MAGNETIC OBSEEYATIONS. 361 



for the north station, H^zrH — H', where H and H' are, respectively, the 

 horizontal components of the magnetism of the earth and of the rock, as 

 before. The numerator of the right-hand side of the equation will be the 

 same for both stations, while the niimerical value of the denominator will 

 be less for the north station than for the south. Consequently tan a, and 

 therefore a., will be greater for the north station. 



For great depths of superficial covering, however, these differences 

 become almost imperceptible, owing to the fact that H' is so small that 

 H,. is essentially the same at the two corresponding stations. The tendency, 

 therefore, as h increases is for the dip curve to become symmetrical. 



In the special case in which H' — — H, H^= 0, and the dip needle stands 

 at 90°. This can only take place north of the rock, and may, depending 

 on the strength of H', be found at two stations, one on either side of the 

 station at which H' is a maximum. At the same stations the horizontal 

 needle is not acted on by any unbalanced force, and rests indifferently in 

 any position. 



The dips less than normal which are often observed at stations south 

 of a magnetic rock which lies very near the surface are also easily under- 

 stood by a reference to equation (5). At these stations the resultant pull 

 of the rock is so nearly horizontal that the vertical component V is very 

 small in compai'ison with the horizontal component H'. If V is a negligible 

 quantity, equation (5) becomes 



tan a z= ^t — tt7 • ^^^ ^ 

 ri-j-Il 



In such cases the angle of dip is therefore less than the index error. With 

 north or south dipping rocks, where V is negative, tan a becomes negative 

 when V >H tan 0. 



3. TWO PARALLEL MAGNETIC FORMATIONS. 



The cases so far considered have involved only one belt of magnetic 

 rock, which has been assumed to have a uniform dip in one direction, or, in 

 other words, to be a monocline. In practice, however, owing to complexi- 

 ties of structure and other causes, which will be considered hereafter, it 

 frequently happens that two or more approximately parallel belts are found 

 within the range of one another's influence. Under these circumstances 



