SOUDAN FORMATION. 187 



a relatively small proportion of chert we might thus get as a result of the 

 alteration of the carbonate a ferruginous rock, possibly with alternate bands 

 rich and poor in iron. These presumably served as a nucleus from which, 

 by replacement, were derived the more ferruginous bands and ore deposits. 

 Detailed descriptions of these processes have been given elsewhere by Van 

 Hise," and will not be discussed in this place. The presence of the limonite, 

 hematite, and magnetite in the cherts g-ives us the varieties of the ferruginous 

 chert, as it is commonly called— the red jasper and the black or lean, 

 hungry jasper, respectively. 



Associated with these cherts and ores are the griinerite-magnetite 

 rocks. These are not present in large quantity. In places we find a green 

 rock consisting essentially of chert with griinerite and but few crystals of 

 magnetite; at other places there are rocks in which magnetite is the essential 

 constituent with but little griinerite; and all gradations between exist. 

 This griinerite is very nearly of the composition of the hydrated ferrous 

 silicate which is so abundant in and forms such a conspicuous part of the 

 iron-bearing rocks of the Mesabi range. This material has been described 

 in detail in the monograph on this range by C K. Leith." Altered forms 

 of this same material occur in the iron-bearing Grimflint formation at the 

 eastern end of the Vermilion district. The griinerite may very well have 

 been derived from this material by a simple process of dehydration, or it 

 may have been produced from an iron carbonate by silicification, as it was 

 in the Marquette district, as described by Van Hise." Indeed, since, as is 

 concluded later, on page 191, iron carbonate was the original rock of the 

 iron-bearing formation, it is presumed that the griinerite was for the most 

 part formed by the silicification of the carbonate. 



In those rocks in which the griinerite occurs no traces have been found 

 of the peculiar oval and globular structures so characteristic of the Biwabik 

 and Gunflint rocks. The absence of these rounded bodies is not, however, 

 conclusive evidence that they did not originally exist. These rocks have 



« The Penokee iron-bearing series of Michigan and Wisconsin, by C. E. Van Hise; Mon. U. S. 

 Geol. Survey Vol. XIX, 1892, p. 283 at seq. The Marquette iron-bearing series of Michigan, by C. E. 

 Van Hise; 5lon. U. S. Geol. Survey Vol. XXVIII, 1897, p. 402. 



6 The Mesabi iron-bearing district of Minnesota, by Charles Kenneth Leith: Mon. U. S. Geol. 

 Survey Vol. XLIII, 1903, p. 101 et seq. 



'•The ^Marquette iron-bearing series of Michigan, by C. E. Van Hise: Mon. U. S. Geol. Survey 

 Vol. XXVIII, 1897, p. 367. 



