SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 017 
of the Caledonian Canal is predominant and is finally connected with the basalts of 
Mull, while in the northern region its effects are complicated by its intersection with 
another line, which connects it with the basalts of Skye on the one side, and with the 
well-defined Elgin centre on the other. 
Pursuing the course of this transverse ridge line eastwards from Loch Duich, we 
find that it traverses Skye, The investigation of so highly disturbed a region is full of 
difficulties, and observations made at Portree (“ 1890 Memoir,” p. 123) prove that on the 
basalt itself the local overpower the regional attractions. There are, however, places 
on the shores of Skye and on neighbouring islands at which it is possible to 
plant the tripod on patches of Jurassic or Silurian rocks. These we have selected for 
our stations, and in consequence of this precaution, the results are fairly concordant. 
In the first place they indicate a region of very high Vertical Force between 
Balmeanach and Broadford, towards which the Horizontal Disturbing Forces point 
east from Balmeanach and Loch Sligachan, south from Broadford and a number of 
neighbouring places, and north from L. Slapin. Though the Vertical Disturbances 
are very great, reaching OT metric, or O'Ol C.G.S. unit at Balmeanach, the region of 
high Vertical Force is very narrow, stations of powerful negative Disturbance 
bordering it closely both on the south and north. The southern valley is itself very 
narrow, and separates the ridge from the basalts of Ganna and Eigg, towards which 
tend the Horizontal Disturbing Forces on the valley line stations, such as Soa and 
Ord. The Horizontal Disturbing Forces at Ornsay and Sandag Bay indicate a peak 
to the south of these stations. It may be questioned whether this is an outlier, but 
if not, the ridge line is curved, as shown in ]\Iap 13. 
The distribution of the Vertical and Horizontal Forces at Loch Bay and in the 
Hebrides suggests tliat the ridge line we are tracing passes westward from Balmeanach 
across the Minch. It might be urged that if the ridge runs to the Hebrides as we 
have shown, the Horizontal Force at West Loch Tarbert ought to be directed towards 
it. Inasmuch, however, as its magnitude is only 0'0004 metric or O'OOOOI C.G.S. 
unit, which is less than the average difference between two independent experiments, 
we know practically nothing of its direction. L. Staffin appears to be an anomalous 
station. 
On the whole then, in spite of some doubt as to the minute constitution of some 
of its part.s, we think that the fact that a region of high Vertical Force runs across 
Scotland, from the Hebrides to Skye, Loch Ness and Elgin, is proved, and that the 
apparent divergence of tlie Great Glen magnetic ridge from the line of the fault is due 
to the complications introduced by its intersection with this important cross range. 
Passing next to the southern part of the Caledonian Canal ridge, we find that it 
runs near to but a little to the west of the line of the Canal. It passes between 
Kinloch Eil and Camas-nan-Gall. As these stations are only about six nulos apart, 
its position at this point is very accurately known. 
The Vertical Forces at all the stations near Ben Nevis and Loch Ell are very high 
MDCCCXCVT.—A. 4 k 
