428 THE MARQUETTE IRON-BEAKING DISTRICT. 



appearance wliicli reseuibles that of the brecciated jasper of the Negaunee 

 foi-mation. However, a close examination discloses differences between the 

 two rocks. The jasper laminae in the conglomerate have not the continuity 

 that they have in the true jasper. Some of the flattened areas have a round- 

 ish apjiearance, and slightly different colored jaspers occur close together, 

 whereas in the brecciated jaspers, while the layers may be broken apart, 

 there is usually a suggestion of former continuity and a likeness of character 

 in the fragments. The cracks in the broken pebbles and in the matrix are 

 cemented by magnetite in crystals, which is readily discriminated from the 

 detrital, mashed, lustrous, micaceous hematite. 



A short distance to the north of the main pits of the Jackson mine 

 the relation of the Negaunee formation and Goodrich quartzite is much 

 clearer. The Negaunee jasper is folded into a number of westerly pitching 

 rolls, the strike of the axis of the central fold being nearly N. 75° W. The 

 dip of the jasper or the dip of the axes is 45° W. The overlying quartzite 

 has a north-south strike, and dips to the west at an angle of 20°. At this 

 locality the unconformity between the two formations is apparent, but at 

 the main Jackson pit the severe folding obliterated evidence of this. The 

 conglomerate and recomposed feiTuginous schist pass upward quickly into 

 the plainly fi'agmental feiTUginous quartzite. 



As examined in thin section, the mashed conglomerates simulate to a 

 remarkable degree the mashed and brecciated original jasper. Many of 

 the pebbles were broken by the pressure into a number of angular frag- 

 ments, which are cemented by secondary iron oxide. Other pebbles, where 

 the mashing was most severe, are flattened until they approach the jasper 

 bands in appearance. In the recomposed rock in which all of the material 

 is derived from the Negaunee formation, it would l)e impossible to state 

 from the thin sections that the rock is clastic, but in many of them 

 there appears a subordinate quantity of small, roundish grains of distinctly 

 fragmental quartz, larger than the quartz grains of the Negaunee forma- 

 tion, and evidently derived from some other source, probably from the 

 Basement Complex. These quartz grains show undulatory extinction and 

 fracturing. By an increase of the clastic quartz the rocks pass into the 

 feiTUginous quartzites, the fragments of which are derived almost wholly 



