52 TIN DEPOSITS OF THE YORK REGION, ALASKA. [no. 229. 
varies in width from 12 inches to 27 feet and is richest in tin in the 
deepest levels, where the ores sometimes average 10 per cent cassiterite. 
The richest ore occurs where a number of veins intersect and is said 
to be of a compact, bluish rock, consisting of a mixture of chlorite, 
quartz, and tourmaline, with stringers of cassiterite running through 
it. On the north side it passes gradually into a barren granite. From 
its upper workings, which are in the slate, this mine yielded only cop- 
per ores, but from the deeper levels mined at the present time, which 
are in the granite, only tin ores are obtained. a 
The average richness of the ore from a number of Cornish mines 
for ten years, from 1871 to 1881, is given in pounds of black tin per 
ton, as follows: Dolcoath, 59 pounds; Cook's Kitchen, 43 pounds; Tin- 
croft, 53 pounds, and Cam Brea, 35 pounds. 6 This is approximately 
equivalent to 1.8 per cent, 1.3 per cent, 1.6 per cent, and 1 per cent, 
respectively, in metallic tin. During the half year ending December 
31, 1902, the average product of the Dolcoath mine was 38.28 pounds 
black tin per ton, approximately equivalent to 1 per cent, while dur- 
ing the year ending April 24, 1903, the product of the Wheal Grenville 
Mining Company at Camborne averaged 43.6 pounds black tin per 
ton, c approximately equivalent to 1.1 per cent in metallic tin. 
Many examples showing the nature of occurrence and extent of the 
tin ledges of Cornwall might be cited for comparison, but those given 
above will probably be sufficient for present purposes, and will show 
the general resemblance of the occurrence of tin ore in the York region 
and in Cornwall. 
BOLIVIA. 
The tin mines of Bolivia occur in veins that are regarded as excep- 
tional in that the tin ore is intimately associated with silver ores, bis- 
muth ores, and various sulphides, while the gangue includes barite and 
certain "carbonates. The deposites often occur in trachytes and ande- 
sites erupted during Cretaceous or Eocene time. 
REDUCTION OF TIN ORES.^ 
Tin ore is prepared for smelting by roasting, if necessary, then 
crushing and concentrating to at least 60 per cent cassiterite. This 
may be done with ordinary stamp mills and concentrating machinery. 
In Cornwall both gravity and steam stamps are used. 
The earliest and simplest method of smelting was as follows: A hole, 
about 2 feet in diameter, was dug in the earth, preferably in a bank, 
in which sticks of wood and well-cleaned ore were piled in alternate 
a Phillips, J. A., and Louis, Henry, A treatise on ore deposits, Macmillan Co., 1896, p. 211. 
bFrecheville, R. J., Great main lode of Dolcoath: Trans. Royal Geol. Soc. Cornwall, vol. 10, 1887, 
p. 154. 
cNewland, D. H., Tin, the mineral industry, vol. 11, 1903, p. 595. 
d Louis, Henry, The metallurgy of tin: Mineral Industry, vol. 5, 1896, pp. 533 to 588. 
