366 THE CRYSTAL FALLS lEON-BBAEING DISTEICT. 



SECTION VII. THE INTERPRETATION OF MORE COMPLEX STRUCTURES. 



The existence of two parallel belts of magnetic rocks may be accounted 

 for g-eologically in more than one way. They may represent two distinct 

 formations occurring at different horizons in the same series, or they may 

 represent the same formation either duplicated by folding or faulting or 

 separated into two parts by the intrusion of a sheet of igneous rock parallel 

 to its bedding. Since, then, two magnetic lines, the existence of which has 

 been established by observation, may have more than one interpretation, 

 the discrimination of these cases, when possible, is of special importance. 



The question whether any given case belongs to the first of these cate- 

 gories can generally be settled only by following the lines of attraction into 

 a district which affords a geological section across the formations involved, 

 or by the occasional outcrop of the rocks which give rise to the disturbances, 

 in which case lithological resemblances or differences, the relations to other 

 formations, and the observed structure will decide the matter one way or 

 the other. In the special case in which either or both lines can be followed 

 completely round an anticlinal dome or a synclinal basin, which of course 

 can only rarely happen, the question would be settled affirmatively, even if 

 outcrops were entirely lacking. 



In the other instances the magnetic observations themselves often give 

 means of discrimination, even when the outcrops are so few or so obscure 

 as to be in themselves indecisive. It is characteristic of the folds in the pre- 

 Cambrian rocks of this region that the axes are not usually parallel with 

 the horizon for long distances, but are often inclined to it; in other words, 

 when followed for greater or less distances they pitch. The outcropping 

 edges of any formation involved in an anticlinal or synclinal fold which has 

 been cut by a plane of denudation will be parallel to each other wherever 

 the axis of the fold is horizontal, but will approach each other where the 

 axis is inclined. In an anticlinal fold they converge in the direction in 

 which the axis sinks, while in a synclinal they converge in the direction 

 in which the axis rises. If the formation is a magnetic one, conformably 

 placed between beds of nonmagnetic character, the magnetic lines to which 

 the outcropping edges give rise will therefore run parallel to each . other 

 when the axis is horizontal and will converg-e or diverge when the axis 

 pitches. The convergence or divergence takes place gradually, since the 

 angles of pitch usually are not large. 



