STRUCTURAL GEOLOGY. 69 



Peripheral phases of the gabhro.] 



sition to the muscovadyte. It appears, therefore, that muscovadyte has alliances in 

 opposite directions, and without questioning here the contention that it is a part 

 of the gabbro, and that the Pewabic quartzyte represents a silicified condition 

 of the gabbro incidental to its peripheral position, it is claimed only that it has also 

 undeniable and direct connection with the Keewatin. 



With this observation as a key a considerable revolution has latterly been made 

 in the interpretation of the iron ores which are associated with muscovadyte further 

 southwest, and they have been assigned to the Keewatin, rather than to the 

 Animikie. Their peculiar petrography is due to the gabbro metamorphism, and 

 that will be discussed under the heading 



So-called peripheral phases of the gabbro. There have been mentioned by 

 different geologists a series of modifications in the gabbro, the same being supposed 

 to be confined to the margins of the mass, and due to contacting on the older rocks. 

 These are all connected with the muscovadyte mentioned above, and are, as it 

 appears, only conditions of that rock. In some cases the gabbro is said to become 

 granulitic, and fine-grained, with development of considerable quartz, hypersthene 

 and biotite; it is said to become non-feldspathic, making a pyroxene-olivine rock, or 

 peridotyte, and to be charged, in other places, with titanic iron ore. It exhibits 

 great variations, not only in the relative proportions of the usual minerals, but in 

 the successional order in which they were generated. There is so much variety that 

 the characters of the rock show almost endless change, and no special classification 

 seems possible. The minerals concerned are olivine, quartz, magnetite, labradorite, 

 hypersthene, biotite, augite, diallage. The details in some instances are given in 

 the chapter of this report devoted to the special petrography of the crystalline rocks; 

 and they have been presented in considerable minuteness by Prof. W. S. Bayley.* 

 Quartz and magnetite are present in greater quantities when the locality furnishing 

 the specimen examined is from the immediate vicinity of some of the ore lenses 

 above alluded to as closely connected with the rock muscovadyte. They are some- 

 times wanting, and then the rock presents some of the varieties usually referred to 

 "granulitic gabbro," but there is no distinction that will hold, either structural or 

 petrographic, between the granulitic gabbros, with their variations, and the iron- 

 bearing muscovadyte. They are parts of the same variable rock mass, and belong 

 originally in age to the clastic greenstone member of the Archean. It cannot be 

 assumed that the muscovadyte, when free from quartz and magnetite, had a 

 different origin from the same rock containing those minerals, for these minerals 

 show all stages of increase, from zero to ninety-five per cent. When the rock consists 

 essentially of the former it is a vitreous quartzyte, and when essentially of the latter 



* Journal of Geology, vol. ii, p. 814; vol. iv, p. 1. "The peripheral phases of the great gabbro mass of northeastern 

 Minnesota." 



