small veins which are, as it were, thrown off from the main reefs of quartz.
Rocks—The granitoid schists or gneissoid rocks of South-East Wynaad are, it is probable, as will bs shown
hereafter, only completely metamorphosed sedimentary strata. The
minerals observable are felspar quartz, hornblende, mica, talc,
chlorite, pholerite, and magnetic iron. The ordinary foliated rock is
usually massive or composed of thin compact layers of quartz and
felspar, or of quartz and hornblende. Magnetic iron takes the place of
one or other of these constituents, or accompanies them in some places,
and at and in the neighbourhood of Marpanpadi North Teak, magnetic iron
is largely present in the rock, the decomposed surface stone exhibiting
layers and reniform and nodular masses of sesquioxide of iron. Some
specimens are composed almost entirely of quartz and magnetic iron, and
in others the iron occurs with quartz and felspar; and again there is a
variety composed of translucent quartz, magnetic iron, and an
asbestiform mineral resembling iron amphibole.
Bands also have been observed in which the proportion of iron pyrites in disseminated crystals is very large.
Massive
hornblende rock is found on the south-east, and garnetiferous foliated
gneissoid rock is common towards the north and east.
In
many places the country rock, besides being intersected by large
persistent reefs of quartz, is seamed and veined throughout with
threads and strings of quartz; not in any way conformable to the lines
of foliation ; and where a section of the rock decomposed in situ is
laid bare, these threads and strings are clearly traceable through the
red, reddish brown, and dark brown of the kaolin-like clays resulting
from the decomposition of the stone. These veins vary from one-eighth
of an inch or less to six inches or more in thickness, and they consist
either of white opaque or blue opaline quartz, the latter not unlike
that which forms thin layers in the foliated country rock.
In
more than one locality near Devala, the harder rocks consisting of
dense quartzite or quartz with magnetic iron and a little felspar
appear as large rhombohedrai blocks or as symmetrical hexagonal or
square prisms ; and the faces of the planes are not seldom covered with
thin layers of rather dark blue opaline quartz. Many of the forms are
of singular interest, and a stranger might without examination suppose
that some of the masses were of igneous origin. It is perhaps needless
to say that the shape of these blocks is due to the direction of the
several systems of divisional planes or joints.
The
modes of decomposition of the several varieties of gneissoid rocks in
the Wynaad give hints as to the degree of metamorphism to which they
have been subjected. For instance, where not intensely metamorphosed,
they do not weather into spheroids nor exhibit concentric layers around
a hard core of rock, such as one sees in tracts occupied by granitic,
trappean, and the older igneous rocks. Where, however, there is a
greater development of the crystalline structure, as in some parts of
the country west of DevAla, this well-marked feature presents itself.
East
of Needle Rock and east of Harewood thin micaceous shales occur which
remind one of the typical micaceous lower Silurian rocks; and the lines
of lamination in the former might well seem to correspond with the
cleavage planes of the latter. The strike of these thinly laminated
micaceous shales east of Needle Rock is N. 55* W., being nearly at
right angles to the general direction of the folia of the gneissoid
rocks. It is perhaps correct to say that observation has shown that the
most productive auriferous belts in the Wynaad are those in which these
micaceous and chloritic rock occur; and that as a rule where the hard
dense massive quartzo-hornblendic strata are found, and where the
foliation is indistinct, the veins are either absent, or, where
present, consist of saccharoid quartz with large and small plates of
mica.
The
foliation of the rocks in that part of South-East Wynaad already
described preserves throughout a nearly uniform strike, namely, N. 54°
E.— S, 540 W. There are curves however where the direction varies from N. 40*
