UPPER HURONIAN OF FELGH MOUNTAIlsr RANGE. 423 



little or no chalcedony, the silica being- crystallized quartz, while the latter 

 have a great deal of chalcedonic silica. Also the former contain small 

 amounts of detrital material, which the latter generally lack, but the essen- 

 tial difference between them is one of degree of crystallization only. 



If the silica of the.Mesabi cherts had originally crystallized entirely as 

 quartz, or if after passing- through the stage of mixed chalcedony and quartz 

 it had subsequently crystallized as quartz, there would be no essential 

 difference between the iron formations of the two districts. 



There are, then, at least two possible forms in which the iron and silica 

 of the Grovelaud formation may liave been deposited originally, as indicated 

 by the conclusions of observers who have studied the similar iron-bearing 

 formations in other districts of the Lake Superior i-egion in which these 

 formations are less altered than here. ' Either of these forms — namely, a 

 chert}' iron carbonate, as on the Gogebic range, or a glauconitic greensand, as 

 on the Mesabi range — coiild give rise, under the action of vigorously oxid- 

 izing waters, to rocks of the mineralogical composition of those in question, 

 and since no trace of either original form has been found in the Groveland 

 formation the choice between them may perhaps be regarded as still open. 

 My own opinion, based on the microscopic structure which, as I intei'pret it, 

 shows that the Grroveland formation was in the beginning largely made up 

 of rounded particles having the same general form as the glauconite grains 

 of the Mesabi range, is that the iron and silica were originally present 

 largely in the form of glauconite. 



SECTIOX VIII. THE MICA-SCHISTS AND QUARTZITES OF THE UPPER 

 HUROjVIAN SERIES. 



Through the eastern part of sec. 32, T. 42 N., R. 28 W., and entirely 

 across section 33, runs a belt of mica-schists and thin-bedded ferruginous 

 quartzites which seem to have unconformable relations with the fonnations 

 just described. These rocks are seen on the west in a cut in the North- 

 western Railway in the SE. J of the NE. ^ of sec. 32. At the western end 

 of this cut the strike is northwest and the dip northeast at an angle of about 

 35°. At the eastern end there is a decided bending in the strike to a more 

 nearly east-and-west direction, and the bedding surfaces carry striations 

 which dip east at an angle of 10°, all indicating that these outcrops prob- 

 ably lie on the south limb of a gently eastward-pitching synclinal fold, and 



