200 THE PENOKEE lEON-BEAKING SERIES. 



ferruginous cherts to that of the actinolitic type is a verj' gradual one, 

 extending through a distance of 10 or 12 miles. 



To one visiting the Iron-bearing member, only at two or three points 

 distantly removed from one another, without examining the intervening 

 portions — say, for instance, at Penokee gap and in the vicinity of tlic 

 mines at Bessen^er, Michigan — the difference between the rocks which 

 compose it might be so striking as to give rise to a doubt of the actual con- 

 tinuity of the belt between the two places; biit by one who follows it 

 throughout its whole length no such question for a moment can be enter- 

 tained. Mining developments and natural exposures make the belt practi- 

 cally continuous and one finds a gradual transition ])etween the three types 

 of rock, which take place only in the most gradual manner. Further, he 

 finds immediately beneath the Iron-bearing member the Quartz-slate, and 

 immediately above it the Upper slate.^ 



Microscopical character of the cherty iron carbonates (PI. xxi). — This 

 type of rock has two chief constituents, siderite and chert. In composition 

 the siderite is an iron carbonate, bearing more or less of calcium and mag- 

 nesium and not infrequently passes into a, ferrodolomite. The siderite 

 varies in its character from earthy to well crystallized. When crystallized 

 the small individuals perfectly interlock. It is usually more or less impure, 

 including at times green chlorite, carbonaceous matter, and occasionally 

 numerous minute crystals of magnetite. The chert making n\) the other 

 part of this type of rock is in part amorphous. It is that variety of silica 

 or quartz which has in the polarized light a minute spotty ajjpearance, 

 due to exceedingly small individuals of quartz, mingled with more or less 

 of silica which is apparently amor])hous. Hornstone or iiint gives in thin 

 section the same appearance; and the chert of the iron carbonates and 

 hornstone have a very close microscopic resemblance. That a portion of 

 the silica is really amorphous, as indicated by the appearance of the section 



' At the time the WiBConsiii geological survey of this district was made, there liad been no 

 prospecting along the east end of the Wisconsin part of the range, und the exposures between the 

 Potato and Montreal rivers are very sparse; yet witli tliese few ex])osiircs, fielile magnetic attraction, 

 and the asaistance of topography, the survey was able to locate so accurately the Iron-bearing mem- 

 ber that within it or very close ti> it are all the iron mines yet discoveied M]iciu the Wisconsin side of 

 the boundary. (See PI. xxvi. Atlas, Geology of Wisconsin.) 



