wolfp] ZINC AND MANGANESE OF FRANKLIN FURNACE, N. J. 217 
times a coarse vein-like aggregation of these minerals separates gran- 
ite from ore A large number of the rare minerals come from these 
underground granite contacts, and there can be no doubt that the 
granite is later than the zinc bed and intrusive into it. 
At Sterling Hill there is an analogous structure of east-west 
limbs of the ore bod} 7 , outcropping on the surface in a hook and pitch- 
ing under northeast at both ends, the west vein outcropping about 
GOO feet from the turn, and the east vein about 1,500 feet. 
The west vein has only been worked down a short distance from 
the outcrop ; the east vein in places about 650 feet at an average angle 
of 50° to 65°. 
In the center of the canoe joining the two veins the axis pitches 
50° NE. In the mines the pitch of the ore shoots is said to have been 
generally 65°. At the apex of the trough the limestone is filled with 
various silicates (diopside, jeffersonite, etc.), and was probably 
impregnated with franklinite and zinc ores, now mined out. A little 
farther north a large deposit of calamine was mined, which lay in a 
bowl-shaped cavity on top of the limestone, and was undoubtedly 
hydrated ore derived from the decomposition of the higher lying 
portions of the zinc deposits. 
ORIGIN OF THE ZINC DEPOSITS. 
The descriptions of the structure and relations of these deposits 
speak for their contemporaneity in present form and structure with 
the inclosing white limestone and associated gneisses, and therefore 
for a period of formation earlier than that of the intrusive granites, 
although the difference in time may have been small. The ore 
deposits are often not sharply defined from the limestone foot and 
hanging walls, and the latter are shot through with franklinite, willem- 
ite, etc. Horses of limestone or coarse calcite also occur in the mid- 
dle of the ore deposit. It is believed that the zinc deposits acquired 
their present structure and mineralogical composition contempora- 
neously with the limestone, and that they represented originally a 
local segregation of the zinc manganese and iron minerals in some 
other form which may have been originally that of sulphides which 
were then oxidized to carbonates, and the latter by metamorphism, 
which caused the loss of carbonic acid with or without the substitu- 
tion of silica, assumed the present form. Sphalerite (zinc sulphide) 
has been found very rarely in the ore deposits and only in small 
isolated masses, and the carbonates are equally rare, so that there is 
little positive fact upon which to base a theory. 
