SOUDAN FOKMATION. 199 



formation of clastic sediments, nonclastic sediments were formed without 

 tlie conglomerates, etc., intervening between tliem and their igneous-rock 

 basement; hence we now find them resting upon the greenstones with a 

 sharp line of demarcation between them. Conditions of sedimentat' "^ 

 varied; hence we get a gradual change from mechanical sediments \o 

 organic sediments (iron-bearing rocks). The sediments in their turn were 

 buried by lava flows, which again at a later date were covered up by 

 succeeding sedimentary dejaosits, and so on. So far as we can ascertain, 

 volcanic activity continued only during the time when the lowest sediments 

 were being formed. We have no evidence that volcanic activitj' continued 

 during that later period in which the iron-bearing sediments were deposited. 

 Lastly, through this series of sediments and lavas intrusive masses would 

 be forced, which would, in some places at least, include portions of the 

 sedimentary deposits as well as of the lava associated with them. 



RELATIONS TO THE ARCHEAN ACID INTRUSIVES. 



On Soudan Hill and elsewhere the iron-bearing- formation is intruded 

 by acid intrusives belonging to the Archean eruptive series. At the 

 Eaton explorations at the SE. J of sec. 7 and SW. ^ of sec. 8, T. 62 N., 

 R. 14 W., the jasper and ore are cut by granite-porphyr}^, which carries very 

 large quartz phenocrysts. Also north of Mud Creek Bay, in the SE. J of sec. 

 1, T. 62 N., R. 15 W., the jasper is both cut by and included in a granitic 

 eruptive. On the north shore of the lake, in sec. 18, T. 62 N., R. 12 W., 

 the jasper is cut by granite-porphyry. Granite dikes cut the iron-formation 

 belt south of Ely in sees. 3 and 4, T. 62 N., R. 12 W. The jasper belt 

 extending through the south half of sec. 10, T. 63 N., R. 10 W., is also cut 

 by dikes of granite and granite-porphyry. Similar occurrences could be 

 multiplied, all showing the iron-bearing formation cut by the younger acid 

 eruptives, but it is not necessary to further emphasize this relationship, 

 which is indisputably clear. 



RELATIONS TO OVERLYING SEDIMENTS. 



The iron formation is in places overlain by a series of sediments of 

 clastic origin. Where contacts between these series were observed it 

 was found that the relationship existing was that of two unconformable 

 sedimentary deposits. The ])roof of this is in the fact that the upper. 



