20fi THE PENOKEE IRON-BEARING SERIES. 



means of the iron oxide. In some cases the h'on oxide which marks tliese 

 concretionary ai'eas is so plentiful as to render them nearly or (juite (ijijujiie. 

 From such extreme cases there is every gradation to those sections in 

 which only an exceedingly minute amount of iron oxide remains to separate 

 these areas from the intei'stitial silica, while there are not unfrequent cases 

 in which even a minute amount of iron oxide is al)sent. llie outlining ui' 

 the areas is then perceived only in the polarized light, its silica being either 



* 



nearer to or farther from the amorphous c(indition than that portion of the 

 matrix immediately in contact with it. 



The iron oxide which designates the concretions from the matrix pre- 

 sents itself in the shape of a general stain, composed of minute j^articles, 

 distributed in the shape of a mere border or in concentric rings. It may 

 be any of the three oxides — magnetite, hematite, or ;i ])rown, somewliat 

 hydrated oxide, or a mixture of two or three of them together. In cases 

 where the iron oxide is magnetite, as in the cherts which occin- between 

 Tylers fork and Sec. 10, T. 45 N., R. 1 E., Wisconsin, it may present itself 

 either in the shape of an exceedingly fine dust or as sharply outlined crys- 

 tals of some little size, and these crystals are not unfrequently arranged 

 around the edges of the concretionary area, their sharp angles projecting 

 from its outline. (PI. xxviii, Fig. 2.) It is not to be understood that the 

 iron oxides are completely lacking in the interstitial material; on the con- 

 trary, they are often present either in minute stains or aggregations of par- 

 ticles; but the rule is that they are more plentiful in concretions than in the 

 matrix, while in many cases the matrix api)ears to be almost wholly devoid 

 of them A concretion will often be sharply defined only along a portion of 

 its outline, the remainder being exceedingly vague. This arises at times 

 from lack of sufficient iron oxide stain to differentiate the concretions from 

 the matrix, while the silica of both may be so closely of the same degree 

 of crystallization as not to lielp in the definition when the section is 

 examined with the ])olarized light. In some sections the outlines of the 

 concretions are of such a nature as to suggest very strongly their having 

 been partly dissolved away, while in many cases veiidets of purer and 

 differently crystallized silica of the matrix enter into the mass of the concre- 

 tions. (PI. XXII, Fig. 3; PI. xxiii,.Figs. 1 and 2; PI. xxv. Fig. 3.) These 



