抄録
Yaku Sima, which is a small, round island, is situated at 130° 30' E, 30° 20' N, to the south of Kyûsyû, Japan. The island is composed almost entirely of granite, but along its northeastern margin some sedimentary Mesozoic formations of undetermined age are also met with (see the maps). The topography is chiefly characterised by a continuous and fully developed erosion surface of considerable size, which extends along the coast (See plate, fig. 1 & 2). As the surface is confined, within the investigated region, to the area of Mesozoic rock, we naturally find everywhere gravels of this rock; but we also find large gravels of granite, which must doubtlessly have been derived from the old land behind. This fact alone seems to prove that this surface is very far from what is called “Piedmont Fläche”, as is occasionally suggested by some Japanese geologists. Moreover, upon this surface, there are also found large pumices of more than 80 centimetres in diameter in the best state of preservation. Now, the nearest volcanic island is Kutino Erabu, but, as the distance is at least 30 kilometres, it is almost impossible to imagine that these pumices were transported by volcanic activity through the air. We must think, therefore, that they were brought by means of the sea water, and that, consequently, a part of this erosion surface was once covered by the sea. From these things, we can conclude that this surface is nothing but a real marine abrasion surface.
The nature and origin of this surface having so far been made clear, we have here another confirmation of what was suggested elsewhere(1), namely, of the mode of crustal movement in this island going on since the formation of this terrace.
The conclusions follow:
1. That the erosion surface, which almost encircles Yaku Sima, is not a fluviatile “Piedmont Fläche” but of real marine abrasion origin.
2. That though the main deformation is in the form of warping, some faultings have also occurred as an accessory phenomenon.
3. That Since the formation of the terrace the earth crust has been uplifting, more intensely towards the old land.
4. That there is a greater probability to find marine shells and sediments upon other erosion surfaces of Japan, many of which, though they have hitherto been supposed to be of subareal denudation, will be explained by marine abrasion.
