The Nohi Rhyolite is a large volcanic pile of the Cretaceous age, extensively distributed in central Japan and consisting chiefly of silicic pyroclastic flow deposits, mainly welded tuffs. A series of geological survey has been carried out by the authors in the western part of the rhyolite mass who recognized nine welded tuffs and two sedimentary units as shown in Table 1 and correlated these units with previously established succession in other parts of the mass. As a result, it is suggested that the succession of the main part of the Nohi Rhyolite is grouped into five volcanic stages (Stage I-V). In the western marginal part of the mass, the Ako Welded Tuff belonging to the Stage Ib filled up the depression (graben) which elongates in a NW-SE direction. Volcanism of the Stage II began with the subsidence in the western part of the mass evidently apart from the above-mentioned depression of the Stage Ib as well as in the northern part, which was followed by the sedimentation of coarse-grained lacustrine deposits (Futatsumoriyama Conglomerate). Successively, enormous pyroclastic flow deposits, ranging from rhyolite to rhyodacite in an ascending order, filled up these depressions and a part of them overflowed these areas. Finally, several stocks of granodiorite porphyry which are supposed to be co-magmatic with the main part of the pyroclastic flow deposits of the Stage II, especially with the Higashimata Welded Tuff, intruded the center and the periphery of the depression. Volcanism of the Stage III is also inferred to have begun with subsidence of the central part of the mass, judging from the abrupt lateral change in thickness of the lacustrine Atera Formation which are supposed to have filled up this depression, and have been followed by successive deposition of the pyroclastic flows (Takadaru Welded Tuff and others), mainly of rhyolitic composition. Consequently, it is suggested that a series of events as shown in a following scheme has occurred in each stage of the Nohi Rhyolite: subsidence→sedimentation of lacustrine deposits→deposition of a thick pyroclastic flow→intrusion of granodiorite porphyry (or adamellite porphyry in some cases). It is also noticed that the above-mentioned depression of each stage migrated in a following direction in the Nohi Rhyolite mass: (southern marginal part)→western marginal part→western part (and northern part)→central part→eastern part→eastern marginal part.

View full abstract

The writer has been studying on the evolution of vertebrates by using shark teeth as materials Recent sharks have many specialized features, but they can be regarded as a relict form of the primitive gnathostomata vertebrates on account of their whole body structure. In the phylogeny of vertebrate hard tissues, it is considered that all sorts of basic tissue types were established in sharks and continue to mammals and men. Further, the genesis of the outer layer of shark teeth has been in controversy for more than one hundred years, one school regarded it as enamel, while the other called it a sort of dentin, and its true nature has not been made clear till today. In this paper, the writer described the development of shark teeth and discussed the evolution of teeth from a standpoint of phylogeny. The jaw teeth of living Triakis scyllia were used as materials and their developing process was observed by optical microscopy and scanning and transmission electron microscopy. The results are summerized as follows. 1) The order of tooth hard tissues formation is as follows: Formation of enamel matrix→formation of predentin and calcification of enamel→formation of dentin, formation of osseous tissue of root of the tooth and maturation of enamel. 2) The tooth germ is composed of the epithelial and the mesenchymal elements. The former consists of the inner and the outer enamel epithelia, which differentiated from the dental lamina. The mesenchymal element consists of the dental papilla and its surrounding tissues. The enamel organ which has the stellate reticulum as seen in mammals does not differentiate in the epithelial element of shark tooth germ. 3) The enamel matrix is formed on the dental papilla side of the basement membrane, and contains many micro-tubular structures, collagen fibrils which continue from Korff's fibers, and odontoblast processes. 4) Initially, the enamel crystallites are of needlelike shape, but later they become hexagonal rods which are similar to mammalian enamel crystallites. They do not develop uniformly as in mammals, but are irregular in size during development. 5) The odontblast becomes to contain well developed rough-surfaced endplasmic reticulums and Golgi apparatus concomitant with the formation of the enamel matrix, retreats centripetally extending many plotoplasmic processes, and differentiates into columnar cell. While, in the epithelial element, the inner enamel epithelial cell differentiates into columnar ameloblast, and at the same time the outer enamel epithelial cell differentiates as cuboidal cell. In the ameloblast, large aggregation of glycogen particles is observed at the matrix formation stage, and well developed Golgi apparatus and many granular structures are found at the calcification and maturation stages. The above results lead the writer to the following conclusion. It seems that the mesenchymal element contributes considerably to the enamel formation because of a low degree of differentiation in the epithelial element of shark tooth germ. This suggests that the tooth tissues of vertebrates, even enamel or dentin, are in general formed by the interaction between epithelium and mesenchym. Further, it is considered that the cause of tooth (enamel) evolution in vertebrates is directly controlled by the degree of differentiation potential of the tissues and cells of the tooth germ, but, fundamentally, it should be searched for in the evolutionary processes of whole body structure and mode of life.

View full abstract

Daiichi-Kashima Seamount is located on the junction of Japan and Izu-Bonin Trenches, and the top of seamount is characterized of guyot at 3,700 m in depth. The basement structures of Daiichi-Kashima Seamount seems to continue to Mineoka up-lift zone. The seamount is consisted with four acoustic layers. The broadly outcropping sediment layer (Su. 1) is composed with Cretaceous Limestone, ineluding foraminifera (Orbitorina sp.) and Stromatopora. Generally, it seems to be corresponded to Barremian to Cenomanian. The second layer (SU. 2) presumes to consist of tuffaceous sandstone. The third layer (Su. 3) is the major parts of the seamount bodies and Su. A is the debris layer derived from lower layers. The olivine dolerite, olivine augite basalt, trachyte and trachytic andesite were obtained from the dredging samples at the summit of seamount. Also the materials of surface sediments and core might be originated from volcanic ash on the top of seamount. The clay minerals in the core sample of St. 2 show the same composition as recent sediments in the adjacent sea of Japan. From our biostratigraphic investigation, microfossils of the core were deposited on the latest Pleistocene, and it is indicated that the environment of sea changed to become colder at lower than 40 cm in depth. This seamount was also studied geomagnetically this time as follows. There are some high magnetic anomalies and they would be due to the matters related to the crustal structures about 4-5 km from sea surface. The reason why this seamount is located in such a depth as 3,700 m may be explained by the eustatic movement, rising of sea level, after middle Cretaceous time.

View full abstract

The purpose of this report is to describe the volcanic vent of a new type in the Neogene. The vent is situated in Tatenoyama, Marumori-machi, about 40 km SSW of Sendai City, North Japan. Its inner structure has became visible gradually every year because it was the stonepit. The Ryozen formation consists largely of basaltic lava and its pyroclastic rocks, and the Kaneyama formation consists of sandstone, siltstone and mudstone. Both formations are distributed around the vent, and are correlated with the lower part of the Neogene. In these formations, faults of both directions of NW-SE and WNW-ESE are widely developed, and have been of great significance to the formation of volcanism and sedimentary basin in the early Miocene. Volcanism of the vent occurred in the following order. 1st stage: The volcanic activity started with the opening of fissures that take directions of NW-SE and WNW-ESE, which occurred in connection with the fracture movement, namely, the formation of a fanshaped pit crater. 2nd stage: The successive explosive activities were accompanied with the outburst of a large amount of coarse ash, spherical lapilli and ball bomb. 3rd stage : The intrusion of lava and vent breccia, last stage: The emission of a little amount of ash, lapilli, breccia and ball bomb. Macroscopically, the lava speciments comprise two varieties. One of the two varieties is black in colour and very hard, and the other is redish purple in colour and remarkably porous. The volcanic products of the vent are petrographically basalt with olivine phenocrysts, and rarely with microphenocrysts of augite, groundmass of intergranular texture which consists of plagioclase, olivine, augite, magnetite, ilemenite and rarely interstitiary silica minerals. Formerly, the writer has described numerous volcanic vents and necks in the Neogene. These are circular-shaped or elliptical-shaped in horizontal cross section, and are constructed of a marginal part which consists of pyroclastic rocks containing enormous granitic fragments derived from the basement, and of a central part which consists of intrusive lava including a lot of xenoliths and xenocrysts. These vents are connected to volcanic products accumulated on the surface of the basement. The above-mentioned facts may show that magma injected the basement, while violently destroying it. On the other hand, the volcanic vent described in detail in this report easily erupts basaltic magma along the fissure which occurred owing to the fracture movement. The volcanic activity of this vent ends in simple style, and never occurs again from the same crater.

View full abstract