In this paper, the petrology of the granitic rocks from the district of San-in,
especially from the province of Izumo, is described with some references on the mineral resources associated in these rocks. The granitic ro:ks of which the Chugoku-Mountainland in the Izumo province is mainly composed may be classified mineralogically and chemically into next seven kinds. Their names and the respective average silica contents are as follows: (1) Hornblende-biotite granodiorite·········SiO(2) 66.18% (2) Hornblende-biotite granite········· SiO(2) 69.28% (3) Biotite granite·········SiO(2) 74.91% (4) Fine-grained biotite granite and
biotite granite porphyry·········SiO(2) 73.47% (5) Aplite ·········SiO(2) 75.96% (6) Gabbro and diorite·········SiO(2) 56.05% (7) Quartz porphyry· ·SiO(2) 73.79% Among the various mineral resources found in the granitic rocks in this region, the iron sand and the molybdenum ore seem to have more ecconomical value and are indeed wrought respectively in several localities in farely large scale. The iron sand is collected either from the decomposition product of the hornblendebiotite granite (2) and from that of the diorite (6). The former, so-called "Masa",
is superior in quality for the production of steel, but it is inferior in its quantity. On the other hand, the latter, so-called "Akome", can be gathered in quantity in larger amount than the former, but it is inferior to the former in quality. For this reason, both sorts of sand are in practice mixed in some definite amounts. The ore deposits of molybdenite contained in the quartz vein traversing the granitic rocks are found in several localities and some of them are wrought. In such cases the parent rock of the ore seems to be invariably confined to the rock type of the fine-grained biotite granite or of the aplite. Besides the rocks stated above, some descriptions are given to the rocks, such as quartz porphyry and basalt, which are found in several places in the granite region. However they seem to have no genetic relation with the underlying granites.