USEFUL METALS 



213 



for superheated gases. Magnetite, which several investigator* 

 have shown might be precipitated from iron silicates by reaction 

 with lime, replaced the limestone, and, in part, the porphyry m:t.-s. 



Te SAHl Al II, MIOWIHQ IMl qtOWTM OP TWt MOLMI OM &. 



THI miMMT gTATI. 



FIG. 113. A group of ideal sections showing the probable history of the iron 

 ores of Pilot Knob, Missouri. (After Nason.) 



There are other occurrences of iron ores in the United States 

 that are of commercial importance, as shown in Figs. 113 and 

 114. 



SlCTIOh THK004M TMC FHOJ11I IWMMINt ,NtAKO>wtC,01(aCM. 

 (MOPiriCD FROM PUTHAM.) 



FIG. 114. Section through the Prosser mine, near Oswego, Oregon. 

 . (Modified from Putnam.) 



Numerous masses of igneous rocks belonging to the gabbro 

 and norite families are found in the southern part of Norway and 

 Sweden (see Fig. 115). In these masses are found some of the 



