SOUDAN FORMATION. 197 



Within the greenstone are areas of jasper of very irregular shape and 

 size which do not possess the regular banding seen in the jasper of the large 

 areas of the iron formation. Such small areas of jasper are not uncommon 

 in the northern half of sec. 21, T. 62 N., R. 14 W., from 1,150 to 1,260 

 jDaces north, 950 paces west of the southeast corner; again, in sees. 1, 2, 

 and 3, T. 61 N., R. 15 W. These jasper areas are of irregular shape and 

 appear to owe their origin to a process of infiltration similar to that which 

 forms veins. This is clearly the mode of origin of these irregular masses of 

 jasper which occur in the midst of the ellipsoidal greenstones, filling the 

 angular interstices between the ellipsoids (see page 139). Some of these 

 irregular masses may be remnants of the iron-bearing formation deposited 

 in irregularities of the underlying rocks, but this mode of origin could not 

 be proved for any occurrence. There is a further possibility that some of 

 these masses may be inclusions of the iron formation in the eruptive green- 

 stones. This explanatio]! has been ofiFered by H. V. Winchell and others 

 for a large part of the iron formation of this district and has been cited as 

 proof that the iron formation was older than the greenstones. However, as 

 already shown, the presence of the conglomerates cleaidy disproves this age 

 relationship for the greater part at least of the iron formation. Infolding 

 may as readily explain the intimate character of the relationship between 

 the greenstone and the iron formation as the suggested intrusive relationship. 

 For instance, when we find small isolated lenses of the iron formation lying 

 in the greenstone, with the surface only exposed, or when, as not uncom- 

 monly happens, narrow bands of the formation are bounded on two sides 

 by the greenstone, their lateral extension being concealed in the other two 

 directions, the exposures are too impei'fect to enable one to determine the 

 exact relation of each mass of the iron-bearing formation to the greenstone. 

 Nevertheless, when considei'ed in connection with other instances, such as 

 have been mentioned and described, where the relations of the large masses 

 of iron formation to the greenstone are clearly those due to infolding, it 

 will be readily admitted that the relations of the other doubtful cases are 

 also best explained as due to this same thing. Especially are we inclined 

 to this conclusion when the close folding to which the rocks of the district 

 have been subjected is fully recognized. Admittedly some of the greenstones 

 may be younger than some of the iron formation; for instance, the sills 

 and dikes forced into the iron-bearing belts toward the close of the period 



