MAGNETIC OBSEEVATIONS. , 359 



sides of the meridian, the difference is increased. It is to be noted that the 

 iniiuence on both instruments of the direction and angle of dip of the rock 

 becomes weakened with an increase in surface covering. 



2. THE MAGNETIC ROCK STRIKES EAST AND WEST. 



When a vertically dipping magnetic rock strikes east and west, or 

 nearly so, the traverse lines must be run north and south so as to cross it 

 as nearly as possible at right angles. In approaching such a belt from the 

 south the instruments give little warning. The readings of the horizontal 

 needle show either no deflections, or else very slight deflections, from the 

 magnetic meridian. Past the middle of the formation the horizontal needle 

 is strongly deflected, often through an angle of 180°, so that it may point 

 due south. But as the magnetic rocks having this strike which were 

 encountered in our work were not deeply buried, and had also quite irregular 

 upper surfaces, generally the needle pointed either east or west of south on 

 account of the weight which the nearness to the surface gave to the adja- 

 cent material, either from the irregular distribution of magnetite or from 

 the protrusion of small masses above the general level. Continuing north, 

 the horizontal deflections gradually diminish and eventually disappear. 



The behavior of the horizontal needle is explained in the same way 

 as in the preceding cases. The position of the needle at any station is deter- 

 mined by the resultant of the horizontal components of the two forces — the 

 earth's force and the rock force — that act upon it. South of the magnetic 

 rock these two components act in the same direction and essentially in the 

 same line, since the magnetic meridian practically coincides with the true 

 meridian. The resultant, therefore, is equal to their sum and coincides with 

 them in direction, and consequently there is no deflection. North of the 

 magnetic rock the two horizontal components act in opposite directions, and 

 when they are in the same line the needle takes up its position in the direc- 

 tion of the greater, which determines that of the resultant; when H' is 

 greater than H (which often happens near and north of the rock) this direc- 

 tion is due south. When the two components do not act in exactly the 

 same line, the needle will point east or west of south at an angle which 

 • depends on the angle between the two forces and their ratio. 



Still farther north the horizontal component of the rock force dimin- 

 ishes rapidly and we consequently first pass through a zone of large and 



