568 THE MARCJUETTE IKON-BEAlilNG DISTRICT. 



rocks of the Marquette series have, as it Avere, been pushed over the rocks 

 of the Basement Complex on the north and south sides of the area. The 

 outer Algonkian formations are closely plicated into a series of overturned 

 and in some places isoclinal folds, the dips on both the north and south 

 sides being toward the center of the trough and away from the Basement 

 Complex (fig. 1). These secondary east-west folds are usually only dis- 

 covered by tracing the contact between two formations. In passing, on the 

 plain of denudation, toward the center of the trough, one first passes from 

 a lower fonnation to a higher formation; then apparently above this he 

 may again find the lower formation; and this infolding in extreme cases is 

 repeated several times (Atlas Sheet XVIII). However, on the whole, the 

 great syncline controls, so that finally the inferior formation is not again 

 found. In passing inward toward the center of the Marquette area the 

 minor folds become more open in their character, and in the center have 

 a symmetrical shape (fig. 1). We then have a structure in this district 

 in some respects like the fan-shaped folds of the Alps, with, however, the 

 great difl^erence that the area as a whole is a synclinorium instead of an 

 anticlinorium; that is, the oldest rocks are found on the outside of the 

 fan-shaped areas and the youngest rocks in the center of the area. The 

 significance of this type of fold, which I have named an abnormal syncli- 

 norium, is fully discussed by me in another place.^ 



The overfolds on the outer borders of the Marquette belt are best dis- 

 covered in places where, as a consequence of the pitch given by north-south 

 folds, an east or west termination of the formation appears. A few of the 

 best illustrative areas may perhaps be mentioned. West of Goose Lake, in 

 sees. 22 and 23, by reference to the maps (Atlas Sheets IV and XXXV), 

 it will be seen that there are four Ai-chean areas, separated by Algonkian 

 rocks both in an east-west and north-south direction. Their separation in 

 an east-west direction is due to the secondary north-south folding, and 

 then- separation in a north-south direction is due to the isoclinal northwest- 

 southeast overfolds. The latter folds are the closer; consequently the 

 majority of the strikes are northwest and southeast, and the dips are mostly 



'Principles of North American pre-Cambrian geology, by C. R. A'an Hise: Sixteenth Ann. 

 Eept. U. S. Geol. Survey, Part 1, 1896, pp. 612, 615-621. 



