UPPER HURONIAN. 379 



shapes to much of the Clinton ore. All stages of the alteration of this green ferrous 

 silicate rock to the ferruginous cherts and iron ores are to be observed. Scarcelj^ a 

 slide of the cherts does not show some traces of the granules. The alterations have 

 been for the most part characteristic of surface conditions and have consisted in the 

 decomposition of the ferrous silicate, the oxidation of the ferrous iron to the hydrated 

 hematite form, and its segregation from the silica. Where metamorphosed by the 

 Keweenawan gabbro the alteration of the granules has consisted in the development 

 of a variety of amphiboles, including actinolite, griinerite, cummingtonite, and 

 perhaps others, of which griinerite is the most abundant, and the partial oxidation of 

 the ferrous iron to the magnetite form. 



Ill the Grunflint formation the rocks very commonly have structural 

 characters indicating their development from ferrous silicate granules in 

 the manner characteristic of the metamorphism by the gabbro — that is, 

 traces of the granular structures still remain; but the characteristic min- 

 erals are magnetite and the amphiboles resting in a chert matrix. In 

 addition to the rocks that give a good indication of the kind of rock 

 from which they were produced, there are others tliat give no such clue. 

 They are without characteristic structural features. We know that fer- 

 ruginous carbonates form a part of the iron-bearing formation, and it is 

 presumed that some of these metamorphosed products have been derived 

 from such carbonates. It is impossible, however, to give any quantitative 

 estimate of the relative abundance of the ferruginous carbonate and ferrous 

 silicate rocks; so that we can not say which of these has been most impor- 

 tant in furnishing material for the rocks of the Gunflint formation. 



In general, the least metamorphosed of the Grunflint rocks are thin 

 bedded and consist of bands of nearl}'- pure chert alternating with cherty 

 and granular quartzose bands containing varying percentages of iron car- 

 bonate, bands of jasper and magnetitic chert, and others consisting of quartz 

 as a basis with actinolite and griinerite crystals, with which minerals are 

 always associated more or less ferruginous carbonate, magnetite, hematite, 

 and limonite. A description of the least altered Gunflint beds lias been 

 given by Irving and Van Hise in their monograph on the Penokee iron- 

 bearing series." The cherty ferruginous carbonates occur in better develop- 

 ment just outside of the Vermilion district in Canadian territory, on the 

 north shore of Gunflint Lake, than in the Vermilion district proper. The 



aMon. U. S. Geol. Survey Vol. XIX, 1892, pp. 260-268. 



