Summary: Twenty-five new analyses of 10 spilitic pillows from the British Isles offer evidence of considerable variability in distribution of components, notably Si, Na, Mg.Fe²⁺, Ca, Fe³⁺, and water. Selvedges of pillows have compositions strongly contrasted with but essentially complementary to those of cores. Redistribution of components is believed to have occurred during devitrification. Geosynclinal spilites are regarded as basalts which have suffered such redistribution under conditions of diagenesis or low-grade metamorphism.