The Seikan Tunnel, connecting Honshu with Hakkaido by railway, is the longest undersea tunnel in the world, upon completion it will pass cross the Tsugaru Straits Undersea. The Straits is underlain by volcanic, pyroclastic, and sedimentary rocks (the Fukuyama, Kunnui, Yakumo, and Kuromatsunai formations) of Neogene age.
Clay mineralogical investigation was carried out on 70 core samples obtained from the pilot and service tunnels at the Hokkaido side of this tunnel by means of X-ray diffraction technique, thermal analysis, and electron microscopy. Some of the core samples were observed under petrographic microscope.
It is found that these core samples contain di-octahedral and tri-octahedral smectites, chlorite, interstratified mineral of chlorite/smectite, and mica clay mineral as clay minerals. Among them, di-octahedral smectite occurs most dominantly and is widely distributed in pyroclastic and sedimentary rocks, whereas tri-octahedral smectite is found only in basalt. The lower part (Kn-2) of the Kunnui formation is characterized by the presence of interstratified mineral of chlorite/smectite. This is in good agreement with the results obtained by Yoshimura and Kawahara et al. In addition to clay minerals, zeolites such as clinoptilolite, analcime, and laumontite are found. Regarding zeolites, characteristic distribution enough to classify the formations is not recognized.

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The measurement of exchangeable cations was carried out on smectites in core samples obtained from the pilot and service tunnels at the Hokkaido side of the Seikan Undersea Tunnel by means of atomic absorption spectroscopy.
It is found that exchangeable cations of these smectites mainly consist of Sodium, Calcium, and Magnesium, and that these smectites can be classified into three types, i. e., (1) Sodium-Magnesium type, (2) Calcium type, and (3) Intermediate type of (1) and (2), based on the exchangeable cation compositions. Sodium-Magnesium smectites are in cation exchange equilibrium with ambient seepage waters corresponding to Present seawater or Magnesium-rich seawater, and Calcium smectites are in equilibrium with those corresponding to Calcium-rich saline water or Sulfate-rich ground water.
From this investigation, it is recognized that the exchangeable cation compositions of smectites are strongly controlled by the chemical compositions of the ambient seepage waters.

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Many sericite deposits occurreing in granitic rocks are found in Mitoya-cho, Iishi-gun, Shimane Prefecture. These sericite deposits characterized by pale-green or white mica clay mineral are found along the nearly vertical fissures which strike N15-20°E and N50-70°W. The sericite ores mainly consist of mica clay mineral, calcite and quartz accompanied with a small amount of kaolinite, chlorite, smectite and occasionaly pyrite. The polytypes of mica clay mineral are 2M₁, 1M and 1Md, and those crystals consist of thin hexagonal plate, lath-shaped particle and irregulary plate respectively. Mica clay minerals are iron and/or magnesium rich dioctahedral species (phengite analogue). The strikes of ores consisting of the mica clay minerals of 2M₁ polytype trend northwest, while those of 1M and/or 1Md trend northeast. Since calcite in the sericite deposits seems to be genetically related with the mica clay mineral, filling temperatures in calcite were studied. The range of value of filling temperature is 200-300°.
The modes of occurrence and mineralogical characteristics of mica clay mineral in the sericite deposits are closely similar to those of mica clay mineral in the clay veins generally found in granitic rocks in Chugoku province. Mica clay minerals in the veins were formed by the hydrothermal solution (Kitagawa & Kakitani, 1978, 1981). These results may be suggested that mica clay minerals in the sericite deposits were formed by the hydrothermal solution, whose temperature is 200-300°, subsequent to the post granitic activity. Mica clay minerals in the sericite ores which strike northwest were generally formed at higher temperature than those in the sericite ores which strike northeast.

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The shear strength of clay increases with time of aging after remolding by thixotropy phenomenon. As this phenomenon is controlled by many complicated factors, the present state is that very little explanation has been made of the mechanism.
It this paper, the following items have been studied experimentally. 1) The thixotropic behavior of remolded aged clays at low water content. 2) The fabric change of montmorillonite increase with time of aging.
Results which were obtained from this study are as follows: 1) The thixotropy phenomenon of remolded clay at low water content appears remarkably as the change of secant modulus E₅₀ and failure strain εf more than that of unconfined compressive strength qu. And remolded aged clay at low water content showed approximately reversible properties as in the case of sample at high water content. 2) According to scanning electron microscope, the increase of strength with time of aging of montmorillonite may be caused due to the fact that filmy structure changes into ped increase with time of aging. This assumption was also confirmed by the fact that secant modulus E₅₀ from the different test increased as ped was formed more clearly.

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Ootani-yaki ware is famous for very large stone ware (for example a jar with 2m hight and 1m diameter). Ootani clay and Himeda clay (used for Ootani-yaki body and glaze respectively) were elutriated and classified into four size fractions. Identification of clay minerals and a study of their plasticity are conducted for each fractions by means of thermal analysis, x-ray diffraction, electron microscopy, infrared spectroscopy, specific surface area analysis and Pfefferkorn plasticity test. The results were;
(1) Main components of clay minerals in Ootani clays consisted of metahalloysite and illite/smectite mixed layer clay minerals. Himeda clay consisted of chlorite, illite, metahalloysite and illite/smectite mixed layer. Both clays were accompanied by quartz and feldspar as non-clay minerals.
(2) The smaller the elutriated size fractions of both clays, the larger became the Pfefferkorn plasticity index (PI), water retentivity (WR: the difference in percent water loss between the bodies heated at 40° and 100°) and ignition loss (IL). However the characteristic values of plasticity (CV=100°WR/PI) did not increase monotonously. The CV of Himeda clay was smaller than that of Ootani clay and increased with the decrease of its particle size. The CV of Ootani clay was 1.5 to 2 times larger than that of Himeda clay and remained substantially constant at any size fractions. The behaviour of CV was qualitatively explained by the clay mineral assemblages.
(3) Thicknesses of water film after drying for 3 days at 40°(T_40°) were nearly the same for both clays at any size fractions. Those measured with Pfefferkorn plasticity index (TPI) were large for Himeda clay and small for Ootani clay.
(4) The CV was theoretically identical with water film thickness ratio (T_40°/TPI), and the measurement of CV is much easier than that of water film thickness ratio, because it does not require specific surface area measurement. Therefore, it is recommended to calculate this value to estimate the plasticity of various clayc minerals.

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