40 GOLD AND TIN" DEPOSITS OF SOUTHERN APPALACHIANS. 
the merest stringers to several feet, and the metamorphic action of even the smallest is 
intense. The quartz of the veins is rather finely granular and under the microscope the 
grains are found to interlock in the manner characteristic of the granitic texture of igneous 
rocks. Small, many-sided prisms of deep-brown tourmaline occur scattered and in bunches 
through the quartz. In thin section the zonal structure of these crystals is readily seen. 
The interior is usually pleochroic in blue and violet colors, while the exterior is brown. 
Flattened individuals of ilmenite are in some places abundant. Sulphides, of which pyrite 
is most common, but including also pyrrhotite and chalcopyrite and perhaps a little arseno- 
pyrite, are common though not abundant constituents in certain of the veins. In many 
instances the various sulphides are intergrown. Careful assays have shown that some of 
these veins carry gold. Further reference to this subject is made on pages 69-70. 
The metasomatic processes which these veins have induced in their walls resemble as 
closely those of igneous masses as they do those of fissure veins. The formation in the 
country rock of garnet, brown biotite, and probably some magnetite might be attributed 
to contact metamorphism, while the impregnation with sulphides and quartz is more closely 
related to vein action. Tourmaline is common to both. 
These veins are later than either the aplitic or the pegmatite dikes, and induce in them 
changes which are somewhat similar to those caused in amphibolite. In the granite or 
aplite the tourmaline has so nicely replaced feldspar a that it might easily be mistaken for 
an original constituent of the rock. 
The texture and mineral constituents of these veins and their effect on their walls indi- 
cate that they were formed at great depth, and their intimate association with the granite 
and pegmatite shows almost beyond question that they are genetically related to those 
rocks. 
TIN DEPOSITS. 
It may be well to state at this point that the tin deposits of the Carolinas can m no way 
be considered veins. They arc, instead, dikes of an igneous rock — pegmatite — which have 
solidified from an originally molten slate. Placer deposits of comparatively little importance 
have been derived from the breaking down of these dikes. 
DISTRIBUTION OF CASSITERITE IN THE DIKES. 
The tin-bearing mineral is an exceedingly variable constituent of the pegmatite. Probably 
in the majority of cases where pegmatite lias been found no cassitertite can be seen in it, 
and while it is possible that the mineral is gen- 
erally disseminated in minute grains through- 
out the rock, such is probably not the case. 
On the other hand, certain bodies or portions 
of bodies of pegmatite contain a very large pro- 
portion of cassiterite, and some masses weigh- 
ing several hundred pounds are made up 
almost wholly of cassiterite. There are all 
gradations between these limits. Whib the 
distribution of the tin mineral is thus very 
irregular, there is in most cases a certain sys- 
tem in this irregularity. The tin is usually con- 
centrated along certain lines or zones, which 
thus constitute ore shoots. These ore shoots 
also are irregular, something after the manner of the dikes themselves. They pinch and 
swell, branch, die out, and reappear in an erratic way. Nevertheless, the determination 
of the presence and direction of an ore shoot is a great aid to development. "Pockets" 
Fig. 4.— Sketch of narrow pegmatite dike, 
showing distribution of cassiterite. Natu- 
ral size. 
"Cf. Lindgren, W., Genesis of Ore Deposits, New York, 1904, p. 533. 
