356 THE CRYSTAL FALLS IRON-BBAEING DISTRICT. 



More general information as to relative depths of burial is also given 

 by the dip curves. It is easily seen that where the superficial covering is 

 small the vertical component of the rock force must remain small, except 

 immediately over the rock. This condition is, therefore, indicated by steep 

 slopes in the dip curves. On the other hand, where the depth of covering 

 is considerable, the vertical component increases slowly and steadily, begin- 

 ning at stations at a distance from the rock, and the resulting dip curve 

 approaches the maximum with gentle slopes. 



7. SUMMARY. 



(1) The strike of a magnetic rock is given by the line joining the points 

 on successive traverses, at which the horizontal needle is not deflected from 

 the local meridian between the converging arrows, or at which the dip 

 angles are a maximum. When the rock is vertical, this line lies in the middle 

 plane of the rock and fixes its position. It may be called a line of mag- 

 netic attraction. 



(2) The dip of a magnetic rock is toward the nearer horizontal 

 maximum. 



(3) The thickness of the magnetic formation must, if buried, always 

 be less than the distance between the maximum points. 



(4) Where the superficial cover is not very great, a change in the dip 

 of a magnetic rock from moderate or high angles to low angles is attended 

 with a rapid decrease in the values of the horizontal component, with a 

 corresponding decrease in the deflections of the horizontal needle. 



SECTION VI. APPIiICATIOJiTS TO SPECIAL CASES. 



In the preceding section certain g-eneral conclusions have been estab- 

 lished with regard to the relative positions of the stations at which the 

 horizontal and vertical components of the force of a inagnetic rock have 

 maximum and zero values. The deflections produced by these components 

 from the positions which the magnetic needles assume under the action of 

 the earth's force have maximum and zero values at the same stations at 

 which the components have maximum and zero values, and therefore the 

 conclusions as to the relative positions of these points are true for any 

 angle of strike. But certain numerical relations between the deflections 

 depend upon the orientation or strike of the magnetic formation and upon 

 the direction of dip, and these will now be considered. 



