KEWEENAWAN GABBRO 505 



labradorite ; augite, which is often diallagic; olivine, and magnetite, 

 with occasionally hypersthene, biotite. hornblende, and minor accessory 

 minerals. In general, the mass is of fairly uniform composition. Varia- 

 tions, however, take place mainly in three directions : First, by increase 

 of feldspar the rock becomes ananorthosite ; second, by increase of feld- 

 spar and olivine a forellenstein is formed ; third, by increase of magnetite 

 masses of titaniferous magnetic iron ore originate. Along its northern 

 limit the gabbro, while at times assuming a finer grain, usually preserves 

 its distinctly coarse grain and granular texture to its contact with the 

 underlying rocks.* 



Although this large mass of rock has been regarded as the earliest 

 flow or series of flows of Keweenawan time, there has been a recent trend 

 of opinion toward the conclusion that the gabbro is not of extrusive, but 

 more probabl} 7 of intrusive nature, and that it has the form of a lacco- 

 lite. There is, so far as known, no separation of this enormous mass 

 into distinct beds or flows similar to the other flows of Keweenawan age, 

 nor have any of the characteristic textures of surface rocks been reported 

 from the gabbro proper. It must be said, however, that the southern or 

 upper side of this mass has not been studied in detail. Moreover, the 

 marked metamorphosing effects which the gabbro has had on the under- 

 lying rocks along its northern border — effects described below — also point 

 to the non-extrusive character of the mass. 



Contact Metamorphism produced by the Gabbro 

 general character of the metamorphism 



This metamorphism is very noticeable, and consists of a partial or a 

 complete recrystallization of the adjacent rocks. Complete recrystalliza- 

 tion is the rule near the contact, and in places this extends 500 feet from 

 the contact. A partial recrystallization is at times noticeable for a dis- 

 tance of a quarter of a mile or more from the present surface limits of 

 the gabbro. 



In northwestern Cook county, Minnesota, the gabbro is in contact with 

 the uppermost or graywacke-slate member of the Animikie. In going 

 westward this igneous mass cuts across the strike of the Animikie, touch- 

 ing the black-slate and the iron-bearing members, then strikes the Kee- 

 watin, and finally reaches the Archean. In the rocks of each of these 

 series certain characteristic contact phenomena can be seen. In Lake 

 county the gabbro, except for isolated patches of the iron-bearing member 



*The petrography of the gabbro has been described by W. S. Bayley (Journal of Geology, 1893, 

 vol. 1, pp. 688-716), and by N. H. Winchell and the writer (Geological and Natural History Survey of 

 Minnesota, Final Report, vol. 5, in press). 



