364 THE MARQUETTE IRON-BEARING DISTRICT. 



Tlais aualysis shows that the rock is composed almost wholly of silica 

 and iron oxides. 



The iron ores in the Marquette district comprise many varieties, among 

 which the more prominent are hematitic, granular magnetite; magnetitic, 

 specular hematite, and soft, red hematite, which is very often limonitic; and 

 all gradation phases. The magnetites and specular hematites are called 

 hard ores by the miners, and the red hematites are called soft ores. 



The hematitic magnetites vary from very coarsely granular magnetite 

 to finely granular magnetite. With the magnetite there is always more 

 or less of hematite, in many cases a large part of this resulting from the 

 alteration of the magnetite. The hematite varies from a subordinate to an 

 important amount. Also, at many places, with the magnetite are varying 

 quantities of pyrite and garnet, and the alteration product of the latter, 

 chlorite. The magnetites vary in color from pure black to gray. 



By an increase in the quantity of the hematite the magnetites pass 

 into specular hematites. The specular hematites vary in texture from very 

 coarse to aphanitic. In the coarse hematites each individual of hematite 

 resembles a flake of mica. Such ores are frequently called micaceous 

 hematites. The flakes are largely arranged with their greater dimensions 

 parallel, thus giving the ore a marked schistosity or rift. The coai'se 

 hematites are usually strongh^ magnetitic, there being lietween the flakes 

 of hematite many crystals of magnetite. In the finer-grained specular 

 hematites the particles are so small that the eye does not easily discriminate 

 them. In many cases such ore cleaves like slate or schist, giving a smooth, 

 brilliant surface. These ores are frequently called slate ores. While mag- 

 netitic, they are usually less strongly so than the so-called micaceous ores, 

 although in them in many cases may be seen numerous small crystals 

 of magnetite. Another variety of the fine-grained, specular hematite is 

 steel-gray, dense, very hard, and breaks with a conchoidal fracture. 



Many of the specular ores contain a greater or less quantity of red 

 hematite, which gives them a mottled appearance. This class of ores, 

 which is abundant, is either very slightly magnetitic or not at all so. These 

 ores are gradation phases between the pure specular hard ores and the soft 

 red hematites. 



