Phase-equilibrium studies of this system, at and just below temperatures where a liquid phase is present, show one ternary compound, the iron analog of cordierite (2FeO. 2Al₂O₃. 5SiO₂), which decomposes at 1210°±l0°C. to mullite, tridymite, and liquid. The fields of stability of corundum, mullite, hercynite, iron cordierite, cristobalite, tridymite, fayalite, and wüstite have been delineated. The following invariant points (the first two eutectics and the remainder reaction points) were located:
Fayalite+wüstite+hercynite+liquid 1148°±5°
Fayalite+iron cordierite+tridymite+liquid 1083°±5°
Fayalite+iron cordierite+hercynite+liquid 1088°±5°
Hercynite+iron cordierite+mullite+liquid 1205°±10°
Iron cordierite+mullite+tridymite+liquid 1210°±10°
Corundum+mullite+hercynite+liquid 1380°±5°
Cristobalite+tridymite+mullite+liquid 1470°±10°
Iron cordierite crystallizes with some reluctance, and the metastable invariant points fayalite+tridymite+spinel+liquid at 1073°±5°, and mullite+hercynite+tridymite+liquid at 1205°±10° can be realized. No ferrosilite or almandine garnet could be crystallized from the melts at any temperature even when melts were seeded with these crystalline phases. Natural almandine from Botallack, England (91. 3 per cent almandine), when heated begins to decompose at an appreciable rate as low as 900°C. and yields a mixture of hercynite, iron cordierite, and fayalite. The bearing of these results on petrology and slags is discussed.

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This area which occupies a part of the backbone ranges between Iwate and Akita Prefecture consists largely of marine Lower and Middle Miocene, terrestrial Upper Miocene and Lower Pliocene sediments. These are underlain by the Permian sediments and granodiorite, moreover overlain by the Recent volcanics. The Cenozoic sediments of this area are sub-divided as shown in the text.
In the eastern part of this area, the Orose, Mizuyama, and Ohira formations make a centroclinal fold with low angles. These formations are also broken by a system of radial faults conversing to the center of the area and make some minor folds with the N-S trend along the western most part. In the central part of the area, the Maekawa and Orose formations make a system of asymmetrieal folds with axes forming broad arcs with their concave sides facing towards the east and with dip angles of 10-20° along every western wing and of 15-30° along every eastern wing. The western part of the area consists of the Oidegawa and Isawagawa formations besides the subjacent pre-Tertiary rocks. An anticlinal structure traverses along the middle portion of this part, and the Maekawa and Oidegawa formations dip at angles varying from 20° to 40° towards the east along the eastern wing, and the Iwaigawa formation and the pre-Tertiary rocks are exposed along the crest of the anticline. The Maekawa and Oidegawa formations which build the western wing of the anticline dip westwards at angles of 30°.
After deformation of the Permian sediments, intrusion of granodiorite and subsequent prolonged subaerial denudation, the first intense extrusion of basic lava and its pyroclastics of Neogene age took place in this area. This volcanism was followed by the first marine transgression of the Neogene and it was at this time that the initial phase of the Tertiary geosyncline in North Japan was born. The Lower Miocene Isawagawa formation is a product of such a condition. The subsequent intense volcanic activity of acidic lava and its pyroclastics occurred during progression of the aforestated marine transgression. Thus, the middle Miocene Oidegawa formation of shallow water origin were formed.
When these volcanisms became weakened and marine transgression progressed further, the siliceous shale besides conglomerates of the Maekawa formation deposited. The aforesaid conditions were followed by marine regression in this area which resulted in the deposition of coarse-grained pyroclastic sediments of the Orose formation. Before the sea was completely withdrawn from this area, gradual uplift along the western borderland occurred. Thus, following complete regression, the Orose was well as the subjacent formations became exposed above sea-level. The dacite and its tuffs of the Mizuyama formation were deposited in a depressed basin fomed in the Upper Miocene in this area. With the opening of the Pliocene, the lowland of this area was invaded by a new marine transgression. This transgression is responsible for the building of the Ohira formation. After the deposition of the Ohira formation, this area was uplifted and all of the Neogene formations stated above were subject to a subaerial denudation until Recent date. Some volcanic cones, sub as Yakeishi-dake, etc., were formed in the late Pleistocene, and the several river terraces and fans were constructed along the eastern margin of this area almost simultaneously with the neo-volcanisms or somewhat later.

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