Fossil diatoms are remarkably abundant, both as to genera and as to species, in the Tertiary sediments and in the present lakes, seas, and oceans, However, the significance of these fossils in the stratigraphy of the Tertiary formations has been somewhat neglected due to the fact that they are cosmopolitan and invariable., The purpose of the present article is to find out if the Tertiary Diatomaceae of Japan are of stratigraphic significance, and to show the relationship existing among the Tertiary deposits in certain parts of the Peninsula and the vicinity of Kanazawa city., The chief localities selected by me in these districts are : Iizuka, Tsukada, Wakura, Yamatoda, Awara, Hijirikawa and Mitsukoji., The diatom-bearing mudstones in the six localities, except Yamatoda, are characterized by marine species for the most part, and they represent the same geological horizon., The ages of these mudstone beds are somewheve in the late Miocene or early Pliocene according to the abundant data of other geologists., Among the fossil diatoms the genus Coscinodiscus is a dominant form., The other fossils include sponge spicules, Radiolaria and, Silicoflagellata., The species Dictyocha fibula and Dictyocha speculum belonging, to Silicoflageliatae are characteristic of the recent cold water in the Northern Japan Sea., The diatom-bearing mudstones of these six localities are of marine origin, and all of them were deposited by cold currents., The Yamatoda mudstone is of brackish origin., My studies of the constitution of diatom plankton leads to the conclusion that the Yamatoda mudstone beds were deposited in a half enclosed bay.,