68 THE GEOLOGY OF MINNESOTA. 



[Muscovadyte. 



possible from a petrographic point of view. It became evident that the name had 

 been applied to fragmental rocks and to igneous rocks, and that in the light of the 

 close alliance of muscovadyte with the gabbro it would be best to restrict the term 

 to a "peripheral phase" of the gabbro. With such interpretation Muscovado lake 

 was named, which lies well within the gabbro area, because the most of its shores 

 are composed of this rock. About this time the muscovadyte seen at Gabimichi- 

 gama lake was traced through a slow gradation into the "greenstone" which occurs 

 a few miles further east, and this transition has been observed since at other points. 

 In several other cases the muscovadyte was noticed to be quite siliceous, and, acquiring 

 a distinct sedimentary structure, was seen to change into ordinary greenwacke. 

 Through greenwacke, therefore, it is linked with graywacke and the whole fragmental 

 series. It was noted also, at numerous places, that, where the gabbro in bulk 

 approached the eastern extension of the Animikie iron range, as that range was then 

 understood and defined, the rock embracing the ore frequently was a form of mus- 

 covadyte, and that, in extreme metamorphism of the iron-bearing rock, the curious 

 association was seen of the minerals olivine, quartz, magnetite. To these were added, 

 usually in subordinate but varying proportions, biotite, diallage, augite, hypersthene 

 and sometimes cummingtonite. Sometimes olivine was poikilitically related to 

 quartz and magnetite, embracing both. Sometimes magnetite served the same office, 

 and sometimes biotite, but usually hypersthene was latest to form, and hence sur- 

 rounded all the other minerals in large crystals and in favorable situations, as in 

 vugs and veins, hypersthene reached the size of several inches. Prior to this the 

 name Pewabic quartzyte was applied to the supposed base of the Animikie, where it 

 exhibited this curious petrography; and under that name the supposed iron-bearing 

 base of the Animikie was traced westward along the northern periphery of the gabbro 

 to Birch lake and southwest from Birch lake to where the real Animikie appears. 

 But the most interesting and important observation was made in the fall of 1897 at 

 Disappointment lake. The iron ore, which occurs on the south shore and has been 

 referred to the Animikie, is embraced in this rock muscovadyte. It is at this 

 point and along the southwestern shores of the lake that had been observed the 

 transition of muscovadyte into mica schist. This transition was again observed. 

 This mica schist is conspicuously conglomeratic and occurs extensively about Snow- 

 bank lake, about a mile further northwest. The muscovadyte is also conglomeratic, 

 losing this character, however, where it embraces the ore. There is hence a link 

 which cannot be broken binding this ore with the older formation (Archean), and it 

 hence bears exactly the same relation to the Keewatin as the jaspilyte seen in the 

 conglomerate of the Upper Keewatin between Moose and Wood lakes. The gabbro 

 is immediately adjacent and has an irregular superposition which resembles a tran- 



