1966 年 1966 巻 84 号 p. 6-13
The pre-Tertiary igneous rocks, hitherto taken for "quartz porphyries" after the former prevailing conception, was revealed by the writer's reexamination, to be the altered pyroclastic deposits characterized by the rhyolite welded tuff, and was newly named "Okunikko rhyolites." The petrographical characteristics and mode of occurences in the field were described in the present articles. Most of the eruptions are considered to have taken place along the sheared zone of N-S trend within the basement composed of the upper Paleozoics. The rhyolites were intruded and thermally metamorphosed by the late Cretaceous granites. The age of the rhyolites is supposed, judging from the similarity with the igenous activity of Cretaceous age in the Inner Zone of Southwest Japan, to be Cretaceous likewise. The original areal extension is approximately estimated at 1,400 km2, and the former total volume might be not less than 1,000km3.