210 



ECONOMIC GEOLOGY 



some actinolite, forming an actinolitic magnetite schist which 

 meteoric waters were unable to transform into a productive ore 

 body. The productive portion remained through this period as 

 a slightly altered cherty iron carbonate. When in subsequent 

 times the district was folded and erosion was carried to the extent 

 that the iron-bearing formations were exposed to the action of 

 meteoric waters, the siderite was altered to ferruginous slate, 

 ferruginous chert and iron ores. A soft hydrated ferric oxide 

 constitutes the main part of the ore. Hard slaty ore is not un- 



FIG. 112. Map of a portion of the Iron Springs, Utah, district, showing 

 occurrence of iron ore in limestone near andesite contact and also in the 

 igneous rock. After Leith and Harder. (By permission of the Macmillan 

 Company, from Ries' Economic Geology.) 



common, and the oxides of manganese occur in a few deposits. 



3. The Cordilleran Section. There are many widely scattered 

 occurrences of iron ores in the Cordilleran section. Many of 

 these are not worked owing to the long distance from the rail- 

 road, or a limited demand for iron, or the character of the ores 

 (Fig. 112). 



In the southern part of Gunnison county, Colorado, there is 

 a quantity of magnetite in the Cebolla district. According to J. 

 T. Singewald, Jr., the ores occur (1) in the basic igneous rocks. 



