fractionating basalt magma at a depth approximating 6 km. The bulk composition of the blocks is such that the resultant liquid fractions are enriched in silica. Fractional crystallization may be an important factor in the evolution of some calc-alkaline suites.