346 THE VERMILION IRON-BEARING DISTRICT. 



chlorite flakes, and particles of iron ore. Grant" has described cordierite 

 occurring in similar- rocks on Grobbemichigamma Lake, to the southwest 

 of the area above referred to. Very possibly the material forming- these 

 white .spots is cordierite, but no conclusive proofs of this were obtained. 



THICKNESS. 



It has already been stated that in portions of the Vermilion district 

 the Knife Lake slates show a very great width. From this fact alone one 

 not taking into consideration the intensely plicated condition of these rocks 

 and hence the possibility, or even the certainty, of more or less repetition, 

 might be led to infer that these slates are of enormous thickness. 



This folding, of course, points to a probable reduplication of the beds. 

 While the slate area can by no means be described as homogeneous, 

 nevertheless it is true that clearly recognizable key rocks are wanting. 

 Consequently there must be numbers of anticlines and synclines which it 

 has not been possible under the existing conditions to recognize. These 

 facts render it exceedingly difficult to make any authoritative statement as 

 to the thickness of this series. At one place, however, the structure is 

 fairly simple. Between Ogishke Muncie Lake and Gobbemichigamma 

 Lake, two of the large lakes in the eastern part of the district, there are 

 two small lakes. Fox and Agamok lakes, which occupy the low ground. 

 Noi-th of these there is a high ridge occupied by Archean greenstone with 

 the Ogishke conglomerate on its southern flank. South of this string of 

 lakes there is a second ridge of Archean greenstone, forming a very marked 

 topographic feature. Between these two ridges we find the Knife Lake 

 slates, showing a number of good exposures. Traverses across this area 

 from north to south gave at first a series of south dips, gradually becoming 

 flatter and flatter, imtil the bottom of the syncline just south of the lakes 

 mentioned as lying in the bottom of the depression was reached, where the 

 flattest dips were found. Continuing across as we ascend the south ridge, 

 this succession of dips is repeated in reverse order — i. e., the steepest dips are 

 nearest the ridge, and the dips are to the north. Moreover, it was possible 

 to observe a repetition of the rocks. The slates here evidently occupied a 

 distinct syncline, and moreover this syncline seems to be a simple one. 



«Geol. and Nat. Hist. Sui-vey of Minnesota, Final Eept., Vol. IV, 1899, pp. 657-658. 



