MAGNETITE AND PYRITE. 



specific gravity, are also in the sands. Many are too high in 

 titanium to be of use, but there is no more difficulty in concentrat- 

 ing them than artificially crushed ore. In Brazil and New Zea- 

 land they have attracted attention. 1 



2.03.14. On the Origin of Magnetite Deposits. It is important 

 to note that magnetite deposits are almost always in metamorphic 

 rocks, which owe their character to regional metamorphism, or to 

 the neighborhood of igneous rocks (Pennsylvania and Utah). 

 Gneisses form the commonest walls, but so-called norites, or gab- 

 bros, and crystalline limestones also contain them. Where there 

 is lamination, or bedding, the magnetite conforms to it. As the his- 

 tory of the metamorphic rocks is so often uncertain, the magnetites 

 share the same doubt. In igneous rocks magnetite is the most 

 widely occurring of the rock-making minerals. In all explanations 

 the prevailing lenticular shape, the general arrangement in linear 

 order, and the existence of great beds must, be considered. The 

 shape is very similar to that of deposits of specular hematite, with 

 which magnetite is often associated. (Examples 9 and 10.) The 

 following hypotheses have been advanced as to their origin : 1. As 

 intruded (eruptive) masses. This supposes an origin for the lenses 

 on the analogy of a trap dike. Though formerly much advocated, 

 it is now generally rejected. 2. As excessively basic portions of 

 igneous rocks. This supposes that large amounts of iron oxide 

 separate in the cooling and crystallizing of basic magmas. There 

 are such occurrences, although seldom, if ever, pure enough or 

 abundant enough for mining. The titaniferous magnetite of the 

 Minnesota gabbros has been alluded to (2.02.25), and also the 

 Brazilian ore and the Cumberland Hill (R. I.) peridotite. (See 

 also Dakyns and Teall, Q. J. G. &, XLVIIL, p. 118.) Should 

 such igneous rocks be subjected to regional metamorphism and 

 the stretching action characteristic of it, the ore masses might be 

 drawn out into lenses. 3. As metamorphosed limonite beds. This 

 idea has been most widely accepted in the past. It presupposes 

 limonite beds formed as in Examples 1 and 2, which become buried 

 and subjected to metamorphism, changing the ore to magnetite 

 and the walls to schists and gneisses. Igneous rocks have ap- 

 parently changed limonites to magnetite at Cornwall, Penn., and 

 in Utah, but such changes by regional metamorphism are less easy 



1 T. S. Hunt, Geol. Survey Canada, 1866-69, 261, 262; Canad. Nat., 

 VI. 79. A. A. Julien, " The Genesis of the Crystalline Iron Ores," Acad. 

 Nat. Sci., Phil., 1882, 335 ;, Engineering and Mining Journal, Feb. 2, 188^ 



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