THE IRON-BEAEIXG MEMBER. 221 



arrange themselves in the centers of areas whose outer portions ai-e made up of more 

 minute particles. The magnetite, however, also shows some sliglit tendency to a con- 

 cretionary arrangement, while the Tniiiute actiuolife blades are aggregated into little 

 clusters, which appear in tlie main to lie in the spaces between the concretionary 

 areas or in the outer portions of tlu'se areas. The magnetite occurs in bunches of 

 crystals, and in curvilinear aggregations, and also in single individuals scattered in- 

 discrinuuately, though sparsely, throughout the section. Another noticeable ditfer- 

 ence between these rocks and those from Penokee gap consists in their lack of the 

 highly developed lamination which the latter rocks sliow, both in tlie tliin section 

 and in the hand specimen. There is no trace of evidence that the quartz of these 

 rocks is in any measure of a fragmental nature. In fact, its concretionary arrange- 

 ment and the interlocking of the tWfterent individuals puts such an origin entirely 

 out of the question. In slide .'5147 stains of brown ircui oxide occur hei'e and thei'e, 

 usually mingled witli tlie clusters of the minute actinolite blades. In the section of 

 this specimen there is seen also a single rhombic crystal section, which, on examina 

 'tion with a higli ])()wer, proves to be made up chiefly of a nunil)er of nctinolite l)lades, 

 which on the border of tlie crystal are deeply stained by lirowii inm oxide. The 

 shape of this piece suggests the probabibty that it was originally a single individual 

 of iron carbonate. (PI. XXVlil, Fig. 1.) 



17. Ferruginous quartz-schist, from a low horizon. Specimens 9(il 7 (slide .3100) 

 9623 from 117.> X., ll'O VV., <)(>1S (slide .314.SJ, from ll'4.j N., 18.j W., Sec. 3.3, T. 45 N., 

 K. 1 W., Wisconsin. 



These specimens resemble tliose of It! in all res]>ects, exce|>t that iiunicntus spots 

 of reddish jasper arc seen iipun them. These stand nut in ;i prdinincnt fasjiiou and in 

 such a way as to suggest n IVagniental origin lor them. In the same lecjge, liowever, 

 are large bands of the same jaspei' (!MilS), directl\ iiiteiiaiiiniated wiih tlie dtber 

 materials; Specimen 9<>1 7 sIkiws the jieculiar cuneretionary, or partly fragmental, 

 character described under Id. Sp. f^r.. <if 9(iI7. -•7(>; of '.HIIS, 2-!)2. 



The thin sections show that these rocks ditfer tVom Hi in containing no actinolite- 

 but a more interesting ditlerence lies in the very nnuii more striking development 

 which they show of a. concretioiiar\ structure. This appearanre shows not nieieh in 

 the qiuirtz background itself, as seen in the polarized liglit, but ia the arsaageuient of 

 the i)articles of iron oxides, which include both hematitt^ and magnetite, and in smaller 

 quantity the brown oxide. In some cases the magnetite and hematite are aggiegated 

 into opa<iue, round or o\ al areas, the edges of wiiich show the piojeci Jng ('orners of 

 numerous individual crystals of magnetite; but there are more plentiful areas in which 

 these oxides are seen to be disposed in concentric baiiils, made up of more or less sp])- 

 arated particles of the oxides, and having between them the ordinary cpiartz back- 

 ground of tlie roeU. Will Ml these areas are examined in the polarized liglit they are 

 f<mnd to lie in the quartz background in such a fashion as to suggest their having 



