THE IRON BEAKlNd MEMHKH. 187 



proximity of exixisures nt' rocks characteristic oi tlu- Kcwcciiav; series 

 ami nt' the yrauitcs l)Hhmgiii<)- I)eneath the Peiiokee series appear to 

 indicate tlie teriiiination in that direction, tor some distance at h-ast, 

 ol' the Penokee rocks. I'o the east of the easteinmost point indicated as 

 readied l)v tlie continnons iron l)eh of tl)e Penokee ranj^je, the NE. ^ of 

 Sec. "21, '^r. 47 N., P. 44 W., Michigan, exposures of ferruginous rocks allied 

 to those of tlie Iron-ljearing member are met with for a distance of G or 

 7 miles, and to within 3 or 4 miles of Gogebic lake, but these occur in a 

 peculiarly disturbed and difficult area, whose geologA', whatever l)e the 

 true interpretation of it, is unlike that of the Penokee l>elt pro])er. The 

 iron-bearing- rocks of this area receive special attention in ( 'haptei- VIII. 



The widtli of the belt of country occupied l)y tlie Inm-bearing mem- 

 ber is surprisingly uniform from the westernmost exposure as tar east as the 

 central portion of T. 47 X., P. 4(! W., ^lichigan. Tln-oughout this very 

 considerable distance tliis width rarely falls below SOO feet and as rarely 

 exceeds 1,000 feet; its yariations being frequently explica])le l)y chang'es 

 in the dip, though apparently some part of the variation may be due to 

 actual difference in thickness. Still, through most of this distance the 

 thickness itiust be taken as more nearly constant than the width of the 

 belt occupied. When R. 46 W., Michigan, is reache(l, iiowever, a distinct 

 widening of the belt becomes perceptible, and in the eastern pait of that 

 township this liecomes so ra[)id tliat when Black river is readied it has 

 become as nnich as 2,400 feet. From Black rivei- eastward there is a still 

 more rapid increase, the width on the east sidi^ of Sees. 7 and IS, T. 47 N., 

 R. 4.5 W., Michigan, lieing fully 4,700 feet. A \n\vt of this great increase 

 in width is plainly due to a very unusual flattening in tlie degree of north- 

 ward dip, l)ut this will not serve to explain all of the widening, a part of 

 which may be due to an actual hicrease in thickness, but is owing in part at 

 least to the presence of interbedded greenstones. Innnediatdy east of the 

 last iiame(l point the overlying Keweenaw rocks, whose southern l)oundary 

 has lieen for some time rapidh' ajtproached l)y the northern eilge of the 

 iron-bearing belt, are reached b\ it. ( 'oiitinuing to the east, the iron belt 

 is in part cut off by the Keweenaw series, so that at one place its surface 

 width is not much more than oOO feet. Not far east of Sunday lake a 



