MAGNETIC OBSBKVATIONS. 363 



If the two 23arallel formations are not vertical, but dip in the same 

 direction at the same angle, the resulting curves are somewhat different. 

 PI. LXVIII, figs. 3 and 4, show two cases in which the elements are the 

 same, except the depth of covering and the thickness of the intervening non- 

 magnetic material. Here the rocks dip at an angle of 7 1 ° 34 ' . and the width 

 at the rock surface is 6.3 for each. 



In fig. 3, PL XLVIII, where 7i=2 and the width of the nonmagnetic 

 bed is 10.7, and the covering is, therefore, relatively small, the presence of 

 two rocks is distinctly shown by the curves of both components, and the 

 chief result of their interaction is to introduce an additional point of no hori- 

 zontal deflection between them, on each side, of which the horizontal arrows 

 diverge. The positions of the maximum and of the other zero points are 

 hardly disturbed, and consequently the direction of dip is very clearly 

 indicated. 



In fig. 4, PI. XLVIII, where /; — 4 and the formatioias are separated by 

 nonmagnetic material 4.7 wide, there is but one zero point, nearly over the 

 middle of the upper formation, toward which the pointings of the hori- 

 zontal needle converge. West of this are two points of maximum eastern 

 deflection, between which a faint minimum represents the backward pull of 

 the lower formation. 



If the two magnetic formations are parallel in strike, but dip toward each 

 other at equal angles, the resulting curves of the two components are shown 

 in PL XLVIII, figs. 5 and 6. Fig. 5 illustrates the eff'ects on a syncline 

 with steeply dipping sides, the superficial covering being relatively shallow. 

 These conditions result in a point of no horizontal deflection over the mid- 

 dle of the trough with diverging arrows on each side, and besides a point 

 of no horizontal deflection over each rock, toward which the aiTOws con- 

 verge. The positions of the two maximum points for each rock, and of the 

 zero between them, is nearly the same as if the other rock were absent, and 

 conseqiiently the fact that the rocks dip toward each other is clearly 

 indicated by the luisymmeti'ical distances. 



In fig. 6, PL XLVIII, the depth of the rock surface is much greater 

 relatively to the inside distance between the legs of the syncline, and the dip 

 is flatter. In tliis case there are but two points of maximum deflection, one 

 on each side of the syncline, and but one point of no deflection, over the 

 middle of the trough. The maximum points represent the outside maxima 



