950 THE GEOLOGY OP MINNESOTA. 



[Siderite. 



itic condition of the ore of the Mesabi Iron range. The general question of the 

 origin of the rocks as such will find place in the next succeeding chapter. 



The writer finds that in certain parts of the Mesabi Iron range the iron of the 

 iron-bearing member is largely sideritic. This condition prevails about Gunflint 

 lake and continues to a greater or less extent westward as far as to the vicinity of 

 the Mallmann mine in T. 60-13 W. It gradually changes, and the oxides of iron are 

 substituted for the prevalent carbonate. The change is not complete, for some 

 carbonate is found at the western end of the range (No. 158S) in the same manner 

 that the oxides also are found about Gunflint lake. With this exception, and with 

 the further exception that the iron ingredient is more abundant toward the west, 

 the iron-bearing member is, itself, essentially a uniform terrane. There is this 

 further difference, viz.: Toward the west the Fokegama quartzyte generally sepa- 

 rates the iron-bearing rocks from the Archean, while toward the east, i. e., at Gun- 

 flint lake, the iron-bearing member lies sometimes directly on the Archean, and no 

 representative of the Pokegama quartzyte has ever been seen there in situ. It 

 appears, therefore, that such a variation from siderite to hematite must be due to 

 some variation in the nature of the conditions attending the formation of the rock 

 itself, such that oxide of iron was formed more abundantly in one part of the state, 

 while in the same terrane, in another part, a carbonate of iron prevailed. 



This carbonate of iron about Gunflint lake makes small rock masses, and it has 

 been called sometimes limestone (Nos. 312 and 1310). One of the first samples 

 collected (No. 312) is represented by the photograph, natural size, seen in plate VI. 

 It here embraces many angular masses of flint, or chert, which the writer regards as 

 devitrified glass of volcanic origin, originally of basic composition, but now composed 

 largely of quartz. Such flint is abundant about Gunflint lake, not only as fragments 

 in this siderite, but in horizontal thin beds that are intimately interstratified with 

 some Animikie "slate," and taconyte (Nos. 1276, 1277). In other words, the flint, the 

 volcanic glass and the taconyte are intimately associated, and their elements are 

 variously involved in the siderite (No. 1289). 



Again, the sideritic rock (plate VI) exhibits sometimes a parallel, streamed 

 (or sedimentary?) structure seen in No. 1 of the plate referred to, and in No. 158S, 

 and in other parts this structure is replaced by a more confused sideritic rock. The 

 siliceous flinty pieces are placed in all positions in this confused part of the mass, 

 but in the streamed parts they tend to parallelism with the direction of the stream- 

 ing. This structure may be due to an original lava under flow, carrying many pieces 

 of obsidian previously hardened. 



That the sideritic rock was once a lava and is simply changed by becoming fer- 

 rated and carbonated or is a product of leeching from it, is indicated by a series of 



