H. D. Irving — Is there a Iluronian Group f 257 



Passing now over a good deal of intervening territory, where 

 the same two formations, however, continue to display them- 

 selves, I ask your attention next to the Penokee region of 

 Northern Wisconsin and Michigan. 



Here we find a most notable instance of an unconformity be- 

 tween a series of highly tilted but unfolded strata above the 

 break, and a deeply folded series below. Indeed, there are in 

 this region two striking stratigraphical breaks ; one between 

 the iron-bearing series and the folded gneissic formation to the 

 south of it, and another between the unfolded but highly 

 inclined iron-bearing series and the equally highly inclined 

 Keweenaw series to the north. The accompanying map and 

 sections* show the distribution and structural relations of the 

 several formations and the several principal kinds of rock in 

 this region. The Keweenawan and iron-bearing rocks both 

 being highly inclined, the map is in fact what the vertical sec- 

 tion is where the upper strata of an unconformity are essen- 

 tially horizontal. The region is one which is only now begin- 

 ning to be at all easy of access and it is still in the main 

 covered with a dense forest growth. Notwithstanding these 

 difficulties, and the further difficulty that in places the drift- 

 covering is considerable, the general structure of the region has 

 been by a great deal of painstaking labor pretty thoroughly 

 worked out. 



The lower one of the two unconformities of this region, i. e. 

 that between the gneissic formation on the south and the iron- 

 bearing series next north of it, is established by the same phe- 

 nomena as those appealed to in the case of the region of Lake 

 Huron. These are (1) the traversing of the lithologically dis- 

 tinct areas of the older or basement formation by the continu- 

 ous belts of the higher series, as is most beautifully shown by 

 the accompanying map; (2) the intersection of the schistose 

 rocks of the older series by granite masses and areas which 

 have yielded fragments to the higher series ; (3) the occurrence 

 in the higher series of fragments derived from the stratiform 

 members (gneiss, mica schist, etc.), of the lower complex, these 

 fragments occurring in beautifully developed basal conglomer- 

 ates at the contact of the two sets of rocks ; (4) the striking 

 lithological contrasts of the two sets of rocks, the bedded mem- 

 bers of the lower set having arrived at a nearly complete 



* Not here reproduced. The map displayed when the paper was read was a 

 colored one some twelve feet in length, showing the distribution of the iron-bear- 

 ing rocks, the older schists, the granites invading these schists, and also in part 

 of the Keweenawan Eocks of the region. This map with its accompanying sec- 

 tions, has already been published in a reduced form in this Journal for April, 

 1885. In a very much improved form it will appear also in the seventh 

 annual report of the TJ. S. Q-eol. Survey, and in a forthcoming memoir on the 

 Penokee-Gogebic region. 



Am. Joue. Sci — Third Series, Vol. XXXIV, No. 202.— Oct., 1881 



17 



