426 THE MARQUETTE IRON BEARING DISTRICT. 



become an iron ore. Tliese iron ores nsually have a grayish or greenish 

 color, and are known to the miners as the liard gray ores. As looked at in 

 hand specimen, the detrital hematite and tlie heavily ferruginous fragments 

 have a micaceous appearance, due to mashing-. The larger part of the 

 ore is, however, magnetite in crystals, or its alteration product, martite. 

 Associated with the latter is a great deal of a green mineral. As these 

 ores become impure there are observed in them fragments of jasper and 

 grains of quartz, which increase in quantity until the rock is no longer an 

 ore, but a heavil)' ferruginous quartzite, or ore and jasper conglomerate. 

 There are all gradations between a conglomerate which consists almost 

 wholly of fragments derived from the Negaunee formation and one in 

 which the detrital material is mainly grains of quartz, in wdiich case the 

 rock becomes a ferruginous quartzite. 



In thin section the general description of the jasper-conglomerate given 

 on pages 413-415 applies fully to the conglomerate of the area. In differ- 

 ent localities the dynamic effects vary greatly. At some places the jasper 

 pebbles are greatly flattened and the ore pebbles are changed to hematitic 

 slate. In other places the dynamic effects are slight, the rock consisting 

 of rounded or angular ore and jasper pebbles, cemented by finer detritus of 

 the same kind and by iron oxide. There is present in nearly all of the 

 conglomerates a small amount of plainly fragmental quartz not derived 

 from the Negaunee formation; and also a small amount of sericite and 

 chlorite have developed, the former especially in the mashed varieties. 

 The gray ore is found to consist of the original mashed hematite, and of 

 crystals and clusters of cr3"stals of magnetite, between which is chlorite. 

 In the impure ores there is a certain amount of fragmental quartz, which 

 is plainly partly replaced by magnetite. Crystals of magnetite project into 

 the roundish quartz grains, just as though notches had been filed out of the 

 clastic grains to give place to the magnetite. Also numerous crystals of 

 magnetite are wholly included within the quartz grains. It thus appears 

 perfectly clear that by some reaction quartz was dissolved and the mag- 

 netite took its place. AVhat relation there is between the disappearance 

 of the quartz and the appearance of the magnetite is not clear, but one 

 seems to be, to some extent at least, conditioned on the other; for if 



