OXIDES lOi 



oxide and silica, the one or the other, according to the inter- 

 mittent character of the permeating solutions, being leached out 

 and redeposited at no great distance in a fair condition of 

 purity.^ A discussion of this subject belongs more properly to 

 economic geology, and need not be dwelt upon further here, 



Limonite (Brown Iron Ore). — Iron sesquioxide plus water. 

 HeFe^Og + FesOg. An earthy or compact dark brown, black, 

 or ochreous-yellow rock, containing, when pure, about two- 

 thirds its weight of metallic iron. It occurs in beds, veins, and 

 concretionary forms, associated with rocks of all ages, and 

 forms a valuable ore of iron. (See Fig. 1, PL 8.) On the bot- 

 toms of lakes, bogs, and marshes it often forms in extensive 

 deposits, where it is known as bog-iron ore. The formation of 

 these deposits is described as follows: Iron is widely diffused 

 in rocks of all ages, chiefly in the form of (1) the protoxide, 

 which is readily soluble in waters impregnated with carbonic 

 or other feeble acids, or (2) the peroxide, which is insoluble in 

 the same liquids. "Water percolating through the soils becomes 

 impregnated with these acids from the decomposing organic 

 matter, and then dissolves the iron protoxide with which it 

 comes in contact. On coming to the surface and being exposed 

 to the air, as in a stagnant lake or marsh, this dissolved oxide 

 absorbs more oxygen, becoming converted into the insoluble 

 sesquioxide, which floats temporarily on the surface as an oil- 

 like, iridescent scum. Finally this sinks to the bottom, where 

 it gradually becomes aggregated as a massive iron ore. This same 

 ore may also form through the oxidation of pyrite, or beds of 

 ferrous carbonate. At the Ktaadn Iron Works, in Piscataquis 

 County, Maine, the ferrous salt, as it oxidizes, is deposited as a 

 coating over the leaves and twigs scattered about, forming thus 

 beautifully perfect casts, or fossils. 



Pyrolusite, Psilomelane, and Wad. — These are names given 

 to the anhydrous and more or less hydrated forms of manganese 

 oxides, which, though wide in their distribution, are found in 

 such abundance as to constitute rock masses in comparative 

 rarity. The origin of such deposits is at times somewhat ob- 

 scure. In all cases they are doubtless secondary. The original 

 source of the material appears to have been the manganiferous 

 silicates of Archaean and more recent eruptive rocks, whence it 



^ J. E. Spurr, BuU. No. 10, Geol. and Nat. Hist. Survey of Minnesota, 

 1894. Also C. K. Leith, Mono. 43, U. S. Geol. Survey. 



