The volcanogenic dark red soils are distributed in the Minamikushiyama-cho, Unzen City, located in the southwest of Shimabara peninsula. These soils developed on the decayed basalts. In the Uwaba plateau, Higashi-Matsuura Peninsula in the north of Saga, decayed basalts are called the "Onjaku", while, in this study area, there are the materials similar to "Onjaku" originated from Suwanoike Basalts of ca. 600 ka. Chemical and physical properties of volcanogenic dark red soils were similar to the soils originated from the "Onjaku" of Uwaba plateau. It is thought that the volcanogenic dark red soils in this study formed from the materials similar to "Onjaku" as parent material. These soils contained a lot of hematite having high crystallinity and large crystal size. This showed that lava of basalt caught reaction by hydrothermal alteration, or an oxidation state of basalts changed at the time of cooling of lava. It is estimated that the soil particles of surface layer in this soil profile become gradually finer and clayey by weathering. Also the soils were formed by erosion, addition of aeolian dusts, and supply of organic matter by growing of some vegetation. It was effective for the classification of volcanogenic dark red soil to identify hematite by using X-ray diffraction analysis of non-treated air-dry fine soil so that Mitsuchi (1985) pointed it out.
