26 THE OEYSTAL FALLS IRON-BEAEING DISTRICT. 



the Areliean is overlaiu b}- the sedimentaries of the Paleozoic, the Cam- 

 brian, and the Sihirian. The connecting Crystal Falls rocks are west of 

 this Archean dome. 



In the Marqnette district the essential structural features have been 

 shown ^ to be a great east-west synclinorium, upon which more open north- 

 south folds are superimposed. At the western end^ of the district the 

 superimposed north-south folds become close, and the Republic trough is a 

 close fold with an axis in an intermediate position. In the adjoining Crystal 

 Falls district there are also two sets of folds with their axes approxi- 

 mately at right angles to each other. The closer folds are represented by 

 the great anticline in the central part of the district. This anticline has its 

 axial plane trending west of north and south of east, and the axis plunges 

 down both at the north and south ends. 



The more open set of folds at right angles to the above set, is repre- 

 sented by the Crystal Falls syncline, with its axis striking to the south of 

 west, and plunging west. Farther south the axes of the folds become much 

 closer and more nearly east and west, thus nearly according in direction 

 with the close folds of the Menominee district. Thus the structural features 

 of the Crystal Falls district merge. into those of the Menominee district, 

 which joins the Crystal Falls district on the southeast, where the great 

 structural feature is a synclinorium similar to that of the Marquette, but 

 with its axis trending north of west and south of east. 



A glance at PI. Ill will show the presence in the eastern part of the 

 northern half of the district of an oval-shaped mass of Archean, and, nearl}^ 

 surrounding this, a number of rock belts. 



The Archean ellipse is 11 miles long and 3 miles wide on the average. 

 The rocks are mainly granite and gneiss. They are cut by rather infre- 

 quent acid and basic dikes. 



Immediatelj' surrounding the Archean is a quartzose magnesian lime- 

 stone formation, to which the name Randville dolomite has been given.^ In 

 the eastern half of the district described by Sm3^th, where more numerous 

 exposures are found than occur in the western half, the formation has an 

 estimated thickness of about 1,500 feet* Not only are the exposui'es 



' Mon. XXVIII, cit., p. 566 et seq. 



^Loc. cit., p. 570. 



3 See Part II, Chapter IV, by H. L. Smyth, p. 431. 



<See Part II, Chapter IV, Sec. Ill, p. 433. 



