chloride of iron will precipitate the iron as the oxide (hematite , 

 according to the equation: 



Fe 2 CJ,+3CaCO 3 = Fe 2 O 8 +3CaCl 2 +3CO 2 . 



r riir iron ore replaces the lime- 

 stone in the metasomatic de- 

 posits molecule by molecule. 



Thomas and Mac A lister 

 divide the iron ores of metaso- 

 matic origin into two classes: 

 (1) Contemporaneous, in 

 which the replacement oc- 

 curred either during or im- 

 mediately after the deposition 

 of the original rock. (2) Sub- 

 sequent, in which the replace- 

 ment occurred after the depo- 

 sit ion and consolidation of the 

 original rock. The former in- 

 cludes the bedded deposits 

 and the latter the irregular 

 patches and veins. The oolitic 

 ('linton iron ore of central 

 New York, Pennsylvania and 

 Alabama is a representative 

 of the contemporaneous de- 

 posits. The ore lies above the 

 Medina sandstone in lime- 

 stones of Silurian age. 



The Lake Superior iron ores 

 lie upon the Archean complex 

 and in the Keweenawan ter- 

 ranes. They form the best 

 American representative of 

 subsequent replacement. (See 

 Fig. 29.) 



The source of the iron seems 



to have been the ancient igneous rock of the Lake Superior 

 region. The iron was precipitated as siderite. The siderite 

 then passed into hematite and insensibly into ferruginous 

 quartz schists, jaspers, magnetite and limonite schists. In the 



