Studies of the mountain development of Japan have been made on the following assumptions deduced from the Davician cycle theory. They are as follows ; ( I ) The alrnost areas in Japan Island were flattend or peneplained in late Tertiary or early Pleistocene age. (2) The outline of topography of Japan Island was graduaily formed by tectonic movements which followed such stable period. Undulating low relieves of ridge or accordant ridges have been thought to be remnants of erosion surfaces or peneplains. (3) On the other hand, the chronological studes on mountain have been made on the hypothesis of mutiple cycle theory. Terrace-like erosion surfaces recognized for the reason of accordance of ridge have been explained to be results of alternation of uplift and stable age. Mairfy, the procedure of such study is frstly to class~y erosion sufaces and next to determine the ages of them. The author attempts to reconsider if the basic assumptions of study as stated above is fruitful means of research. He examined the history of study on erosion surface in Japan and United States of America and he reconsidered, by exarning Tertiary sedirnents in the Japan island, in order to make clear whether there existed in tertiary age stable period in which lands have been once of low relief or flattened. As the results, the author suggests the following conclusions ; The assumptive ideas stated above are impracticai. Ridges of low relief and accordant ridges, regarded as remnants of erosion surface, are of differnt origin to be respectively examined. It is almost impossible to assumpt such stable age in Tertiary age or early Pleistocene age that the Japan Islands were of low relief or flattened. Tectonic movements have been so continuous through Cenozoic era that fault movements acted to compound topographies in different ages. In other words, new tectonic movements took place before pre-existing landforms were eroded down into low relif or flat. Therefore, we need to classify mountain masses and mountain ranges according to age of formation and to accord them to stresses in different tectonic fields in respective age. It is possible, by law of superposition, to classify mountains according to age of formation. The outline of the geomorphology of Japan are deterrnined by the transition or change of tectonism in late Cenozoic era. The author suggests that the important problem to be solved is to make clear order of mountain formation in relation to Cenozoic tectonic history.
