ECONOMIC GEOLOGY 



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species and varieties. The following tabular arrangement shows 

 the natural grouping of the species : 



Chemical name Mineralogical species and common names 



( Ferric and ferrous oxides. g 



i Magnetic iron ore. 



.Titaniferous iron ore. 



Proto-seequioxide of iron. 



72.4 % of iron. 



/-Hematite. 

 I Red hematite. 



. , j Anhydrous ferric oxide. | Specular ore. 



Carbonates 



Sesqui oxide of iron. 

 70 % of iron. 



Hydrated ferric oxide. 



Sesquioxide of iron. 

 60 % of iron. 



Ferrous carbonate. 



i 



| Clinton ore.— Fossil ore. 

 [ lied ochre. 



< 



[ Limonite. 

 Brown hematite. 



Brown ochre. 



I Bog iron ore. 



f Siderite. 



j Carbonate ore. 



Carbonate of iron. Spathic i Clay iron stone. 

 48 % of iron. Iron ore 



I 'White Horse.' 



A general law of occurrence of iron ores is that certain species 

 occur in, or are characteristic of, definite geological horizons. 

 For example, the magnetic. iron ores and the red hematite are 

 found in the crystalline rock areas of the Pre-cambrian ; the 

 fossil ere, the limonite or brown hematite and the carbonate are 

 found in the Palaeozoic rocks; and the bog iron ore in the more 

 recent formations of Tertiary and Post Tertiary ages. There are, 

 as might be expected, many exceptions; but in the greater num- 

 ber of these apparently exceptional cases, the surface alteration, 

 due to weathering or other atmospheric agencies, explains the 



occurrence. 



This relation between the geological formation and the mineral- 

 ogical species or lands of iron ore indicates the areas in which 

 they may occur, and determines roughly their limits. Hence, a 

 geological map «xf the state shows approximately correct bound- 

 aries of the several iron-ore districts, and is, as it were, an iron 



