564 THE MAEQUETTE IRON-BEAEING DISTEICT. 



of the district remained for a time above the sea, and therefore at first 

 received no deposits. We thus partly exphiiu the very considerabh' thick- 

 ness of the quartzite in the Ishpeming- and Negaunee areas, its dying- down 

 to an exceedingly narrow stratum in the western end of the district, the 

 considerable thickness of the Negaunee formation about Ishpeming and 

 Negaunee, and its thinning or disappearance at the west end of the district. 



The first deposit of the advancing sea was a conglomerate, the detritus 

 of which Avas derived mainly from the immediately subjacent Negaunee 

 formation. Hence it is that the basal formation is so frequently jasper- 

 conglomerate, chert-conglomerate, and, where the detritus is finer, recom- 

 posed chert and jasper, ferruginous slate, etc. However, the detritus was 

 derived not wholly from the Negaunee formation, but in part from the 

 various lower formations. This shows that either within the district under 

 discussion or adjacent to this district erosion had cut into the inferior forma- 

 tions, and even down into the Basement Complex. This is well illustrated 

 by the Palmer belt of the Groodrich quartzite, where the conglomerate con- 

 tains not only fragments of the Negaunee formation but of the Ajibik 

 quartzite and of the Basement Complex. 



Following the basal conglomerate, which is from a few feet to several 

 hundi-ed feet thick, came a sand deposit. This sand was largely composed 

 of simple, pure grains of quartz, which could not have been derived from the 

 iron-bearing formation, but must have come fi-om lower formations outside 

 of the district discussed. This probably implies that adjacent to the district 

 erosion by this time had removed large areas of the Negaunee formation. 

 Mingled with the coarse simple grains of quartz are also fine complex frag- 

 ments of chert and jasper, which shows that in places the Negaunee iron 

 formation was still being cut. This sandstone has been subsequently 

 changed to a quartzite. 



Early in the time of sand deposits along the southern pai-t of the dis- 

 trict, an east-west fissure was formed near Clarksburg, and a major and 

 probably at least two minor volcanoes were developed. As a consequence 

 there was piled up the Clarksburg formation, a mountainous mass of mate- 

 rial, consisting of lavas and tuft's, some of which were rearranged by water, 

 and of volcanic materials interstratified with ordinary sedimentary rocks. 



