SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. G39 
magnetic ridge lines are indicated to which we refer in the present discnssion. It is 
easy to see where the others occur by comparing with Map 13. 
We have to thank our colleague, Pi’ofessor Judd, Sir Archibald Geikie, 
Director-General of the Geological Survey, and the late Mr. Topley F.P.S., for 
much valuable advice and assistance. 
Magnetic Disturbances '^produced by Dykes, Trap Sheets, and Isolated Masses of 
Basalt. 
There are two principal types of basaltic plates to be considered, viz. ; vertical 
dykes and sheets, which, like the great Whin Sill, are nearly liorizontal. 
We have examined four specimens of basalt from the Boulmer Dyke, north of 
Alnmouth in Northumberland, and find the average susceptibility to be O’OOIS, 
which is nearly the same as that of the Mull rocks (O’OOIG). 
The Boulmer Dyke is vertical and is about 100 feet wide. 
Assuming these data, it is easy to show that even if the out-crops of the dykes 
and sheets were infinitely long, and if the vertical dimensions of a vertical dyke, and 
the horizontal extension of a horizontal sheet were infinite, their magnetization by 
the earth’s field could not produce Disturbing Forces which could be detected beyond 
a couple of hundred yards from their edges. 
Thus, if the upper or horizontal surface of a rectangular vertical dyke, of suscepti¬ 
bility O'OOIG, and 100 feet wide, were 25 feet below the surface, the Forces due to it, 
expressed in terms of 0*0001 metric unit, would be as follows :— 
Distance in feet 
from line on surface 
vertically above 
centre of dyke. 
Disturbing Forces.* 
Horizontal. 
Vertical. 
0 
0 
329 
50 
208 
202 
150 
98 
10 
250 
59 
350 
42 
450 
33 
If the planes of the sides were in the magnetic meridian, or if they were 
perpendicular to the magnetic meridian, but inclined to the horizontal at the angle of 
dip, the above would be the Disturbances produced. 
The Vertical Disturbance would become too small to be detected only 100 feet 
* It will be remembered that in terms of the units employed in this table 30 i.s a very small 
Horizontal Distui'bance, and that two independent determinations of the Vertical Distui’bance differ on 
the average by 60. 
