This is a critical review of the geology of the region. The stratigraphy is discussed in dividing the terrain into three tectonic units as shown in fig. 1, and schematized as indicated in the table on page 6. The best Palaeozoic sequence can be seen in the west zone. There however, the Devonian system is unknown, but present in the east flank of the Pak Lay anticline in the median zone. The Khorat series in the Khorat plateau in the east which contains dicotyledonous plants must be Cretaceous, instead of Jurasso-Triassic. It is quite probable for it to extend easterly into the Indosinias supérieures of South Laos which yielded Senonian Reptiles and non-marine Trigonicides faunule and southerly into Grès continentales of Cambodia which is also Upper Cretaceous, if not Palaeogene. Thus it is certain at the least that marine Jurassic and Triassic formations should be eliminated out of the continental Khorat series. The Triassic formation in the median zone is accompanied by eruptive rocks, like the Pahang volcanic series, but it is known paralic in North Laos. In the west zone there are Triassic and Jurassic formations devoid of volcanics. They are strongly folded, while the Cretaceous (?) red sandstone of Tenasserim is gently monoclinal. It is probable that the old Cimmerian folding in the median zone has confined the flooding of the Jurassic sea to the west zone and that crustal movements have taken place in the still older periods on the west side of the Indosinian massif. Nevertheless it is quite warranted that the folded mountains through the middle-west of Thailand has not been established by the Cretaceous period. Older rocks are intruded by granitic rocks, but not the Khorat series as well as the Tenasserim sandstone. They are both continental, and slightly undulated or simply inclined with low angles. Thus the vast terrain turned out land in the Palaeogene. The influence of the Himalayan orogeny was weak in this part of Asia.
The hot water veins have been detected as negative self-potential anomalies, having the maximum gradient of 2.8 mV/m. (Fig.3), and the potential profiles of the anomaly zone were equal independently from both the topography and the geology, at the Oshuku hot springs in Iwate Prefecture. (Fig. 4) Accordingly the negative anomalies there are surely caused by the chemical reactions due to the intrusion of hot mineral water. In addition, a structural line, which may be hardly detected by surface geological exploration, has been found out by the electrical method. The bore-site, decided after the electrical prospecting, was supposed to be in the outside of the outcrop of hot water vein, and the bore was expected to penetrate the another vein, different from that of the existing hot springs, at the depth of 71.5 m. below the ground surface. These suppositions have been proved by the actual practice of well-boring. Namely the temperatures of the bore-hole was never beyond 30°C down to the depth of 70 m., in spite of the short distance of only 12 m. from the nearest hot spring, and they raised suddenly just from the depth of 71.7 m. up to 56.5°C at the depth of 79.3 m. (Fig.6) And the pumping test proved that the well yielded the hot water of 53.8°C and the pumping gave no influence to the pre-existing hot water sources.
The author has investigated several mineral springs at Kushimoto and its environs. This paper is the short description on those springs. Some of them were found utilizable for the purpose of bat hings.