With the purpose for explotation of petroleum and knowledge on the geological structure of the Tertiary rocks lying below the Chokai volcano and its detritus in the region bordering both Akita and Yamagata Prefectures, the writers made geological observations during the sammer of 1951. As a result, five formations were recognized; these in ascending order are the, Nikko, Nadakayama, Harigaoka, Uguisugawa and Nigorikawa formations. These are thought to correspond to the so-called Green tuff of authors, the Onnagawa shale-, Funakawa shale-, and Nomura grey shale formations and the Wakimoto formation of siltstone, which are developed in the oil-fields of northwestern Japan.
The geological structure of the surveyed area is shown in Figure l and 2 in the text.
The antic ?? e developed in the vicinity of Yunosawa is cleaved with many faults in different magnitudes and thus its structure is not simple, and, northwards the anticline dies out and its structure is no longer recognized, owing to being blanketed by the volcanic detritus of the Chokai volcano.
Although the development of the Onnagawa and Funakawa formations, which are generally accepted as being the source rocks for petroleum, is good, the lack of favorable structures for petroleum, makes it desirable to encounter test drilling. Since in the Yunosawa anticline are found both gas and oil seepage, the probability for oil production is not altogether improbable. However, owing to the northern part of the anticline being blanketed by the Chokai volcano detritus and to the structure being unrecognizable, further studies are required necessary.

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