On Some Pebbles collected from the Floor of the Japan Sea.

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Many pebbles of igneous rocks, both plutonic and effusive, and of sedimentary rocks were collected from a submarine ridge known as the Yamato Bank in the Japan Sea. They are round or subangular with well polished surface, and vary in size from sand-grade to gravel over 50cm. in diameter. If these pebbles had been found only in a limitted area lying 011 the track of steamships, their presence might be attributed to ballast thrown overboard from passing vessels. But the distribution of the pebbles is evidence of its improbability. Neither is it likely that ocean currents transport pebbles of this size so far from the present shores. On the other hand, there can be no wearing down of the surface of hard rocks on the sea-floor with a depth of 500-600 in. The only alternative is that the sea-floor that yielded these pebbles was at one time either a land-surface or a shallow sea near the shore, when they were formed by erosive action of the water flowing over their respective parent rock-masses, t'le shores submerging later to their present depths. All the pebbles described in this paper have the general petrographic characters common to their respective rock-types now exposed in the Japanese Islands. Hence it is inferred that the pebbles are derived from land which is now below the sea and which is geologically connected with the Japanese Islands, or that it was transported from some part of the present Japanese Islands before its submergence to where they lie on the sea-floor at present. The granite pebbles does riot show characteristics that could be correlated with any Japanese granite whose geological age has been established ; while the andesite arid basalt pebbles have characteristics common to the late-Tertiary and Quaternary volcariics in the Japanese Islands. It is therefore highly probable that the sea-floor where these pebbles exist was a land-surface or a very shallow sea near the shore during the post-Neogene. If the latest land-submergence had occurred in a very remote geological past, the pebbles could riot have remained for long, geologically speaking, in the condition in which they are found at present without the aid of some auxiliary agent such as earthquake disturbances at the sea-bottom, since a sea-floor with a depth of 500-average along its outer margin, and the lower one, which is far less incomplete in form than the other, extends down to 720m. These two are regarded by him as the prolongations respectively of the T arid PN terraces on the present land-surface of the Japanese Islands. Thus, the present discovery of the pebbles and the inferences drawn from a petrographic study made of them may be strong evidences in support of Dr. Yabe's views concerning the epeirogenetic or eustatic movement of the Japanese Islands in recent geologic times, regardless of the cause, the magnitude, or the mechanism of the movement. Further studies of these pebbles may supply data that will afford more weighty evidence in favor of some particular age-span for the latest land-stage of the seas now bordering the present Japanese Islands. In conclusion the writer desires to express his sincere thanks to Lieutenant-Commander Asahina, who supplied him with a piece of the granite pebble, and also to Mr jSTiirio, through whose kindness the writer obtained many pebbles from the collection of the Imperial Fisheries Experimental Station of Tokyo.
東京帝国大学地震研究所,Earthquake Research Institute, Tokyo Imperial University,地震研究所の論文
1932-00-00

On Some Pebbles collected from the Floor of the Japan Sea.

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東京帝国大学地震研究所,Earthquake Research Institute, Tokyo Imperial University,地震研究所 | 論文

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