26 Report S.A.A. Advancement of Science. 



As soon as the mean variation with latitude and with longitude 

 has been found in this way, it is possible to determine at the actual 

 stations what the values of the elements would be had the variation 

 been uniform and not variable as it actually is. The difference 

 between the actual and the values calculated according to this method 

 may be called the magnetic anomaly. It is from these anomalies 

 that the conclusions have been derived. 



The nomenclature adopted is that of Thorpe and Rlicker modified 

 for convenience. 



A Ridge line is a line of country which attracts the South Pole 

 of a magnet, a valley line one which repels it. A peak is a point 

 towards which the anomalous horizontal forces converge. 



Natal-Zululand Ridge Line. — A ridge line extends from the 

 east coast, starting between Umhlatuzi and Ginginhlovu railway 

 stations, passing north-west to a point whose approximate lat. and 

 long, is 31° 30 E and 28° 55' S respectively, then turning south- 

 west, and continuing in that direction to a point a little south of 

 Boston ; there its course changes and passes northwards between 

 Boston and Bulwer, to the west of Fountain Hall, Estcourt and 

 Modderspruit. It then goes east, turning north again near Wasch- 

 bank, and passing to the west of Dannhauser. On this line there 

 is a peak between Umhlatuzi and Ginginhlovu, in a district of maxi- 

 mum vertical force. The next peak is between Boston and Bulwer, 

 approximate lat. 29° 40' S., approximate long. 29*^ 20' W. There 

 another ridge line goes off from the one already referred to and 

 passes to the north of Bulwer and of Underberg. 



The directions of the horizontal disturbing forces at Bulwer, 

 Underberg, Boston and Fountain Hall, point towards this peak. A 

 valley line starts between Modderspruit and Waschbank, and passes 

 south to intersect the first ridge line about Dalton, thence continuing 

 piobably to the east of Kranzkloof. 



Griqualand East and South Natal Valley Line. — Another valley 

 line passes between Rietkuil and Kokstad N.E. by E., south of 

 Richmond, Krantzkloof and the Bluff, and north of Ibisi Bridge, 

 Umzinto and Illwo River; it then continues in a more northerly 

 direction to the east of Mount Morland, Stanger and Ginginhlovu. 



Griqualand East and South Natal Ridge Line. — Another well- 

 marked ridge line passes from a little south of Mount Frere in a 

 north-easterly direction, keeping south of Kokstad, Ibisi Bridge and 

 Umzinto ; there it turns to the north, and is continued east of Illwo 

 River and probably through Durban. 



The Natal-Zululand ridge line runs practically perpendicular 

 to the magnetic meridian from Ginginhlovu to Boston ; its origin 

 is probably due to magnetic matter at no great depth and not very 

 widespread at that depth. 



It has already been stated that a ridge line is one' which 

 attracts a south-seeking pole. Hence when such a region extends 

 roughly east and west, the horizontal intensity to the south of it 

 will be less than the average, and to the north greater. In the case 



