MAGNETIC EOCKS OF CORNWALL. 



281 



both from St. Just in the west, and Polyphant in the east, 

 whose magnetic powers far surpass that of the Oanna stone, as 

 will be seen by the following table: — 



Percentage 

 of Affinity 

 for Magnet. 



13-4 



11-4 



7-6 



6-7 



5-9 



Botallack 



Polyphant 



Oanna 



Cataeleus 



Blackhead, Lizard . . 



I selected from my collection of Cornish rocks 4 samples 

 (from which I cut 4 slices of each) and tested their various 

 affinities, the results of which is appended. 



The St. Just sample I obtained from Capt. James Bryant ; 

 it is from the celebrated Botallack mine, and was raised from 112 

 fathoms below the sea. This class of rock passes down obliquely 

 from the lichen-covered ridges of the hillside, far out beneath 

 the sea, and its magnetic power has hitherto baffled all attempts 

 of the engineer to work out his explorations by the aid of the 

 compass only.* 



The sample of Polyphant stone was obtained from one of 

 the quarries, near Launceston, about 20 ft. from the surface. The 

 magnetic affinity of the rock has, I believe, been hitherto 

 unknown, the quarries being situated several miles inland, far 

 away from the mariner's course, and in a part where mining 

 operations are very rarely conducted. In connection with this 

 rock, I found that in the shallower parts of the quarry much of the 

 magnetites, through atmospheric influence, were changed into 

 amatite, therefore in many cases it had lost much of its influence 

 on the magnet. 



Fresh samples of the Cataeleus stone (from the neighbour- 

 hood of Padstow) are very difficult to obtain, it being a long 

 time since any of it has been quarried for building purposes (that 

 used for road-metalling is a different material) ; I was therefore 

 compelled to fall back for my test sample on fragments of the 



* A sad catastrophe has recently occurred at Wheal Owles Mine, St. Just, by 

 the tapping, at a great depth, of an old mine adjoining, thereby flooding Wheal 

 O-wles and drowning 20 of the miners. I have but little doubt that this sad mishap 

 will some day be traced to the miner having been misled in his explorations by 

 the deflection of the Magnetic Needle, 



