The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 61, Issue 721
Displaying 1-7 of 7 articles from this issue
  • Akiho MIYASHIRO
    1955 Volume 61 Issue 721 Pages 463-470
    Published: October 25, 1955
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    A pyralspite garnet from a cavity in rhyolite at the Thomas Range, Utah, U., S., A., was analysed as shown in Table 1., It is high in Mn content, being mainly composed of the spessartine and almandine molecules., Pyralspites occurring as porphyritic minerals in volcanic rocks are poorer in Mn than those from cavities and lithophysae in volcanic rocks (Fig., 1)., Pyralspites from granites are generally poorer in Mn than those from pegmatites (Fig., 2)., According to a crystallo-chemical consideration, pyralspite poor in Mn is stable under a narrower range of physico-chemical conditions than those richer in Mn., The physico-chemical conditions under which porphyritic minerals and granites were formed, were more favourable for the generation of pyralspite than those under which cavities, lithophysae, and pegmatites were formed., Therefore, pyralspite poor in Mn could be generated as porphyritic minerals and in granites, but not in cavities, lithophysae, and pegmatites.,
    Download PDF (863K)
  • Shiro KANEKO
    1955 Volume 61 Issue 721 Pages 471-480
    Published: October 25, 1955
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Tertiary deposits in the Itsukaichi Basin, Tokyo Prefecture, rest unconformably upon the greatly deformed Chichibu-Torinosu System, which has suffered subsequently, intensive disturbance after the deposition of the top conglomerate (T5) of the Tertiary rocks., In the Tertiary deposits is recognized a dislocation zone consisting of four areas (Fig., 4) with steep dips and a general trend of NW-SE., This fact suggests some genetic relationship between the formation of the abnormal dislocation zone in the Tertiary rocks and revival in activity of the previous fault lines of the basement., In the southwestern part of the basin, the dislocation is accompanied with an interesting mode of deformation and shearing which simulates plastic deformation., (Figs., 5∼7)., This is considered important in interpreting the process of dislocation of the Tertiary deposits., The numerous minor faults found in the Tertiary deposits are classified into three types, namely, A, B and C, according to their fault pattern (Fig., 8)., The distribution and characters of the fault patterns (size, shape, internal structure and external relation) of each respective type are discussed in detail., From field data it is inferred that the factors causing the different structural phemonena (dislocation, deformation and fault pattern) in the Tertiary deposits are the process of disturbance in the basement rocks and the difference in the physico-geological conditions of the Tertiary rocks.,
    Download PDF (1225K)
  • Yoshio KATSUI
    1955 Volume 61 Issue 721 Pages 481-495
    Published: October 25, 1955
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The extinct volcano Mashu is situated on the ESE wall of the Kutcharo caldera in eastern Hokkaido; it belongs to the south-western part of the Kurile volcanic zone., In the late Pleistocene, after the piling up of the huge stratovolcano of Kutcharo, a tremendous amount of pumice and welded tuff of dacite was erupted., This was followed by the depression of the Kutcharo caldera (Krakatoan type of WILLIAMS) which measured 26×20km., in diameter., After the depression, the alluvial volcanoes of Mashu, Atosanupuri and Nakajima were formed along the fissure which trends NNW across the caldera., The somma of Mashu was constructed by alternate eruptions of lava and pyroclastic ejecta, which are mafic to intermediate pyroxene-andesites (SiO2=52., 78∼60., 05%), belonging to the pigeonitic rock series of KUNO., Slightly after the effusion of the somma lavas, an enormous pumice eruption took place., That pumice is felsic augite-hypersthene-andesite (SiO2=65., 53%)., Following the pumice eruption, the Mashu caldera of the Krakatoan type, measuring 7., 5×5., 5km., in diameter, 36O∼560 m., in depth and 9 km3., in volume, was formed by depression., The volume of the vanished cone was about 13., 5 km3., However, it may be estimated that the actual volume of liquid magma discharged by this eruption was considerably less than that of the vanished cone, similarly to the case of the Crater Lake and other calderas., After the formation of the caldera, a steep-sided cone and a lava dome, named Kamuinupuri and Kamuishu respectively, erupted in the caldera along the NNW fissure above mentioned., The post-caldera volcanoes consist of lavas of felsic augite-hypersthene-andesite and dacite (the latter SiO2=72., 96%), all of which are included in the hypersthenic rock series of KUNO., The rocks of the volcano Mashu are chemically characterized by poorness in alkalies and MgO, and richness in CaO and FeO+Fe2O3., The alkali-lime index is estimated to be 65., 5, indicating a very calcic rock suite., Normative quartz is always calculated as fairly large, up to 39% in the most siliceous lava., Petrological studies lead to the conclusion that the somma lavas of Mashu were formed mainly due to fractional crystallization of the new tholeiitic magma without direct relation to the magma which furnished the pumice and welded tuff of the Kutcharo caldera; while the pumice ejected from the pre-Mashu caldera are considered to be of contaminated origin, similar to the case of the Towada double caldera., A part of the residual magma which supplied the pumice was erupted after the formation of the Mashu caldera, forming a viscous lava dome and a steep-sided cone.,
    Download PDF (1763K)
  • Teiichi KOBAYASHI
    1955 Volume 61 Issue 721 Pages 496-501
    Published: October 25, 1955
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Download PDF (666K)
  • Seizi SUGIURA
    1955 Volume 61 Issue 721 Pages 502-503
    Published: October 25, 1955
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Download PDF (430K)
  • Yukio YABE, Shiro KANEKO, Saburo AKAGI
    1955 Volume 61 Issue 721 Pages 503-505
    Published: October 25, 1955
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Download PDF (793K)
  • Fusao ARAI, Tokio SHIKAMA
    1955 Volume 61 Issue 721 Pages 505-507
    Published: October 25, 1955
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Download PDF (627K)
feedback
Top