170 REVIEWS 



bearing formation proper, which consists of siliceous rocks, largely white cherts — 

 though varying in color from white, green, yellow, and purplish to black — with red 

 jasper and carbonate-bearing chert, griinerite-magnetite-schist, hematite, magnetite, 

 and small quantities of pyrite. These iron-bearing rocks are clearly of sedimentary 

 origin. They do not now present their original characters, but are presumed to have 

 been derived from rocks that were largely carbonate-bearing, ferruginous cherts. The 

 relation of the iron formation to the adjacent greenstones is clearly that of a sediment- 

 ary overlying an igneous series. The few basal conglomerates of the iron formation 

 that have been found consist of pebbles derived from the underlying greenstone, 

 showing conclusively their relationship. This relationship is obscured, however, in 

 most places, by the absence of the conglomerates, and by the fact that the iron formation 

 has been very closely infolded in the greenstone. In consequence of the extreme 

 folding and of the impossibility of determining different horizons in the iron formation 

 it has been impracticable to ascertain its thickness. The iron-ore deposits of the 

 Vermilion district show a striking analogy with those of the Marquette district. Like 

 them, they mav occur in two positions with respect to the iron-bearing formation. 

 They are found, first, at the bottom of this formation, and, second, within it, the ores 

 in both cases being the same in character. The Ely deposits are typical of the 

 deposits occurring at the base of the formation. They are found at the bottom of a 

 closely compressed syncline of the iron formation where it lies in the relatively imper- 

 vious greenstone. The source of the iron was, in the first instance, the Ely greenstone. 

 From this it was removed through the action of water and collected in the Archsean 

 sea to form the sedimentary deposits of the Soudan formation. After the folding of 

 the formation this disseminated iron was carried by downward percolating waters into 

 places favorable for its accumulation, such as the bottom of this synclinal trough, 

 where it was precipitated by oxygen-bearing waters coining more directly from the 

 surface. Pari passu with this precipitation silica was removed, affording space for 

 the accumulation of the iron to form the ore deposits as now known. The Tower and 

 Soudan deposits differ only in detail from the Ely deposit. 



Granites, intrusive into the Archaean, occupy a wide area, and are named from 

 the topographic features with which they are conspicuously associated. That these 

 intrusives are older than the Ogishke conglomerate (Lower Huronian), which succeeds 

 in age the Soudan formation, is shown conclusively by the fact that pebbles derived 

 from them occur in this conglomerate. The general period of intrusion of all of these 

 acid igneous rocks is placed between the time of the deposition of the latest sediments 

 of the Archaean and that of the deposition of the earliest sediments of the Lower 

 Huronian series. 



The Lower Huronian occurs in two detached areas, one of which, known as the 

 Vermilion Lake area, extends from the western limit of the area mapped, in the vicin- 

 ity of Tower, to within about eleven miles of Ely on the east, and the second of which, 

 known as the Knife Lake area, begins about seven miles west of Ely, and extends 

 eastward to the eastern limit of the area mapped. At the base of the series there lies 

 a great conglomerate, known as the Ogishke conglomerate, containing pebbles and 

 finer detritus from all of the rocks of the Archaean. Above this conglomerate, in the 

 eastern portion of the district, there are found in a few localities small masses of the iron- 

 bearing Agawa formation. This formation is petrographically the same as the Soudan 

 formation. In it, however, there is in places a development of the carbonate-bearing f acies. 

 No iron ores have been found in it. Overlying the Ogishke conglomerate, in the 



