Sanukitic andesite, glassy andesite and obsidian are found from six sites, including Kozaki, Fujio-yama, Iwagami, Nagamasu, Minamikata and Amagoidai sites, in Yamaguchi prefecture; they range in age from palaeolithic to Jomon period. These volcanic rocks are carried from a distance. The authors undertook speculative research to determine the original localities of these rocks using three petrological methods: megascopic judgement, polarized microscopic judgement, and comparison of main chemical compositions by X-ray fluorescence analysis.
With megascopic judgement, it was possible to identify the source of rocks only in some special cases. Examples are the greyish white obsidian from Kannon-zaki, Hime-shima island, Oita prefecture. This obsidian was found at six sites.
Polarized microscopic judgement is an effective measure for the andesitic rocks.
With its use, we identified sanukitic andesite from Hari-yama, Hiroshima prefecture, in five of the six sites (all except Amagoidai), another sanukitic andesite from Taku, Saga prefecture, in the Fujio-yama and Minamikata sites, and glassy andesite from Himeshima island, Oita prefecture, in all six sites.
X-ray fluorescence analysis is a convenient method for identifying black obsidian. It enabled us to identify obsidian from Kumi, Oki-Dogo island, Shimane prefecture, in the Kozaki and Fujio-yama sites, another obsidian from Matsuura, Nagasaki prefecture, in the Iwagami site, and an obsidian from Koshi-dake, Saga prefecture, in all six sites.