Abstract
About 350_??_500 pebbles of a size of 1_??_3cm were gathered by sieving at 20 sautpling stations chosen in the river bed. of the Watarase-gawa system, northere Kanto District (Fig. 1). These pebbles were petrographically classified in order to know the mixing rate of pebbles of various rocks and to understand how the percentage of each rock varies from upstream to downstream (Fig. 2). The principal conclusions are as follows: 1) Pebbles of Palaeozoic rocks show the highest percentage at almost all stations. This is due to the fact that Palaeozoic rocks have the most extensive distribution along the river course and that they are liable to disintegrate into small pieces. 2) Percentage of granite pebbles is generally small. The maximum percentage of them appears at st. No. 7, but it decreases rapidly downstreams. This infers that the granite pebbles disintegrate to sizes less than 1cm while they are being transported by the river water. 3) Percentage of pebbles of quartz-porphyry, diorite and diabase, etc., as a whole varies little in the upper, the middle and the lower course of the area studied. This reveals that pebbles of these rocks do not break up into smaller pieces so easily. 4) This study has demonstrated with firm numerical data the fact that, the petrographical varieties of pebbles in a river bed have a close relation to the rocks outcropping along its course, and their size varies with their manner of disintegration.
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