MAGNETIC OBSERVATIONS. 



865 



§ y^ . s 



6 -5 ^ R 



the zone of retardation and a larger over the point of no horizontal deflec- 

 tion, which correspond respectively to the two magnetic rocks. 



If the two formations are parallel in strike, but dip away from each 

 other, the curves of the horizontal and vertical components for different 

 angles of dip and different relations of thickness and depth of covering are 

 shown in figs. 19 and 20. In fig. 19 the formations are widely separated, 

 h is relatively small, and the angles of dip are equal and low; the inter- 

 action of the two rocks therefore extends over a narrow zone only, and the 

 curves of the components clearljr indicate the presence of two formations 

 and the direction of dip of each. In fig. 20 the anticlinal is so truncated 

 that magnetic material occupies the whole space on the rock surface 

 between the outer boundaries 

 of the two formations. The 

 angles of dip are equal, and are 

 higher than in the preceding 

 case, while the depth of cover- 

 ing is relatively much greater. 

 The horizontal component is 

 zero in the axial plane of the 

 anticlinal, and has maximum 

 values at two points, one on 

 each side of the zero. The vei-- 

 tical component is a maximum 

 at one point, also in the axial 

 plane. The deflections pro- 

 duced by these conditions could 



not be distinguished in practice from those produced by a single vertically 

 dipping formation. 



In general, -therefore, when two magnetic formations lie within range 

 of each other's influence, the deflections are determined by the relative 

 magnetic strengths of the two rocks, by their distance apart, by their strike 

 and dip, and by their depth of burial. It is evident that for certain given 

 relations among these factors the special cases above described will occur, 

 and it is found that they really do occur in practice. For other relations it 

 is not possible to make a general statement either as to the number or the 

 position of the maximum and minimum points. 



Fig, 20.— Truncated anticlinal fold with steeply (lipping limbs. 



