562 THE MARQUETTE IRON-BEAEING DISTRICT. 



doubtless life. As the iron carbonate came down into the open water it 

 was peroxidized and the iron precipitated as hydi'ated oxide. When this was 

 bui-ied with organic matter the decomposition of the latter produced carbon 

 dioxide, and the iron was reduced to the protoxide by the organic matter. 

 The two combined and reproduced iron carbonate. Whether the area of 

 deposition of iron carbonate was an arm of a large sea or an almost inclosed 

 lagoon, there are no means of ascertaining, but the widespread distribution 

 of this inferior iron-bearing formation in the Lake Superior region suggests 

 that the areas of deposition of such material were very large. 



ERUPTIVES OF LOWER MARQUETTE TIME. 



At one locality amygdaloids are interstrattied with the Siamo slates. 

 In others, closely associated with the Negaunee iron formation are volcanic 

 tuffs. It thus appears that in later Lower Marquette time there was vol- 

 canic action. Just how extensive the volcanoes were has not yet been 

 determined, as these rocks have not in all cases been discriminated from the 

 later igneous rocks. 



UNCONFORMITY AT THE TOP OF THE LOWER MARQUETTE SERIES. 



Whether any later formations followed conformably upon the Negaunee 

 iron-bearing formation it is impossible to say, but if so they were subse- 

 quently removed by erosion. Following the deposition of the Negaunee 

 formation and all possible later conformable formations, the land was raised 

 above the sea, gently folded, and eroded. In general the discordance between 

 the Lower Marquette series and the succeeding series is not great, being meas- 

 ured frequently by 5° to 10°, at other times by 10° to 15°, and it is only 

 rarely that the plications of the lower series are such as to make the beds 

 abut perpendicularly against those of the overlying series. In these cases 

 the truncated layers are those of the minor plications rather than the major 

 folds (figs. 20 and 21). Erosion cut deeper in the Lower Marquette sei'ies 

 in some places than in others. At the east end of the area it left a very 

 considerable thickness of the iron-bearing formation, but in places to the 

 west this formation is quite cut out. Indeed, in places erosion cut through 

 the Siamo slate and the Ajibik quartzite, and in some places even into the 



