518 Prof. Riicker. On the Magnetic Permeability of 



groups, according as they were () non-magnetic, (/3) magnetic but 

 not basaltic, (7) basaltic. 



Of the first group, we tried a number of specimens, many of which 

 were a, priori certain not to be magnetic. Some of them, however, 

 were just as likely to be conductors as the magnetic rocks ; and the 

 fact, therefore, that they have been tried, and produce no effect, 

 strengthens the view that i;he measurement of the permeabilities was 

 not affected by the conductivities of the specimens. Among those 

 submitted to experiment were limestones, sandstones, mica- and horn- 

 blende-schists, granite with tourmaline, red granite, trachyte, felsite, 

 rhyolite, gabbro, muscovite granite, luxullianite, various diorites, and 

 haematite. 



Two specimens of Archrean gneiss, brought by Dr. Thorpe from 

 Loch Maddy, in the Outer Hebrides, were found to be practically non- 

 magnetic. 



We have also, tthrough the kindness of Professor Judd, had the 

 opportunity of testing the specimens of Silurian rocks and red 

 sandstones obtained from the Palaeozoic ridge by deep borings near 

 London. Fragments from Kentish Town, Richmond, Meux's 

 Brewery, and Ware were tried and found to be non-magnetic. 



Turning next to specimens of other than basaltic rocks which were 

 found to be magnetic, we obtained the following results : 



Phonolite "00070 



Dolerite 94 



Trachyte . . ' 39 



39 

 24 

 104 



Meiaphyre 

 Tourmaline granite 

 Syenite 



be noticed that several of these are rocks of the same kind 

 as those of which other specimens were found to be non-magnetic. 

 This is an example of the fact that the permeabilities of different 

 portions of the same rock are very various, and that no conclusion 

 can be drawn unless a large number of specimens have been 

 examined. 



Special attention having been given in the recent magnetic survey 

 to the magnetic disturbances produced by the Malvern Hills, it was 

 thought that a detailed investigation of their magnetic properties 

 would be interesting. Mr. Highfield, therefore, paid a visit to 

 Malvern for the purpose of collecting specimens. The position at 

 which any specimen was found was marked on the spot on an 

 Ordnance map carried for the purpose. It is, perhaps, hardly neces- 



