Clay separates from the alluvial soils of the Oyodo River and diluvial upland soil at Tsuma-machi in Miyazaki Prefecture were examined by the techniques of x-ray diffraction, electron microscopy, phosphate retention, cation-exchange capacity, and thermal and chemical analyses with decomposition tests for organic materials. The clay separates of the alluvial soils consist mainly of hydrous mica and vermiculite with some montmorin and intermediates between illite and montmorin. It is suggested that hydrous mica has come from illitic shales and sandstones of Neogene formations, and vermiculite and montmorin layers have originated from hydrous mica. That is, the alluvial soils of the Oyodo River in the Miyazaki Plain contain predominant minerals of the illite-montmorin series in the clay fraction. One of these soils at Ikime has an underlying layer derived from ejecta, of which the clay fraction consists mainly of hydrated halloysite with subsidiary hydrous mica. The diluvial tableland of Saitogahara is covered with volcanic ash soils, brown Ando soils, for several feet in thickness. The horizons of this soil profile disagree in clay minerals with each other. The clay fraction of the uppermost horizon consists mainly of allophane, but the horizons of the subsoil have hydrated halloysite, allophane, and gibbsite, and, in fact, the content of hydrated halloysite has a tendency to increase with depth, while the diluvial layer has hydrous mica with hydrated halloysite and gibbsite. This seems to suggest that the clay mineral of the soil is definitely related to the parent material of the soil. It is worthy of notice that the decomposition of organic matter is markedly slower in the soil having allophane as a dominant clay mineral than in the soils of illite or illite-montmorin.
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