84 KEMP'S ORE DEPOSITS. 



the residual clay left by their alteration the ore is found. The gos- 

 san of the neighboring veins of copper pyrites, best known at Duck- 

 town (see Example 16), were originally exploited for iron. 1 



The Tennessee limonite extends across northwestern Georgia, 

 and still farther east the Huronian limestones of North Carolina 

 also enter the State. But as even these Huronian schists and as- 

 sociated marbles have been considered by F. P. Bradley to be 

 metamorphosed Silurian (Cambrian), the ores may also belong un- 

 der Example 2a. The well-determined Siluro-Cambrian rocks 

 form but a narrow belt of no great importance in North Carolina. 2 



The limonites are again strongly developed in Alabama and 

 furnish a goodly proportion of the ore used in the State. They 

 form a belt lying east of the Clinton ores (Example 6), later de- 

 scribed. As in Tennessee, they are associated with strata of the 

 Knox group. 3 



2.01.22. Extensive deposits of limonite also occur in the Lake 

 Superior district, near Negaunee, Mich., as stated above, but they 

 are mentioned again under Example 9a. They are in Huronian 

 strata. 



2.01.23. Origin of the Siluro- Cambrian Limonites. Dr. Jack- 

 son of the First Pennsylvania Survey argued in 1839* that they 

 originated in situ ; that is, by the alteration of the rocks in and 

 with which they occur. Percival, in his report on the Geology of 

 Connecticut, in 1842 (p. 132) attributed them to the alteration of 

 pyrite in the neighboring mica-slate. Prime, in Pennsylvania, 

 in 1875 and 1878 (Reports D and DD), considers that the iron has 

 been obtained by the leaching of the neighboring dolomites and 

 slates, it being in them either as silicate, carbonate, or sulphide ; 

 that the ore has reached its position associated with the slates, be- 

 cause, being impervious, they retained the ferruginous solutions ; 

 and that the potash abundantly present in the slates probably as- 

 sisted in precipitating it. 5 Frazer, in 1876, 6 in studying the beds of 



1 J. M. Safford, Geol. of Tenn , p. 448, 1869. B. Willis, Tenth Census, 

 Vol. XV., p. 331. 



2 F. P. Bradley, " The Age of the Cherokee County Rocks, North 

 Carolina," Amer. Jour. Sci., iii., IX. 279 and 320; B. Willis, Tenth Cen- 

 sus, Vol. XV., p. 367. 



3 W. M. Chauvenet, Tenth Census, Vol. XV., p. 383. For other ref- 

 erences to Alabama iron ore deposits, see under Example 6. 



4 Ann. Rep. First Penn. Survey, 1839. 



5 M.E., II. 410. 



6 Second Penn. Survey, Rep. C, p. 136. 



