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150
GOLD IN INDIA.
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E. to nearly east and west. The dip is southerly and south easterly from 60° to vertical.
Only
an extended examination of the country, such as would be made in the
course of a minute geological survey, would enable the observer to
offer suggestions as to the probable origin of these metamorphic and
metamorphosed rocks. It is apparent wherever they are well exposed
that there is one system of planes very like strike-joins which, but
for the direction of the dip, might be supposed to represent planes of
original deposition. The direction of these in some degree accords with
the general direction of the main quartz veins, and it might be
surmised on a first view that the formation of the latter was due to
the same forces which operated in altering the strata they intersect;
that in other words, the quartz in the veins was segregated during the
enormous period which elapsed from the time of the first slight
alteration of the original sedimentary rocks until they were
metamorphosed as we see them now.
In the Manual of the Geolopy of India it
is stated that " this Nilgiri strike is noted as distinctly that of the
lamination and bedding of the gneiss as well as of the foliation," and
therefore it is the more difficult to conjecture to what forces the
direction of the quartz veins is due, .coinciding as it does, not with
the foliation, but rather with the system of joints- above referred to.
While
the strike of the rocks over a large part of Australia is nearly
meridional, the reefs also have generally a north and south direction.
The
stronger and more persistent veins as they appear at present may
represent what were once lines of least resistance, and some
speculations of a strictly geological character might follow this
suggestion if this report were not confined to questions of a practical
character.
Much
valuable information is to be obtained respecting these rocks from the
reports and map prepared by Mr. King. A sketch of a geological map of
South-East Wynaad on the scale of four miles to one inch, published
with the records of the Geological Survey of India (May 1875), and
which I had not the opportunity of seeing until after much of this
report was written, shows alternating bands of felspathic gneiss and
chlorite gneiss running northeasterly and south-westerly, as well as a
large area occupied by quartzo-hornblendic gneiss (Nilgiris) and the
smaller areas of granitoid gneiss at Yeddakilmullay and Munnaynvulla.
There
is an absence of intrusive rocks in South-East Wynaad. There are no
dykes or masses of porphyry, no basalts or recent volcanic rocks; and
it is only at one point, as far as is known, where greenstone occurs,
namely, on the Hamsluck estate. The rock consists mainly of hornblende
with a small proportion of felspar (oligoclase).
Near
the spot where this rock is exposed there are veins of granite, or
perhaps, to speak more correctly, veins of quartz which are essentially
granitic. In several places, more particularly at Giidalvir,
Cherambiidi, Moopenaad, and Velliry-mulla there are masses of quartz
with large transparent plates of mica. The micacisation of the quartz
is observed most frequently (but not invariably) where the veins are
very thick; and from the observations which have been made up to the
present time it might be inferred that gold, in such proportions as it
is found in veins which are free from mica, is rarely present in these
micacised reefs.
It
is not yet certain that the " country rock " is commonly less silicious
in those places where the quartz veins are numerous, but this
peculiarity is to be noted in the neighbourhood of Pevala.
General character of the auriferous quarts veins.—The
quartz veins of the Wynaad differ in some respects from those
intersecting the almost unaltered lower Silurian rock of Australia, but
they are usually as thick, or thicker; and quite as persistent. The
auriferous veins, those which have yielded well both on the large scale
and by tests in the laboratory, are laminated and more or less
pyritous, and those which up to the present time are regarded as less
auriferous are composed of saccharoid, often snow-white opaque quartz
with transparent particles
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