SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 31 L 
It now remains to investigate the question whether any connection can be established 
between these and their geological characteristics. 
It is well known that certain varieties of crystalline rocks are often more or 
less magnetic, and that when they are permanently magnetised the poles are sometimes 
very irregularly distributed. 
We have, however, thought it worth while to investigate the magnetic state of 
some pieces of diorite and basalt, brought from Malvern and Canna respectively. 
The fragments were cut into small rectangular blocks, delicately suspended and 
tested. We have to thank Dr. Hoffert, Demonstrator in the Phj^sical Laboratory 
of the Science Schools at South Kensington, for undertaking this part of the work. 
The observations were very tedious, and were, for the most part, made by Messrs. 
Gray, Anderson, and Wilkinson, students in the laboratory. 
Of several blocks from Malvern only one showed any polarity in its natural state, 
but when placed between the poles of an electromagnet it became magnetised by 
induction, so that the time of oscillation was reduced from 72* to 56*. The stone 
brought from Canna was part of a basaltic column and its upper, lower, east and 
west ends were marked. It showed polarity, but the upper end was a north-seeking 
pole, so that it was magnetised in a direction opposed to that which would be induced 
by the magnetic field of the earth. The moment due to the permanent magnetism 
was calculated by three different methods, viz. : (1) by the difference of time of 
oscillation when the direction of the field (about twelve times as strong as that 
of the earth) was reversed, (2) by the deflection when the stone was placed 
E. and W., and (3) by the difference of times when the position of the stone was 
reversed. The results obtained were 
•0023 -0027 and ’0019 C.G.S, unit. 
The periods of oscillation were 93® and 86® in the earth’s field and the artificial 
field above described. The general conclusion arrived at was that, as the volume was 
about 1 C.C., the permanent intensity of magnetisation was about 0'002 C.G.S. unit, 
and that in a field of strength F the induced intensity was about O'OOIS F. 
We have examined the relations between the magnetic disturbing forces and the 
geology of the area of the survey by means of a geological map which has been 
specially prepared for us under the kind superintendence of Professor Judd (Plate XIV.). 
Details are disregarded, but the principal masses of basaltic and non-basaltic crystal¬ 
line rocks and the main groups of the sedimentary formations are clearly distinguished 
from each other. This may be compared with Plate XIII., in wdiich the magnetic 
disturbing forces and the ridge and valley lines are shown. 
As the ridge lines are drawn according to an arbitrary rule, they are only intended 
to draw attention to the districts in which loci of attraction probably exist. We 
have no real knowledge of their distances from the stations between which they run. 
In the geological map, therefore, the stations on each of the principal ridge lines have 
