368 THE CRYSTAL FALLS lEOFBEARINa DISTRICT. 



component acts parallel to the axis. The direction and amount of the deflec- 

 tion depend upon the direction of strike and pitch of the synclinal. 



Let us suppose, first, that the axis of the synclinal strikes north and 

 pitches north. In this case Section I, in fig. 17, is the most northern, 

 Section V the most southern. 



The traverses along- Sections I and II display the usual phenomena 

 for two parallel belts. East of the eastern limb and west of the western 

 the horizontal needle will be deflected toward the syncline. Between the 

 two limbs there will be at least one j)oint of no deflection, and frequently, 

 depending- upon the relations between the depth of burial and the thickness 

 of the intervening nonmagnetic material, either two other points of no 

 deflection or two zones of retardation, one on each side of this middle zero. 



Along Sections III, IV, and V there will be but one point of no 

 deflection of the horizontal needle, which will correspond with the axial 

 line of the fold. Since this axis is north and south, and so coincides with the 

 magnetic meridian, the horizontal component of the rock force coincides in 

 direction with the horizontal component of the earth's pull, and consequently 

 there is no deflection of the horizontal needle. For other stations east and 

 west of the central station the deflections are toward the west and east, 

 with the usual maximum points. 



The deflections on successive sections south grow smaller, since the 

 angle between the two hoiizontal components progressively diminishes. 

 The relative value of the horizontal component of the rock force also 

 progressively diminishes, since the thinning of the magnetic material due 

 to the rise in the axis of the fold brings the buried north poles into promi- 

 nence. Therefore the deflections of the horizontal needle after the mag- 

 netic rock has been left behind very soon become imperceptible. 



The dip needle deflections for the northern sections, I and II, reach 

 their maximum values at the usual points, over the central zero and near 

 the outside zeros or points of retardation. For the southern sections the 

 dips grow less, since the horizontal restoring couple due to the rock has 

 always a positive numerical value, and also because the vertical component 

 of the rock force diminishes, -owing to the nearness of the south poles. As 

 the section approaches the limits of the magnetic material the points of 

 maximum dip become less and less clearly defined, and the dip curve passes 

 into an irregular line, slightly depressed below the line of no deflection. 



