270 THE VERMILION IRON-BEARING DISTRICT. 



the granite of Saganaga Lake (tliey are deiived directly from it) that they 

 were actually taken for that granite, altered, however, and were supposed 

 to have been intruded in thin sheets parallel to the fine-grained beds 

 lying in alternation with them. This was Grant's idea upon his- first 

 visit, when he decided that" the granite of Saganaga Lake was for this 

 reason younger than the adjacent sediments." A subsequent visit caused 

 him to change his view to the correct one, upon recognition of their true 

 relations. The sedimentary characters of these bedded arkoses were noted 

 by N. IL Winchell,* and likewise the resemblance of these arkoses to 

 the granite of Saganaga Lake, the main mass of which lies east of and 

 beneath these sediments, and the relations were interpreted by him as 

 evidence of progressive downward metamorphism, the arkoses having 

 been fused and transformed into the granite of Saganaga Lake. Examined 

 under the microscope, these bedded rocks are seen to be made up of 

 fragments of quartz and feldspar and flakes of mica. None of the minerals 

 are well rounded, but neither do they show the same relationships to each 

 other that the same minerals always exhibit in unquestionable granites 

 Moreover, the well-marked sedimentary banding in them and the gradation 

 from coarser- to finer-grained rocks show that these are without question 

 fragmental rocks or arkoses derived from the granite and consisting of the 

 same constituents as the granite from which they were derived. These 

 fragments of minerals have not been very much worn, and, since they 

 were deposited here through the action of water, have been cemented 

 together so as to form a rock that is strikingly like a granite, especially to 

 one making a superficial macroscopic examination. A microscopic study, 

 and indeed a close macroscopic field study, immediately discloses the 

 characters above mentioned, and shows that they are different from the 

 granite. Excellent exposures of the arkose occur on the portage from Oak 

 Lake to the west bay of Saganaga Lake, and good exposures of the frag- 

 mentals showing distinct bedding may be seen on the north shore of this 

 bay, on the point just east of the portage. After passing along these 

 outcrops of sediments one finally comes to the clearly recognizable typical 

 massive granite-porphyry of Saganaga Lake. These rocks have the 

 distribution shown upon Sheet XVI in the accompanying atlas. 



«Geol. and Nat. Hist. Survey of Minnesota, Twentieth Ann. Kept., 1893, pp. 90-95. ^Loc. cit. 



