100 AQUEOUS EOCKS 



(1) OXIDES 



Here are included those rocks consisting essentially of oxygen 

 combined with a base, though usually other constituents are 

 present as impurities. 



Hematite. — Anhydrous sesquioxide of iron. Fe203 = oxy- 

 gen, 30% ; iron, 70%. In nature nearly always more or less im- 

 pure through the mechanical admixture of argillaceous silicates 

 or calcareous matter, manganese oxides, sulphur, phosphates, 

 etc. Several forms are recognized, the distinction being based 

 mainly upon physical properties. Specular hematite is a mica- 

 ceous or foliated variety with a black, metallic, often splendent 

 lustre; this variety is mainly a metamorphic form, and prop- 

 erly should be classed with the metamorphic rocks. Compact, 

 columnar, fibrous, and earthy forms also occur, the latter often 

 known as ochre, as are similar forms of limonite. Although 

 classified here under the head of aqueous rocks, it does not 

 follow that the hematites have all originated in precisely the 

 same manner. To a limited extent the specular variety is found 

 about volcanic craters and fumaroles, where it was orginally 

 deposited by a process of sublimation. Through a process of 

 oxidation, beds of magnetic iron become locally altered into 

 hematite, giving rise to pseudomorphous granular, octahedral, 

 and dodecahedral forms, to which the name martite is given. 

 Many extensive beds undoubtedly arise from the dehydration 

 of dynamic agencies — the folding and metamorphosing of the 

 enclosing rocks — of beds of limonite. Others, like the fossil 

 and oolitic ores of the Clinton formations, arise in part from a 

 process of chemical precipitation and subsequent segregation, 

 the ore being originally disseminated throughout a ferruginous 

 limestone, and having accumulated as an insoluble residue as 

 the lime carbonate was carried away through the action of car- 

 bonated waters. The extensive hematite deposits of the Lake 

 Superior region of Michigan are regarded as oxidation prod- 

 ucts from pre-existing carbonates (siderite), the oxide having 

 been precipitated from solution in synclinal troughs, and subse- 

 quently crystallized by metamorphism.^ The ores of the Mesabi 

 range, on the other hand, are regarded by at least one writer 

 as having originated through a somewhat complicated process 

 of oxidation and metasomatosis, whereby a ferruginous silicate 

 (greenalite) became converted into an admixture of free iron 



^ Van Hise Monograph XFX, U. S. Geol. Survey, 1892. 



