TIN DEPOSITS. 41 
of tin ore arc usually found to be simply lens-like portions of an ore shoot that are of especial 
width or richness. Development has not yet been sufficiently extensive to show whether 
or not there is any regularity in the position of these shoots in the dikes. They have as 
a rule a much elongated form, measuring only a few feet along the dike at a right angle 
to their greatest dimension. Some of the ore bodies, however, have a considerable extent 
in two directions and a few of them persist as far as present explorations have gone. 
In general they pitch steeply, though the angle of pitch may vary in the same shoot. In 
some places there seems to be a tendency for the shoot to occupy the full width of the 
dike, while in others it is confined to one side or the other or to the middle of the dike. 
Fig. 4 illustrates concentration near the middle of a narrow dike. 
THE ORES. 
The principal tin-bearing mineral in nearly all the tin-producing regions of the world is 
cassiterite, the dioxide Sn0 2 . This is the only tin mineral known to occur in the Carolina 
region. It is present in the pegmatite dikes in sizes ranging from tiny grains, barely vis- 
ible to the unaided eye, to individuals weighing as much as 2 pounds. The mineral is 
crystalline, but only rarely possesses crystallographic outline. Where crystal faces are 
developed they are uniformly those of the low unit pyramid. Twinning is rather common, 
as shown both by the form of the crystals and by the behavior of sections under the micro- 
scope. An imperfect cleavage parallel to the prism face is often observed. Owing both 
to this cleavage and to some peculiarity of crystal development, many individuals of cas- 
siterite are flattened. The fracture is uneven. The mineral is extremely brittle and friable, 
particularly where occurring near the surface, and on this account seems in many cases 
much lower in the scale of hardness than its actual value of 6-7. On account of its rela- 
tively high specific gravity, 6.8 to 7.1, it is rather easily separated by panning or other 
means of gravity concentration from most or all of the minerals which ordinarily accom- 
pany it. The color of the Carolina cassiterite is in most cases very dark, ranging from 
deep brown to black. Lighter colored or even colorless varieties have been reported, but 
none have been seen by the writer. The brown specimens have a resinous appearance; 
the blacker ones show a metallic luster. The streak and the powder range in color, depend- 
ing on the intensity of color of the specimen, from nearly white to chocolate brown, and 
are in no case black. This fact serves to distinguish cassiterite from many minerals of 
similar appearance which occur in and about the tin region. Owing to the presence of 
included impurities which can not be mechanically separated, the Carolina cassiterite falls 
somewhat below the theoretical content of 78.6 per cent metallic tin. Probably 74 to 75 
per cent would be an average for the carefully selected material. 
In thin section the cassiterite generally gives evidence of imperfect crystal form, and 
in some specimens the effect of twinning is shown in the outline. Twinning is parallel to 
the pyramidal face (1, 1, 1) and is in some individuals repeated polysynthetically. Pris- 
matic cleavage is readily discernable in some instances, and indication of a second cleavage 
parallel to the pyramid has been seen. The mineral is usually deeply colored, but the 
distribution of the color is uneven, giving a flaky or patchy appearance. The color is 
probably due to the solution, in the cassiterite, of a small amount of an iron mineral, possibly 
hematite, since the deeper-colored varities carry most iron. Dichroism is decided. The 
common absorptions are £=deep red or red-brown, &3=light yellowish brown to nearly 
colorless. Sometimes s— coffee brown, co= bluish green. The refractive index and the 
double refraction are characteristically high. Some minute included flakes of hematite 
and grains of ilmenite are visible. Partial analyses a of specimens of cassiterite from Kings 
Mountain made in 1883 indicated the presence of about 1 per cent of tungstic oxide. A 
qualitative analysis of concentrates from the northern part of the tin belt, made by Dr. 
W. F. Hillebrancl, fails to show the presence of tungsten, but reveals an appreciable amount 
a Dabney, C. W., jr., Jour. Elisha Mitchell Sci. Soc, 1883-84, p. 81. 
