64 THK PENOKEE IRON-BEARING SERIES. 



reoxidized by I'rco. coufcact witli the atmosplicri' and i)r('ci[)itat(il in tlic insohilili- form, 

 and (liiis accuiimlates in hods. Bof;' ore is now bciny (k'positwl in lids manner, and 

 tlu" ores of tlic Clinton and Ooal periods are j;en('rally attribnted to sinnlar action. 

 Little hesitancy wonld be felt in referriufj the Htironian deposits to the same ajjeney 

 if there weri' any indcpeiideut evidence of the prevalence of land \ ei^etation. There 

 is, ai^ we shall see, independent evidence of life, but it has not usnally been tiionjiiil 

 to have been terrestrial. Lowland or marsh vegetation would probably fnrnish the 

 requisite conditions, and there is no reason for dtmbting its existence, except tlie 

 want of direct evidence of it in this and the succeeding; formations. Notwithstanding 

 this doubt, no equally satisfactory explanation of the origin of the massive iron ores 

 has been ]»roposed. 



Slates, schists, (did dioritcs. — Upon the nuxguetic schists there repose a series of 

 black, ini<'abearing slates, alternating with diorites (plar/i-lioni) and .schistose quartz- 

 ites, including several horizons which are concealed by superficial material and whose 

 character is therefore unknown. Among these there appear to Ije included those 

 horizons which in the Marcjuette region bear the rich iron ores. They are here 

 doubtless concealed because of their softness, owing to which they have been more 

 dce))ly eroded by denuding agencies. Whether these horizons are iron-bearing here 

 remains to be determined by actual removal of the drift. The mica-slates were 

 originally (^lay beds, probably containing some carbonaceous matter. The schistose 

 quartzites weri' siliceous sandstones or (juartzose clays. What the diorites were 

 originally is yet an open question, it being maintained on the one hand that they 

 iiii' metamorphosed basic clays, and on the other that tliey are ancient lava flows, 

 niodilied by long-continued chemical action. This series reaches a total thickness of 

 about 3,500 feet. 



jl[ir<( -schists. — Above this is founrl a .still thicker series of mica-schists, which 

 were probably once mixed clayey sediments. This series now measures nearly- 8,000 

 feet in thitikness, making the entire group of the region embrace, as above stated, 

 about i;5,000 feet of strata. 



It will be obseived, in glancing over the whole, that tlie great mass of the series 

 was formed from the ordinary sediments arising ti'om rock disintegration, and that 

 they were unciuestionably derived from the luljaceut Laurentiau laiul. The excei)- 

 tions to this statement are found (1) in the limestone, probably derived from tlie 

 remains of marine life; (2) in the iron ores, a i)ortion at least of which probably 

 arose through organic action; and (o) possibly the diorites, which may have had an 

 igneous origin. (I'p. 80-84.) 



Succeeding the period of Iluronian sedimentation, whether immediately or 

 somewhat delayed, there was an era of njiheaval and metanioiphism, analogous to 

 that which occurred at the ch)se of the Laurentiau era. It produced analogous, but 

 less extreme, ettects. 



