228 THE PENOKEE JKUN-BEARING SERIES. 



ground of this rock, as exiiniined iu the iiolarized light, does not a])pear to have been 

 much affected by concretionary action, except that here and there the larger indi- 

 viduals are bunched in irregular oval areas. Two or three slender veins of quartz 

 traverse the section, in these the individuals of quartz are larger than in the general 

 background of silica. Uere and there the nuclei of the concretionary areas referred 

 to are seen to be composed of a number of unusually large sized quartz individuals 

 (PI. XXII, Figs. 1 audi'.) 



30. Ferruginous chert, from a h)w horizon. Specimen 9047 (slide 2775) ; from 

 405 N., 1095 W., Sec. 27, T. 40 N., R. 2 E., Wisconsin. 



The rock is a brown chert similar to 29, but having a darker color and contain- 

 ing some sjjots which are bright red and jaspery. 



Tlie thin section is composed of a finely divided but still wholly crystalline 

 silica, the larger sized individuals being nnu-h more plentiful than in 29. Irregularly 

 blotching this background are areas of nearly opaque hematite, which are, witliout 

 any perfect concretionary arrangement, which arrangement is also only faintly indi- 

 cated in a portion of the siliceous background itself. 



From the Oermania mine. 



31. Ferruginous chert or flint, from a very low horizon. Specimen 9015 (shde 

 3103); from 200 N., 1575 W., Sec. 24, T. 46 N., R. 2 E., Wisconsin. 



A dark brownish gray, aphanitic, cherty rock, carrying irregular seams of 

 hematite. 



This section shows an almost pure chert, quite closely resembling that whicli 

 forms the background of 29. A few concretionary areas only faintly marked with 

 iron oxide are seen. 



From the Montreal river aection. 



.32. Ferruginous chert or flint, at the base of the Iron-bearing member, Ashhvnd 

 mine. Si)ecimen 7019 (slide 230S); from 1965 N., 1925 W., Sec. 27, T. 47 N.. R. 47 W., 

 Michigan. 



The thin section is made up almost wholly of silica in an exceedingly finely 

 divided and even amorjihous state, at times showing faintly the radial chalcedonic 

 arrangement. Brownish and reddish iron oxides occur here and there in irregular 

 ]iatches and spots. They also appear in very distinctly outlined rhombic sections, 

 which suggest a derivation from the oxidation of an iron carbonate; a suggestion 

 which is borne out by the i)rcsence in some of the rhombic sections of areas of umil- 

 tered carboinite. 



33. Black banded flint and siderite, from a high horizon. Specimens 9007 (slide 

 2916), 9009 (slide 2765); from 160 N., 100 W., Sec. L'l, T. 47 N., R. 47 W., Michigan. 



A tVesli fracture of these rocks shows an interbanding of dark gray and black layers 

 ranging from about one-eighth to one-half inch in width. The black bands are com 



