360 THE OEYSTAL FALLS lEON-BEAEING DISTRICT. 



diminishing deflections to tlie east or west, depending on the side of the 

 meridian on which this component falls ; and finally, when it becomes 

 insensible, the needle rests again in the meridian. 



In the case of a rock striking east and west, the points at which the 

 horizontal component of the magnetism of the rock has maximum values 

 become indeterminate by the methods hitherto described, from the fact that 

 throughout the traverse the two components act in or nearly in the same line, 

 and the deflections from the local magnetic meridian, therefore, do not 

 indicate the relative strengths at different stations of the horizontal com- 

 ponent of the rock force. 



The dip-needle readings for an east-and-west-striking rock are as fol- 

 lows: At some distance south of the rock the angles are constant at the 

 index error. As the rock is approached, the angles of dip depend upon the 

 depth of burial. If the surface covering is considerable, an increase in the 

 dip angles begins at a considerable distance away, and progresses continu- 

 ously as the magnetic belt is approached. If the rock is near the surface, 

 the dip needle shows either the constant index error or else angles of dip 

 less than the index error for all stations south except those very near the 

 southern margin of the rock. The maximum reading is attained north of 

 the middle plane of the rock, at a distance from it which also depends upon 

 the depth of covering. Farther north the dip angles decrease slowly and 

 are in general greater than at the corresponding stations south. The form 

 of the dip-cui-ve, therefore, shows a steeper slope south of the mag-netic 

 rock than north of it. The reasons for these differences will be evident 

 from the following considerations. 



Let it be supposed, for the sake of simplicity, that throughout the 

 north-and-soiTth traverse the two horizontal components act in the same line 

 in the meridian. At any station south of the magnetic rock they act in the 

 same direction, and their resultant will be their numerical sum. At the 

 corresponding station north they act in opposite directions, and their result- 

 ant will be their numerical difference. The aug'le of dip is given by 



eqiiation (5): 



_V'+ H tan . 



tan a^- 



H,,. 



For the two corresponding stations, V will be the same. The other 

 quantities are all constants except H^. For the south station H^ ^ H' -f H ; 



