55G THE MARQUETTE IKON BEARING DISTRICT. 



before a groat thickness of the Lower ^[arquette series was deposited the 

 sea had entirely overridden tlie ^[ar([iiette district. 



Till. LOWER MARQX'ETTE SERIES. 



THE TRANSGRESSION HORIZON. 



Toward the ckise oi the pre-Marquette (hnuidation tlie sea reached the 

 northeast border of the Marquette district. Advancin<i' iqton it. perhaps in 

 part as the result of depression, but largely as a consequence of subaerial 

 and marine erosion, the fraginental sediments of the ^lesnard foriuation were 

 laid down. This advance steadily continued from the northeast toward the 

 southwest and west, the first deposits being everywhere fraginental sedi- 

 ments, at the base usualh' a coarse conglomerate, and higher up a sandstone 

 which subsequently was changed to the Mesnard quartzite. Long- before 

 the seashore reached the western end of the district other formations were 

 deposited in the eastern half of the su'ea, so that we have some measui'e of 

 the time required for the transgression. The formations thus deposited 

 above the Mi'snard quartzite before the sea advanced to Michigamme Lake 

 were tlie Ivoua dolomite and the We we slate. It follows, then, that in 

 passing from the east to the west end of the district there are in the Lower 

 ]\[arcpiette series fewer and fewer formations. At the east end is the full 

 succession; at the extreme western are only the two upper members. Litli- 

 olog'icalh' the whole transgression horizon is one formation, marking as it 

 does a continuous belt of conglomerate and metamorphosed sandstone 

 immediateh- above the Basement Comple.x. Chronolog'ically, however, 

 ditiereiit parts of it are to be equated with several formations, that part oi 

 it onlv being called the ^lesnard quartzite which was deposited before the 

 beginning of the deposition of the next higher member, the Kona dolomite, 

 and hence it is necessary in the chronological scale to subdi^^de this lower 

 conglomerate and quartzite between the \arious formations from the Mes- 

 nard quartzite to the Ajibik quartzite. It is not possible to do this accu- 

 rately in the mapping in all places, and the manner in Avhich on the maps 

 one formation feathers out against the shore-line, to be succeeded by the 

 next one, is more or less arbitrary, although it so happens that there is 

 no considerable ditficulty in this particular for most of the disti'ict. This 



