[Introduction] Petrological notes were supplied to Davis, Day and Waterhouse for use in their paper (1938) on the ecology of the Five Islands. Subsequent work necessitates some modification of the views then expressed. Islands I and II. Harper (1915, p. 303) considered that Islands I, II, III, and V are composed of either the Dapto-Saddleback flow (popularly called dolerite) resting on red tuffs, or entirely of tuffs. Islands I and II, however, consist entirely of Dapto dolerite showing a pronounced rusty red colour on the weathered surface. From examination. of this rock in the Port Kembla Government Quarry, Browne and White (1929) enumerated the following types in descending order: A. Dapto-Saddleback trachybasalt. B. Deuterically altered trachybasalt. C. An intrusion from the same magma crystallizing under different conditions, with no completely unaltered type to be seen. D. A rock usually pinkish in colour and heavily impregnated with carbonates., This is type C in an advanced state of alteration. The first two types are seen on Island II and the whole four occur on Island I. The field relations are the same as in the. quarry, though not so well exposed. In the following descriptions all numbers prefixed by DR refer to specimens in the Australian Museum collection, while other numbers denote rocks in the Mining Museum. Textural terms are used according to Johannsen (1931).