MAGNETIC OBSEEVATIONS. 



367 



In the case also of a single formation which stands on edg'e and has 

 been split by the intrusion of a sheet of eruptive rock parallel to the bed- 

 ding planes, the magnetic observations will often show two parallel lines, 

 which, at the extremities of the eruptive rock, where it wedges out, merge 

 into one. 



In general, therefore, two parallel magnetic lines which represent two 

 distinct formations preserve their identity, and do not pass into each other; 

 when, however, they represent the same formation, they will often come 

 together if followed far enough. The principles which have already been 

 applied to the analysis of simpler cases are useful in discriminating among 

 the three cases of convergence. 



I. PITCHING SYNCLINES. 



Let us first consider a pitching synclinal fold, which is represented in 

 plan and by successive cross sections in fig. 21. It is evident that on the 

 lines of traverse along Sections 

 I and II the deflections of the 

 needle will observe the usual 

 sequence for two parallel belts, 

 the details depending upon 

 separation and depth of cover- 

 ino- while on lines alonsr Sec- 

 tions III, IV, and V the phe- 

 nomena will be those caused 

 by a single belt of magnetic 

 rock. Also, on account of the 

 rise in the axis, the south poles 

 of the rock are brought continually nearer the surface on these successive 

 cross sections, and therefore the two components of the rock force will be 

 smaller for each traverse than for the one preceding. Since the magnetic 

 material comes to an end at A, it is no longer true that there is as much 

 magnetic material on one side of these sections as on the other. Conse- 

 quently the horizontal component due to the pull of the rock does not 

 become zero at anj^ point along these sections, but for every station has a 

 positive numerical value and acts in the general direction in which the syn- 

 clinal pitches. At the station in the plane of symmetry of the fold this 



Fig. 21 . —Plan and cross sections of a pitching syncliue. 



