260 ECONOMIC GEOLOGY 



known ore deposit, occur at Mine Hill, near Franklin Furnace, 

 and at Sterling Hill, near Ogdensburg. The ores consist of vary- 

 ing proportions of franklinite, zincite and willemite admixed with 

 calcite, and in some instances with the silicates rhodonite, garnet 

 and tephroite. In some parts of the vein franklinite is the only 

 important mineral present. In some instances it is accompanied 

 by willemite; in others only by zincite, and some cases by both 

 minerals. Occasionally the ore consists of zincite set in a matrix 

 of coarsely crystallized calcite. 



At Mine Hill the zinc content ranges from 23 to 29 per cent. 

 The iron ranges from 19 to 22.5 per cent. The manganese from 

 6 to 12 per cent. The zinc content of the Sterling Hill ore is 

 somewhat less than the per cent, given above. The ore at Mine 

 Hill comprises a layer varying in thickness from 12 to 100 ft. 

 or even more, bent upon itself to form a long trough with sides of 

 unequal height. The outcrop of the ore is about 2,600 ft. in 

 length. 



A. C. Spencer regards the Sterling Hill deposits also as com- 

 prising a trough. The layer ranges in thickness from 10 to 30 

 ft. In some parts of the layer the ore consists largely of frank- 

 linite and in others of zincite. The sides of the trough are of 

 unequal height and strike in a northeasterly direction. The dip 

 of the veins range from 45 to 60 degrees. 



According to A. C. Spencer, the deposits must have been intro- 

 duced either before or during the metamorphism of the containing 

 limestones and the igneous rocks which are now gneisses. He 

 regards the main ore body at both Sterling Hill and Mine Hill 

 as injected bodily into the limestones like igneous intrusions, 

 and the leaner ores of Sterling Hill as deposited by magmatic 

 waters which permeated and replaced the associated limestones. 

 According to J. F. Kemp, the ore was deposited from solutions 

 stimulated by granitic intrusions subsequently metamorphosed 

 into gneisses. According to J. E. Wolff, the ores are contempora- 

 neous in form and structure with the enclosing limestones, and 

 therefore older than the granites. 



(2) Central States. In the Central belt the Joplin district is the 

 most important producer of zinc. In fact it is one of the most 

 important zinc-mining camps of the world. The geological sec- 

 tion consists of Mississippian cherts and limestones overlaid with 

 Pennsylvanian limestones, shales, sandstones, with occasional 

 beds of coal. All these terranes are of Carboniferous age. 



