The material used was a tusk of a young Indian elephant (Elephas maximus LINNE) without any abrasion on its surface. The tip was covered with a thin enamel layer on the inner and with a thin cementum layer on the outer. Histological examinations by lightmicroscopy and scanning-electronmicroscopy revealed certain characteristics which differed from those of the molar teeth. Hunter-Schreger bands were developed in the inner two-thirds of the enamel layer, although the outer one-third was a prismless enamel layer with distinct Retzius lines. The Hunter-Schreger bands formed more regular rows in the tusk than in the molar teeth. Each band was some 30μm wide and was composed of 10-15 enamel prisms in a parazone. The enamel prism was 4-5μm in diameter and showed a key-hole pattern like that of human enamel. The gingko-leaf patterns characteristic of molar enamel could not be observed in the tusk enamel. Processes of dentinal tubes several μm in length were present in the enamel layer just adjacent to the dentino-enamel junction, although no large enamel spindles were observed. The above-mentioned histological features of Indian elephant tusk enamel are similar to those of the molar enamel of Stegodon, which is considered to represent the ancestral genus of the Indian elephant. The coronal cementum of the tusk was 1-2 mm thick and contained a number of cementocytes and Sharpey's fibers. This structure resembled to that of the root cementum of the molar teeth. The coronal cementum adjacent to the dentine was divided into inner and outer layers by an acellular cementum layer. These two layers were distinguished according to their arrangement patterns of Sharpey's fibers. The cemento-enamel junction formed a rather smooth line in the tusk, whereas an irregular line with many resorption lacunae was observed in the molars.

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Quaternary strata distributed in the Sasagami Hills are stratigraphically divided into the Yamadera, Sasagami, Aganogawa, Anchi and Nihonmatsu Formations in ascending order. Moreover, debris (Gozu Debris flows) which flowed down from the Gozu mountains are subdivided into seven beds and interfinger with the three latter formations. The Aganogawa, Anchi and Nihonmatsu Formations are terrace deposits which are composed of gravel, sand and silt beds. The complete sequence of the Yamadera Formation comprises sand and gravel beds (the lower member), silt or silty sand beds (the middle member) and sand beds (the upper member). The total thickness is about 80 meters. This Formation unconformably covers the Pliocene Dainichi Formation with a basal gravel bed. This unconformity is named the Yamadera Unconformity. The Sasagami Formation is composed of irregular alternation of gravel, sand and silt beds (the lower member), regular alternation of sand and silt beds (the middle member) and alternation of gravel, sand and silt beds (the upper member). The total thickness exceeds 70 meters and unconformably covers the Yamadera Formation. This unconformity is named the Sasagami Unconformity. Gozu Debris flow deposits widely cover the Yamadera and Sasagami Formations with unconformity (Gozu Unconformity). The Yamadera and Sasagami Formations are correlated with the Pliocene-Lower Pleistocene Uonuma Group in Cenozoic Niigata sedimentary basin. The Aganogawa, Anchi and Nihonmatsu Formations are correlated with deposits of "high", "middle" and lower terraces in Japan, respectivly. The Yamadera and Sasagami Unconformities are closely connected with the birth and growth of the sedimentary basin of the Uonuma Group. Distribution of these unconformities are limited to the marginal area of the basin. The present-day topography of this area is principally controlled by upheaval movement of the Gozu massif at the stage during the Gozu Unconformity was formed.

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The Lower-Middle Silurian Okuhinotsuchi Formation (KAWAMURA, 1977, 1980) in the Setamai Subbelt of the Southern Kitakami Belt characteristically contains intermediate to acidic volcaniclastic rocks in its lower member. The volcaniclastic rocks are welded tuff, non-welded tuff and tuffaceous slate. Also the upper member of the Formation intercalates thin layers of "tuffaceous" sandstone and pebbly limestone, which include epiclastic materials derived from acidic volcanic rock. The welded tuff constitutes the basal part of the lower member together with the basal pelitic rock which unconformably overlies the Okuhinotsuchi granite mass. And between the two, "lithic or pebbly part" carrying fragments of acidic volcanic rocks and granite is observed. While, the non-welded tuff is overlain by pale green limestone which yields some marine fossils, and interpreted as an intercalation of the middle part of the lower member. The welded tuff shows eutaxitic welded texture composed of devitrined glassy matrix and deformed essential lenses. Crystal fragments are only of plagioclase. The non-welded tuff shows vitroclastic texture consisting of devitrified vesicular glass shards. Average modal compositions of welded and non-welded tuffs are as follows; glass+matrix+essential lens, 86% and 78%; rock fragment, 6% and 11%; crystral fragment, 8% and 11%, respectively. So these are classified as vitric tuff. Crystal and rock fragments are somewhat rich in the non-welded tuff. Welded tuff is considered to be subaerial pyroclastic flow deposit, and non-welded tuff is probably formed as ash fall deposit derived from land volcano. Bulk chemical compositions of the tuffs show the approximate contents of SiO2 66-70%, FeO+Fe2O3 4%, MgO 1-1.5%, CaO 0.4-0.7% and K2O+Na2O 6-7%. The tuffs are dacitic to rhyodacitic in chemical composition. K2O/Na2O ratios are variable, but total amounts of K2O + Na2O are rather constant. This compensative relationship of alkalis is inferred to be the result of chemical exchange during the diagenetic or comparable processes. Low content of CaO is probably due to the same cause. The Okuhinotsuchi Formation represents the lowest part of the Paleozoic (Silurian to Permian) sequence in the Southern Kitakami Belt. Early Silurian intermediate to acidic land volcanism as indicated by the tuffs of the Okuhinotsuchi Formation is interpreted to be the forerunning activity for the Devonian to Early Carboniferous island arc volcanism of the Southern Kitakami Belt.

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[in Japanese]

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