44 



Report IS. A. A. AdvancivMkst of ScirNX"!:. 



intensity ;iie given : their vuidisturbed direction is indicated by a 

 dotted line ; the actualh^ deteiinined directions are shown by continuous 

 lines. 



A glance at fig. 1 shows how markedly the disturbance in that 

 district — whatever the cause may be— atfects the distribution of hori- 

 zontal intensity. Had there Ijeen no disturlmnce the course of the line 

 along which the intensity has the value -17(300 would be approximately 

 that given to tlie dotted line 1)eginning a little to the south of Port 

 Elizcxbeth and passing in a north-easterly direction towards Port 

 Alfred and East London. The effect of the disturbance is to cause this 

 line to deviate far to the west, running north of Coeniey and Glen- 

 connor as far as Wolvefontein, then turning back on itself to the north 

 of Alicedale and Grahamstown. The effect on the numerical value is 

 to make it considerably less than otherwise it would have been. The 

 result would be the same were a mass of matter — (jii tlie wholes -nega- 

 tively magnetised intruded in this part of the country. 



In fig. '2 the liiK^s of ecpuil dip are shown. The undisturbeil course 

 of these lines is approximately the same as the line fi'oui Klipplaat to 



rL<i Z i^howinc Unti cf ecu.il at,p 



XJ-£ Lt'f 



Witmoss. Jt will be, noticed that in this case also the lines passing 

 near Alicedale and Cirahamstown are deviated considerably from tlie"- 

 naturally undisturbed course. Tht' effect on the value is in tliis 

 instance an increase in the value of thfe dip, since as we go west the 

 value of the <lip increases, and the disturbance in this case is eijuivalent 

 to pushing the lines of ecjual dip- which in the undisturbed condition 

 would be in the more easterly part — into a regiim rounrl (grahamstown 

 and Alicedale, considerably to the west. This increase in the value of 

 the dip is in accoi'dance with the sujjposition made abov*' that in this 

 pai-t theiT is an intrusion of negatively magnetist-d matter. 



'J'here is finallv to be considei-ed what can bf dfMivcfl fioin the 



