650 
MR. A. W. RtrCKER AND DR. T. E. THORPE ON A MAGNETIC 
relative importance must be greater, and it is more likely that it will be closely 
associated with a locus of attraction. In this case, however, difficulties of an 
opposite kind are encountered ; the local disturbances being so great that, although 
ridge lines may be traced up to the mass, it is difficult to follow them across it. 
All therefore that can be expected is that we should be able to show that far- 
reaching underground extensions of iDasaltic rocks would be competent to produce 
the observed disturbances, and that in the case of very large exposures of basaltic 
rocks the regions in which they occur are in general clearly marked centres of 
attraction. 
We will take these points in order. 
The first has been dealt witli in the paper already referred to, but the application 
of calculation to the case of Antrim will serve as an additional example. 
The basaltic surface of North-East Ireland may be roughly represented by a square, 
each side of which is 50 miles long. The length from north to south is somewhat 
greater, and the breadth less than 50 miles. It forms the centre of a magnetic 
district which extends some 50 miles from the edge of the basalt. 
The calculations made above are not applicable when the distances invmh’ed are so 
ofreat that the curvature of the earth has to be taken iuto account. In this case, 
however, they are sufficiently accurate to illustrate the principle. 
The thickness of the Antrim basalt after denudation is only about 1,200 feet. 
Disturbances directly due to the visible mass would therefore be unimportant a very 
few miles from its edge. 
Even if we supposed the basalt to extend from the surface to the depth at which 
it would cease to be magnetic (say 12 miles), the ratio of the thickness of the block 
to the length of an edge would not exceed one-fourth, and it has been shown that 
such a mass would produce no effect at a distance from the neai’est face equal to the 
length of an edge, that is, at 50 miles, whereas at Letterkenny and Stranorlar, which 
are about 40 miles distant, the attractive Forces, though not large, are appreciable. 
We can explain, therefore, the Forces at these places by their proximity to Antrim 
only on the assumption that beneath the basaltic districts there is a magnetic mass, 
the upper face of which is separated from, but approaches relatively near to the 
visible mass (say, within a mile or a mile and a half) and slopes away from it towards 
the west. 
It would then approximately fulfil the conditions assumed by the second 
calculation given above. Its upper surface would be 6 miles below the sea level at 
50 miles from the edge of the visible basalt, but the Horizontal Force would be 
0'00036 instead of only 0'00007 C.G.S. unit. As a matter of fact, the Horizontal 
Disturbance at Letterkenny is 0'00035 and at Stranorlar O'OOOIl C.G.S. unit. The 
close agreement between these numbers and the calculated Force is, of course, 
accidental, but they are sufficient to show that, although masses with steep sides and 
of moderate thickness would produce very small Forces at distances from these edges 
