380 THE MAEQUETTE lliONBEAKING DISTRICT. 



and sometimes hrecciated jasper may be particularly well seen on the 

 so-called jasper 1)1 uff southeast of Ishpeming (Pis. XXV and XXVI). In 

 the exposures, and particularly in the open pits and waste-dump material 

 of the mine, may be seen all stages of the processes of replacement of the 

 siliceous bands of the ferruginous chert and jasper by iron ore. 



At Negaunee a section from the Jackson mine to the southeast (Atlas 

 Sheets XXVIII and XXXI) gives the fullest known succession from the 

 jasper above to the comparatively little altered griinerite-siderite-slate below. 

 At the Jackson open pits exposures of the Negaunee formation is beautiful 

 typical banded jaspilite (Pis. XXIV and XXVII, fig. 1). To the south the 

 red cj^uartz is somewhat suddenly replaced by the white c^uartz, and in place 

 of the jasper Ave have the ferruginous chert (Pis. XXI and XXII). This 

 jasper and ferruginous chert, while having a general northward dip, shows 

 minor cremilations, faulting, and brecciation, becoming not infreciuently a 

 genuine reibungsbreccia. As the ridge of greenstone is neared in the 

 southeast part of sec. 1, the rocks of the iron formation change gradually 

 from the typical ])roken ferruginous chert to a somewhat regularl}- lami- 

 nated ferruginous slate, in which a large part of the oxide of iron is limonite. 

 The change frouT this ferruginous slate to the ore is very beautifully shown 

 at the Grand Rapids mine. To the south of the greenstone ridge there at 

 once appears the sideritic gTiinerite- magnetite slate. While the section 

 is not complete, no one can study this locality without becoming con- 

 vinced that the eveidy banded sideritic slate (PI. XVII, fig. 1) to the south 

 is the rock from which the regularly laminated ferruginous slates and 

 griineritic slates have developed, and that from these the ferruginous chert, 

 jasper, and ore bodies have been formed by combined dynamic action, 

 metasomatic change, and infiltration. In thin section the rocks of the 

 Ishpeming and Negaunee area include all phases of the ferruginous cherts 

 and jaspers found in the eastern part of the district. To describe them 

 would be but to repeat the general description of these rocks. 



Area southeast of Ishpeming. — In the broad area south of Negaunee and east 

 of Ishpeming (Atlas Sheets XXVIII, XXIX, XXXI, and XXXII), very 

 largely composed of greenstone, there are everywhere found, in the 

 valleys between the greenstones, exposures of sideritic slates, sideritic 



