On the Zinc Mines of Franklin^ N. J. 259 



considerable size, fragments of two or three pounds being the 

 largest. It forms a sort of paste, in which the crystals or grains 

 of Franklinite have become imbedded, and it is required to be 

 separated from the Franklinite by the process of burning, and 

 pounding, and the use of magnets. Explorations at a greater 

 depth must determine whether the proportion of the Franldinite 

 or red oxide increases relatively with the depth of the mine. I 

 know of nothing in relation to other mines, that can lead to any 

 well founded opinion on this subject. If the red oxide should 

 continue to form one half of the bulk of the bed, or even if it 

 should not hold out so much at a greater depth, it will neverthe- 

 less prove immensely valuable for reduction, because the expense 

 of separation by rotary magnets, including burning and pounding, 

 will not exceed four dollars for each ton. Now, if we suppose 

 the average proportion to be one half red oxide, we may form a 

 tolerably correct idea of the quantity of metallic zinc within any 

 given number of cubic feet of the beds at Franklin and Sterling, 

 by ascertaining the weight of each cubic foot of ore, from its 

 known specific gravity. 



Taking the specific gravity of the red oxide at 5-420, we have 

 340 lbs. (very nearly) the weight of each cubic foot. One half 

 of this is 170 lbs., or the quantity of red oxide in each cubic foot 

 of the ore, as it averages. Oxide of zinc contains 81 per cent, 

 of pure metal ; consequently, 170 lbs. of the ore contain 137 lbs. 

 pure metallic zinc* The length of the bed visibly exposed at 

 Sterling is more than six hundred feet ; its depth we will assume 

 to be one hundred feet, which will be as deep as it can be con- 

 veniently drained.! I^s average width cannot be less than four 

 feet. Now 600x4x100=240,000 cubic feet of ore, each of 

 which contains 170 lbs. of red oxide. Again, 240,000x170 = 

 40,800,000 lbs. of red oxide, which, yielding 81 per cent, of pure 

 metal, gives of it, 33,048,000 Ibs.J This, multiplied by six cents, 



* I am aware that I do not here take into account the portion of manganese and 

 iron combined with the red oxide, but the proportion of red oxide in what ap- 

 peared to be an average sample of the ore, after the Franklinite had been sepa- 

 rated by the magnet, amounted to considerably over one half of the whole bulk. 



t There can be no doubt that it is really co-extensive in depth with the lime- 

 stone ; and it probably penetrates deep into the substratum on which the lime- 

 stone rests. The maximum thickness of the limestone is supposed by Prof. Rogers 

 to be upwards of 2000 feet. 



t This estimate supposes the whole cubic contents of the bed of red oxide, to be 

 entirely without cavities ; a character which it has thus far uniformly maintained. 



