374 THE MARQUETTE IRON-BEAEIXG DISTRICT. 



profoundly inetamorpliosed than they are farther east. Therefore the 

 unusually modified character of the rocks of the Negaunee formation 

 accords Avith what would be expected from a study of the other formations. 



On account of the opacity of the iron ores, comparatively little is 

 learned by a study of their thin sections. 



The magnetites are perfectl}' opaque in transmitted light, and in 

 reflected light give the characteristic spotty appearance of that mineral. 

 Where not pure the usual minerals contained in the iron formation appear 

 with their ordinary relations. Those most plentifully seen are quartz, 

 griinerite, muscovite, and biotite. Occasionally garnet, and chlorite as an 

 alteration product, are abundant. Bordering the included material, the 

 maonetite invariably shows crystal outlines. As a result, each area of 

 included minerals lias a serrated form. 



The coarse specular hematites are made up mainly of large, closely 

 fittino- flakes of hematite, the majority of which take an imperfect polish, 

 and have, therefore, a gray, sheeu}^, spotted appearance. The flakes which 

 are parted along the cleavage reflect the liglit like a mirror. The large 

 number of individuals of this kind is appreciated only by rotating the 

 sections. This brings successively different flakes of hematite into favor- 

 able positions to reflect the light into the microscope tube. In some sections 

 cut transverse to the cleavage the schistose character of the rock is apparent 

 in reflected light, innumerable lamince of hematite giving fine, narrow, 

 parallel, dark and light bands, which are comparable in appearance to the 

 polysynthetic twinning bauds of feldspar. As both the magnetite and 

 the hematite are usually opaque, the two minerals in general can not be dis- 

 criminated, although in some cases the crystal forms of magnetite are seen, 

 and a small part of the hematite, much of it in little crystals, shows 

 the characteristic blood-red color. The important accessory minerals are 

 quartz, griinerite, feldspar, and muscovite. Some of the small, detached 

 areas of quartz and feldspar appear to be fragmental. The muscovite 

 occurs mainly in small, independent flakes, but some of it is apparently 

 secondary to the feldspar. 



The fine-grained specular hematites differ from the so-called micaceous 

 hematites chiefly in that much more of the hematite is translucent, and 



