SOUDAN FORMATION. 223 



jasper to the intricate infolding' and subsequent truncation of the two 

 formations. In other places the evidence is fairly conclusive that we have 

 to deal with dikes which were intruded both parallel to the balnding of the 

 jasper and across this banding. In both cases folding subseqtient to their 

 intrusion has brought about structural relations similar to those existing 

 between the jasper and the basement greenstone. The rock forming the 

 dikes which cut the iron formation is now uniformly green, except where 

 discolored by the hon. Its chief component at present is chlorite, but 

 certain facts — for instance, the presence of quartz phenocrysts, which were 

 observed and which will be considered in detail at a more fitting place — 

 show very conclusively that at least some of these dikes were originally 

 acid intrusives. The alterations that these acid rocks have undergone have 

 produced schistose rocks, which now are strikingly similar in macroscopic 

 characters to some of the greenstones derived from rocks of an originally 

 basic character. These dike rocks have been classed as basic rocks — 

 greenstones. In one case — that of the schist a,t the Lee mine — Smyth and 

 Finlay" state specifically that it has been derived from a quartz-porphyry. 



DEPOSITS OCCXXRRING AT BOTTOM OF THE IRON FORMATION. 



We have no conclusive evidence that any of the deposits of iron ore 

 at Soudan or Tower rest upon what is the actual basement greenstone. 

 Such an occurrence, if recognizable, would be found to be similar in all 

 essential characters to the occurrence of the ore at Ely. We would find 

 the ore- at the bottom of a synclinal trough of the iron formation, with the 

 greenstone forming the impervious bottom and sides of the trough. It is 

 impossible to recognize with certainty the true basement greenstone on 

 account of the intrusion of acid sills and dikes in the iron formation in the 

 mines in the vicinity of Soudan and Tower; on account of the intricate 

 and intimate relationship existing between these two kinds of rocks, due to 

 this intrusion and heightened by the subsequent infolding of the eruptive 

 and the iron-formation rocks; and also on account of the resemblance of 

 the altered acid rocks to the greenstone. The deposits at the east end of 

 Soudan Hill, which have been mined out, are supposed to have been laid 



eThe geological structure of the western part of the Vermilion range, by Henry Lloyd Smyth 

 and J. Ralph Finlay; Trans. Am. Inst. Min. Eng., Vol. XXV, 189.5, p. 639. 



