The welded pyroclastic deposit of moonstone rhyolite exposed in southwest Toyama Pref. has been considered to be the uppermost member of Futomiyama formation being covered by the basal conglomerate, Nirehara formation, of Neogene formations unconformably. The result of geological study by the writers shows, however, that the rhyolite is embeded in Nirehara formation representing a product of the earliest Neogene volcanism prior to the eruption of a large amount of andesite in the following Iwaine stage. The result is consistent with that of K-Ar dating which gives the mineral age of the moonstone as 24-25 m. y.
K-Ar age determination was carried out on two moonstones from rhyolitic welded tuff which consists of the upper part of the Futomiyama formation. The measured ages gave 24 and 25 m. y. respectively, corresponding to the lower Miocene age. The result agrees well with the stratigraphic succession of the Futomiyama formation (Yamasaki and Miyazima, 1970).