The Akiyoshi folded mountains on the continental side of the Japanese islands are composed of geosynclinal sediments, their metamorphosed and granitized facies and the metaorogenic basin deposits. The first of these consists chiefly of the Middle Palaeozoic rocks of the intrageosynclinal volcanism in the lower part, in the middle of the Upper Palaeozoic of the limestone-rich Akiyoshi facies and the chert-rich Yamaguchi facies, the two facies indicating the differentiation of the geosyncline by the embryonic folding and in the upper part of the Permo-Triassic formations in the Maizuru zone. The embryonic Akiyoshi geanticline was bisected in the Permian by the Maizuru syncline into the Sanin anticline in the west and the Hokuroku anticline in the east which were aligned en echelon. The Akiyoshi mountains which reveal the best display in the inner side of West Japan consist of the axial metamorphosed zone and the Yamaguchi folded zone with the Nagato tectonic line at their boundary. The Sangun metamorphic complex in the former was produced from the Pre-Permian rocks by regional metamorphism. Subsequent to the metamorphism the axial zone was granitized, causing the dislocation of the median line in front of the granitized regime. In the Sanin anticline the zone was divided into the Hida gneiss zone on the rear side and the Sangun schist zone on the fore side. In the Hokuroku anticline or Hida plateau region the axial zone was largely granitized and dislocated along the median line in front, leaving its northeastern part where the Sangun complex exists. Because the Moscovian of Itoshiro which is located far inside of the median line is a roof pendant on the Hida gneiss, the Hida plutonism cannot be older than Carboniferous. It can be distinguished into three stages, i.e. the Amo injection, the Shimonomoto subconcordant intrusion and the discordant intrusion of the Funatsu granite, the last of which took place along the median line, intruding the Permian. Therefore its age must be either late Permian or Triassic. In the early and middle Triassic period sea flooded into the palaeo-Maizuru bay from the Pacific side, as far as Ussuri. The Ladinic stage is absent in the Maizuru zone and the base of the Carnic Nabae series marked by a strong discordance, but the zonal structure of Maizuru was not completed by the Carnic because the Nabae is folded in within the zone. The Shidaka and Nariwa series, Noric or late Carnic in age, were deposited in basins in the zone and behind it. The mountain-building was completed earlier on the west side of the zone where the Ladino-Carnic Atsu and later Triassic Mine series are all metaorogenic sediments. On the east side of the zone the Noric of Myogadani, Mt. Ibuki is folded together with the Paraeozoic and the Liassic Kuruma series deposited in a basin along the Nagato line is tremendously thick. It is a remarkable fact that the Maizuru zone as well as the folded arcs on its two sides are distinctly truncated by the Ryoke gneiss zone on the rear side of the axis of the Sakawa folded mountains. No evidence showing either Pre-Cambrian age of the Hida gneiss or its being an ancient massif has as yet been discovered.
A critical study of the geological map, 1 : 50, 000, Sheet “Yoshinoyama” by K. Hirayama and F. Kishimoto, published by the Geological Survey of Japan, 1957, led the writer to the following conclusion : I. The Median dislocation line of Southwest Japan, in the original sense of the writer, is divisible into three sections. The east section which extends from the south of the lake Biwa to the southern side of Mt. Kongô, lying north of Gozyô, Nara Prefecture, separates in its greater part the terrain of the Ryôke complex (Ryôke metamorphics, with grantitic rocks in an intimate association) in the north, from the terrains of the Nagatoro metamorphics and semi-to non-metamorphosed sedimentaries of the Paraeozoic Titibu system in the south. West of Wasika, Ogawa-mura, Nara Prefecture, the former terrain comes in direct contact with that of the Upper Cretaceous Izumi group on the south. This east section marks the front margin of a southerly, resp. southeasterly overthrust of the Ryôke complex. II. The middle section of the Median dislocation line, which divides the terrain of the Izumi group in the north from that of the Nagatoro metamorphics in the south, runs almost straight ENE-WSW, resp. E-W, from near Wasika successively through the western part of northern Kii Peninsula, off the south coast of Awazi-sima, and across the northern part of Sikoku to Saganoseki, Kyûshû, beyond the Kitan and Bungo Straits. The crustal disturbance along this section is dated post-Cretaceous or early Tertiary, since it displaced the Upper Cretaceous Izumi group on one hand, and preceded on the other the deposition of the Middle Eocene member with Eofabiania of the Kuma group of the Isizuti-yama, which rests almost horizontally on both the Izumi group and the Nagatoro metamorphics over the tectonic boundary. III. These terrains on either side of the middle section of the Median dislocation line are overlapped near Wasika and thence eastwards by the Ryôke complex displaced by the southerly, resp. southeasterly overthrust cited above. The behavior at the junction near Wasika of the two dislocation lines bordering the south and north sides of the Izumi terrain can be well understood in this way. IV. Close to the east section, the Ryôke complex is fringed by a tectogenic rock-complex, the Kasio gneiss, of various mylonites and porphyroids, etc. The boundary between the Kasio gneiss and the Ryoke complex is often quite meager, while that between the former and the Izumi group, Nagatoro metamorphics and semi-to non-metamorphosed Paraeozoic rocks are in general more distinct. The Kasio gneiss is not known along the middle section. V. The zone of mylonitization is a name applied by S. Tsuboi to the linear extention of the Kasio gneiss, and he believed its earlier genesis than the Median dislocation line in the original sense of the present writer ; T. Kobayashi also distinguished early Cretaceous Kasio phase from the post-Cretaceous Itinokawa phase of the tectonic disturbance of the Median dislocation line, based on Iwao's observation on the Izumi group lying over the Kasio gneiss at the southern side of Mt. Kongô, where later R. Sugiyama found their relation to be discordant, and emphasized the absence of the pebbles of the Kasio gneiss in the conglomerate of the Izumi group there exposed, while those of the Ryoke complex are common in it. In the present peper, it is maintained that the Kasio phase, if this term is acceptable, is rather later and never older than the post-Cretaceous movement, the Itinokawa phase of Kabayashi, of the middle section of the Median dislocation line.