Relative abundance of intermediate members of the oceanic basalt-trachyte association

The relative abundance of analyses of intermediate and salic lavas of the oceanic basalt-trachyte association supports Daly's suggestion, based originally on field study, that in the oceanic islands rocks intermediate in composition between basalt and trachyte are much less common than trachyte. The sample distribution of SiO2 shows a clearly defined central minimum, but the effect is considerably stronger in CaO. It is perhaps best shown by the Thornton-Tuttle index. A bimodal frequency distribution of any major oxide or normative parameter is difficult if not impossible to explain as a consequence of crystal fractionation alone. Partial melting, the most reasonable alternative to crystal fractionation, is also open to a variety of objections. A satisfactory explanation of the oceanic basalt-trachyte association will require both new data and new thinking.