SURVEY OE THE BRITISH ISLES FOR THE EPOCH JANUARY 1 , 1891 . 647 
Hoi'izontal Disturbance. 
Depth 
Distance from nearest 
edge. 
face. 
= 0-U016. 
-v- = 0'0024. ' 
I 
1 
13 or — 1 
18 or - 2 
1 
2 
X 
2 
.51 or 9 
76 or 13 
1 
4 
1 
5 or — 2 
7 or - 3 
4 
24 or — .3 
36 or — 5 
The two values given in each case are for corresponding positions on opposite sides 
of the mass. The Forces due to the horizontal faces are in each case the same, but 
those pioduced by the vertical faces change sign according as the point is magnetic 
north or south of the rock. 
If we suppose that instead of being vertical the nearer face of the rock has a slope 
of 1 119 8, the Forces at distant points are increased. One example will suffice. 
Depth 
Distance from nearest 
edge. 
face. 
% 
Horizontal Disturbance. 
1 
4 
1 
24-1 
36-1 
It is evident from the tirst set ot figures that the Forces at distances such as those 
for which the calculations are made, diminish very rapidly with the vertical thickness 
of the plate. 
thus, if a mass of basalt, a mile and a half square, of susceptibility 0‘0024, were 
three-eighths of a mile, i.e., say 2000 feet thick, the Horizontal Disturbance a mile 
and a half from it would be quite negligible, amounting to only 0-00007 O.G.S. unit 
towards the rock on one side, and 0-00003 awai/ from it on the other. 
If the susceptibility were not greater than that of the Mull specimens, the columnar 
mass might be 4000 feet deep, and yet produce at points a mile and a half distant 
Horizontal Disturbances far below our limit of the accurate determination of direction. 
It will be remembered that this limit is 0-00030 C.G.S. unit, whereas the forces in 
question are 0-00013 and 0-00001 unit. 
These calculations throw light upon observations made near Titterstone Hill in 
Shropshire, as they explain a negative result obtained near to it. The hill consists 
of two masses of basalt, of which the larger is about two miles long and one mile 
wide. 
Basaltic pipes or feeders may extend downwards to an indefinite depth, but the 
sheet is of small thickness and has been pierced by the shafts of coal mines. 
