196 THE VERMILION IKON-BEARING DISTRICT. 



is frequently found in contact with tlie iron-bearing rocks and tends to 

 separate the formation into a number of small belts whose continuit}^ can 

 not be traced on account of the rarity of exposures. The greenstones 

 which occur in this iron-formation belt are of different kinds, and it is 

 supposed that they may represent flows of greenstone geologically con- 

 temporaneous with rocks of the iron formation. The exposures are so 

 isolated, however, that it is not feasible to connect them and separate the 

 rocks into individual flows or sills. Furthermore, it is in these belts of the 

 iron-bearing rocks that the above-mentioned clastic rocks derived from the 

 greenstones and underlying conformably the iron-bearing formation have 

 been found. In view of these facts, we are led to believe that some of the 

 rocks of the' iron-bearing formation, while resting upon the basement of 

 greenstones, are likewise overlain by greenstones, or, in other words, that 

 some of the iron formation is interbedded with greenstones which are of 

 volcanic origin. 



Thus the clastic sedimentary deposits derived from and overlying the 

 lava flows may grade up into nonclastic sediments. The conditions of 

 sedimentation var)^ from place to place in the area, hence we get a gradual 

 change from mechanical sediments to organic sediments (cherty ferruginous 

 carbonates). Where the conditions were not favorable for the formation of 

 the clastic sediments the nonclastic sediments were deposited without the 

 conglomerates and gra3'wackes intervening between them and their igneous 

 rock basement. Hence we now find them resting upon the greenstones with 

 a sharp line of demarcation between them, or at most with a narrow zone 

 of schistose greenstone intervening. These sediments were in their turn in 

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

 by succeeding sedimentary deposits. These processes continued through- 

 out a shorter or longer period. It is due to this fact that such intimate 

 relationship exists between the greenstones and the associated — in the main 

 younger — iron-bearing formation. As a result of this intimate relationship 

 it has been impossible to logically separate the two in a more marked way 

 than has been done in the above pages. While of a distinct method of 

 origin, their formation took place within essentially the same period of time. 

 In general, however, it is possible to recognize the greenstone as the true 

 basement rock of the district, correlative with the Archean rocks of the 

 other Lake Superior iron-bearing districts. 



