258 i?. -D. Irving — Is there a Huronian Group f 



re-crystallization, while the upper series is composed mainly of 

 but little altered fragmentals ; (5) the highly foliated, folded 

 and contorted condition of the lower rocks as contrasted with 

 the unfolded and unfoliated condition of the higher. Any one 

 of these would alone raise a strong presumption in favor of the 

 existence of a great break at the junction of these two series 

 of rocks ; together they seem to amount to a demonstration 

 of chronological distinctness. 



North of the narrow belt of country occupied by the iron- 

 bearing series in this region, and all the way to the shores of 

 Lake Superior, the rocks of the Keweenaw series are at sur- 

 face, with the exception of a belt of country in the vicinity of 

 Chequamegon Bay, which is underlain by the Potsdam sand- 

 stone, and a small area in the vicinity of the Presqu'Isle 

 Eiver, which is again occupied by the same formation. This 

 sandstone, as is well known, traverses the, upturned edges of 

 the Keweenaw series in many places. It will not be possible 

 now for me to review the nature and general structural charac- 

 ters of the Keweenawan rocks of this region. I can only say 

 that the series consists of a great succession of eruptive flows, 

 alternating with which are beds of detrital material, and follow- 

 ing which is an immense thickness of sandstone. In the vicin- 

 ity of the iron-bearing rocks these layers stand at high angles, 

 and present in strike and dip a general conformity with the 

 layers of the iron series itself. But a closer inspection renders 

 it evident that the upper surface of the iron-bearing series has 

 suffered a deep erosion previous to the spreading upon it of the 

 great flows of which the lower portion of the Keweenaw series 

 is made. This is shown by the very striking manner in 

 which these flows are found in contact with all members 

 of the iron-series as the contact line is followed from east to 

 west. 



Thus in the Marquette, Menominee and Penokee districts 

 there are great discordances between a lower set of gneisses and 

 other crystalline schists, intruded by granite, and an upper set 

 of detrital rocks, carrying iron. In all three districts, moreover, 

 the two sets of rocks seem so manifestly the same, being, indeed, 

 very probably directly continuous through all, that we can 

 hardly doubt that in all of these districts we are dealing with the 

 same discordance. Above the uppermost of these series suc- 

 ceeds the great Keweenaw complex, while the Potsdam sand- 

 stone traverses indifferently the eroded edges of all three of 

 these groups of strata. 



If now, grouping these three districts in one region, we com- 

 pare their succession with that of the Lake Huron region, we 

 find such a striking similarity as to the relations and natures of 



