50 TIN DEPOSITS OF THE YORK REGION, ALASKA. [no. 229. 
or about $1.05 per ton, and the dressing and smelting was covered by a 
yield of 0.5 per cent tin oxide, equal to about 10 pounds of black tin 
per ton." 
Nearly all the ore obtained in Tasmania is smelted at the Mount 
Bischoff Company's smelting works in Launcester, Tasmania. 
CORNWALL. 
The tin-bearing district of Cornwall 6 is at the extreme southern end 
of England and has a length of about 100 miles and a width of from 
10 to 30 miles. The bed rock consists of metamorphosed clay slates, 
called "killas," of Devonian age, intruded by large masses of granite. 
Both the granite and the slates are cut by dikes of quartz-porphyry, 
called "elvan courses," whose outcrops form a fringe around the 
granite areas. Five granite areas of this kind are shown on geological 
maps of the region, while the Scilly Islands, about 20 miles to the 
southwest, form a sixth. 
The tin gravels of Cornwall were exploited as early as Roman and 
Grecian times, when the British Islands were called Cassiterides. At 
present the original tin-bearing gravels have long been exhausted and 
abandoned. What is called u stream working 1 ' at the present day is 
merely the extraction of tinstone from the tailings of the stamp mills 
collected in the valley depression. In 1891 about 6 per cent of the total 
tin production of Cornwall came from the washing of these poor slimes. 
The tin-bearing lodes of Cornwall have been worked for many years 
and afford the best examples of lode mining for comparison. These 
lodes occur in the granites, slates, or elvans, or in the contacts between 
them. Nearly all of the mineral Avealth occurs within 2 or 3 miles on 
either side of the boundaries between the slates and the granite. 
The granites, especially in their outer portions, are usually more or 
less altered, and the name greisen is often applied to them. Typical 
greisen consists principally of quartz and lithia mica, with tourmaline, 
zircon, topaz, fluorite, and cassiterite in small amounts. In some 
cases the rock consists very largely of tourmaline and quartz, with 
fluorite in varying quantities. 
The common minerals associated in the veins with the cassiterite are 
quartz, feldspar, chlorite, and tourmaline, with fluorite, lepidolite, 
topaz, copper pyrites, and copper glance in varying proportions. Sev- 
eral of the mines have produced both copper and tin ores, and in some 
cases mines which were opened as copper mines have become tin 
mines in depth by a gradual increase in the amount of tin ore and 
corresponding decrease in copper ore. 
aRolker, C. M., Production of tin in various parts of the world: Sixteenth Ann. Rept. U. S. Geol. 
Survey, pt. 3, 1895, p. 505, quoted from Min. Res. of Tasmania, Nov., 1894. 
&De la Beche, H. T., Report on the geology of Cornwall, Devon, and West Somerset, with map: 
Gt. Britain Geol. Survey, London.. 1839. 
