IRON-BEARING MEMBER OF THE ANIMIKIE 507 



hornblende, and occasionally gninerite, and cummingtonite. These 

 rocks, like the rocks from which they are derived, are beautifully banded, 

 the separate bands being composed of quartz, or of magnetite, or of 

 silicates, or of a mixture of any two or more of the minerals. The tex- 

 ture of the different b>ands is granitoid as a rule, but in some cases one 

 mineral, notably the hypersthene, is developed into large plates, which 

 include the other minerals in a *poikilitic manner. These contact rocks, 

 which on account of the abundance of magnetite and olivine are some- 

 times called the olivinitic iron ores, are so peculiar that attention has 

 been called to them by several geologists. As they have been referred 

 either to the Lower Huronian or to the Upper Huronian (or Animikie) 

 or to the gabbro (Keweenawan) itself by different geologists, it seems 

 best to present briefly the evidence proving their real age.* This 

 seems best also because, on account of the abnormal mineral composi- 

 tion of these rocks, there may be some hesitancy about accepting their 

 original sedimentary nature. 



The facts which show the Animikie age of the rocks in question may 

 be stated as follows : 



1. The outcrops, while not absolutely continuous, are of sufficient 

 frequency to allow the gradual tracing of the quartz-magnetite-amphibole 

 slates along their strike into these highly crystalline rocks. 



2. In following these outcrops a gradation in crystalline and mineral 

 nature can be traced from the slates to the rocks in question. 



3. Certain structural features of the region, the details of which can 

 not be given here, are most readily explained by assuming the Animikie 

 age of the olivinitic iron ores. 



4. The rocks underlying and those overlying the olivinitic iron ores 

 are the same as those underlying and overlying the less altered slates — 

 that is, the stratigraphic positions of the two are similar. 



5. The rapid alternations of bands of different compositions in the two 

 rocks are exactly similar. 



6. The anomalous mineral composition of these highly crystalline 

 rocks can be explained as the direct result of the mineral composition 

 of the Animikie rocks from which they were derived. The peculiar 

 mineral character consists in the presence of so many and such large 

 amounts of minerals which are not commonly found in metamorphosed 

 sediments, but which are characteristic of basic igneous rocks — that is, 



* Of the geologists who have studied the rocks in question the writer understands that J. M. 

 Clements, A. H. Eli'tman, C. K. Leith, C. R. Van Hise, and H. V. Winehell agree with the interpre- 

 tation here given, namely, that these rocks are parts of the iron-bearing member of the Animikie 

 metamorphosed by the gabbro. N. H. Winehell, while formerly holding this interpretation, now 

 regards these rocks as of Keewatin age. W. S. Bayley has described them, or at least large parts 

 of them, as peripheral phases of the gabbro. 



