48 TIN DEPOSITS OF THE YORK REGION, ALASKA. [no. 229. 
made of riffles being used. After running for some time, the water 
is shut off and the material in the boxes is cleaned up. This material is 
further concentrated by hand panning in flat wooden bowls, which 
resemble the American gold pan. The final process is cleaning by 
hand picking, by which the magnetic iron and other impurities are 
removed. Ore treated in this way will average from 60 to 70 per cent 
tin. In one instance hydraulic monitors are used, but the greater 
part of the tin ore from this region is produced b}^ the more primitive 
methods. 
BANCA. 
In the island of Banca/' which is under the Dutch Government, the 
geological conditions resemble those of the Malay Peninsula. The bed 
rock consists of granite masses Hanked by Silurian slates. Tin ore 
has been found occurring as impregnations in the granite and also as 
veins in the slate, but these deposits are not worked. The tin wash 
consists mainly of fragments of granite, "schorl," 6 and sandstone. 
The bed rock nearly always consists of granite more or less decom- 
posed. A section of an average stream-tin deposit shows above the 
bed rock 3 feet of tin-bearing gravel, overlain by red sand, followed 
by red clay, then coarse sand with pockets of clay, layers of tine sand 
with a little fine tin ore. The average overburden is from 25 to 35 
feet; shallow diggings are prospected by pits, deeper ones by system- 
atic borings. In 1891 and L892, according to the United States Bureau 
of Statistics, 7,982 men were employed in the mines of Banca and pro- 
duced 5,753 tons of tin, a yearly product per man of seventy-two one- 
hundredths of a ton. There is water for working in the lower valley 
diggings but eight months each year, and for only five months in the 
upper diggings. 
BILLITON. 
In Billiton, also under the Dutch Government, the geologic condi- 
tions resemble those in Banca. There are granite masses surrounded 
by quartzites, schists, and slates of Silurian age. Some tin is obtained 
from ledges that occur both in the granite and in the quartzite, but 
the greater part of the tin comes from alluvial deposits. In 1891-92 
8,690 men were employed here, the output averaging per man a little 
over seven-tenths of a ton of tin. The prospecting is done very 
systematically, and is in charge of a corps of European engineers 
who test the fields in advance of the mining operations by boring first 
at intervals of, say, 100 yards, and supplementary holes are made from 
20 to 25 yards apart to ascertain the course, average thickness, and 
aRolker, C M., Production of tin in various parts of the world: Sixteenth Ann. Rept. U. S. Geol, 
Survey, pt. 3, 1895, p. 484. 
b "Schorl" is an old name for rocks composed mainly of tourmaline and quartz. 
