PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 687 



Magnetite. Quartzyte and muscovadyte.] 



In the siderite patches are other substances, and particularly pyroxene{V) which in 

 occasional elongated grains has a transverse cleavage and parallel extinction, as if it 

 were orthorhombic, while its high polarization is like epidote. Other grains show 

 conclusively that this mineral is not orthorhombic. Its optic plane is perpen- 

 dicular to the scant cleavage. It is scattered in finer particles throughout most of 

 the siderite patches. At the same time there is much magnetite in form of a tine 

 powder. These give the siderite patches, in common light, the form and manner of 

 distribution of the glauconite (so called) of the rock taconyte, and the rock might be 

 called taconyte, were it not that the patches are largely of siderite and epidote. 

 One portion of the slide is free from epidote and iron oxide, and is then simply a 

 sideritic taconyte, or "cherty carbonate." One section. 



Age. Animikie (probably iron-bearing member). 



Ju-iiKirlc. This close association of basic igneous rock with the sideritic taconyte, 

 well known about Gunflint lake, seems to link them in some measure in a common 

 history or in a succession of chemical transformations having a common cause, artd 

 is quite significant. N. H. w. 



No. 1308. MAGNETITE. 



Near the north shore of Gunflint lake, about on sec. 13, T. 65-3 W., if the United States system of survey 

 were extended to the Canadian shore. 



Ref. Annual Report, xvi, pages 72, 73, 120 ; Bulletin vi, pages 117, 131, 422. 



Meg. Fine-grained, compact, nearly pure magnetite. No section. 

 Age. Animikie (iron-bearing member). u. s. a. 



No. 1309. QUARTZYTE AND MUSCOVADYTE. 



East end of Gunflint lake. 



Ref. Annual Report, xvi, pages 72, 73, 120 ; Bulletin vi, pages 117, 131, 422. 



Meg. A condition of " muscovadyte," near contact with the gabbro. 



Mic. Essentially quartzyte at the present time. The grains are of uniform 

 size, wholly reformed, but without interlocking. They fit closely to each other, filling 

 all the interstices, but do not interlock. They contain numerous globular inclusions 

 which extinguish independently and which are always at or near the centre. They 

 are separated from each other by a yellowish film which is chloritic or serpentinous, 

 but occasionally this is replaced by iron ore, which spreads irregularly about the 

 grains of quartz. When the iron increases thus the greenish-yellow substance also 

 increases, and it then occasionally shows the polychroism of adinolite, into which 

 secondary quartz sometimes has insinuated itself in the manner of a micropegmatyte. 

 This change goes further, for in connection with the increased occurrence of mag- 

 netite and of actinolite is also considerable of the epidote mentioned in No. 1307 and 

 of augite, the latter even forming large but much altered crystals, while a yellowish 

 substance resembling bowlinyite may be the residuum from olivine. 



