222 THE PENOKEE IRON-BEARING SERIES. 



been painted upon it, u siiiylc baud of tlie iron oxide traversing Tuultitudes of quartz 

 individuals, to whoso arrangement it bears little or no relation. As usual, the (juartz 

 in tliese locks is plaiidy tiie last separated ingredient, and it seems that its conere 

 tionary form had sonicthing to do with the ('oneentric arrangement of the iron oxide 

 also. From what follows later, it is probable that the bands of iron oxide are the 

 remains of iron carbonate areas which liave altered to iron oxide in part, but have 

 partly been replaced by silica, tliese piocesses producing the concretions. Besides 

 these concretionary areas, there are other areas wliose outlines in the polarized light 

 suggest their being fragments. These are mainly made up of silica like that of the 

 groundmass, except that iu some cases they are possibly finer grained and have the 

 iron oxide particles scattered through them irregularly. This is particularly noticeable 

 in slide 3148, which appears with a low power, a* if it might have been a fragmental 

 rock whose fragments had all been silicifled, the same silica tilling the spaces between 

 the fragments. In this section also some of the apparent fragments are jaspery — tiiat 

 is to say, a luixture of red iron oxide and ([uartz — and appear therefore quite different 

 from the rest of the rock. Of particular interest is the occurrence iu this slide of quite 

 an amount of iron carbonate. This substance, which is usually more or less stained 

 with brown iron oxide, but is very plaiidy recognizable as a carbonate, appears l)oth 

 in single individuals scattered through the groundmass and in clusters of individuals. 

 These clusters often lie in groups, between which are areas of the groundmass, in such 

 a fashion as to render it evident that they were once counected with one another and 

 have since been sev>arated by the insertion of the (juartz. In a few cases some of the 

 apparent fragments above alluded taare mainly made uj) of this brown stained car- 

 bonate, little veins and streaks of the ipiartzose groundmass entering into them or even 

 entirely traversing them. This occurrence of the carbonate is of great interest, 

 because it seems to indicate that all of the apparently fragmental and concretionary 

 areas which are now made up of (juartz and iron oxide were originally composed of 

 this substance. Whether the fragmental appearance which these rocks have is due to 

 a real fragmental character, or is caused by the same processes which have formed the 

 rounded and oval concretions (i. e., is of a secondary nature), will be discussed later. 

 IS. Kcrrugin(ms(|uart/.-scliist, from a nuddle horizon. Specimen itOl'.j (slide 3150); 

 from l'J85 N., 230 W., Sec. .3;i, T. 45 N., R. 1 W., Wisconsin. 



A nearly bla<'k aphanitic tlinty rock in which the only ingredient recognizable 

 with the magniticr is hematite in minute metallic lustered scales. ■ Sp. gr., 2-!)0. 



Tlu- thill section of this lock, examined with a low power and in the ordinary light, 

 presents a much more striking irregular appearance than those immediately above 

 described (except slide 3148), being made up of round or oval areas, which differ from 

 the whiter (|iiite sparse matrix in l)eing dotted over with a fine black dust. These 

 areas are usually outlined by a border of magnetite crystals, but otherwise there is 



