MAGNETIC OBSEEVATIONS. 351 



deflections both of the horizontal and dip needles caused by the same rock 

 should be less in amount, ceteris paribus, where that rock dips at a low 

 angle than where it dips at a high angle. 



In the course of the field work certain peculiar deflections of the 

 needles were encountered in traverses across rocks dipping at moderate or 

 low angles. These were not thoroughly understood at the time, but the 

 cause was believed to be connected with the angle of dip of the rock. For 

 example, it was found along traverses crossing certain north-and-south- 

 striking rocks, which were known to 'have a westward dip that may have 

 been either high or low, that the two points of maximum deflection of the 

 horizontal needle were not situated at equal distances from the point of no 

 deflection betweeir them, but that the distance of the western maximum 

 was much the shorter. It happens in this I'egion that no east-dipping rocks 

 occur which are so far removed from other magnetic formations as to be out 

 of range of their possible influence, but, so far as they go, traverses across 

 these showed that the nearer maximum was situated on the eastern side of the 

 point of no deflection. It therefore seemed probable that the cause of the 

 inequality in the distances from the zero point to the maxima was the dip 

 of the rock, and that the dip was in the direction of the nearer maximum. 



If the magnetic formation has a surface width z= h, is uniformly buried 

 to the depth 7^, and dips at the angle z/, then, if A ^ tan ^, it may be 

 shown that the horizontal and vertical components at any point P, the hori- 

 zontal distance of which from the lower edge of the formation is x, are 

 given by the following equations: 



H' A2 , h^-\.^^ , 2A 



~c -1-f A^ '^^h^J^(x-hy ' 1-f A^ 

 tan-'^ ^y— -^ — tan 1- — —I (10) 



— zr2 < tan - — tan — = — 

 CO ( II h 



, A T h' + x' , ^ 



+ 1-^2 log 7.2 , Z M 2 + 



1_|-A2 ^h^^(x—hy ' l + A'' 

 tnn-i ^ (1 + A') - A (1 +^) _ tan-^ h-^\ h-x , . . 



