234 THE pejStokee iron-bearing series. 



lu slides 3137 and 3087 the (juartz is more coarsely crystalline, and the iron 

 oxide is mainly the brown hydrated variety. The iron oxides in them are arranged 

 in concretionary and brecciated forms, and the silicii is affected, although less plainly, 

 by the same arrangement. Some of the larger concretions are complex, one of them 

 perhaps including several smaller concretions. The areas in 3137 are very irregular, 

 and suggest a mechanical brecciation or a brecciation caused by the processes of 

 alteration. 



45. Ferruginous chert, from a low horizon. Specimens 7554 (slide 2298), 7555 

 (slide 1983); both from 1G80 N., 1909 W., Sec. 13, T. 47 N., R. 46 W., Michigan. 



Slide 2298 is an exceedingly finely and regularly laminated ferruginous chert, 

 quite analogous in general character to the l)lack bands of 33; while 1983 is a bre<t- 

 ciated chert closely analogous to 34, which in turn is directly iuterstratitied with .'53. 

 In other words, we have here a representation of the phenomena presented by the 

 exposure on the Montreal inver, from which 33 and 34 came, except that here all of 

 the iron carbonate has been removed. 



40. Siliceous siderite, from a lower middle horizon. Specimen 9504 (slide 318(j), 

 from 1725 N., 1900 W., Sec. 13, T. 47 N., R. 46 W., Michigan. 



A very fine grained, dark gray, evenly and finely laminated earthy rock. Sp. 

 gr., 2-07. 



The thin section shows an intimate mixture of minutely crystalline, with perhaps 

 some amorphous silica, with minute grayish rhombohedra of siderite only slightly 

 altered here and there by oxidation. These rhombohedra are generally single, but 

 are at times aggregated closely into bunches. 



47. Ferrngiuous chert or jasper, from an upper middle horizon. Specimen 9500 

 (.slide 3182), from 1840 N., 1975 W., Sec. 13, T. 47 N., R. 46 W., Michigan. 



The rock is an aplianitic chert or jasper, the colors varying in irregular blotches 

 tlirough dark gray to bright red. 



The thin section is a brecciated concretionary chert like others above described; 

 rounded areas of a finely crystalline and amorphous silica, mingled with more or less 

 brown and red iron oxides, being embedded in a silica which is usually more coar.sely 

 crystalline. Occasionally, instead of the rounded areas, there are long tabular pieces 

 showing the lamination of the original chyt. In both kinds of areas there are 

 numerous places where the rhombohedral outlines of the original carbonate reproduced 

 in the inm oxide may be seen. Besides this are to be seen within the chert frag- 

 ments rhombic areas, often of large size, composed of silica similar to tlie rest, but 

 outlined distinctly by brown iron oxide. These again are taken to be substitutions 

 for original carbonate crystals. 



48. Ferruginous chert, from an upper midtUe horizon. Specimens 9501 (slide 

 3183), 9.502 (slide 3184); both fn.ni 1840 N., 1975 W., Sec. 1.3, T. 47 N., R. 46 W., 

 Michigan. 



