Nineteen sixty-one was the year in which Onishiyama rock fall broke out and its debris blocked the Koshibu river. This fall of mylonite took place on the west-side of the Median Tectonic Line.
Creeping of crystalline schist or its soil, on the east-side, is essentially the major mechanism for very slow down-slope movements. We cannot find out here a rapid and chaostic disaster except for toe erosion or heavy stormy rain.
This paper deals with geology and mass movement at Kashio area, Oshika-mura, with a radio-isotope calculation of 222Rn. Its density in soil shows a 50-to-60 day period of repeated mass strain.

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Niigata Prefecture is of the most frequently occurrence with landslide in Japan, especially the thematic areas are well known as densely distributed areas with landslide.
Here the writers discussed on the relation between landslides in these areas and their geological structures and rock facies.
Many superior studies on origins and basic factors of landslides in these areas have been reported hetherto by many eminent investigators, but researches on some relations between geological environment of these areas and occurrence of landslide are very few.
Considering from the relationship between landslide distribution in these areas and their rock facies and geological structures, the writers concluded that the basic factor of landslides in these areas will be rock facies, and geological structure must be the secondary factor.
The report is divided into two part; as
(I) On Naka-Kubiki and Higashi-Kubiki area,
(II) On Kariha and Mishima area (will be reported in the next number)

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Explanation of landslide mechanism is described from every minute records of internal strain meter. Analysis of the onset time of these records makes it clear that rupture occures at lower part in the upper landslide slope and is propagated to upper slope at a few meters per an hour, and rupture occures at upper part in the middle landslide slope and is propagated to lower slope at a few meters per an hour. The records close to the landslide surface clarify that rise time in landslide is a few hours. The displacement at the landslide surface is propagated to the ground surface with a velocity above a half meter per a minute.

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Various methods are used to analyse a slope such as the friction circle method, Fellenius and Bishop method; method of slices, Jambu' s method and others.
The topographic profile of landslides are often complexed so that the analysis method is the more simplified, the better. Method of slices is fitted to these requirements but it still needs amounts of manual calculation. These days, we take the advantage of micro computers, already installed in our laboratories.
It takes long to analyse a slope though we use a micro computer when we adopt the high density slice method. Auther thinks method of slice called simplified method should be more improved one. Digesting the method of slice, it was found integration method is adoptable for micro computers. Interpreting the expression (integration equality) for a computer and running it, it is clalified that the integration method is quite fitted to computers, results are theoretically accurate and executing time is shorter than the other methods.

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This paper presents the results of detailed studies on the hydrogeologic characteristics and the behavior of melting snow in landslide ground. The investigated area, located in the Aburuma river basin, is well known as the concentrated zone of landslide.
The results of the investigation are summarized as follows.
Geology
1) Base Rocks-Tertiary-
Base Rocks concerned with the landslide phenomena of this area are Tertiary and are divided into two horizons. The lower one mainly consists of green rocks and the upper one consists of alternating perlitic tuff and mudstone. These strata trend east northeast and dip 20 to 30°northwest.
2) Ejecta of Sumon-dake-Quaternary-
Ejecta of Sumon-dake are divided into two horizons, The lower one is composed of weathered tuff and debris with volcanic ash, sand and weathered andesite breccia. The upper one, Sumondake Lavas, constitutes Sumon volcano. This is not related directly to the landslide of this area.
3) Landslide ground
Field evidence and boring data shows that the landslide ground exists at several horizons and they are divided into two groups. One is old land failure deposits and the other is recent landslide ground. The former is recognized in the both area of Tertiary and Ejecta of Sumon-dake. The latter ranges in thickness from a few meters to 6 meteres and its permeability is higher than the former.
Groundwater
Continuous observation of water level, temperature, flux and electric conductivity of groundwater are executed by using bore hole and collective well. The outline of the discussion about the behavior of the melting snow is as follows.
i. Infiltration of melting snow is occured within a period of May or April in natural condition.
ii. Infiltration speed is accelerated by the artificial agent (collective well).
iii. In early stage, groundwater is supplied from melting snow in the neighborhood area of landslide but in process of time and as a depth increases, the groundwater is changed place by the water which is supplied from distant area.

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