60 GOLD AND TIN DEPOSITS OF SOUTHERN APPALACHIANS. 
It will at once be noticed that there is a striking similarity in structure between these 
quartz veins and the pegmatite dikes of the tin belt. (Compare figs. 6, p. 50, and 14, 
p. 102.) It is believed that this similarity is due to the fact that, although the materials 
are different, the receptacle is the same. In other words, the form and position of both the 
quartz veins and the pegmatite dikes are almost wholly dependent on the structure of the 
surrounding rocks. The explanation of the structure of the quartz veins is therefore the 
same as that for the pegmatite, which has already been given at some length (pp. 35-37) 
and need only be summarized here. It is believed that the veins occur along planes which 
represented places of weakness in the rocks. The interfoliated veins, or those conformable 
with the sorrounding rocks, are irregular because these places of weakness, the planes of 
schistosity, were uneven, discontinuous, and irregular. The material from which the veins 
were deposited, because of its probably greater fluidity than that from which the dikes 
solidified, sought out even more irregularities, and consequently the veins are less regular 
than the dikes. In short, it is believed that the bodies of quartz which now exist did 
not solidify in open spaces of corresponding dimensions which were ready to receive the 
solutions; but that the solutions, pushing their way along what may in many cases have 
been the merest fractures, actually forced the walls apart and made the receptacles in 
which their load was deposited. The force of crystallization « may have aided somewhat 
in this expanding of the openings, but it is believed that the principal factor was the pressure 
under which the solutions reached this zone of deposition. This pressure, which must 
have exceeded that resulting from the weight of the overlying rocks, was transmitted 
from a greater depth, where the weight of the overlying rocks was greater than above. 
An idea of the structure of these interfoliated veins is given by figs. 12, p. 100, and 14, p. 
102. The cross veins, though deposited at the same time as the others, are more regular 
than the conformable veins, since they occupy later fissures which are definite cracks 
breaking across the schistosity. 
Faulting has not been important. Slight faults with throws of a few inches are sometimes 
seen in those portions of interfoliated veins which consist of many irregular stringers, but 
it is not certain in all these cases that the faulting has been later than the formation of the 
vien, for it is possible that the vein materials simply filled fissures which in places were 
faulted. In a few cases more important faults are known, but in general there is very little 
evidence of movement since the period of vein deposition. 
As regards internal structure of the veins, it may be said that the quartz is usually dense 
and massive, but in a few places possesses a granular texture which has led to the miners' 
term "sugary quartz." In general, the ore minerals occurring in the quartz gangue are 
irregularly disposed, although in certain deposits there is a more or less decided tendency 
toward accumulation or concentration at or close to the walls. In a single vein only, the 
Little Wilson, has any indication of banding in the interior of the vein been seen, and even 
in this case the narrow streaks of pyrite parallel to the vein walls give little support to the 
idea of vein banding in the sense of successive deposition or trustification. Druses and comb 
structure of original formation have not been observed in a single instance in the mines of 
this region. 
REPLACEMENT DEPOSITS. 
Large bodies of ore wholly different in outward aspect from the quartz veins occur in 
several places in the eastern part of the area. While the fissure veins are more numerous, 
this type of deposit has been more productive. These deposits occur, with only one or two 
exceptions, in volcanic rocks of the quartz-monzonite-porphyry group and are most com- 
mon in the fragmental varieties or tuffs. The porous nature and easy alterability of these 
volcanic rocks, particularly the tuffs, have allowed widespread penetration and replacement 
by ore-bearing solutions. While minute quartz veinlets are locally present, the great mass 
of the ore bodies has been formed by replacement of the country rock, chiefly by silica and 
pyrite; and it seems probable that many of the veinlets themselves are simply cases of 
extreme replacement and not deposits by filling. 
a Dunn, E. J., Reports on the Bendigo Gold Fields, Victoria Department of Mines, 1896, p. 25. 
