NOTES ON THE LIZARD ROOKS. 397 



grains of bright iron, the process removed the dark margin of 

 magnetite that might be visible in a slide, and left the strong 

 bright core, which was malleable, and could be flattened with the 

 hammer. 



The Magnetism of the Lizard Rooks. 



The bad repute of the Manacle rocks first led me, out of 

 curiosity, to apply the magnet to them, while preparing my 

 slides for microscopic work, and then my study of the Lizard 

 district induced me to test the magnetic influence of the rocks 

 along the principal parts of the coast, from Porthallow to near 

 the Lizard Point (Penvose), and the results are so widely 

 different, I consider them worthy of record. 



The first means employed in endeavouring to arrive at the 

 quantities of magnetism at the various points, was by suspending 

 a bar magnet (12-in. long x lj-in. wide, J-in. thick) with a silk 

 thread, over a card arranged similar to that of the mariner's 

 compass. The samples tested were of unequal sizes, and 

 engendered a suspicion that this was the cause of the very varied 

 affinities indicated, and no percentage of power being properly 

 arrived at, the results I considered unsatisfactory. 



Another mode was then adopted, viz : by cutting thin slices 

 of the rocks, about 2-ins long and f-in. wide, weighing them 

 accurately with a fine balance, and while each slice was in the 

 scales, fixing the magnet horizontally a short distance over, until 

 the slice immediately raised to it ; while adhering, the scales 

 were raised again to an even beam, the weights removed from 

 the opposite pan until the affinity between the slice of rock and 

 magnet was overpowered. By this means I am in a position to 

 make the following assertions. 



The Manacle rocks, of ill repute with mariners for their 

 supposed magnetic influence, have no title to such a character, 

 their affinity for the magnet being only equal to 1 ton in 3,000. 

 The olivine gabbros of Dean Point and Coverack, the brown 

 serpentines of the Black Head, and the black serpentine of 

 Cadgwith, as will be seen from the illustration herewith, were 

 the principal deflectors, the black serpentine having an attraction 

 equal to 1 ton in 19, or over 155 times greater than the 

 rocks at the Manacle Point 



