Rocks and Regional Magnetic Disturbances. 507 



respectively, and if k, k^, and k 2 are the susceptibilities of the rock 

 and the two liquids, k may be calculated from the formula 



aj, K i 



Xn /Co ~~ K 



* = *i + ;r:i:T x (V-*i). 



My thanks are due to Professor Judd, F.R.S., for the large number 

 of rock specimens which he has placed at my disposal. Nearly all 

 which have been used have been provided by him. This aid has 

 been especially important from the fact that the attention which he 

 has paid to the geology* of the West of Scotland has made his 

 collections rich in basaltic rocks gathered from that district, 

 which is within the area of the recent magnetic survey, and is re- 

 markable for the magnitude, not only of the local, but also of the re- 

 gional, magnetic disturbances of which it is the seat. He has also been 

 good enough to have sections made of a number of the rocks examined 

 by the induction balance. Some of the results thus obtained are 

 referred to below, but we hope to extend this part of the enquiry in 

 the immediate future. 



PART I. On the Magnetic Susceptibility of Rocks. 



The apparatus used for determining the absolute susceptibilities of 

 the mixtures is shown in fig. 2. 



Primarily it consisted of a magnet attached to a mirror which was 

 delicately suspended in a glass case by means of a quartz fibre. Two 

 large solenoids wound upon glass tubes were placed at equal distances 

 east and west of the needle. A smaller tube, which could be filled 

 with any of the mixtures, was arranged so as to slide into and out of 

 either solenoid. The deflections when the tube was inserted first 

 into one solenoid and then into the other were noted, and from these 

 the susceptibility of the mixture could be calculated when the strength 

 of the field in which the magnet was suspended was known. The 

 necessity for the determination of this datum was obviated by also 

 deflecting the magiiet by a small solenoid, the moment of which was 

 calculated from the number of coils, the length, and the strength of 

 the current flowing through it. In the figure, M represents the 

 magnetometer, and SS the two large solenoids into which the tube 

 containing the liquid is capable of sliding. The ends of the solenoids 

 projected for some 7 or 8 cm. beyond the ends of the tube, which was 

 always brought up to the same position within the solenoids by means 

 of fiducial marks. The sensitiveness of the magnetometer was adjusted 

 by a control magnet placed above it, acting so as to reduce the earth's 



