MAGNETIC OBSERVATIONS. 369 



The reasons for this are, of course, obvious from what has been said 

 above. 



In the case of a synchnal fold pitching south. Section I (fig. 17) 

 becomes the most southern, Section V the most northern, line of traverse. 

 Sections I and II present the same general phenomena as before for both 

 needles. In Sections III, IV, and V the horizontal component due to the 

 rock has a positive value for all stations as before, but in this case acts in a 

 generally opposite direction to that of the horizontal component of the 

 eai'th's force. Therefore, on these sections we should expect at first greater 

 deflections of the hoi'izontal needle, which would diminish rapidly as the 

 sections approached and passed beyond the northern limit of the magnetic 

 material, but which, for corresponding sections, would be greater than for 

 the northerly pitching fold. The deflections of the dip needle would also 

 be greater for the same reasons. 



For a synclinal pitching west. Section I is the most western, Section V 

 the most eastern, traverse. In this case, along I and II, the deflections of 

 the horizontal and dip needles are dependent for their details upon the ratio 

 of depth to distance of separation, but if far enough to the west will show 

 clearly two belts of magnetic material, approximately parallel, and striking 

 approximately east and west. For Sections III, IV, and V, in which the 

 distance of separation is either nothing or relatively small, the phenomena 

 will indicate but one belt. On these sections, owing to the fact that the 

 horizontal component of the rock pull is nowhere zero and has everywhere 

 a general westerly direction, the deflection of the horizontal needle will be 

 westerly throughout, and will reach a maximum north of the east-and-west 

 axial plane of the material, where the angle which it makes with the magnetic 

 meridian is more than 90°. 



In accordance with the general principles stated in the discussion of a 

 single belt with the same strike, the angles of dip are in general smaller 

 south of the syncline than north, and the maximum dip is reached at a 

 point north of the axial plane. On sections farther to the east, near the 

 limits of the rock and beyond them, the dip-needle deflections, like those 

 of the horizontal needle, rapidly diminish and soon become imperceptible. 

 These facts are well shown in fig. 22, which represents a series of north- 

 and-south traverses across the Oroveland basin, the limits of which are 

 defined by outcrops on the eastern side. 



MON XXXVI 24 



