SOUDAN FORMATION. 181 



one can distinguish the extremely altered greenstone fragments and the 

 matrix derived from the greenstones, both consisting now largely of 

 actinolite, chlorite, and epidote, with quartz. As the sediments get finer 

 the clastic characters disappear as the result of the extreme alteration, and 

 one can only surmise the mode of origin of these rocks by their intimate 

 association with and gradations from the coarse elastics, and, when no 

 gradation is visible, by the presence of banding and false-bedding lines. 

 The microscopic examination shows these sediments to be made up of 

 chlorite, actinolite, epidote, sericite, spheiae, quartz, carbonaceous material 

 (graphite), and some iron oxides, in vairious proportions, so tliat these 

 metamorphosed slates vary from absolutely black, greasy-feeling, graphitic 

 slates to dark-green and fairly light greenish-gray rocks. The graphitic 

 slates consist essentially of graphite and quartz in exceedingly fine grains 

 and in some cases in very small quantity. In one specimen the place of 

 the quartz seemed to be taken by feldspar, which is altered to sericite, so 

 that the sediment consists of graphite and altered feldspar. 



MACROSCOPIC CHARACTERS OF THE IRON-BEARING FORMATION PROPER. 



The iron-bearing formation proper (that is, that portion in which the 

 ore bodies occur) consists of cherts of various colors — green, Avhite, 

 yellow, black, and purplish — red jasper, carbonate-bearing cliert, slaty rock, 

 showing in some cases intimate association with the clastic formation proper, 

 griinerite-magnetite-schist, hematite, magnetite, and some pyrite. To the 

 formation as a whole the miners and prospectors apply the name "jasper," 

 although only a portion of it falls strictly under this designation. These 

 various kinds of rock occur in bauds of varying thickness, rarely exceeding 

 5 or 6 inches, and commonly in extremely thin laminae. llsuall}- the 

 individual bands appear to be homogeneous. Occasionally there is a 

 banding within the bands, which is due to the arrangement of the mineral 

 constituents. In one place such a banding simulated the false bedding- 

 of normal clastic sediments (PL V, B). 



The alternate bands of material of different color combined with the 

 complicated folding ma,ke the formation a very striking object, which on 

 exposures almost always attracts the attention of the traveler, even if he is 

 not accustomed to closely noticing rocks. 



