448 THE CRYSTAL PALLS IROK-BEARING DISTRICT. 



angular bowlders evidently derived from the Groveland formation are found 

 in the zone of magnetic disturbance, but no outcrops have been discovered. 

 There can be little doubt that these disturbances roughl}^ outline the position 

 of the Groveland formation in the axial region. 



Except in Michigamme Mountain, the most elevated point of the dis- 

 trict, the Groveland formation is not topographically prominent. In the 

 Fence River area it produces a more subdued and somewhat lower-lying 

 surface than the underlying formation, but the difference is slight and is of 

 little moment in comparison with the confusing effects of glaciation. 



FOLDING AND THICKNESS. 



In the Fence River area there is no reason to suppose that the Grove- 

 land formation contains within itself minor folds of any importance. Our 

 knowledge of its attitude is supplied almost wholly by the magnetic obser- 

 vations, and these indicate that it has a general eastward dip like the under- 

 lying members of the succession. Here and there it may be divided into 

 two or more parts by sheets of intrusive material, and also may be slightly 

 crumpled, but on the whole it must be regarded as a single persistent sheet, 

 having a general eastward dip. 



At Michigamme Mountain the Groveland formation caps the hill in a 

 well-marked syncline, the axis of which runs northwest and southeast. 

 The structure is distinctly shown by the attitude both of the ferruginous 

 rocks and of the underlying phyllites. At the Interrange exploration half 

 a mile south, a secondary embayment of the same syncline, but more open, 

 is found. These are the only folds of the Michigamme Mountain area 

 sufficiently deep to include the iron-bearing rocks. The thickness of the 

 formation can only be guessed at, as no complete section is exposed, and 

 the data for determining its uipper limit are decidedly shadowy. The mag- 

 netic observations indicate a breadth of from 400 to 600 feet, and as in the 

 Fence River area it is certainly much thinner than the two lower forma- 

 tions, its thickness may be approximately^ 500 feet. 



PETROGRAPHICAL CHARACTERS, 



In general aspect the iron-bearing formation in this area is strikingly 

 like that of the Felch Mountain range, and all the varieties there found are 

 represented here also. It is therefore unnecessary to repeat the detailed 

 descriptions already given. By way of broad comparison, however, it may 



