5^0 Prof. Riicker. On the, Magnetic Permeability of 



limestone and Silurian rocks, which are bent upwards on their flanks. 

 The collection of specimens was composed entirely of the crystalline 

 rocks: it was begun at the extreme north end of the range, and 

 continued as far south as the Herefordshire Beacon, a distance of 

 about 5 miles. In most cases two specimens were taken at each spot, 

 and in the table these are indicated by the same number followed by 

 different letters. 



As the susceptibilities of the rocks varied so considerably, Professor 

 Judd was good enough to have sections made of some of those which 

 differed most widely. The following report made by him shows a 

 satisfactory agreement between the indications of the induction 

 balance and microscopic examinations : 



Specimen. 



K. 



Remarks. 



llo 



126 



-00139 



113 



30 



12 



zero 



Large amount of magnetite, well crystallised; much 



pyrites. 

 Eather smaller quantity of magnetite, and in smaller 



crystals. 

 Magnetite, small in quantity, and sporadically dis- 



tributed. 

 Magnetite, very small in quantity (possibly only 



titanoferrite present). 

 Magnetite, very small in quantity (possibly only 



titanoferrite present). 



As was to be expected, basic rocks proved to be the most strongly 

 magnetic ; but it is well at once to emphasise the fact that powerful 

 permanent magnetisation affords no proof of high permeability. 

 Thus a specimen of rock from the Peak of the Island of Ascension, 

 kindly supplied to us for examination by Captain Creak, P.R.S., 

 strongly attracted and repelled the pole of a compass needle. Its 

 susceptibility, however, was only moderately large, being '00122. 



We have collected in the following table the results of our measure- 

 ments on basic rocks : 



