676 THE GEOLOGY OF MINNESOTA. 



[Gabbro. Taconyte. Flint. 



Mic. Ophitic, and finely holocrystalline, with secondary calcite. The rock con- 

 tains a few spots, occupied by a greenish-yellow felted substance, which are not areas 

 of magma glass, but amygdules or pseudamygdules filled subsequent to the consoli- 

 dation. The augite is in rather fine particles, but sometimes is cut by the feldspars 

 which are wholly microlitic. One section. 



Aye. One of the Logan sills of the Animikie. N. H. w. 



No. 1275. GABBRO. 



A mile west of the entrance of the river to Gunflint lake, north shore. 

 Ref. Annual Report, xvi, pages 65, 66, 119. 



Meg. Rather medium-grained gabbro. 



Mic. The striking feature in this rock is that the formation of the feldspar* 

 preceded that of the olivine, there being an ophitic relation between them like that 

 characteristic of augite and feldspar in diabase. The feldspar is labradorite-bytownite 

 as shown by extinction of 28 on a section exactly perpendicular to n g as the acute 

 bisectrix. The angitr, which is purplish, is also ophitic toward the feldspars. While 

 this is the general rule, there are still instances in which all three of these minerals 

 appear to have formed nearly cotemporaneously, since these relations are reversed, 

 and augite and olivine crowd upon the boundaries of the labradorite-bytownite. 

 One section. 



Age. Cabotian. 



Remark. It is but rare that this ophitic relation between the olivine and the 

 feldspars in the gabbro is met with. Compare Nos. 757 and 1842, where it is also 

 mentioned. N. H. w. 



No. 1276. TACONYTE. 



North shore of Gunflint lake. 



Ref. Annual Report, xvi, pages 66, 119. 



Meg. Immediately overlain by gabbro (No. 1275). 



Mic. The green glauconitic grains are rounded and at their centres are 

 crystallized into minute actinolites, which also is -somewhat distributed elsewhere. 

 These green bodies lie in the midst of interlocking fresh secondary grains of quartz, 

 which also permeates the green masses in still finer granules. Sometimes a little 

 magnetite in fine powder is scattered through the green substance, but generally not. 

 So far as can be determined the green substance is perfectly amorphous. One section. 



Age. Animikie. \. H. w. 



No. 1277. FLINT. 



North shore of Gunflint lake (Canada). 



Ref. Annual Report, xvi, pages 66, 119 ; Annual Report, xviii, pages 34, 62 ; Bulletin vi, pages 114, 123, 422. 



