We can see remarkable differences between the topography of the granitic rocks area and that of the paleozoic rocks area in lower relief mountains (200-400m in height), of the seacoast of Setouchi District.
The writer has had a interest in that difference and tried to measure the drainage-density on the topographic maps of those regions and also had many field observations and field experiments (infiltration-capacity, etc.). Moreover, he made several lavoratory experiments on water-content, grain-size analysis, liquid-limit, plasticity-index and shear-test with the eighty samples of surface soil in order to find out the differences of each rock area. The results are summarized as follows:
1. On the drainage-density, there is no statistically significant diffe-rence between the Hofu region (Yamaguchi Prefecture, western. part of the Setouchi) and the Kojima region (Okayama Prefecture, eastern part of the Setouchi), both of which are situated in the same Setouchi clima-tic region, but significant defferences are recognised between the granitic and the paleozoic lower mountain area in those regions. The fact that the drainage-density on the granitic area is higher than that on the Paleozoic area was brought out clearly by his actual field observations.
2. One of the most remarkable differences that have been lighted by the field observations over those regions is the difference of the vegeta-tion; that there is denser vegetation in the Paleozoic area than in the granitic.
The water-content, the infiltration-capacity and the effective-cohesion (after shear-test), in general, are higher in the paleozoic than in the granitic. In the values of the grain-size analysis and the liquid-limit, there are no differences between the Hofu and the Kojima regions, but the significant differences between the granitic and the paleozoic soils of each region; the latter has smaller value of median-diameter and higher liquid-limit than the former.
3. It has been reported that. the characteristics of water-content, effe-ctive-cohesion, liquid-limit, and so on, respectively, are mainly due to the grain-size distribution. The writer thinks, therefore, that the diffe-rences of each of the above-mentioned properties are due to the differe-nce of the grain-size distribution between the granitic and the paleozoic soil; the coarser granitic soil is less in the liquid-limit, perhaps also in the effective cohesion and the ability of the soil supporting vegetation etc., than the finer paleozoic soil. In other words granitic soil has higher susceptibilities than the paleozoie to erosion.
In short, the granitic area of this study in this district has a higher in the erosibility of soil and the drainage-density than those of the Pa-leozoic. The higher drainage-density seemes to be mainly due to the higher erosibility.