314 
MR. A. W. RUCKER AND DR. T. E. THORPE ON A MAGNETIC 
On comparing the geological map with Plate XIIL, on which the disturbing forces 
are shown for the whole country, a difficulty arises from the fact that the Vertical 
disturbing Forces appear, on the whole, to be greater over the districts in the east and 
south of England, where the later sedimentary lucks occur, than over Wales and 
Ireland, where, from tlieir absence, it would primd facie appear probable that the 
downward attraction would be greater. 
Of course many hypothetical explanations could be offered of the fact, such as that 
the primary rocks in England might possibly contain larger cjuantities of ferruginous 
matter, &c., but we must be content with observing that if a fairly uniform increase in 
the disturbing Vertical Force were to take place from east to west, it is very doubtful 
whether we should have detected it. Probably it would cause a disturbance of the 
first order, the terrestrial lines would be deflected, and the disturbances at distant 
points would not be comparable. 
If the true Vertical Force isomagnetics could be prolonged beyond Wales into 
another district in which the tertiary strata re-appeared, a southward trend in Wales 
would indicate an increase in the force. On looking at Plate VIII. we think it wall be 
admitted that the slope of the lines of equal Vertical Force is greater in Wales than 
in England, which is in harmony with the existence of a southward bend, but the 
fact that they are extremely irregular and terminate on the west in the sea makes 
any certain deduction impossible. We must, therefore, regard the distribution of the 
Vertical Force disturbances as presenting some difficulty, and must emphasise the 
necessity of using them only to compare neighbouring stations. 
IVie Causes of Local and Regioncd Magnetic Forces. 
It lias long been known that distortions of the isomagnetics occur chiefly in the 
neighbourhood of crystalline rocks, and it has been generally assumed that this is due 
to so-called rock magnetism, the rocks being magnetised either permanently or by 
the inductive action of the earth’s field. 
Er, Naumann, in the work already quoted (‘ Die Erscheinungen des Erdmag- 
netismus,’ Stuttgart, 1887), has recently opposed this view. 
The arguments which may be brought against it are : (1) That rocks brought from 
considerable, depths do not exhibit magnetic qualities until they have been for some 
time upon the surface ; (2) That the effects which rocks or mountains produce on the 
magnet, even if very great when the distance is small, diminish so rapidly as the 
distance increases that they are quite insufficient to account for the widespread effects 
which are attributed to them; (3) That extensive local magnetic disturbance is 
associated rather with geological faults than with the presence of igneous rocks; 
(4) That the cause of the phenomena is to be looked for in the effects produced on 
