SOUDAN FORMATION. 189 



into the jasper. On the noi'tli side of the exposure the gTeenstone is 

 massive, but as it nears the jasper it becomes more and more schistose, and 

 with this increasing schistosity there is a more or less imperfect banding, 

 brought about by an alternation of bands of the greenstone discolored with 

 iron with those which are not so colored. Farther south the true jasper 

 appears. Here apparently is a fine-grained mechanical sediment derived 

 from and immediately overlying the greenstone, a deposit which is 

 essentially indistinguishable from the greenstone so far as its composition is 

 concerned, its mode of origin being indicated by the imperfect banding- and 

 the presence of ferruginous material. Here there is no well-marked clastic 

 deposit between the greenstone and the iron formation, and this is one of 

 the localities where the quiet conditions controlling the deposition of the 

 iron formation had already set in, while in other parts of the district 

 elastics were being formed. For instance, in sec. 10, T. 63 N., R. 10 W., 

 west of North Twin Lakes, bands of the jasper lie parallel to the strike of 

 the edge of the greenstone in contact with it, as though the banded 

 rocks were a sedimentary deposit laid down upon it as a base. But no 

 clastic sediments were found in this case between the jasper and the green- 

 stone, their absence being due probably to the fact that here, likewise, 

 the conditions Avere those of quiet deposition, during which no elastics 

 could be formed. From the fact that the rocks of the iron formation are 

 conformable with the clastic sediments near their contact one might be 

 inclined to infer that the iron-formation rocks also are of mechanical 

 origin. But microscopic examination, however, shows that the cherts and 

 jaspers and ores do not possess the minute textures indicative of mechanical 

 sediments, although one case has been noted in which three possibly clastic 

 quartz grains were associated with the chert, thus showing that the condi- 

 tions fluctuated from those suitable for the formation of clastic sediments 

 to those giving rise to organic sediments, as shown also by the microscopic 

 occurrence of interbanded elastics with the cherts. 



In recapitulation it may be said that the banding possessed by the 

 ii'on formation and the slat}^ bands associated with it, which show true 

 sedimentary characters, and which were evidently originally of detrital 

 mud, give the clearest proof of the sedimentary origin of the iron formation 

 itself. The question which may next be raised concerning its mode of 

 origin is whether it could not have been a chemical deposit. N. H Winchell 



