The dependence of Si-O distances on the electronegativity of the modified ion has been
examined for X₂YSi₂O₇ melilites. The Si-O distance depends, in part, on the electronegativity of the Y ion and the two groups of variation trend are observed. One is Mg²⁺-Mn²⁺ series and the other is Al³⁺-Be²⁺-Zn²⁺-Co²⁺ series. In the former the Y sites are occupied by the lower electron density ions, whereas in the latter the Y sites are occupied by the higher electron density ions.

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The basic rocks from the Miho area can be classified into three groups, cortlandite, anorthosites and eucrites. The cortlandite is characterized by high MgO and FeO^*, and low CaO, Al₂O₃ and SiO₂ contents. The anorthosites are characterized by high Al₂O₃ and low FeO^*. The eucrites are intermediate in chemistry between cortlandite and anorthosites, and are rich in TiO₂.
The cortlandites from the Miho area show an intermediate FeO/MgO ratio among those from the Ryoke metamorphic belt, and are characterized by low K₂O and slightly high MgO, MnO and Na₂O contents. The anorthosites of the area are similar to that of the Kaji-shima, Ehime Prefecture. The eucrites of the area are higher in CaO and lower in K₂O than gabbroic rocks from the other localities in the Ryoke belt.
The basic rocks of the area are considered to be derived from the primary tholeiitic magma and emplaced into sedimentary rocks prior to the regional Ryoke metamorphism, and form the layered intrusion. It is suggested that coarse-grained basic rocks of the northeastern part of the Ryoke belt are mostly derived from the tholeiitic magma, and those of the western part (the Kinki and Setouchi districts) are from the calc-alkaline basalt magma.

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Geological survey on Chokai volcano in northeastern Japan has been carried out to construct the volcanic history and structure of the volcano. During this survey a detailed geological map of the volcano has been completed. The volcanic history of Chokai volcano can be divided into three stages as follows. Stage I: In the middle Pleistocene eruptions of major lava flows and associated pyroclastic flows and falls started from a central vent to form a large stratovolcano, circa 1, 000m high above the base. and 47km³ in volume. After completion of the stratovolcano, the north sector of the volcano seems to have collapsed, leaving dry avalanche deposits on the northern foot. The rocks are mainly talc-alkaline andesite with augite, hypersthene and rarely olivine phenocryst, which is accompanied by a small amount of basaltic rocks. Stage II: After the collapse, eruptions of lavas and pyroclastics occurred from a number of vents aligned E-W for 14km. The E-W trending of the crater alignment and normal faults cutting the volcanic edifice may be related to the regional stress field. The volcanic products covered the western and eastern flank of the volcano, although no prominent cone was built, The estimated volume of the volcanic product is 22km³. Finally, a part of the western flank collapsed, resulting in formation of a horseshoe caldera. Calc-alkaline andesite with abundant olivine phenocrysts including chromian spinels was erupted in the early period of this stage, which is followed by eruptions of hornblende-bearing andesite. Stage III: A small stratovolcano was built on the eastern flank, and minor lava flows covered the western flank. About 2600 years B.P. the northern part of the stratovolcano collapsed, leaving a horseshoe caldera opening to the north and dry avalanche deopsits on the northeastern foot. Subsequently, two lava domes were built in the caldera, one of which was formed as lately as 1802. The erupted materials in this stage amounted roughly to 4km³. They are calc-alkaline andesite containing augite, hypersthene and frequently olivine phenocrysts.

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