292 THE MARQUETTE IKON liEAlJING DISTRICT. 



areas, which were behaved to l)e due to spheroidal fracturing, are clearly 

 shown to be of this character in the thin section. Instead of having- smooth 

 exterior boundaries, as would be expected in waterwom pebbles, there are 

 minute irregularities, such as would be produced by fracturing. The sphe- 

 roids are found to be pure vitreous quartzites, which are wholly cemented 

 by the enlargement process, or, more rarely, by this combined with finely 

 crystalline, interstitial quartz. These i)ebble-like areas rest in a background 

 composed of quartz grains, which are set in a matrix composed of finely 

 crystalline quartz, iron oxide, and sericite. It is apparent that the individ- 

 ual grains of this part of the rock were broken apart, and thus allowed the 

 secondary materials to enter, whereas in the uncrushed pebble-like areas 

 the space was fully occupied. It is clear that before this rock was lirecciated 

 it was indurated by the enlargement process. 



In the macroscopical description a locality was mentioned where 

 the lowest horizon of the Ajibik quartzite bears slate fragments. Here the 

 lower beds consist of iuterstratified slates, graywackes, and conglomerates, 

 which quickly pass up into ferruginous quartzite, and this into the ordinary 

 vitreous rock. The slates are composed of iuterstratified coarse and fine 

 materials, which differ chiefly from each other in that the coarser layers 

 contain numerous large fragmental grains of quartz, usually sim])le, but 

 sometimes complex, and sometimes chei'ty. The matrix is claye}' material, 

 so fine that it is difficult to determine the constituents, but sericite, quartz, 

 chlorite, feldspar, and ferrite are present. The conglomeratic layers also 

 bear fragments of the underlying Wewe slate. However, these fragments 

 when closely examined are seen not to be sharply outlined, as is usual with 

 ordinar}^ pebbles, but are greatly elongated and have minutely irregular 

 borders, the projections of Avhich fill the interspaces of the quartz graius. 

 This suggests that the underlying slate was not much indm-ated at the time 

 it yielded the fragments to the quartzite, being rather a C(^mpacted clay 

 than a solid rock. 



For those parts of the area where the Ajibik fonnation is overlain by 

 the Negaunee iron formation the lower formation grades into the higher, or 

 beds which belong lithologically in the two formations are iuterstratified. 

 In passing from the lower to the higher formation ; where the lowest rock of 



