688 tHE GEOLOGY OP MINNESOTA. 



[Limestone 



The foregoing describes one of the slides. Another slide, having the same 

 number, has much less of quartz, or none, but granular augite wad feldspar , with a 

 liberal sprinkling of magnetite, compose the rock. Here frequently the finer mag- 

 netite grains are clustered near the centres of the feldspars. This is a typical musco- 

 vadyte. Two sections. 



Aye. The quartzyte is supposed to be of the age of the Animikie, and the other 

 seems to be a part of the gabbro. The first section described shows a transition from 

 the muscovadyte to quartzyte, in a manner represented by Bayley (Nineteenth Annual 

 Report), which requires that both slides be referred to the gabbro. 



[NOTE. In the light of the latest conclusions as to the nature and relations of 

 the muscovadyte to the gabbro, such curious combinations as this are explained by 

 referring both the quartzyte, the muscovadyte and the magnetite, with all their asso- 

 ciations, primarily to the Keewatin, the quartzyte and magnetite appearing abundant 

 when the Keewatin happened to contain a jaspilyte lode which suffered the meta- 

 morphosing action of the gabbro revolution, the normal gabbro itself being the last 

 term in the series of rock genesis, formed by complete refusion where the original 

 Keewatin contained no jaspilyte. | N. H. w. 



No.- 1310. LIMESTONE in-it/i tuff'r > 



North shore of Gunflint lake. 



Ref. Annual Report, xvi, pages 72, 73, 121 ; Bulletin vi, pages 115, 121, 129, 130, 422. 



Meg. Breccia of flint, etc., in limestone. 



Mir. The sections consist of the well-known granular limestone, in the main, 

 but contain chloritic and finely siliceous portions, evidently of foreign nature, which 

 remain mainly dark between crossed nicols, and which, in one case, show a rhyolitic 

 structure, with quartz and magnetite. It appears, therefore, possible, if not probable, 

 that it is of the nature of a lapillus from an adjoining volcanic source. The rock 

 also contains more siderite than calcite. Two sections. 



Age. Animikie (probably the iron-bearing member). 



Remark. In several instances it has been stated that there were evidences of 

 local volcanic action in the rocks of the bottom of the Animikie in the vicinity of 

 Gunflint lake and North lake, but in nothing has this been indicated more strongly 

 than in this slide. From this it is reasonable to infer that the flinty, angular frag- 

 ments which characterize this limestone in the vicinity of Grimflmt lake are generally 

 of the nature of devitrified volcanic glass. 



Two additional sections were made of this breccia, cutting some of the siliceous 

 masses, and the black strings, but showing nothing decisive as to the nature and 

 origin of these inclusions. The siliceous masses are apparently not of clastic struc- 

 ture. Special conditions are responsible, apparently, for the carbonate of iron here, 

 in the iron- bearing member, instead of the usual oxide. N. H. w. 



