The stratigraphic succession of the Cenozoic strata developed in the Iwamura basin is as follows; Age Terrace deposits unconformity Hirooka gravel Tazawa sand and gravel I₂ Seto group Higashihora formation H₂ Mizunami group Toyama formation Agi formation F₂∼F₃ unconformity and fault Granite and Quartz-porphyry The Mizunami group represents a cycle of sedimentation and its lower part or the Agi formation may be freshwater in origin while the upper or Toyama formation, which consists of five members is marine., The Mizunami group is probably middle Miocene (F₂∼F₃) in age as inferred from the molluscan fossils., It forms a synclinal structure controlled by the fault of NE-WS trend.,

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Observations were made on the mode of sedimentation in a settling pond of the Joban Coal Mine, Ibaragi Prefecture, with regard to the development of cyclic sedimentation and ripple marks., From the observed facts, the following three important features were found with regard to the mechanisms of cyclic sedimentation and development of ripple marks: 1) In a steady flow, the cycles all tend to become thinner and also coarser grained from the lower towards the upper., 2) The sediments accumulated in the centre of two vortexs in a turbulent flow tend to decrease in their volume and become coarser in composition than elsewhere., 3) Among five kinds of of ripple marks which developed in the above pond, the wave ripple marks distribute so as to surround each vortex and their branching tips point to each centre., Other ripple marks are developed around the margins of each vortex.,

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In general, the middle Miocene formations in the so-called "Green tuff" region were deposited on the eroded land surface in an early stage of Miocene transgression., Thus, the sedimentary environment was variable, as shown by the refletion of the faunas and facies., In this paper, the relationship between the faunas and facies in the Yatuo group, and the sedimentary environment are discussed., The Yatuo group consists of four formations, namely, Nirehara, Iwaine, Kurosedani and Higasibessho, in ascending order., The distribution, stratigraphy and rock facies of these formations in the Yatuo area are shown in figures 1, 2 and 3., The litho-facies of the Nirehara and Kurosedani formations is variable as shown in the figures., Generally speaking, the maximum size of the grains in the deposits of the two formations is in the lower portions of the conglomerates which are distributed in the central part of the mapped area., The grains decrease in size in all directions therefrom., The rock facies and fossil occurrence reflect the sedimentary environment of the Kurosedani formation, as is shown by the terrestrial boulder conglomeratic facies containing drift-wood fragments, and which grades into a marine muddy facies characterized by marine animals which live in a deeper sea of normal salinity., The alternations of sediments between the terrestrial boulder conglomerates and marine mudstones contain abundant brackish shallow sea forms mixed with terrestrial ones., From the above mentioned facts, it can be said that the boulder conglomerates of the Nirehara and Kurosedani formations are ancient deltaic deposits., Close resemblance is seen in the sedimentary environments of the two formations., The Iwaine formation consists of thick volcanics as a result of rapid accumulation of volcanic products., The faunas and facies of the Higasibessho formation show that it was deposited in a deeper sea of normal salinity., An evolutional change is observed in the structural movement between the Kurosedani and Higasibessho.,

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It is suggested by the present writer that Mg-enrichment is very common in the altered halos of some hydrothermal ore-deposits in Japan -with particular reference to gypsum deposits in Tertiary formations as well as to silica deposits in the bedded chert of the Chichibu Palaeozoic formation., Stoichiometric studies reveal that some of the Mg-enrichment is due to the supply of magnesium from ascending solutions, while others merely to local concentration of that component in the rocks., Magnesian-chlorite (including leuchtenbergite) is characteristic to the products of magnesium enrichment, and associated with extremely variable amount of either sericite or montmorillonite, giving rise to clayey zones in the altered aureoles., From these mineral associations, the oreforming temperatures can be estimated as having not exceeded about 450°C., The elucidation of variation in crystallinity of those minerals, however, seems to be neccesary for a more detailed temperature estimation., Attention is given to the petrochemical characteristics of the "green tuff" of Miocene age of regional development in Japan., These rocks are enriched in magnesium relative to other components particularly to iron, and are impoverished in SiO₂, probably due to submarine alteration.,

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