506 U. S. GRANT — CONTACT METAMORPHISM OF BASIC IGNEOUS ROCK 



of the Animikie, is in contact with .still older rocks, and in eastern Saint 

 Louis county the Animikie again emerges from beneath the gabbro.* 



GBAYWACKE-SLATB MEM HER OF THE ANIMIKIE 



Where the gabbro is in contact with the uppermost member of the 

 Animikie there is a marked contact zone, in which the sedimentary 

 rocks are much hardened and have a massive aspect, appearing at a dis- 

 tance like igneous rock, while close to the gabbro they are completely 

 recrystallized. Near the contact the more silicious strata have been 

 turned into completely crystallized quartzites. The less pure silicious 

 rocks consist of granitic aggregates of quartz, feldspar, biotite, and raus- 

 covite in varying proportions, with occasionally cordierite. It is to be 

 noted that the minerals common to the gabbro, especially pyroxene and 

 olivine, which are so abundant in the metamorphosed rocks of the iron- 

 bearing member of the Animikie and of the Archean, described below, 

 are absent from these silicious rocks at the gabbro contact. A short dis- 

 tance from the contact the recrystallization becomes less complete, al- 

 though biotite and muscovite have been developed. 



BLACK-SLATE MEMBER OF THE ANIMIKIE 



The known exposures do not show any contacts of the gabbro on the 

 next lower or black-slate member of the Animikie, but in a few places 

 somewhat removed from the contact and in the immediate vicinity of 

 sills of diabase cordierite has been developed in these slates. A frag- 

 ment referred to these black carbonaceous slates and included in the 

 gabbro is now a completely crystallized aggregate of quartz, graphite, 

 and biotite, with large plates of beautifully pleochroic hypersthene, 

 which incloses the other minerals in a poikilitic manner. 



IRON-BEARING MEMBER OF THE ANIMIKIE 



It is in the next lower or iron-bearing member of the Animikie that 

 the most interesting contact phenomena are exhibited. This member 

 is the one in which, farther west, the immense hematite deposits of 

 the Mesabi range occur. The original rock is regarded as a glauconitic 

 greensand,in which there is more or less iron carbonate. This rock has 

 been altered to a quartz-magnetite-am phibole slate, the amphibole being 

 in the form of actinolite, griinerite, cummingtonite, and hornblende. 

 This quartz-magnetite-amphibole slate, commonly known in the Lake 

 Superior region as actinolite schist, has been profoundly changed by 

 the gabbro, and the resulting rock is a coarse grained aggregate of quartz, 

 magnetite, olivine (which is frequently fayalite), hypersthene, augite- 



*Cf. (.col. and Nat. Hist. Survey of Minn., Final Rept., lh'.t;». vol. I. pis. 67, 68,09. 



