streams,
where the latter have deepened their channels by erosion, but it is
more frequently below the water level, in one case as far as 120 feet.
To a large extent, it underlies swamps and rice fields. The working of
the available localities has been carried on so far and so long that
many of them are wholly exhausted and the rest are being rapidly
reduced.
The
corundum appears in several varieties, and the interesting fact is
noted that while the crystals before described, from the Ilaldummulla
and neighboring localities, in connection with the sillimanite rock are
prismatic, those from the gem gravels are usually bipyramidal. The
varieties found are the following:
1. Korundugal—opaque and only used as an abrasive.
2. Dalam—semitransparent. inferior; sold by the pound. 3. Nila —blue sapphne.
4. Ratuke or Arunal—asteriated.
6. " Topaz,"—Oriental topaz, yellow sapphire.
7. " King topaz."—clear pink or flesh-colored corundum.
The
topaz of Ceylon is yellow sapphire; true topaz is not rare in the
gravels, but it is never yellow, being either white, pale-green, or
brownish-yellow. (See under Topaz.)
The
methods of working the gem gravel are described in detail, and are
closely similar to the native methods used in mining for rubies in
Burma and for diamonds in Borneo.
The Illam generally rests upon decomposed rock in place, called " malana." In some cases the lower portions of it are cemented by iron oxide into a sort of conglomerate. In the present beds of
rivers in the gem district, especially those that are fast and shallow,
the actual surface gravel is gem bearing, forming a modern Illam, doubtless largely derived from the older one, which represents the accumulation of ages.
The
principal region where these gem gravels occur is the Sabaragamuwa
province (Ratnainira district), and some parts of the southern province
of the island. .
SAPPHIRE.
MONTANA.
The
sapphire workings at Yogo Gulch, Montana, are being gradually
developed into a great and permanent mining industry. They have been
noticed frequently in the reports of this Bureau for the last ten
years, and a general statement of the disposition of the properties of
the two companies engaged upon the gem-bearing dike was given in the
report for 1901.a The English company, known as the New Mine
Sapphire Syndicate, has been thus far the largest producer and the one
most prominent before the public; but the other, the American Sapphire
Company—frequently called the Yogo Mining Company— has been engaged in
extensive prospecting and developing work, and is now preparing to
begin active production on a larger scale than any heretofore
attempted in Montana.
Taken
as a whole, the Yogo dike is judged by qualified experts to be perhaps
the greatest gem mine in the world. It extends some 4 miles in length
on the surface, and being a true igneous dike, descends to an unlimited
depth. If estimated down to 2.000 feet, below which possible working
becomes questionable, and at an average width of only 6 feet—although
it is often much wider—the entire content of sapphire-bearing rock
would approximate 10,000,000 cubic yards.
a Mineral Resources C S. for 1901, U. S. Geol. Survey, 1902, p. 736.
