SOUDAN FORMATION. 177 



The Vermilion district, therefore, appears to be one of the best regions 

 in the workl to ilhistrate complex folds, or folding in two directions at right 

 angles to each other, and the formation that best exhibits this folding is 

 the Sondan. This is due to the very marked banding of that formation, 

 by means of which the position of bedding is readily determined, and to 

 the fact that for the most part it does not take on any secondary structure. 

 Furthermore, it frequently is found in contact with the Ely greenstone, 

 which also gives the pitch of the cross folds. 



This remarkabl}^ complex folding partly explains the distribution of 

 the Soudan formation with reference to the FAy greenstone. Naturally, 

 where the formation is thick it is found along the boi'der of the greenstone. 

 However, since upon the major folds are superimposed secondary and 

 tertiary folds, numerous patches of the jasper occur in the greenstone. 

 Moreover, because of the cross folding, these patches may be very narrow 

 at one place, widen out very rapidly so as to make a thick formation, and 

 again narrow. When the extraordinary complexity of this folding is 

 understood one has only to premise an erosion extending to different 

 depths in the Soudan formation before the Lower Huronian was deposited 

 in order to see how in the greenstone the jasper may range in size or extent 

 from patches a few feet in width and length, to the great continuous forma- 

 tion about Tower and Ely. Moreover, sxich premise full}' explains the 

 extraordinary variation in width of the jasper belts at some places and their 

 persistency and uniformity at others. 



Occasionally there is associated with the iron formation and inter- 

 banded with the jasper some bands of slaty material. In places the 

 amount of this slaty material is so great that where folding has taken 

 place a slaty cleavage has developed in these layers. This cleavage, 

 however, does not pass through the bands of iron oxide or chert. These 

 bands with the slaty cleavage afford excellent opportunities for making 

 observations upon the relations of cleavage to the direction of pressure. 

 In these bands this development of slaty cleavage is seen to obey the laws 

 of slaty cleavage, as explained by Van Hise." PI. VII, a representation of 

 a specimen taken from the folded jaspers, shows this cleavage so clearly 

 that textual explanation is scarcely needed. 



"Principles of North American pre-Cambrian geology, by C. E. Van Hise: Sixteenth Ann. 

 Rept. U. S. Geol. Survey, Pt. I, 1896, pp. 363-369. 

 MON XLV — 03 — —12 



