MAGNETIC OBSEEVATIONS. 353 



distant maximum, and, foui-thly, that the distance between the maximum 

 points is nearly the same for the inclined rock as for the vertical. 



In PI. XL VII, fig. 2, the constants have the same numerical values as 

 before, except li, which now —4 instead of 2. The rock is thus buried to 

 twice the depth of the former case. The same conclusions are true for this 

 case as for the first. The zero point is still nearer the upper edg-e of the 

 rock, and the maxima are farther apart. ! 



Let it next be assumed that AzzO.5 (or that the rock dips at an angle 

 of about 26° 34'), /j = 2, and a=6. These data lead to the curves of 

 PI. XLVII, fig. 3, in which, as in the case of the rock ot higher dip, the 

 maximum points are imsymmetrical to the point of no deflection, the nearer 

 lying on the dip side. 



In PI. XLVII, fig. 4, we have a rock of the same thickness and dip at 

 a depth /tii:4; and the same conclusions hold true. 



From these four curves, which represent formations dipping at high 

 and moderately low angles, and buried to depths which are in the one case 

 small and in the other great, relative to the thickness, it is probably safe 

 to draw the following general conclusions: 



(«) The direction of dip of a magnetic formation is toward the nearer 

 and (for north-and-south-striking rocks) the numerically smaller maximum. 



(&) The point of no deflection between the converging maxima is not 

 situated over the middle plane of the formation, but is nearer the upper 

 edge. But with increasing depth and diminishing angles of dip, this point 

 may pass beyond the upper edge. 



(c) With slightly inclined rocks, for moderate depths of surface cover- 

 ing, the disturbances are spread out over a much wider zone on each side, 

 and the maxima are less sharp, particularly the maximum on the dip side. 

 Under these circumstances irregular and anomalous deflections would be 

 expected in practice, as will be seen in the following sections. 



(d) The curves of the vertical component show maximum values near 

 the zero value of the horizontal component only in the case of the rock of 

 high dip. In the case of the rock of lower dip, the vertical component has 

 a negative value, or is directed upward over a wide zone on the side of the 

 rock opposite to the dip side. Over this zone the readings of the dip needle 

 will be less than normal, or even negative if V >> H tan 9. This is in 

 accordance with the facts of observation. 



MON XXXVI 23 



