K-Ar age determination and paleomagnetic measurement have been carried out on the Miocene rocks distributed in Fukui Prefecture, the northeastern part of Southwest Japan. Thirteen K-Ar whole rock ages ranging between 17.1 Ma and 7.3 Ma were obtained from the Miocene rocks, which are virtually consistent with their biostratigraphic ages. Paleomagnetic samples were obtained from sites whose ages were determined by the isotope or biostratigraphic age-determination. Their stabilities were examined by applying alternating and thermal demagnetization, and it is confirmed that 17 sites give us reliable paleomagnetic data.
The new paleomagnetic results, together with those published previously, made clear, first, that Southwest Japan was rotated clockwise through 14° with respect to Eurasia in a period between 26 Ma and 17 Ma, second, that a rapid rotation of Southwest Japan was initiated at about 17 Ma, and ceased at 15 Ma resulting in the 36° clockwise rotation. The rate of the rapid rotation is 18°/Ma. This clockwise rotation can be explained by the drift of Southwest Japan associated with the spreading of the Japan Sea during the Miocene.

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A new method is proposed for the determination of mineral compositions of granitic rocks. This method is performed on a personal computer (NEC, PC9801) with a color image processing unit (edec, ED1351, 256×256 pixels) for stained rock slabs. The minimum area to give an accurate modal composition, which depends on grain size of rock, is obtained. It requires only a few minutes to complete the modal analysis by this method. The modal analysis method is more swift and easier and the result is more accurate than the ordinary method using thin section.

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Metamorphosed pelitic rocks were studied in the contact aureole around the Bushveld mafic layered suite, northeastern Transvaal, South Africa. The vertical variation along the cross section at the Penge area indicates that the metamorphic grade increases from chloritoid-chlorite through andalusite-cordierite or andalusite-staurolite-garnet to cordierite-orthopyroxene-sillimanite zones with approaching the contact. Except for the chloritoid-chlorite zone, biotite is ubiquitous in low- to high-grade pelitic hornfelses. Ti content in biotite coexisting with ilmenite also increases toward the contact. Using available geothermometry, metamorphic temperatures vary gradually from 750±50°C at the contact to 420±20°C at Penge. Metamorphic pressures at the Penge area are estimated to be 2.1±0.4 kbar at 5.4km and 2.4± 0.9 kbar at 8.9km apart from the contact. The lateral variation of mineral assemblages in the Lower Shale Unit of the Timeball Hill Formation, Pretoria Group, shows that the metamorphic grade increases from east (chloritoid-chlorite at Penge) to west (andalusite-staurolite± garnet at Malipsdrift). This variation reflects the reduction of the distance from the contact between the mafic layered suite and Pretoria Group, because the intrusion plane cuts gradually into the lower horizons of the Group from east to west.
The metamorphic conditions along the cross section at Penge suggests that the contact metamorphism recorded the gentle geothermal gradient (80±20°C/km) which is several times smaller than that due to the Duluth Complex, northwestern Minnesota, U.S.A.

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