982 THE GEOLOGY OP MINNESOTA. 



[Structural field relations. 



"The shores of the island in N. W. J^ sec. 6, T. 64-5 W. [i. e., in Gabemichigama lake] are of coarse 

 ijiililiro, except for the southern shore, which is composed of fine-grained, granular Keewatin rock, not stratified 

 as far as seen. In one place the gabbro was seen within ten feet of this rock, and the former still retained its 

 coarse grain. At the southwest corner of the island is the locality figured several years ago. The fragments 

 included in the gabbro show no distinct stratification lines. The gabbro is still of coarse grain, even when in 

 small stringers. The included fragments, represented by No. 847G (which also well represents this rock all along 

 the southern shore of this island), are of various shapes and sizes, and many have rounded outlines." 



The granulitic structure of the rock in these angular fragments is a " peripheral 

 phase," so called, of the gabbro. This shows that that phase is not dependent on the 

 peripheral action of the gabbro mass, as has been presumed. Again, rock No. 2001, 

 a diabase dike, though having been shattered by contact (evidently) on the walls of 

 a fissure, has a distinct angularity in its minerals, indicating that such friction was 

 not the cause of the fineness and roundness of the grains in muscovadyte and noryte. 



The same is indicated by rock No. 8M, which is a finely granular mica schist, pro- 

 duced by metamorphism of the Animikie. It has never been in a fluid state, but 

 shows a similar granular structure, and outwardly resembles some muscovadyte. 



At the south shore of Disappointment lake are many obvious transitions from 

 the coarser gabbro to the finer. In some cases the finer rock might be called noryte, 

 or granular gabbro, containing considerable orthorhombic pyroxene (Nos. 2201, 2202, 

 2203). These transitions are both gradual and abrupt. The rocks are favorably 

 exposed and can be seen for many rods without much hindrance. On a single knob 

 of rock five feet across, standing a little isolated from the main gabbro mass, can be 

 seen on one side a well-developed coarse gabbro, embracing indistinct pebbly forms 

 in smothered outline. Passing gradually across the front of the knob the grain can 

 be seen to grow gradually finer, till, at the opposite side, the rock is quite different, 

 and could be called granulitic gabbro, and embraces distinct pebbly forms. In the 

 field, however, in this instance, this was called pebbly muscovadyte. 



The foregoing is probably sufficient to show that in the field the coarse gabbro 

 and the granulitic gabbro (or noryte, when hypersthene is abundant) pass into each 

 other by insensible gradations. 



It would now be in order to show that the muscovadyte becomes, when less 

 alteration has taken place, the clastic greenstones of the Keewatin. This is easily ascer- 

 tained by examining the greenstone belt about Gabemichigama lake and eastward to 

 Flying Cloud lake. One may follow the greenstone eastward from the former lake 

 by a series of small lakes and portages. It forms a conspicuous hill range which, 

 eastward still further, merges into the Giant's range, and westward it culminates in 

 the Twin peaks, south of Ogishke Muncie lake. The route gradually approaches the 

 northern limit of the gabbro, which here has a more northward trend. Finally, 

 making a more careful examination of this greenstone belt, one finds it presents a 

 decided approach toward muscovadyte (Nos. 1778, 1780, 1781) and really becomes 

 the muscovadyte of the region, the change being very evident on following the 



