OUTLINE OF MONOGRAPH. 25 



abundant type. Certain porphyritic fades of the granite also occur. This granite 

 varies from a normal mica- and hornblende-granite to an augite-granite, and by 

 loss of quartz to a syenite. The Cacaquabic granite is somewhat more interesting 

 than the preceding ones, in that it is one of the rather exceptional augite-soda-granites. 

 The main mass of this granite is developed as a medium-grained gray or pink to red 

 gi'anite, whereas on the periphery of the granite area a finer-grained granite and 

 also a granite-porphj-r}^ facies of the rock are developed. In addition to this there 

 are various granite and granite-porphyry dikes whose immediate relationship to the 

 granite massives already described could not be traced in the field. A section is 

 devoted to a brief description of certain basic and intermediate intrusives of doleritic 

 and lamprophyric character, which bear the same relations to the various adjacent 

 forrhations as do the acid rocks previouslj^ described. 



Chapter V. This chapter treats of the Upper Huronian (Animikie) series. 

 This sei'ies is found in the extreme eastern portion of the district, where it underlies 

 a relatively small area. It is known, however, to have enormous development 

 to the east, immediately beyond the limits of the Vermilion district, and also to the 

 south-southwest, in the adjacent Mesabi district. This Upper Huronian series may 

 be readilj' divided into two facies of rocks that are quite different petrographically. 

 At the bottom of the series occurs an iron-bearing formation known as the Gunflint 

 formation. Above this occurs a great slate-gra3'wacke formation to which the 

 name Rove slate has been given. The Gunflint formation is correlated with the 

 Biwabik formation of the Mesabi district. It has a verj^ limited development in 

 the Vermilion district, and its most interesting phases are especial!}^ well developed 

 in the vicinity of Akelej^ Lake. In general the rocks of this formation have a 

 monoclinal dip to the south-southeast at a low angle, but variations in the strike 

 and dip indicate clearlj^ that the structure is not so simple as it appeal's to be. 

 Minor folds have been traced. Petrographically the rocks of the Gunflint formation 

 are peculiar. Where least metamorphosed, they consist of thin bands of nearly 

 pure chert alternating with cherty and granular quartzose bands containing varying 

 percentages of iron carbonate, bands of jasper, magnetitic chert, and other bauds 

 consisting of c^uartz as a basis with actinolite and griinerite crystals. With these 

 minerals are always associated more or less ferruginous carbonate, magnetite, 

 hematite, and limonite. In these rocks we find developed the peculiar oval, crescent- 

 shaped, and rounded granules which are so characteristic of the Biwabik formation 

 of the Mesabi range — granules made up in their freshest condition of a hydrous 

 ferrous silicate of varying shades of green. These rocks have been extremely 

 metamorphosed by the Duluth gabbro. Where most metamorphosed the iron-bearing 

 Gunflint rocks are composed of coarsely crystalline bands of quartz, of varying 

 width, alternating with coarsely crystalline bands of magnetite ore reported to vary 

 from 1 inch up to 10 or 12 feet in thickness, and of bands of dark-green, brown, or 

 black rocks that consist of combinations of quartz, augite, hypersthene, hornblende, 

 olivine, and magnetite as the principal minerals, but associated occasionally with 



