48 VoL XLIII., Art. 6. — s. Tsuboi: 



(II) Surface Sculpture of the Mountain Slopes. 



There is no river in Oshima, nor is it believed that there 

 ever has been one. Meteoric water in flowing down the natural 

 slopes of the ground as temporary streams (consequent streams) 

 excavated sliallow channels or gulches, but later volcanic 

 products often filled these up. Exposures as shown in Fig. 29 

 are frequently n^t^t with on some road cuttings, for example, 

 midway along the road from Nomashi^^ (Ic) to Sembasaki"^ (Id). 



These reveal the alternately re- 

 peated accumulation and dissec- 

 tion. 



As a whole, Oshima is young 

 =^^"^ — = in dissection, the radial gullies ex- 



Fig. 29. Ejecta layers showing cavated as draining channels being 



alternate excavation and deposition. 



mostly not deep. 



In general, the depth of valleys depends on the thickness of 

 the surficial deposit of éjecta overlying the solid lava, since the 

 streams have excavated the ground till the underlying lava surface 

 has become exposed. Most of the valleys in Oshima (not all) have 

 not yet reached a stage of development where the solid lava is 

 excavated deeply. 



Thus different parts of the island are variously dissected. The 

 difference in dissection, however, will be understood when the 

 geologic condition of the island and the surficial agencies acting 

 on it are considered. 



In the northwestern half of the somma, i.e. on the northern 

 and western flanks, comparatively shallow radial valleys are 

 rather regularly developed. This is quite natural as the inclination 



1) m^ 2) ^v^<m 



