MAGNETIC OBSERVATIONS. 



371 



Therefore, in general, the buried north poles of the mag-netic formation are 

 not brought nearer the surface; and this, together with the fact that the 

 material continues on in the line of the axis, produces characteristic phe- 

 nomena in the magnetic sections. 



These phenomena, the details of which can be easily followed out for 

 an}^ given direction of pitch, and need not here be described, show in gen- 

 eral two lines of attraction merging into one, which continues in the same 

 direction as a strong line, showing, as it is followed, the peculiarities due to 

 an increasing depth of burial. The points of maximum deflection of the 

 horizontal needle continue to 

 separate from each other on suc- 

 cessive sections. The dip curve 

 shows a definite maximum 

 closely corresponding, except for 

 due east-and-west strikes, to the 

 point of no horizontal deflection. 

 Where the axis of the fold is so 

 oriented that these points can 

 be established, they indicate the 

 nature of the fold. If the strike 

 is east and west, in which case 

 they become indeterminate, the 

 continuity of the line and its 

 very gradual decrease in power may give an excellent basis for inference 

 as to the nature of the fold. 



^^7r\ 



CROSS SECTIONS 



-Plan aud cross sections of a pitching anticline. 



3. FORMATIONS SPLIT BY INTRUSIVES. 



When a sitigle formation has been split into two by the intrusion of a 

 nonmagnetic igneous rock, there are in the area in which the igneous rock 

 occurs two parallel magnetic formations, which give rise on cross traverses 

 to phenomena the precise features of which depend upon the strike and dip 

 of the formation and upon the relation which the width of the intruded 

 mass bears to the depth of burial. To describe these would involve a mere 

 repetition of what has been said before. Such intruded masses always have 

 a definite limit in length, which is usually not very great. When the limits 

 are reached, the two parallel lines pass into a single line which continues on 



