The Larger Foraminifera found from the Tertiary sediments in the Tethyan, America and Indo-Pacific provinces have been studied intensively during the past fifty years. That some elements of the Larger Foraminifera are common as similar faunas in each province indicates that their mutual occurrence is probably due to their migration and transportation during the younger stages by warm ocean currents. For regional correlation based upon the Larger Foraminifera, the chronostratigraphy of them is by the Letter Stage. The Letter Stage has been a means for dividing the Tertiary of the East Indies and now is extended to cover the entire Indo-Pacific region. The purpose of this paper is to consider the Letter Stage based upon an assemblage of Larger Foraminifera from the known stratigraphic distribution in the western Pacific Ocean and to clarify previous confusion. Here, discussions are based upon the Oppel-zone composed of an assemblage of selected genera and/or species of the Larger Foraminifera, the Phylo-zone of the genus Nephrolepidina, and the common association with the Larger and Planktonic Foraminifera. The Letter Stage established in the East Indian region of lower latitudes can be applied satisfactorily to the vicinity of 42° north latitude in the western Pacific Ocean (see Fig. 1). Accordingly, Letter Stage Tb is determined by the first appearance of Spiroclypeus vermicularis, Biplanispira absurda, B. mirabilis, B. fulgeria, B. inflata, Pellatispira orbitoides and P. provaleae, and corresponds to P14 of the planktonic foraminiferal zonation of Blow as judged from the consideration of IDA (1974). Based upon the first appearance of Nummulites fichteli and Gypsina globulus, and the disappearance of Pellatispira and Discocyclina, this stratigraphic positiond (=Tc) corresponds to P17 of the planktonic formainiferal zonation of Blow. Td is determined by the first appearance of Eulepidina papuaensis, Lepidocyclina isolepidinoides and Cycloclypeus oppenoorth. It probably corresponds to P19 of the planktonic foraminiferal zonation. Tel-4 is determined by the disappearance of Nummulites fichteli, and corresponds to N1 of the planktonic foraminiferal zonation. Te5 can be determined by the horizon nearer to the disappearance of Heterostegina borneensis andSpiroclypeus margaritatus, and it probably corresponds to N 4 of the planktonic foraminiferal zonation. Tf1 is determined by the appearance of Nephrolepidina rutteni, and Tf2 by the disappearance of N. verbeeki, N. japonica, N. douville, Miogypsina globulina, M. polymorpha, M. borneensis, Flosculinella bontangensis and Austrotrillina howchini. They correspond to N6 and N9 of the planktonic foraminiferal zonation, respectively. Tf3 is determined by the disappearance of Nephrolepidina rutteni in the lower latitude areas, but in the middle latitudes, Tf3 cannot be determined by the Larger Foraminifera. The former may be pointed to N13 to N14 of the planktonic foram iniferal zonation. Tg is determined by the disappearance of Nephrolepidina rutteni in the lower latitude areas, but by the appearance of Alveolinella quoii and Borelis schlumbergeri in the middle latitudes. The former may be pointed to the horizon nearer to the middle part of N15 of the planktonic foraminiferal zonation. In Tokunoshima island, Ryukyu-retto, Japan, in the vicinity of 27°30' north latitude, the Itokina Formation at the basal part of the Ryukyu Group is represented by the larger foraminiferal fauna such as Borelis pulchrus, Archaias sp., Peneroplis pertusus and Cycloclypeus carpenteri. Therefore, Th should be positioned before the appearance of these assemblages. Also, according to IKEBE et al. (1972), the base of the Ryukyu Group corresponds to the basal part of N22 of the planktonic foraminiferal zonation.
This paper presents the basic data for tephrochronology of the Middle Pleistocene events of South Kanto, Central Japan. The tephra preceding to the last interglacial age (the Tama tephra) is found in highly dissected terraces (Fig. 1) much thicker than the younger tephra formations (Fig. 2). It is subdivided into five formations from T-A to T-E in order of increasing age (Table 1 and Fig. 2). This classification is based not on the eruptive history of the volcanoes but on the sequence of terraces. The relationship between these two is shown in Fig. 2. Of the four columnar sections (Fig. 6), section 1, synthesized from many sections in the Ooiso Hill (Fig. 3), 25 km east of the Hakone volcano, provides a standard section for South Kanto. There are so many tephra layers of the Tama stage (about 150 m in thickness) preserved in a relatively fresh state, that they are expected to cover a long time in the Middle Pleistocene. A detailed description is therefore needed for specifying any particular tephra bed. 75 marker beds have been selected, and described in terms of lithological and petrographical characteristics as in Table 2 and Fig. 4. The most valuable for characterizing marker tephras are the refractive indices of orthopyroxene and hornblende. The distribution and the petrographic character of tephras indicate that the tephras mainly comprising two pyroxene phenocrysts might come chiefly from the Old Somma of the Hakone volcano (Fig. 8-1 and 8-2) and partly from the Ashitaka volcano. Whereas such salic tephras with abundant hornblende, biotite and other phenocrysts as GoPi and TE-5 might originate from distant volcanoes (Fig. 8-3). As a result of precise examination, tens of the marker beds have been traced eastward to the environs of Tokyo and Yokohama (Fig. 6), where the standard Quaternary sequence had been established. Furthermore, several marker beds have been recognized within the Middle Pleistocene marine sediments in the Boso Peninsula. By use of these the Quaternary stratigraphy already established in South Kanto is significantly revised (Table 3 and 6).
On the wall of interior cave Fugoppe, many carvings consisting of reliefs were inscripted. Recently intensive crumbling of the cave wall were observed. In order to preserve the wall carvings, the investigations on basal rock of cave were carried out. The entire cave consists of Tertiary tuff and tuff breccia. Weathering of the exposed rock potion is severe and crumbling in the form of sand granules has occurred. There are a number of joints in the bed rock and at some parts blocks of rock have fallen out along the lines of joints. For the purpose of obtaining a concept of the state of weathering and crumbling of the cave walls, on-the-spot investigations and experimental works consisting of microscopic examinations of rocks, measurements of void rations and monoaxial compressive strengths, measurements of elastic wave velocities (Vp) by Ultrasonic methods, freezing and thawing tests, vibration experiments and measuring of crumbling of outcrops were carried out in the present study. As a result, the peculiar weathered condition of the bed rock, especially the increase and enlargement of voids accompanying weathering process was pointed out. However, when the cave has been closed off for a long period, the cave walls were preserved with relatively little weathering, and attention should be paid to this feature.