FORMATION OF MAGNETITE. 845 



texture is produced. Thus are formed the specular hematites. Where the 

 movements are of extreme intensity the little flakelets of hematite may 

 become flat and parallel, and thus resemble mica. Such a rock is a 

 hematite-schist. Where more or less of a reducing agent is present at the 

 time of the dehydration, magnetite may form and be associated with the 

 hematite. 



MAGNETITE. 



Magnetite, with quartz and various silicates as subordinate constitu- 

 ents, may compose considerable rock bodies. Such material occurs as 

 important members of the iron-bearing formations of the Lake Superior 

 region, and in the gneisses of the eastern metamorphic region of the United 

 States — in the Adirondacks, in the highlands of New York and New Jersey, 

 and in the southern Appalachians, notably at Cranberry, N. C. But the 

 greatest of the eastern magnetite deposits is that of Cornwall, Pa., in lime- 

 stone of Lower Silurian age, underlain by Triassic intrusive trap. It is 

 uncertain whether or not the gneisses with which many of these magTietitic 

 rocks are associated are of sedimentary origin, but presumably many of 

 them are. If these rocks be sedimentary it is more than probable that the 

 magnetite members are also sedimentary. The most natural hypothesis as 

 to the origin of these rocks is that they were iron-bearing- carbonates meta- 

 morphosed as described on pages 834-841. The deep-seated metamorphism 

 changed the original iron-bearing carbonates to magnetite, with the simul- 

 taneous development of quartz and silicates. 



The production of magnetite from siderite may be supposed to be that 

 of imperfect oxidation, the reaction being : 



3 FeC0 3 + O = Fe 3 4 + 3C0 2 . 



But it has been seen (p. 828) that where iron carbonate is produced 

 iron sulphide also is often found. Magnetite and pyrite, also, are often 

 associated. In the deep-seated zone of anamorphism it can not be supposed 

 that oxygen is usually present. It therefore appears to me that the more 

 frequent reaction is that producing magnetite from siderite and pyrite. 

 The change may be written thus: 



2FeC0 3 + FeS 2 + 2H..0 = Fe 3 4 -f 2H,S + 2C0 2 . 



These changes would result in a decrease of volume of 50.32 per cent 

 for siderite to magnetite, or of 46.67 per cent for siderite and pyrite. 



