抄録
The observation of the profile and collection of the samples were made on the soil formation covering thick gravel bed of the upper terrace (Fig. 1) near the appex of the dissected fan of Azusa River, extending eastwards across the fault basin of Matumoto in the northern part of the Central Highland of Japan Proper. The Azusa River comes from the eastern side of the central range of the northern " Nippon Alps " to the Matumoto basin. The climate of this basin resembles that of the inland river valleys of the North Japan Proper, having 10.3℃ of the average annual temperature and ll2 cm of total annual precipitation. The profile (Fig. 2) consists of the following three holizons : (1) the Surface Soil (± 4 dm) consisting of loose clay rich in humus and with grayish brown colour, (2) the Upper Subsoil (± 9 dm) consisting of soft clay very poor in humus and with faintly reddish, light brown-yellow colour, and (3) the Lower Subsoil (± 8 dm) consisting of soft clay very poor in humus and with light brown-yellow colour. The three holizons pass each other without distinct boundaries. The subsoils are destitute of colour patches or stripes. The reaction of the soil is faintly to weakly acid. The original material of the soil is the volcanic ash of two-pyroxene andesite with biotite and hornblende. The chemical analyses of the fine soils and clay fractions (<0.002 mm) show that the soils are rich in alumina, rather poor in silica and extremely poor in lime. The molecular ratios of SiO_2 : Al_2O_3 and Fe_2O_3 : Al_2O_3 in the parts decomposed by boiling hydrochloric acid are as follows : - [table] If we assume that the more than half of ferric oxide is in the combination 2SiO_2 : Fe_2O_3 (in Mol.), the molecular ratio SiO_2 : Al_2O_3 in the surface soil approaches that of allophane, and the ratio in the subsoil becomes less than that of allophane suggesting the possibility of the presence of free alumina. The surface soil absorbs the acid fuchsine faintly and the methylene blue very distinctly. The subsoil absorbs the former distinctly and the latter rather weakly to distinctly. Those results indicate that the soil belongs to the amphiphilous group of our volcanic-ash soils, which is found frequently in the northern part of Kanto Plain and in the southern part of the Central Highland. The morphological feature of the profile shows that the soil formation belongs to the brown soil in wide meaning, though the degradation is slightly going on. From the above mentioned data the soil can be regarded as the sub-allitic one. The writer might designate this soil as the weakly acid sub-alltic brown soil. It occupies the intermediate stage between Tokyo loam (sub-sialltic), and the Onji soil (allitic) of the Southwest Japan in the progress of weathering.
