Sixty-one of 1676 lunar rock fragments examined were found to be anorthosites, markedly different in composition, color, and specific gravity from mare basalts and soil breccias. Compositional similarity to Tycho ejecta analyzed by Surveyor 7 suggests that the anorthosites are samples of highlands material, thrown to Tranquillity Base by cratering events. A lunar structural model is proposed in which a 25-kilometer anorthosite crust, produced by magmatic fractionation, floats on denser gabbro. Where early major impacts punched through the crust, basaltic lava welled up to equilibrium surface levels and solidified (maria). Mascons are discussed in this context.