TUl'] EEl>UJ]UC TKOUGH. 531 



oolitic structure, such as is so beautifully shown in the more modern and 

 less altered iron formation of the Mesabi range. In the Michioannne 

 jasper of the Menominee district, whi<;h is re<^'arded, on strati<--raphical 

 grounds, ns the etjuivalent of the Lower Marquette iron formation, an 

 original oolitic and concretionary structui-e is common. 



The question of the nature of the rock from which the iron-bearing 

 member has Ikhmi derived is fully discussed by Professor Van Hise else- 

 where in this memoii-. Whetlier, as seems probalde, the various phases 

 which the ii-on-bearing member now presents, have been derived from a 

 single original rock of sensibly uniform character or not, it is very evident 

 that much of the differentiation is of long standing and occurred before the 

 Upper Marquette transgression. That this is so appears from the jjresence 

 of pebbles from both the magnetite-griinerite-schists and the specular 

 jaspers in the basal conglomerate of the Upper Marquette series. In the 

 Republic and adjacent areas at least, the specular jaspers occur at a definite 

 stratigraphical position in the highest horizon in the Lower Marquette 

 series. Tliey are present only in those phices where large thicknesses of 

 the lower series remain, as at Republic Mountain and in the range along the 

 northwest side of Lake Michigamme. Where the lower series has been 

 more deeply eroded before the deposition of the Upper Marquette rocks the 

 specular jaspers are far less continuous and of less common occurrence than 

 the magnetite-griinerite-schist phases of the iron-bearing member. These 

 facts appear to bear strongly agahist the view that the specular jasjjers are 

 due to later metamorphic processes which acted along the contact with the 

 Upper Marquette quartzite after the latest folding, while they are what 

 ^vould be expected if these two chief phases existed in substantially their 

 present condition before the Upper Marquette series was laid down. 



Another fact is also significant. It has been said that the griinerite, 

 quartz, and iron oxides of the iron-bearing member have a very distinct 

 banded arrangement and yet are not original minerals, and that this banding 

 is parallel to the ui)per and lower boundaries of the formation. It is prob- 

 able that a set of pai-allel structural planes has controlled the segregation 

 of the present constituent minerals during the changes through which the 

 rock has passed, and that these planes must have been original bedding 



