358 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



west of nortli, the eastern station shows a higher dip than the corresponding 

 station on the west. Grenerally stated, then, the stations on that side of the 

 magnetic rock on which the angle between the strike of the rock and the 

 hne of traverse is obtuse show greater dip angles than the corresponding 

 stations on the side on which this angle is acute. As the angles of dip are 

 represented graphically by a continuous curve, this is the same thing as 

 saving that the dip curve is steeper on the side of the acute than on that of 

 the obtuse angle. 



. These facts are easily explained by the following considerations. The 

 vertical components tend to lower the needle, and would carry it to a ver- 

 tical position except for the action of the horizontal forces, which tend to 

 keep it horizontal. At any station on the acute-angle side of the magnetic 

 belt the resultant of the two horizontal components is larger than at the 

 corresponding station on the obtuse-angle side, the two being represented 

 by the longer and shorter diagonals of a parallelogram. Since the vertical 

 forces are the same at the two stations, it follows that on the obtuse-angle 

 side the angle of di^D must be larger than on the acute-angle side. Or, 

 expressed algebraically, since H,. is the only variable on the right-hand side 

 of equation (5) it is evident that tan a, and therefore a, the angle of dip, 

 increases with a decrease in H,.. 



If the rock dips at an angle less than 90°, these results are either 

 intensified or greatly modified, depending upon the direction of dip. It 

 was shown in the last section that the horizontal component of the rock 

 force is smaller on the dip side. If the strike and dip are both toward the 

 same side of the meridian (e. g., if the strike is northwest and the dip south- 

 west), it is evident that the numerical difi"erence between the deflections of 

 the horizontal needle on the two sides of the rock will be still greater than 

 if the rock were vertical. On the other hand, if the strike and dip are 

 toward opposite sides of the meridian (e. g., if the strike is northeast and 

 the dip northwest), the difference between the deflections on the two sides 

 is less than for a vertical dip, or may even be reversed. 



The deflections of the dip needle in the case of rocks dipping at angles 

 less than 90° are also greatly influenced by the direction of dip. If strike 

 and dip are toward the same side of the meridian, the difference noted 

 above in angle of dip on the two sides of the rock is neutralized, and the 

 dip curve tends to become symmetrical; while if they are toward opposite 



