PRINCIPLES CONTROLLING ORE DEPOSITS 7 $5 



appreciable." 1 This deep ore is mainly copper-bearing pyrites. 

 Douglass tells us that in depth every copper deposit of the entire 

 Appalachian region of the United States shows only cuprif- 

 erous pyrrhotite. An excellent illustration is Ducktown, 

 Tenn., where at the level of groundwater was a very rich 

 deposit of chalcocite but a few feet thickness which rapidly 

 changed into very low grade cupriferous pyrrhotite. 2 In Aus- 

 tralia down to the level of groundwater are high values in native 

 gold ; below the level of groundwater are auriferous pyrites 

 bearing relatively small values of the precious metals. 3 Some 

 of the superintendents say where ounces of gold are found above 

 the level of groundwater only pennyweights are found below. 4 

 In the Sierra Nevada of the United States, according to Lind- 

 gren, 5 above the level of groundwater the gold values ran from 

 $80 up to $300 per ton ; but below the level of groundwater where 

 there are sulphurets the values average from $20 to $30 per ton. 

 Notwithstanding the fact that occurrences such as those men- 

 tioned are typical of the ore deposits of many districts of the 

 world it has been believed by very many practical mining men 

 that ore deposits become richer upon the average with increase 

 of depth ; but it must be admitted that the facts do not justify 

 this sanguine expectation. In fact nine mines out of ten, 

 taking the world as a whole, are poorer the second 300 meters 

 than they are the first 300 meters, and are poorer the third 

 300 meters than they are the second 300 meters. In fact, many 



J The Ore Deposits of Butte City, by R. C. Brown : Trans. Am. Inst. Min. Eng., 

 Vol. XXIV, 1895, p. 556. 



2 The Persistence of Lodes in Depth, by W. P. Blake: Eng. Min. Jour., Vol. 

 LV, 1893, P- 3- 



The Ducktown Ore Deposits and Treatment of the Ducktown Copper Ore, by C. 

 Henrich: Trans. Am. Inst. Min. Eng., Vol. XXV, 1896, pp. 206-209. 



3 The Alterations of the Western Australian Ore Deposits, by H. C. Hoover : 

 Trans. Am. Inst. Min. Eng., Vol. XXVIII, 1899, pp. 762-764. 



4 The Genesis of Certain Auriferous Lodes, by J. R. Don : Trans. Am. Inst. Min. 

 Eng., Vol. XXVII, 1898, p. 596. 



sThe Gold-quartz Veins of Nevada City and Grass Valley, California, by Wal- 

 demar Lindgren : Seventeenth Ann. Rep. U. S. Geol. Surv., 1895-6, Pt. II, p. 

 128, 1896. 



