1. The contact metamorphic effects produced by the intrusion of quartz-monzonite into the Silurian limestone of the Marulan district, New South Wales, are described.

2. The rocks are divided broadly into marbles, hornfelses, skarns, and hybrid diopside-rocks. The first two groups are interpreted as being due to the recrystallization of limestone containing impurities which have reacted with the lime of the sediments to give lime-silicates and certain other minerals. The view is taken that only a very minor amount of pneumatolytic metamorphism followed the main thermal change. The skarns are shown to be due to the metasomatic replacement of recrystallized limestone by magmatic solutions rich in iron and silica, which were also responsible for the deposition of iron-oxides in cracks.

The hybrid rocks are explained as developing from limited marginal assimilation of limestone by the quartz-monzonite magma.

Skerryvore consists of acid gneiss with dark bands containing biotite and hypersthene. The mica was probably formed by the recrystallization of chloritic matter by heat from an underlying plutonic mass. The islet is cut by a 2 foot north-south dyke of olivine-dolerite.

Dubh Artach forms part of a sill of ophitic olivine-dolerite and is probably of Tertiary age.

Sule Skerry consists of andesine-anorthosite into which dark streaks consisting of hornblende and quartz have been injected at a later date to form a banded gneiss.

The classification and nomenclature here adopted is that of Clark (1925), which should be consulted for the early literature; only those references which give recognizable figures are here quoted, although an attempt has been made to interpret the identifications given by Fourtau. The investigation has been facilitated by the collection of recent echinoids made by Dr. Macfadyen in the Farsan Islands; and my thanks are also due to Mr. C. C. A. Monro for permission to examine material in the British Museum (Natural History).