T he rock-specimens here to be described were collected by the Rev. R. Baron during his extensive travels through Madagascar. They do not by any means represent the whole of his collection; but care has been taken to make the collection include all the more important types of crystalline and volcanic rocks. The paper is divided into two parts, the first dealing with the older crystalline rocks, and the second with the comparatively recent volcanic eruptions. I. T he O lder C rystalline R ocks The mountainous portion of Madagascar, extending from north to south through the eastern half of the island, is a ridge of old crystalline rocks, on the western flanks of which volcanic rocks have been erupted. Mr. Baron's collection comprises both foliated rocks (gneisses) and rocks in which there is no parallel structure visible in the hand-specimen (granite, gabbro or norite, pyroxene-granulite, and pyroxenite). In the following pages the petrographical characters of these rocks will be described briefly, the order as here given. 1. G neiss Considered petrographically, the specimens of gneiss, collected by Mr. Baron, may divided into an acid and a basic series, the former being characterized by the presence of abundant quartz with orthoclase as the dominant felspar, the latter by the subordination of the quartz and the predominance of plagioclase felspar. a. Acid Series (Granite-Gneiss). To this division belongs a type of gneiss very common in Madacasgar and occuring, for instance, in the immediate neighbourhood of the capital. It is a medium-grained rock with a granitoid texture, foliation