GEOLOGY OF THE TIN BELT. 39 
figure on the material available. If the mineral is biaxial, as is probably the case, the 
intersection of the cleavages is parallel to <r. This is also the direction of least absorption. 
The refractive index is probably between 1.60 and 1.65. The double refraction is high, 
probably in the neighborhood of 0.060. Analyses by W. T. Schaller gave the following 
average composition for purpurite: 
Average analysis of pvrpurite. 
Fe 2 3 15. 89 
Mn 2 3 29. 25 
P 2 3 47. 30 
H 2 at 100° 3. 31 
H20 above 100° 1. 95 
CaO 1.48 
Na 2 84 
Li 2 Trace. 
Insoluble .52 
100. 54 
It is probable that the soda and lime exist as impurities, or inclusions of foreign material, 
in the mineral. The lithia is doubtless a remnant of that of the lithiophilite. 
Black manganese oxide, probably also derived from the decomposition of lithiophilite, is 
present in many places as thin botryoidal coatings on the other minerals. Some apatite 
occurs in narrow vein-like masses of bluish color and was at first thought to be fluorite. 
In isolated crystals of small size it has a bright-green color. 
The cassiterite, the constituent of these rocks which is important from the economic 
standpoint, may better be described under the head of the ores (pp. 41-42). In thin section 
under the microscope it is found to include, besides tiny flakes of hematite, numerous 
minute dark grains which resemble black iron ore. The fact that a carefully panned sample 
of tin concentrates gave Doctor Hillebrand decided reactions for iron and titanium indi- 
cates that this material is ilmenite. Wolframite has been reported as accompanying the 
cassiterite, but careful search has failed to reveal its presence. It ought readily to be 
distinguished in the pan by its dark color, even in fine powder, its good cleavage, and its 
specific gravity, which is greater than that of cassiterite. It seems probable that the 
tungstic oxide reported in early analyses a of Carolina cassiterite may have been confused 
with the rarer elements columbium or tantalum, which have been found recently, although 
it may be that tungsten does in some cases exist as wolframite, but included within the 
grains of cassiterite, as is the ilmenite. 
Chalcopyrite 6 and arsenopyrite, c or mispickel, have also been reported from the pegma- 
tites, but it is doubtful if they occur as original constituents of these rocks. 
CONTACT METAMORPHISM. 
A general idea of the changes induced in the surrounding rocks by the intrusion of peg- 
matite has already been given. A study of this metamorphism is, of course, futile in the 
zone of surficial decomposition. Unfortunately, at the only place along the belt where 
development has extended into the unaltered rock (the Jones mine), a dike of granite or 
aplite is in close connection with the pegmatite. The granite is probably older than the 
pegmatite and the changes at its contact have certainly been more intense. Its effect has 
therefore partly forestalled and partly overshadowed that of the pegmatite. The develop- 
ment of clear brown biotite and less commonly of garnet and pyrite in the amphibolite can 
in some places, however, be seen to result from the action of the pegmatite. 
Quartz veins closely associated with dikes of both granite and pegmatite exhibit very 
remarkable metasomatic alteration in the adjoining rocks. These veins vary in width from 
oCf. Dabney, C. W , jr., Jour. Elisha Mitchell Sci. Soc., 1883-84, p. 81. 
b Furman, J. H., Trans. New York Acad. Sci., vol. 8, 1889, p. 144. 
cLedoux, A. R., Eng. and Min. Jour., vol. 48, 1889, p. 521. 
