I. Introduction. In the years 1910 & 1911, the officers of the Scottish branch of H.M. Geological Survey, while engaged on a survey of that portion of the Island of Mull which lies south of Loch Scridain, were impressed by the great number of minor intrusions of a tholeiitic and andesitic character which penetrate the western part of the Tertiary lava-field. These rocks were described as being, for the greater part, of one type: namely, olivine-free dolerites or tholeiites, of which many have pitchstone centres, and in which a sheath-and-core structure is frequently developed. 1 Mr. E. M. Anderson noted the occurrence within them of xenoliths of apparently fused or baked sandstone, and also recorded certain nodular masses of bytownite and some ferromagnesian mineral which he compared with similar masses, collected by the late C. T. Clough and described by Dr. J. S. Flett, from a tholeiitic intrusion at Traigh Bhan an Sgoir 2 (Traigh Bhan na Sgurra, Locality 11, fig. 1, p. 230). In 1912 Mr. Anderson discovered in the bed of a tributary of Abhuinn nan Torr (Locality 2) an exposure of a rock that contained abundant small plates of a deep-blue mineral. The rock was submitted to me for examination, and, after isolation, the blue mineral proved to be corundum of the sapphire-variety. Further work on this locality, and some excavation, yielded a series of most interesting specimens that proved clearly the abnormal and intensely xenolithic character of the rock in question; but the relation of the xenolithic mass