PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 365 



Magnetite. Taconyte.] 



colorless, and is sometimes slightly pleochroic, a being almost colorless, b light 

 yellowish and c nearly like b but sometimes showing a tinge of greenish. It has a 

 strong double refraction and is repeatedly twinned, and is referred to grunerite. It 

 is massed in small grains, hardly fibrous, which show the prismatic cleavage well 

 developed. Cross sections sometimes show one or two prism faces partly developed, 

 but the mineral is usually completely allotriomorphic. The magnetite is in small 

 grains and in large, granular, porous masses. The calotte is quite common, although 

 not nearly as abundant as the other minerals. It is in small grains, quite frequently 

 in connection with the magnetite, being between it and the grunerite or enclosed in 

 the magnetite. This seems to be calcite rather than siderite, which mineral might 

 be expected in this rock. It does not show the high index of refraction, rough black 

 cleavage lines and the pleochroism which are common to the siderite of the iron- 

 bearing rocks of Minnesota. Moreover the hand sample effervesces very readily in 

 acid. Three sections. 



Age. Animikie. 



Remarks. This rock is part of the iron-bearing series of the Animikie. It is 

 supposed to owe its crystalline character to the influence of the great gabbro mass 

 just to the south. u. s. G. 



No. 438. MAGNETITE. 



Sec. 15, T. 59-14 W. 



Ref. Annual Report, viii, pages 150, 151; Annual Report, ix, page 108. 



Meg. Magnetite similar to No. 436. 

 No sectfon. 



Age. Animikie. u. s. <j. 



No. 439. TACONYTE. 



Same locality as No. 438. Rock associated with the ore. 

 Ref. Annual Report, ix, page 108. 



Meg. A fine-grained, hard, siliceous rock, gray in color, but becoming brownish 

 and porous on weathering. It is seen to contain rather indistinct crowded granules 

 in an aphanitic groundmass. 



Mir. The section is too thick for careful study. It, however, shows a taconyte 

 with numerous rounded granules in an almost colorless groundmass. These granules 

 are quite distinct in ordinary light, being stained yellowish. In polarized light the 

 granules are hardly distinguishable, the whole section breaking up into a fine-grained 

 aggregate of quartz. Usually the grains in the granules are a little larger than those 

 of the groundmass. Magnetite and siderite are also present, and there are some tufts 

 composed of very minute radiating fibres. These fibres are colorless alone, but when 

 massed together are yellowish brown. They are perhaps of actinolite. One section. 



