Abstract
On the 10 th of August, 1958, three months before the big eruption of Mt. Asama Volcano, the author surveyed the eastern slope of Asama Volcanic Cone in order to clarify the composing materials and the grain compositions.
The grain compositions are explained in this paper. The spots where the sampling station exploited are shown in Fig. 1. Samples were exploited every vertical 50 meters from the summit down to 1, 742m both on the ground surface and 50 cm under the ground.
Samples smaller than 2mm in diameter were sorted with sieves of the 1/2 scales that he had prepard experimentally.
The features of the grain compositions are shown in Fig. 3, Fig. 4 and Tab. 1. The conclusions are as follows:
1. On the 10 th of August, 1958, the surface strata on the eastern slopes of Asama Volcanic Cone were composed of andesite breccias and underlaid coarse grained pumices.
2. The Mdφ, Qdφ and Pdφ of each of them do not vary in height above the sea-level, but qreat differences are recognized between the samples on the ground-surface and 50cm under ground even at the same spots.
3. Fine grained materials smaller than 2mm which are mixed with andesite breccias on the groundsurface and coarse grained pumices 50cm under the ground are distinguished to be quite dif erent as the result of sieving.
4. Considering the differences of grain compositions shown in Fig. 3 between the groundsurface and 50cm under the ground in comparison with the fact that the strata near the ground-surface are separated into two layers, andesite breccias and coarse grained pumices, the author suggests that the varieties of the scales of volcanic activities of Mt. Asama resulted in the establishment of discontinuous surface 50cm under the ground.
