The theory which presented itself to us on the spot was, that the lower part of the great mass of strata which now forms the centre of the island was formerly at the bottom of the ocean; and that the western part, now a firm breccia, had been a beach of shingle produced by the action of the waves on the upper strata, which may have formed a coast above the sea: the granite of the eastern end of the island had been forced into its present situation from beneath the strata, with sufficient violence to dislocate and contort the beds nearest to it, and to inject the liquid granite into the rents formed by the heaving action of the strata as they were raised up. It is natural to suppose that the ragged edges of the strata forming the sides of these cracks would be subjected to a grinding action, from which the strata more remote might be exempted; and in this way we may account for the extraordinary twisting, and separation of masses along the whole course of the granite dyke. In the dyke, as well as in the veins which branch from it, there are numerous insulated portions of schistus. That this last was softened, seems to follow from the frequent instances which occur of its being bent back upon itself without producing cracks. The same heat, generated by the melted granite in the neighbourhood, and which appears to have been just sufficient to soften the schistus, may be supposed to have reduced the shingle beach to a state of semi fusion by the aid of some flux contained in the sand scattered amongst the fragments. We could not discover any circumstance by which the relative antiquity of the two dykes mentioned above could be inferred. The junction of the granite and schistus above described, resembles very much the well known junction at the Lowrin mountain, in Galloway, described by my father, Sir James Hall, in the 7th vol. of the Edinburgh Transactions. It is also very like the junctions at the Cape of Good Hope, described in the same volume. The same theory has been found to explain them all. Specimen 7. Fine-grained granite, composed of white quartz, white feldspar, and olive-green mica. This rock (7) forms the eastern end of the island; the schistus next described (8) the centre, and the breccia mentioned immediately afterwards (9) the western end. 8. Fine-grained compact micaceous schistus: some of the specimens appear to contain plumbago. The strata lie north-west and south-east, dipping only a few degrees from the horizontal line. 9. Breccia, composed of angular and contorted fragments of micaceous schistus, and angular pieces of feldspar and quartz. This rock forms the western end of Hutton's Island[24]: it rises in high rugged cliffs. The angular pieces of schistus are of a similar rock to that described above (8).