MINERALOGY AND PETROLOGY. 951 



Siderite.] 



observations, accompanied by microscopical study of thin sections, made at a point 

 about one mile west of Gunflint lake (S. E. J sec. 24, T. 65-4), and of the taconitic 

 structures seen about Gunflint lake. The former locality is represented by the fol- 

 lowing sketch. The rock seen in the isolated hill at the right consists of sideritic 





FIG. 54. PORTION OF THE ANIMIKIE CONTAINING IRON. 

 S. E. % sec. 24, T. 65-4, near Gunflint lake. 



slates, more or less magnetited, constituting the iron-bearing member of the Animikie, 

 passing upward into a flinty breccia of iron-bearing beds. This breccia descends 

 with the dip to the railroad track (No. 1897), where it also is underlain by unbrec- 

 ciated slates. " It is from five to six feet thick and is composed of Animikie slate 

 and quartzyte, some of the pieces being over two feet long." The cement of this 

 breccia is a greenish fibrous matter, largely actinolitic and also sideritic, and 

 weathers to a rusty surface in the same manner as the sideritic rock No. 312. Indeed 

 it cannot be questioned that the breccia No. 312 (plate VI) was derived from a 

 stratum in or near the bottom of the Animikie, as represented by the above figure. In 

 making an examination of the cement of this breccia, which cement grades into the 

 iron-bearing (sideritic) member of the Animikie, it was found to assume the characters 

 of an igneous rock (Nos. 2052 and 2053), in which, while siderite still exists, yet cum- 

 mingtonite and devitrified glass abound. Asa iava it seems to have given birth on cooling 

 to many nodules of more crystalline rock matter which, on the weathering away of the 

 intervening matrix, stand out on the surface as black balls in a manner like the balls 

 in a surface trap seen on Grand Portage island (No. 544). These balls now consist 

 almost wholly of cummingtonite or some other amphibole,and the intervening matrix 

 which surrounds the balls is a partly devitrified basic lava or chloritic zirkelyte. 



It appears, further, that while the ores, siderite and magnetite, are found in all 

 parts of this breccia, the carbonate is abundant in the matrix of the breccia and the 

 oxide in the fine or flinty portions which form the angular pieces in the breccia. In 

 other words, the carbonate formed later than the oxide, and in such free access of 

 carbonic acid that, in some cases, nearly the whole rock is composed of siderite. 

 There must have been also a free leeching process possible in the concentration of 

 the elements of the lava to siderite, in order to have removed the silica and its bases, 

 while in the concentration to the usual ingredients of a devitrified basic apobsidian 

 the environment in some way restricted the free access of carbonic acid. 



