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gneisses and schists) and
“
Daling
”
(mainly slates and phyllites). The
“
Darjeeling
”
structurally overlies the
and Mallet observed that there is a gradual pas-
sage from one metamorphic unit into another. Daling consists of garnetiferous
biotite schist, biotite schist and kyanite-sillimanite bearing garnetiferous gneiss,
indicating higher grade of metamorphism. Garnetiferous biotite-calc-granulite, calc-
gneiss, quartzite and muscovite-sillimanite-quartz-schist occur as inclusions in the
gneisses and represent
“
Daling
”
the original calcareous, siliceous or other sediments.
According to Mallet,
the gneiss should be the older rock and their inverted on to
the slates and their turn on to the Damuda or that the boundaries should be faulted
one or
“
finally that the relation of these formations to each other should resembles
those of the Tertiaries to Damuda
”
. The problem is that the-
“
Darjeeling
”
-
“
Daling
”
-
“
Gondwana
-contacts are structural stratigraphic discontinuities.
Roy and Sensharma (
1967
) suggested that
”
“
Darjiling
”
and
“
Daling
”
strati-
graphically constitute one continuous unit called
. According to
Ray the boundary between Gondwana and overlying Daling near Tindharia cannot
be drawn with certainty as there is a possibility that the two constitute one con-
tinuous formation in different grades of metamorphism with somewhat different
lithology. Folding has not been commonly recorded in
“
Senchal Series
”
“
”
except in a few
cases in sandstone and shale. These folds are developed on bedding planes. A few
discontinuities fractures occur parallel to the axial planes of the folds, no appre-
ciable recrystalisation or neomineralisation has been observed along this plane.
Successive appearance of chlorite, biotite, garnet and kyanite which apparently
suggest the occurrence of progressive zones of regional metamorphism of Barro-
vian type can be recorded from south to north.
Gondwana sandstones and shales are mostly unaltered. Constructive metamor-
phism has already been observed except for some recrystalisation of small sericitic
micas and biotites; these occur along bedding (Laahiri
1973
). The lithological map
of the concerned study area was collected from Geological Survey of India (GSI),
Kolkata (Eastern Region) and then necessary modi
Gondwana
cations were being incorporated
after thorough
field investigation. Final lithological map was prepared with seven
rock groups and transformed into raster value domain on ARC GIS platform. Each
and every lithological group responds differently whenever it exposes to atmo-
spheric processes and also produces varying magnitude of landslide susceptibility/
Landslide Potentiality. Darjiling Gneiss (A), Reyang formation (E) and Swialik (G)
associated with highly foliated gneiss, mica-schists and occasional bands of flaggy
quartzites and granulitic rocks, slates phyllites with occasional quartzite, quartz-
schists and greywake schists, soft grayish sandstone, mudstone and shales and
conglomerate along with thin bands of marly shales and lignite cover more than
60 % area of the Shivkhola watershed. Chungtung Formation (B) with calc-gran-
ulitie, marble, quartz-granulite and mica-schist; Lingtse Granite (C) with foliated
granite or mylonitised granite with several close space sub-parallel thrust; Gorub-
athan Formation (D) with low grade phyllite and silvery mica-chlorite-schist, grey
sericite, and Damuda Formation (F) with coarse grained hard sandstones, quartzites,
carbonaceous shales and slates, thin seams of crushed and powdery coal share
almost same area (around 8 %) each (Fig.
2.4
).
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