During the half century, since the Hitachi mine was first developed, most of its geologic relations have been discovered. Recent studies by M. Shimada and his colleague have given many results, important from both scientific and economic aspects. Some problems introduced by the advance of science, however, still remain.
First, the geologic age of the formation surrounding the mine has been alluded, as a whole, to the Lower Carboniferous. The discovery of Permian fossils in the limestone pebbles of the Tertiary conglomerate overlying the formation, was introduced the possibility of a Permian age for a part of this formation.
Second, the Paleozoic formations have been thought to be monoclinic, until M. Shimada ascertained an overturned synclinal structure in the country rocks of the ore deposits. This aroused a new concept of repeated folding, the exact nature of which is still to be investigated.
Third, the irregular shape and arrangement of the deposits on the southwestern part should be investigated in relation to this folding, not on the concept of a mere local disturbance.
Fourth, a new assumption that the ore deposits have been formed by the following stages requires consideration in relation to the fundamental geologic history of the area.
1. The original ore deposits were formed in association with preorogenic volcanism, perhaps in the Carboniferous.
2. Reformation of ores into fine-grained granoblastic aggregates, arranged concordant to the schistosity of the wall rocks, was caused by an orogenic movement accompanied by a minor concordant intrusion, perhaps at the end of the Permian.
3. Parts of the ore were further converted into irregular bodies of high temperature aggregates by a post-orogenic discordant intrusion of granitic magma, perhaps in the Mesozoic.
Material migration during metamorphism is also considered from a new standpoint.