The Kosaka Mine tried in vain for a considerable time to discover new ore deposits. By the end of world war II the Motoyama Deposits which had been the only workable deposit in Kosaka were almost exhausted. But, the enthusiastic geological investigation and drillings executed in recent years turned out a success. In the summer of 1959, the Uchinotai Deposits were discovered.
The Uchinotai Deposits are Kuroko or Black Ore deposits like those of the Motoyama, and consist. chiefly of four kinds of ores, that is, black ore (sphalerite-galena-chalcopyrite-tetrahedrite-barite), yellow ore (chalcopyrite-pyrite), siliceous ore (pyrite-chalcopyrite-quartz) and gypsum ore. The geological structure of a NNW-SSE trend, predominant in the vicinity of the Kosaka Mine, is considered. to control ore deposition. The ore deposits concordantly underlie the Akamori pumice tuff formations of Miocene, and their lower parts grade into the Motoyama breccia or the Baramori rhyolite which. are considered to have an intimate relation to the ore genesis.
At the end of 1961, the reserves of the Uchinotai Kuroko Deposits were estimated at about 9, 200, 000 tons of ore (Cu 2.48 %, Pb 1.3 %, Zn 4.2%), as the result of the 116 borings which were drilled at intervals of 50 meters. The fact shows that the Uchinotai has become one of the greatest copper ore bodies in Japan.

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Many subspherical pyritic coal balls are found in the upper portion of the coal seam of Oyake 5-Shaku of the Omine colliery, Chikuho Coal Field. The authors have detected the distinct fossilized wood tissues from the polished specimens of those pyritic coal balls ; the wood tissues consist of the following parts.
a. Slender twigs of a conifer. They are present abundantly, and their original plant tissues are preserved nearly completely. They have no annual rings, so that they must be of a year wood, probably derived from fallen leaves of a tree having feather-like twigs such as common in those of Taxodium.
b. Half decomposed heart woods filling the interspaces of the above described twigs. These have distinct annual rings, consisting of tracheids. They have no vessel structure which is a determinative characteristic of broad-leaved trees, and, moreover, they are found to be similar to those heart woods of the silicified woods popular in the coal seams of the Chikuho Coal Field, and are determined to belong to Taxodioxylon sequoianum Gothan.
c. The interspaces of the woods structures are filled with structureless pyrite.
Presence of nearly or completely identical structures and components have been confirmed in the dolomitic coal balls from the Yubari and the Bibai coal seams of Hokkaido. All of these may suggest the original features of forest swamp of the coal-forming environments of Japanese coal fields.

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1. The geomorphic studies of known ore deposits of Miocene mineralization in Hokkaido and other parts of Japan have led us to the recognition of topographic similarity in their vicinities, represented by the erosion remnants of mineralization.
2. The typical erosion remnants in many places of the Miocene mineralized areas are summarized as follows : (1) square-topped hills having cross-shaped four erosion valleys ; and (2) the erosion valleys surrounding the hills, showing a pattern as seen in anticlined or domed areas.
3. The factors producing such topographic remnant are ; (1) The upheaval movement accompanied by mineralization, and (2) the rock alteration resulting in the selective erosion.
4. The areas of similar topography to the mineralization remnants are marked on 1 : 50, 000 maps, with reference to geological maps, and the reconnaissance routs to reach those areas have revealed presence of many new ore deposits.
5. Thus, the topographic remnants used in locating ore deposits in virgin areas of Hokkaido have proved to be very efficient in the field work of mining geologists.

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This is a brief essay on the geology of mineral deposits, in which the writer emphasizes the necessity of geotectonic and magmagenetic studies in dealing with genetical problems of mineral deposits. Such studies are important not only in the purely scientific investigation but also in the practical researches of mineral deposits. But for the fundamental knowledge of regional geology comprising the tectonic and magmatic history with which the formation, enrichment, and structural behavior of the deposits in the region are closely connected, no perfect researches of mineral resources nor solving the fundamental problems concerning them can be undertaken in a vast, unexplored area, though some mineralogical or structural problems of limited kind or of local ones may be solved by general or empirical knowledge of ore deposits.
The present states of mineral deposits is the resultant of all geological processes to which they have been subjected from the time of their formation to the present, and the degree of enrichment, primary or subsequent, and structural behaviors of the mineral deposits are so closely related to them that no student of mineral deposits can lose sight of them both in fundamental and practical investigations. From this point of view, the writer presents a classification of mineral deposits, bearing much weight on the tectonic and magma-genetic processes that lead to their formation and enrichment.
The writer also pays attention to the general association, of certain types of mineral deposits and those of geological formation, and presents some interpretation of it, besed chiefly on geotectonics.
Lastly, some reference is made to the problem of ore-contamination that seems very probable in the light of recent geotectonic theories and data of lead isotopes in ore materials. It may be certain that ore materials, within the reach of our observation and utilization, have concentrated mainly in the upper earth's crust through the recurrence of diastrophism (crustal movements and igneous activity), and it is presumed that the great majority (or at least appreciable part) of them have been circulating within the upper earth's crust in the course of crustal development since the earliest geological time, though certain amounts of iron, titanium, chromium, etc., might have been supplied from depths by basic or ultrabasic magmas of abyssal generation.

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