612 
MR. A. W. RtlCKER AND DR. T. E. THORPE ON A MAGNETIC 
distinguish betweGii a gradual decrea,sG of the magnetic permeability and a gradual 
increase of the distance V)etween the magnetic rocks and the surface of the earth. 
Enough has perhaps now been said to show that the form of the magnetic contour 
surface (wdiich is directly deduced from our work) must be interpreted with discrimi¬ 
nation, but, if this is clearly understood, we think that the connection between the 
surface phenomena and the forms of the sub-terrestrial magnetic rocks is probably 
sufficiently close to make a nomenclature based upon it, not only the most convenient, 
but also the most suggestive. 
Tn a considerable number of cases, partly discussed in our previous Memoir, partly to 
be treated of hereafter, lines or centres of attraction are connected with the appeal ance 
of magnetic rocks on the surface, or with an anticlinal arrangement of the sedinientai j 
surface rocks, which may be accompanied by a fold in the magnetic rocks below. 
If apjDi’osiIllation to the surface is the predominant cause, w^e should expect that the 
Horizontal Disturbing Forces wnuld be directed to the regions of maximum vertical 
disturbance. This condition is almost universally fulfilled. There is only one region 
(that near Reading) wdiere the distribution of the forces suggests an underground 
table or plateau of magnetic rock, as distinguished from the peaks observed elsewhere, 
and even there the vertical disturbances are very large in the central region. 
Secondly, wm think that the magnitude of the magnetic districts, which are often 
from 50 to 100 miles in width, suggests rather the folds of a primaival rock than 
changes in its nature. It is, of course, theoretically possible that a giadual inciemeut 
in the percentage quantity of magnetite or of iron might continue over an equal 
distance, and it is a matter for geologists to decide whether it is a more probable 
cause than a slow^ change of level. On the other hand, wm may remark that, although 
the Vertical Disturbing Forces can be explained by the existence of underground 
magnetic rocks, similar to those which occur on the surface, there is more difficulty in 
accounting for the Horizontal Disturbing Forces, unless the hidden rocks are more 
magnetic than those wdiich are exposed to view’. 
If, how^ever, all these cautions are borne in mind, the use of the magnetic contour 
surface is so convenient, that wm shall borrow’ terms freely from topography. In 
general wm shall prefix the word magnetic to emphasize the sense in wdiich the terms 
are employed, but sometimes this may be superfluous. 
We proved in our first survey that the Horizontal Disturbing Forces tended 
towards lines of maximum Vertical Disturbing Force ; and, assuming this result, we 
may therefore use the terms ridge and valley lines, and similar expressions, in some- 
wdiat more restricted senses than heretofore. Magnetic ridge and valley lines aie 
such that, if wm cross them at right angles, we pass through a region of maximum or 
minimum Vertical Disturbing Force respectively. In general. Horizontal Disturbing 
Forces tend towards ridge lines. 
Though, as has been shown, this is not a necessary consequence of the theoit of 
magnetic rocks, it is so universally obeyed that stations at wdiich the rule is departed 
from may, in this respect, be considered anomalous. 
