The Tokibariyama Formation of the late Oligocene is divided into the Choshigawa Rhyolitic and Chikaishigawa Rhyolitic and Chikashigawa Andesitic Members in ascending order. The Choshigawa Rhyolitic Member consists mainly of pyroclasitc rocks intercalating some lake deposits−thin alternated beds of sandstone and mudstone−at the upper part．These deposits show a period of weak eruption．The Chikaishigawa Andesitic Member consists mainly of lavas and partly of pyroclasitc rocks． Namely，andesitic lavas flowed out after the explosive eruption of rhyolitic magma．