SPECIMENS FROM HUTTON'S ISLAND, COAST OF COREA.
Latitude 36º 10' north, longitude 126º 13' east.
The following note is taken from the narrative at page 8.
We found the north-east end composed of a fine-grained granite[19]; the middle of the island of a brittle micaceous schistus of a deep blue colour[20]; the strata are nearly horizontal, but dip a little to the south-west. This body of strata is cut across by a granite dyke[21], at some places forty feet wide, at others not above ten; the strata in the vicinity of the dyke are broken and bent in a remarkable manner: this dislocation and contortion does not extend far from the walls of the dyke, though veins of granite branch out from it to a great distance, varying in width from three feet to the hundredth part of an inch: the dyke is visible from the top of the cliff to the water's edge, but does not re-appear on the corresponding cliff of an island opposite to it, though distant only thirty yards. This island is composed of the same schistus, and is cut in a vertical direction by a whin dyke[22], four feet wide, the planes of whose sides lie north-east and south-west, being at right angles to those of the great granite dyke in the neighbourhood, which run south-east and north-west. The strata contiguous to the whin dyke are a good deal twisted and broken, but not in the same degree as at their contact with the granite dyke. The whin dyke is formed of five layers or sets of prisms laid across in the usual way.
Beyond the small island cut by the whin dyke, at the distance of only forty or fifty feet, we came to an island rising abruptly out of the sea, and presenting a high rugged cliff of breccia[23], fronting that on which the granite dyke is so conspicuous: the junction of this rock with the schistus cut by the granite and the whin would have been interesting; but although we must have been at times within a few yards of it, the actual contact was every where hid by the sea.
The whole of the south-west end of this island is formed of breccia, being an assemblage of angular and water-worn pieces of schistus, quartz, and some other rocks, the whole having the appearance of a great shingle beach and cliffs. The fragments of the schistus in this rock are similar to that which forms the cliff first spoken of. (Specimen 8.)
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.