386 THE VERMILION IRON-BEARING DISTRICT. 



some detail and has brought forward evidence in favor of the hypoth- 

 esis that they originated by metamorphic action of the gabbro on tlie 

 Gunflint formation. Grant bases his conclusions as to their mode of 

 origin on the following facts: The magnetite in the Gunflint beds is, as 

 shown by the analyses quoted above, nontitaniferous, whereas that of 

 the gabbro is titaniferous; the large amount of quartz in these beds 

 could not possibly be derived in such quantity from the crystallization 

 of the gabbro magma; feldspar is absent, whereas it is, of course, an 

 essential constituent of the gabbro itself A further fact, which should 

 be considered as evidence against the view that these iron-bearing beds 

 with the bands of ferromagnesian minerals are a contact or border 

 facies of the gabbro, and as favoring the hypothesis that they are an 

 exomorphic contact product of the gabbro — the explanation which is 

 believed to be the correct one — is the coarseness of the beds in comparison 

 with the recognizable border phases of the gabbro itself. These iron- 

 bearing rocks range from medium- to coarse-grained rocks. In general, 

 they are coarser than the border phase of the gabbro. Such a condi- 

 tion is anomalous. Ordinarily the contact is the finer grained the farther 

 it occurs from the main mass of the igneous rocks. If this were interpreted 

 as a contact phase of the gabbi'o, here we would have next to the main 

 mass of gabbro a relatively fine- to medium-grained gabbro and then 

 this coarse-grained facies, which in places is made up of bands of 

 coarse-grained pure quartz and the other bands mentioned. The original 

 rocks from which the iron-bearing rocks, and eventually these rocks, 

 were derived, judging from analogy with the correlated iron-bearing 

 formation of the Mesabi district, are supposed to have consisted largely 

 of chert with a hydrous ferrous silicate, that which occurs in the green 

 granules, with which is associated more or less iron, calcium, and mag- 

 nesium carbonate. From rocks of this composition it is easy to see that 

 the coarse-grained rocks, consisting of quartz, magnetite, olivine, horn- 

 blende, augite, and hypersthene, might have been derived by sim))lo 

 recrystallization, without presuming any transfer of material from the 

 gabbro. We know that farther west in the district, where the gabbro lies 

 in contact with the Lower Huronian slates and conglomerates, it has 

 metamorphosed them extremely, producing in them secondary ferro- 

 magnesian silicates, hypersthene, hornblende, biotite, and augite, with 



