312 
MR. A. W. RUCKER AND DR. T. E. THORPE OH A MAGHETIC 
been connected Ijy lines, the spaces enclosed have been shaded, and thus the districts 
within which the main loci of attraction probably He are clearly indicated. 
It must be remembered that the outlines of these districts depend largely on the 
accidental positions of stations which were selected without reference to them, and 
that we must rather expect such rough indications of relations between magnetic 
and geological facts as may serve to guide future investigations than complete 
and unmistakeable harmony. 
When regarded from this point of view, however, we think the results are very 
suggestive. 
In the Highlands one region of attraction encloses the Caledonian Canal, another is 
evidently in close relation with the basaltic masses in Skye, Glenmorven, and Mull, 
though, as has been pointed out, the main centre of attraction appears to be to tlie 
west of these islands. 
A third region in Scotland encloses the basaltic rocks of Arran and of the Scotch 
Coal-fields. The fact that the Fifeshire basalt lies outside it is probably due onl}" to 
the accidental circumstance that we have no stations between Stirling (47) and 
Dundee (21). 
A fourth region evidentlv consists of the Antrim basalt. There is a fifth in 
O 
Connemara, where the rocks are granite. 
With regard to North Wales, if we consider it as forming one district with Shrop¬ 
shire, we see that a line drawn through the centre of the basaltic rocks would first 
run from east to west, then nearly north, and finally turn to the west between 
Anglesea and Carnarvonshire. The district is thus very irregular and the ridge lines 
do not give much information, but we must point out that the Horizontal Forces at 
the stations which border it all tend inwards towards the axis above suggested. This 
is true of Holyhead (90), Llandudno (106), Llangollen (107), Shrewsbury (138), 
Aberystwith (55), and Pwllheli (128). 
If this suggested relation is hereafter verified, every considerable mass of basaltic 
rock in the kingdom will be closely connected with a region of magnetic attraction. 
Of smaller masses it is to be noted that at Falmouth (80) the disturbing force acts 
southward toward the serpentine of the Lizard, and that the relatively small masses 
in Pembrokeshire and Wexford are within another region of attraction. Our 
measurements do not assign any particular inpaortance to the outcrop of basaltic rock 
near Limerick (185) nor to the dykes in the north of England. It is, however, 
curious that a line drawn through Melton Mowbray and the Wash peak (indicated 
by circles) passes towards Wales through the only basalt in the Midlands. 
Taking the evidence as a whole, we think we are justified in saying that large 
masses of basaltic rock indicate regions of magnetic attraction. 
The other crystalline rocks appear to be much less important magnetically. Thus 
the Malverns, though a strong local centre, do not disturb a district of any consider¬ 
able magnitude. If the effects of the two classes of rocks when they appear on the 
