The first record of pitchstone in Mull seems to have been made by Jameson, who, referring to a locality on the north shore of Loch Scridain, writes: ‘I accidentally discovered a piece of black pitchstone porphyry, similar to that which is found in Glencloy, in the island of Arran.’ Macculloch also reports that it had been found ‘upon the southern shore’ of Mull by Greenough. From that time, although the tachylyte margins of certain dykes were recorded by various observers, pitchstones do not seem again to have been noticed until the detailed mapping of Mull was begun by the Geological Survey. Mr. Lightfoot first noted their association with ‘olivine-free dolerites,’ and also recorded sheath-and-core structure. During the progress of the survey pitchstones have been shown to occur with extraordinary frequency in the Ross of Mull, in an area bounded on the east by Glen Leidle and Carsaig, and extending for several miles to the west. They are also found less abundantly outside this area. The area in question is occupied by plateau-basalts. It is outside the ‘region of complication’ which occupies the centre of the island, and the relation of the pitchstones to the various intrusions of that region is so far uncertain. The pitchstones fall into two classes, which may be easily distinguished by the presence of the porphyric felspar. Those of the non-porphyritic class are by far the more numerous; they form the centres of sills and inclined sheets, the marginal parts of which are crystalline. In many