PART III.
Muang Chieng Kong to Muang Luang Prabang.
Muang Chieng Kong became
our head-quarters for ten days, and from there I made a boat expedition
to the Chieng Sen boundary, north-west; and also one north and east
inland, the object being the examination of the gem deposit, its
extent, character, and, if possible, its value.
From
the Chieng Sen boundary at Hoay Nam Kung, extending for some miles
towards Chieng Kong, is a rapid piece of river tearing through a series
of gneissose and schistose rocks, which form high hills on either bank.
The gem-bearing gravel is not found until several basalt sheets are
encountered below Nam Ngau, a largish tributary flowing in from the
north. The hills on the left bank then become lower and more distant,
and these, consisting of a dark crystalline rock, the exact
mineralogical character of which has not yet been determined, seem to
be the source of all the stone-bearing gravels which are found
deposited in the streams flowing from them. The average thickness of
the gravel is 5 to 20 inches, and consists of quartz and fragments of
the crystalline rock above mentioned. The overburden is a reddish clay
soil of an average depth of 10 feet, through which the Burmese, who are
found wherever there are gems, sink large pits some 10 feet square. A
sharpened bamboo will be often first driven down to ascertain if the
gravel underlies the spot, it having been found very capricious.
Explorations
were made in the neighbourhood for many years before—about two years
ago—the first paying gravel was found; the Burmese relying all the time
on the presence of what is known as nin, small black stones
which have turned out to be black spinel, and are always to be found in
close proximity to the sapphire. When washing gravel in a stream these
little water-worn crystals are found; it will only need industry and
time to find the gem gravel,
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