232 THE PENOKEE IRON-BEAlimG SERIES. 



bands in this groundmass are opaqne aggregations of red iron oxide. They are 

 in part plainly derived directly Ironi an iron carbonate, since the rhombic portions of 

 the crystalline sections aie i)lainly visible, i)articnlarly around the borders of the 

 opaqne aggregations. Thefe is also abundantly in the groundmass vaguely outlined 

 areas representing probably the silicilied areas of the carbonate. These are analo- 

 gous to those occurring in above described sections, but are of rather small size. 



38. Hematitic flint or chert, from a low horizon. Specimen 7G(I0 (slide 2009); 

 from 300 N., 1500 W., Sec. 17, T. 47 N., K. 40 W., Michigan. 



In thin "section the chert background is completely though minutely crystal- 

 line. It otherwise closely resembles the last section described. The hematite aggre- 

 gates, however, show somewhat larger remnants of unalteretl iron carbonate. 



39. Hematitic flint or chert, from a middle horizon. Specimen 7596 (slide 2301); 

 from 453 N., 1300 W., Sec. 17, T. 47 N., R. 40 W., Michigan. 



This section differs from the two last described only in the very much larger 

 proportion of iron oxide contained and that the silica is intermediate in crystalliza 

 tion between the silica of them. The iron oxides appear in the same sort of aggre- 

 gates iu which are often to be seen the rhombic crystal outlines of the original carbon- 

 ate. The texture of the chert in this case also is somewhat ojien, numerous cavities 

 being contained by the section. 



From the Colby mine section. 



40. Siliceous iron carbonate, from a high horizon. Specimen 12508 (slide 5522); 

 from 1125 N., 175 W., Sec. l(i, T. 47 N., li. 4C W., Michigan. 



A very fine grained rock, with an irregular division into light and dark gray , 

 lamina?. Scattered along these laminae are minute shining facets of a carbonate, 

 whicli show however, more plentifully in much larger individuals irregularly blotched 

 all over the specimen without any reference to the arrangement of the bands. The 

 appearance of this carbonate alone is sufBcient to identify it as siderite, an identifi- 

 cation which is confirmed by the deep brown weathering that the specimen shows. 



This section differs from many of those previously described in the very large 

 amount of fresli iron carbonate which it contains. This mineral occurs not oidy in 

 single individuals, but in compact aggregations of individuals, which sometimes 

 occupy quite extensive areas in the section. Irregularly interwoven witli the caibon- 

 ate areas are areas of silica which are largely amori)lious. In this siliceons hack- 

 ground are seen, however, in places the peculiar (-oncretionary areas desciibed as 

 characterizing some of the above sectioiis. These areas very i)lainly originate from a 

 (jhangc of \\\v. caibonate aggregates. Dotted through the section are particles of 

 quartz of varying size. These are partly in the background and partly witliin the 

 masses of carbonate, the larger <mes often ap^tearing as if of fiagmental origin. (PI. 

 XXVII, Fig. 1.) 



