Rocks and Regional Magnetic Disturbances. 533 



the two attractions exerted on opposite sides of the hills towards 

 them. 



Observations have been made at four stations, two (Great Malvern 

 and Malvern Wells) on the eastern, and two (Mathon and Colwall 

 Green) on the western, side of the hills. Great Malvern and Mathon 

 are near the north end of the range, and the sum of the two attrac- 

 tions is 0-00243 C.G.S. unit of force. Malvern Wells and Colwall 

 Green are near the middle of the range, and the sum of the attractions 

 is O'OOllS. It will thus be seen that at what would, primd facie, 

 have appeared the most favourable position the forces are smaller, 

 but the result accords with the fact that the specimens collected at 

 the north end of the range contained the largest quantity of magnetite. 

 Sections across the range have been published by the Geological 

 Survey, and from these it seems that on the eastern side the wall of 

 igneous rock is nearly vertical, while on the western side it slopes 

 more gradually under the sedimentary rocks, and if continued as far 

 from the range as our stations (about a mile and a quarter, or 2 kilom.), 

 it must be at a depth of 2000 feet, or, say, 600 metres. Assuming, 

 then, that our stations were near the edge of the horizontal extension 

 of the igneous rocks, and that the latter extend north and south for 

 8 miles (13 kilom.), I find that the sum of the attractions on opposite 

 sides of the range near its centre, and at points as distant from it as 

 our stations were, is 0*00291, 0*00154, or 0*00067, according as we 

 assume the mean permeability to be 



(1) That of the three most favourable rock specimens ; 



(2) That of all the specimens which possessed measurable perme- 



abilities ; 



(3) That of all the specimens obtained. 



If these are compared with the observed forces, viz., 0*00243 and 

 0*00118, it is seen that, while they are of the right order of magnitude, 

 an exact numerical agreement could only be obtained if we supposed 

 the mean permeability of the range to be somewhat greater than 

 that of its surface, as judged by the specimens collected by Mr. 

 Highfield. 



The results may, I think, be considered to support the view that 

 the igneous rock extends laterally at a moderate depth from the 

 surface, at least on the western side of the hills, to a distance of a 

 mile or a mile and a quarter, but probably not much more, from the 

 axis of the range. 



In conclusion, then, I am anxious that the purport of the calcula- 

 tions above described should not be misunderstood. In particular, I 

 do not attempt to specify the depth at which magnetic matter exists 

 where none appears on the surface, in the east of England or else- 

 where. But, in spite of this uncertainty about every detail, the 



