THE IRON BEARING MEMBER. 263 



at other times the carbonate hiyers contain thin seams of chert finely 

 interhiminated with them. Not unfrequently the chert is in noduhir forms, 

 or a layer of chert which has been continuous for some distance has a 

 somewhat abrupt oval termination (PI. xxv, Fig. 4). The carbonate areas 

 may contain here and there uiiuute particles of cherty material, just as the 

 cherty areas contain rhomboliedra of siderite. In fact, there is every 

 possible gradation between bands of almost pure carbonate and others of 

 almost pure chert. Upon the weathered sui-face these rocks show in a 

 marked degree the peculiar weathering- exhibited by chert}- limestones. 

 Because of the resistant character of the chert the carbonates are more 

 rapidly dissolved, and thus leave the siliceous bands in ridges protruding 

 from the face of the rock, varying in width from those not more than a 

 hair's breadth to those several inches across. 



That this material is of the same origin as the rock which has been 

 regarded as an original one in the Penokee series is manifest. The rocks 

 of the two districts are so remarkably alike, that if the specimens from 

 them were mingled many of them could not be separated by their differ- 

 ence of appearance. Upon the whole some phases ajiproach more nearly 

 to certain of the Ohio carbonates and those near old Livingston manor 

 house, Hudson river, New York, than do most of the Penokee carbonates. 

 That the silica now present in these rocks was there at a very early date is 

 shown by the remarkable brecciation which they exhibit where folded. 

 In general the series is quite uniformly fiat-lying or gently tilted, but the 

 exposures upon the north side of the main body of Gunflint hike exhibit in 

 places somewhat sharp bowing. The laj'ers, where cherty, have not yielded 

 to this bowing uniformly, but have been shattered like a rigid, brittle, 

 inelastic body, and thus conform themselves by a succession of fractures 

 to the general Ijowing taken by the strata. As a result of this peculiar 

 feature the chert and carljonate are in places as finelv and uniformly lam- 

 inated as when found in any sedimentary formations (PI xxv. Fig. 3). 

 From this regularity they suddenly change to rough breccias along zones 

 of fracture. These breccias alone might be taken to l:)e a water-deposited 

 fragmental rock, but, as examined in exposure, their positions at the places 

 of abrupt variation in the bedding of the rock prove that they have 



