Datolite of Somi is white in color and massive, occuring in the druses of metamorphosed rhyolite or dacite of the Miocene Sobosan volcanic rocks.
Optical data are: α=1.630, β=1.657_??_1.659, γ=1.667, 2V=(-)75. Chemical composition and X-ray powder data are shown in table 1 and 2.
It is associated with axinite, epidote, chlorite, grunerite-like amphibole, showing the mode of occurence different from any of the known cases such as limestone contact. Also, genetical problems of datolite from Japan are considered in this paper.

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In the Shichinohe District, mountain iron sand placer bearing Pliocene formations are developed, and temmabayashi Mine, the largest of mountain iron sand placer deposits, lies in the formation and is in operation.
The mountain iron sand placer deposits are composed of Ichinosaka Iron Sand Placer formed in Ichinowatari Formation and Sokota Iron Sand Placer Groups intercalated in Sokota sandstone and conglomerate member. Their sedimentary environment and mechanism are closely related with the tectonic movements of the mountainous land extending at the back, and their outlines are given below.
1. Stratigraphic sequence of the district:
2. Since the Pre-Kotsubogawa stage, upheaval started on the southwestern side of Kamita fualt (in the NW-SE strike trend) and tilting depression took place on its northeastern side. In the central portion of the depression, Shiroishi agglomerate was sedimented.
3. On entering into the Ichinowatari stage, upheaval movements sarted anew around Shiroishi agglomerate as their centre. Ichinowatari and Shimizume formations were caused to sediment respectively in the depressed area due to the differential upheaval and depression movements, and thus differ greatly in their thickness by each of the areas.
4. Shimizume formation of the shallow marine facies interfingers with Sokota sandstone and conglomerate member of the brackish water facies. Sokota sandstone and conglomerate member is a delta deposit formed in relation with the upheaval of Shiroishi agglomerate, and shows specific distribution and rock facies. The member consists of coarsegrained sandstone, conglomerate, fine-grained shale, tuff, lignite and iron sand.
5. Ichinosaka iron sand placer is confined to a horuzon and is on a small scale because of its short duration of shaking movement and its insufficient sources to cause iron sand concentration. Sokota groups are composed of three placers of Tsurukodaira, Sokota and Jimba, intercalates in Sokota sandstone and conglomerate member, and Sokota placer is the most predominant of the three. These placers were formed into ore-beds in the delta sediments having been composed of ironsand grains concentrated on land due to the constant shaking movements of the base rocks caused by the upheaval of Shiroishi agglomerate.
6. Sokota iron sand placer contains warping structure at the western end of the placer caused by the upheaval of Shiroishi agglomerate, while the placer was still in a state of plasticity after sedimentation had been made. The placer shows an 80°E dip on surface and indicates lower dips to the depth, further decreasing its inclination down to 5-10°E at the levels 70-80 meters downward from the surface
7. Shiroishi agglomerate has been continuing its upheaval ever since through Pleistocene age up to this date

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[in Japanese]

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