282 



MAGNETIC EOCKS OF CORNWALL. 



rock that liad been in the walls of Colan church since the year 

 1336, and it yet retains noteworthy affinity for the magnet, which 

 the humid atmosphere of Cornwall has failed to destroy. 



The next sample was obtained from the Blackhead, Lizard. 

 The rocks of this district I very briefly referred to in a former 

 Journal, but since then I have gleaned much fresJi information 

 respecting them and the district in general. I find that the 

 Porthallow banded gneisses and serpentines N. of the Manacles 

 Point are more highly impregnated with magnetic fluid than I 

 had evidence of 12 months since, a condition I find also in 

 the dark serpentines and steatites a little west of the Black- 

 head ; and fresh samples from the Manacles Point have also 

 revealed a higher percentage of power than the former ones. 



In preparing my slices of rocks for the magnet, a difficulty 

 presented itself which had not previously occurred to such an 

 extent, viz. : the great rise in the percentage of power caused 

 by the friction of the saw; in some cases it raised to 25 per 

 cent, above its normal strength, and after a repose of a day or 

 two, tbe power would be found to have considerably relapsed, 

 but would again, after a more extended period, regain their 

 normal strength ; this is undoubtedly throwing a new light on 

 the subject. The slices of Polyphant stone were cut off with a 

 tooth-saw, which did not produce such an amount of friction as 

 would occur in a harder stone cut with a toothless saw and emerj', 

 therefore I did not test for a rise or fall as in the other samples. 



The following table shows the rise and fall in the Magnetic 

 affinity, by friction in sawing, of the St. Just Hypersthene rock. 



After a further repose of 12 months the average affinities 

 ■\7ere the same. 



