The natural gas of the Joban coal field is accumulated mainly in the coal-bearing Iwaki formation and the. Asagai formation of the Oligocene age. Minor accumulations are found in the Goyasu formation of Miocene age. The chief source of the gas is sub-bituminous to bituminous coal of the Iwaki formation. As the Iwaki formation consists mainly of sandstones, the greater portion of the gas has escaped from the coal seams without difficulty, and migrated upward.
Structurally the Joban coal field consists of faulted blocks, and the gas is trapped by the faults.Gas reservoirs are commonly found on the upthrown side of the faults. The reservoir rocks are thick sandstones which are low in permeability. The gas issues chiefly from fissures and fractures of the reservoir rocks. The Takai No. 5 well, which was drilled in the fault zone of the Takai fault, recorded a maximum production of 150, 000 cubic meters per day.

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The Temmabayashi mine is the only mine which is now worked underground, and producing monthlyabout 20, 000 tons of crude ores.
The Sokota Formation of the uppermost Pliocene age, which encloses the iron sand placers, lies on the Pliocene formations with a slight paraunconformity, and is covered unconformably by the Pleistocene sedimentary group.
As a result of sedimentological studies of the area, especially of underground observations and analyses of arill cores, the following features have been recognized.
1. The Sokota Formation consists of loose sandstone, conglomerate and fine-grained tuff of littoralfacies, and interfingers with the Shimizume Formation of shallow marine facies.
2. Besides unimportant placers in the underlying formation, the producing iron sand placers occurin three groups of ore beds, i. e., Tsurukodaira, Sokota and Jimba groups.
3. The Sokota group contains 13 ore beds which are arranged from SW to NE, successively imbrisating each others.
4. The largest ore bed develops as long as 900m. in strike side, and has a width of 400 m. in dipside.
5. Component minerals of the ores are magnetite, ilmenite, hematite, maghemite, limonite, pyrrhotite, etc., and gangiue minerals are quartz, feldspars, hypersthene, hornblende, apatite, etc., each being separate or locked complicatedly.
6. The total iron content of crude ores ranges from 22 to 55 percent, and that of magnetic concentraces is 51 to 60 percent.
7. According to facies changes of the overlying and underlying ormations, cyclic sedimentation of ironsand placers, and relations of ore beds to the enclosing formation, it may be concluded that the oredeposits were formed in a shallow lagoonal environment with clos erelations to wave motion, undercurrentflow and changing rate of subsidence.

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The results of the heating-stage method, of the decrepitation method and of the decrepitoscope method of the liquid inclusion geothermometry are discussed in this paper. From the comparison of the results obtained by the heating-stage method and the decrepitation method about quartz and sphalerite, it is recognized that the overshoot of the decrepitation temperature of quartz varies with the size of sample. The values of the overshoot are 50-60°C for 35-150 mesh and 70-80°C for 65-150 mesh, when the heating rate is 14-16°C/min. while, the overshoot of the decrepitation temperature of sphalerite is about 20-40°C. The filling temperatures of the liquid inclusions of quartz sampled from the Oppu mine are 200-270°C and those from the Yaso mine are 180-270°C.
Opaque minerals, such as chalcopyrite sampled from the Yaso mine and marmatite from the Taishu mine, were measured by the decrepitation method. The decrepitation temperatures of chalcopyrite range from. 200°to 260°C and those of marmatite are about 200°C. As the decrepitation temperatures of chalcopyrite nearly coincide with the lower parts of the filling temperature ranges of the primary inclusions in quartz that coexists with chalcopyrite, the overshoot of the decrepitation temperatures of chalcopyrite seems to be not so large as that of quartz. The decrepitation temperatures of marmatite of the Taishu mine are considerably lower than those expected from the paragenesis of the ore minerals and the geological setting of the ore deposit. It is inferred from the study of the inclusions in quartz from the Taishu mine that the decrepitation temperatures of the marmatite from this mine are related to the secondary inclusions of the marmatite.
It is concluded that the results of the decrepitation method are not so accurate and, if possible, they must be checked by the heating-stage method.

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Many polyphase inclusions composed of liquid, vapor and solids are found in the quartz crystals from the Taishu mine. There are three different kinds of solids in the polyphase inclusions; they are 1) minute opaque mineral, 2) transparent cubic mineral, optically isotropic, and 3) transparent. irregular-shaped mineral, optically anisotropic.
When the inclusions are heated the transparent cubic mineral dissolves at 100°C and it is inferred to be halite. The transparent irregular-shaped mineral is presumed to be calcite as it shows a high interference color between crossed nicols and considerably high relief in the salt solution. It is presumed that the concentration of NaCl was about 256g/l at the formation of the inclusions and that the polyphase inclusions were formed under the high temperature condition and from the high concentrated salt solution.

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