The author has been studying the causal relationship between natural hazards due to geological processes and the effects of heavy rainfalls on weathered granite during past five years. It was recognized that there was a close relationship between the present topography and the condition of weathered rock. The purpose of this paper is to discuss the relationship between a form of rock weathering and the topography of Hiroshima Prefecture and adjacent areas. As a study model, six layers excluding regolith and "idomasa" (colluvium composed of weathered granite) were clearly defined in the horizon of weathered granite: Chemical Weathered Layers I, II and III, and Physical Weathered Layers I, II and III, downwards. The present field investigations indicate that the deep weathered zone is found on the flat surfaces in different elevations such as on the Setouchi, the Sera and the Kibi Surfaces. Further considerations have been extended to the distribution of the deep weathered areas, the formation processes of the present topography and the geological time in which the deep weathering processes were active. It is proposed that the crustal movements which were due to the compression in a N-S direction during the early stage, and in a E-W direction during the later stage have strongly influenced the development of the landforms since the Pliocene.

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The purpose of this article is to investigate the relationship between the formation of geomorphic surfaces and the weathering of bedrock in the Takehara area, Hiroshima Prefecture, where granite rocks are dominantly distributed on the small-relief erosion surface. This erosion surface, known as a part of the Setouchi Surface, is subdivided into three levels; the Higher, the Middle, and the Lower Setouchi Surfaces. The Higher Setouchi Surface is identified with the undulating summits of the small-relief mountains, the Middle one with the piedmont benchlands, and the Lower one with small hills composed of granite or gentle piedmont slopes. The author distinguished the following five horizons in a vertical profile of weathered granite in this area, based mainly on the friability of weathered material in situ and on the degree of preservation of the parental rock structure in weathered material; the Soil layer, the Weathered horizon I, the Weathered horizon II (subdivided into IIa and IIb), the Weathered horizon III (subdivided into IIIa and IIIb), and the basal surface of weathering. Weathering profiles of bedrock in this area can be classified into eight types which are established by combining all or some of the above five horizons. The distribution of these types is closely related to the topography of the region as is shown below: Type 1 is characterized by the deep weathering and distributed in the Higher and the Lower Setouchi Surfaces; Type 2 occupies small eminences projected from the Higher and the Lower Setouchi Surfaces; Type 3 develops on the piedmont benchland of the Middle Setouchi Surface; Types 4 and 5 are distributed in the restricted area near Yomokuro-ike belonging to the Higher Setouchi Surface; Types 6, 7 and 8 occupy projected peaks composed of rhyolite, Paleozoic sandstone, Paleozoic clayslate and fine-grained granite on the Higher Setouchi Surface. The last three types are also found at the knickpoints in valley floors cutting the small-relief erosion surfaces. The above mentioned occurrence indicates that the profile types having the upper weathering horizons distribute surrounding the areas with the profile types having the lower weathering horizons, as is shown in Fig. 5. The relief of the area may have been accentuated by the removal of the upper horizons and the exhumation of the lower horizons. In other words, the relief of this area may have been formed by the exhumation of irregularly rolling basal surface of weathering caused by the difference in susceptibility of bedrocks to the weathering process. The process of the formation of erosion surfaces in this area is such that the small-relief surface was deeply weathered in the stable stage, then it was denuded in the following cycle until the basal surface of weathering nearly parallel to the former geomorphic surface appeared. Based on the above view, it is concluded that the Higher Setouchi Surface and the Lower one may be parts of the irregularly undulating basal surfaces of weathering formed in different stages, and that the Middle one may be rocky peaks formed simultaneously with the Lower one by the exhumation of protruded parts of the basal surface of weathering.

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Urbanization has today brought about the increasing concentration of population in large cities. Under these conditions, a new technique of measurement of urbanization is required instead of the former way, from the viewpoint of international comparisons of urbanization. That is to say that a method of measuring which can express the differences between various sizes of city in order to reflect the trend of concentration of population in large cities should be adopted. In this respect, this new method of measuring is different from that of the degree of urbanization which has been used widely. Jack P. Gibbs was the first scholar who proposed this new kind of method of measuring urbanization ( Su=ΣXY, X: proportion of the city population in each size-class and over to the total city population, Y: proportion of the city population in each size-class and over to the total population ). The present author proposes to Improve Glbbs's formula as follows: Su=Σ^^n_<i=1>RiXi Su: indix of the scale of urbanization Ri: rank number (cumulative index) of each size-class Xi: proportion of urban population of each size-class to the total population of a country The appropriateness of the above "index of the scale of urbanization" was tested by the author by comparing it with the degree of urbanization. The degree of change is shown more strikingly in the "Index of the scale of urbanization" than in the degree of urbanization (Fig. 1). And this shows nothing less than the trend of concentration of people in large cities, because the differences of various city size-classes are reflected in the "index of the scale of urbanization". Furthermore, the "index of the scale of urbanization" is related more closely to the size of population of the country than the degree of urbanization (Figs. 2 and 3). As a result, it may be said that the "index of the scale of urbanization" is more useful for measuring the urbanization of a country than the degree of urbanization. In this proposal of an "Index of the scale of urbanization", the important finding is as follows: Urbanization advances gradually to a stable condition in accordance with the development of the national economy, and the subsequent change in population composition is represented by a function of the size of population of a country and the scale of urbanization. This finding was drawn from the evidence that there is a closer relationship between the "index of the scale of urbanization" and the size of population in the economically advanced countries than in the developing ones (Fig. 4). Accordingly, the country with a large population cannot be said to have a large scale of urbanization until the economy is firmly established.

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