380 



THE VERMILION IRON-BEARING DISTRICT. 



following analyses, made by Mr. Thomas M. Cliatard, of the United States 

 Geological Survey," give the composition of some of these carbonates. 



Analyses of iron-hearing ca/rhonates. 



VII. 



Silica 



Titanic oxide 



Alumina 



Iron sesquioxide .. 



Iron protoxide 



Manganese oxide.. 



Calcium oxide 



Magnesium oxide . 

 Carbon dioxide . . . 

 Phosphoric acid... 



Iron sulphide 



Water at 110° 



Water at red heat . 



58.23 



Trace. 



.06 



5.01 



18.41 



.25 



.38 



9.59 



5.22 



.03 



.14 



.07 



2.01 



46.46 



Trace. 



.24 



.64 



26.28 



.21 



1.87 



3.10 



19.96 



.13 



.11 



.07 



1.15 



23.90 



None. 



.07 



.44 



10.65 



.28 



22.25 



8.52 



32.42 



Trace. 



.13 



Total. 



99.40 



100. 22 



VII (specimen 10575) , iron carbonate from Gunflint beds on eastern side of outlet of Gunflint Lake 

 on international boundary; VIII (specimen 10598), from same beds, but from northern side of Gun- 

 flint Lake; IX (specimen 10588), ferriferous carbonate from another part of north side of Gunflint Lake. 



Under the microscope most of the above-mentioned rocks show nothing 

 of especial interest. "With these one finds cherty ferruginous rocks which, 

 when examined under the microscope, are of interest, since they show the 

 relationship of these rocks to the less altered normal rocks of the iron for- 

 mation of the Mesabi range, concerning which a brief statement was made a 

 few pages back (p. 378). These rocks consist of rounded areas of fine- 

 grained crystalline silica and limonite and rarely hematite — corresponding 

 exactly in shape to those granules which have been mentioned above — 

 which lie in a groundmass of crystalline silica (see PI. XII, A). These 

 areas are surrounded by a border of limonite, hematite, or these oxides — 

 most commonly limonite — are more or less uniformly distributed through- 

 out the granule or occasionally concentrated at the center. Within the 

 border crystalline silica sometimes predominates, although scattered through 

 it there is more or less limonite, sometimes actinolite and griinerite and a 

 ferruginous carbonate. The iron oxide has frequently a definite arrange- 

 ment. It has accumulated in aggregates at the centers of fibrous quartz 



«Mon. U. S. Geol. Survey Vol. XIX, 1892, pp. 191-192. 



