SUEVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 059 
few square miles in area, produce no noticeable magnetic effects at distances 
comparable with their linear dimensions. 
(3.) Theory leads us to expect that the^eftects of faults would be irregular. They 
would only correspond to ridge lines in special cases. There may be large positive 
or negative Vertical Forces in their neighbourhood. If the fault is inclined at a 
considerable angle to the vertical, or if the depth of the magnetic rocks is four or live 
times the throw, little or no effect will be produced. 
Experiment show's that ridge lines are certainly associated with the great faults in 
Scotland and with a fault in Ireland, but that there is no clear connection between 
them elsewhere. 
(4.) Large masses of basalt, such as those which occur in Skye, Mull, Antrim, the 
Scotch Coal Field, and North Wales, are always centres of attraction. 
(5.) Non-basic igneous rocks often produce little or no effect, but sometimes, and 
notably in the cases of Connemara and of the Cheviots and Berwickshire District, they 
are centres of attraction. In both these regions basic rocks occur. 
(G.) In no case does a large igneous mass repel the north pole of the needle. The 
only case in which the suspicion of such repulsion could arise is that of the Wicklow 
Mountains, and the facts would there be equally well explained by supposing the 
magnetic effects of the granite to be negligible. 
(7.) Ihe magnetic ridge lines often occur in districts where there are no magnetic 
rocks on the surface. They appear to be unaffected by the distribution of surface 
rocks ol later date than the coal measures, but there are a number of cases in which 
they are associated with an anticlinal arrangement of the older rocks. To this the 
lower Carboniferous formation of Devonshire is an exception. 
The sixth of these conclusions is of great theoretical importance. If the rocks are 
magnetized by induction they would attract the north pole of the needle. If they 
merely deflected earth currents, there seems to be no reason why the eddies thus 
produced should always circulate in the same direction, which would be necessary 
if they were the cause of the attraction which is always in play in the neighbourhood 
of the rocks. 
Future Applications of the F^esults of the Survey. 
The positions of the isomagnetic lines in the future may be calculated by means of 
Tables XIV., XX., XXL , and XXII. If the epoch for which they are to be determined 
is 11 years alter January 1, 1891, the numbers in Table XIV. must be multiplied 
by 0'2 X 11 and added algebraically to the corresponding numbers in Tables XX., 
XXL, and XXII. 
The values so obtained may be checked by deducing from them the values of the 
elements at the principal magnetic observatories and comparing them with those 
given by experiment. 
As regards the ridge lines, we believe that our survey affords a basis by means of 
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