MISSOURI COPPER MINES. 305 



The Cornwall mines are located ten miles from the town of St. Genevieve, 

 and the ore occurs in two nearly horizontal sheet deposits, in what is pronounced 

 by Mr. Nicholson to be the second of the magnesian limestone of the Lower Si- 

 lurian formation. The principal developments in the Cornwall have been made 

 in the upper deposit, while the lower level is chiefly worked at the Swansea mines. 

 Still, the limited amount of prospecting done makes it possible that there are 

 other levels. 



Mr. Potter says : " The upper sheet-deposit seems to follow very nearly the 

 bedding of the limestone, and varies in thickness from a few inches to three or 

 four feet. Though varying greatly in thickness, the ore- sheet is remarkably con- 

 tinuous. Occasionally it is wanting in a very small area; but such barren ground 

 is easily worked around, and may be utilized for pillars in the support of the 

 roof. A layer of sandstone, quite thin and irregular, seems to be in most places 

 the immediate associate of the ore, and this at times is found to be altered to a 

 hard ferruginous quartzite, carrying more or less copper. Layers and nodules of 

 chert occur in the limestone at times, making the ground a little hard ; but as a 

 rule the latter rock is easy to mine. It is worked out to the parting at the top of 

 the course, which gives an easy plane to break to. This makes the drifts (vary- 

 ing with the position of the ore-sheet in the course) from three and one-half to 

 six and one-half feet high, the average being about five feet. 



" The ore itself consists essentially of the sulphurets, copper pyrites and a 

 small amount of the richer 'purple copper.' These minerals have in the upper 

 deposit been somewhat decomposed by the action of percolating surface waters. 

 There is in this deposit a remarkable absence of iron pyrites, which in the other 

 mines is so common an associate of copper pyrites. This must be regarded as a 

 very favorable circumstance, since iron pyrites degrades copper ore not only in 

 richness but in purity, arsenic, antimony, nickel and cobalt being (one or more 

 of them) always found to some extent in this mineral. The ore is in fact re- 

 markably pure." 



Prof. Potter, in discussing the question to what extent these deposits can be 

 relied upon to supply ore in the future, states that, though they are not fissure 

 veins, they are sometimes, like the St. Joe and Desloge deposits, of very great 

 extent, and developments already made on the upper Cornwall deposits present 

 evidence enough to show that "the ore extends over a very wide area, and is 

 safe to hold out with any demand likely to be made upon it for a long time to 

 come." 



Concerning the cost of mining and treating the ore and the profits of the 

 business. Prof. Potter presents some instructive figures : 



Five tons of dressed ore at furnace , . .$105 55 



Smelting the*same and producing refined copper .... 65 62 



Total cost, per ton, refined copper $17117 



