832 THE GEOLOGY OF MINNESOTA. 



[Gabbro. Mica schist . 



their outlines on the weathered surfaces resemble the balls that weather out from 

 the lava sheet seen on Grand Portage island (No. 544), which is more or less glassy. 

 There seems more reason to refer this rock to the igneous parts of the Animikie than 

 to the fragmental. Its intimate connection with the iron-bearing member of the 

 Animikie, into which it passes, is very significant. The spherical masses in this 

 rock, consisting largely of actinolite, are comparable to the rosettes of actinolite in 

 No. 2052. 



Two additional sections of this rock were made by Dr. Otto Kuntze, one (a) 

 from the balls, in such a manner as to cut two or three balls, showing at the same 

 time some of the intervening rock-matter, and (b) from a part of the rock forming 

 a thin layer (one-fourth of an inch) not embracing the balls, but immediately in 

 contact with the balls. The former shows the intervening rock-matter is isotropic, 

 with a few isolated grains that are evidently rusty siderite, greenish and apparently 

 resulting from devitrification and alteration of a basic glass; the latter (b) is in 

 part of the same material as that intervening between the balls but also contains a 

 notable amount of siderite, and apparently fibres of some amphibole. The glassy 

 part of (b) is in detached masses, which are so crowded with irony oxides as to be 

 nearly opaque. These detached glassy masses also contain a few minute translucent 

 spots that, so far as determinate, appear to be of siderite. 



It hence appears that the balls are globules originating in a basic lava at the 

 time of cooling, and the material that embraces them is a devitrified part of the 

 same lava. It is also plain that this lava has become charged with iron, chiefly in 

 the form of siderite. It appears also that this altered lava contains darker masses 

 of opaque finer lava which are charged with oxide of iron rather than the carbonate. 

 Thus the iron-bearing member of the Animikie at Gunflint lake is intimately asso- 

 ciated with and grades into a basic devitrified lava. Compare Part III. N. H. w. 



No. 2054. GABBRO. 



Same rock mass as No. 2051. 



Ref. Annual Report, xxiv, page 27. 



Meg. Presenting large, dark, crystal surfaces. 



Mic. The rock, though from the same mass as No. 2051, can here hardly be said 

 to be ophitic, and deserves hence the name gabbro. It is not porphyritic with earlier 

 feldspars, but all the feldspars and the augite seem to have formed nearly simulta- 

 neously. Magnetite forms large and conspicuous tabular crystals. Pyroxene is 

 changed to uralite. One section. 



Age. Cabotian. N. H. w. 



No. 2055. MICA SCHIST (with cord-ierite. ) 



From the west end of Loon lake (south of Gunflint); face of an obliquely ascending cliff which faces north, 

 at two-thirds the distance toward the top. 

 Ref. Annual Report, xxiv, page 27. 



